Aerodynamic Development of the Buckeye Bullet 3 Electric Landspeed Vehicle

Home , Center of mass, Land speed record, List of cycling records

A Thesis

Presented in Partial Fulfillment of the Requirements for the Degree

Master of Science in the Graduate School of The Ohio State University

Carrington Bork

Mechanical Engineering Graduate Program

The Ohio State University

2012

Master’s Examination Committee:

Dr. Giorgio Rizzoni

Dr. Mei Zhuang

©Copyright by

Carrington Bork

2012

Abstract

For almost two decades The Ohio State University’s electric motorsports teams have been winning races and setting records. The OSU student designed Buckeye Bullet was the first electric car to reach speeds in excess of 300 mph. The Buckeye Bullet 2 was the first hydrogen powered car to break 300 mph and the team would eventually hold the record for the world’s fastest electric car with the Buckeye Bullet 2.5. The team of students is now setting its sights on reaching over 400 mph using advanced lithium ion batteries in a completely new vehicle, the

Buckeye Bullet 3. At these high speeds aerodynamics play a crucial role in determining the vehicle performance and safety. This thesis presents the aerodynamic development of the

Buckeye Bullet 3 and its role in shaping the future of land speed racing.

Dedication

This thesis is dedicated to all the hard working individuals that have been a part of the

Buckeye Bullet 3 team. Without you this would not be possible.

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Acknowledgments

The work presented in this thesis has had contributions and support from a number of individuals and organizations. Foremost, I must acknowledge the dedication and resourcefulness of all the Buckeye Bullet Team members past and present. I am constantly amazed that a group of students is allowed and able to design and build electric cars that push the envelope of electric vehicle technology. The dedication of these team members is what makes this program, and subsequently this thesis, possible. I have learned a tremendous amount about vehicle design, project management and teamwork from my fellow team members and through my involvement in this program. I am thankful that I have had to opportunity to take part in this endeavor.

TotalSim LLC has been a huge contributor to this work. When I started my master’s program I knew virtually nothing about CFD or aerodynamics. TotalSim was willing take me on as an intern and train me how to use OpenFOAM and their proprietary software. The engineers at TotalSim were instrumental in helping me setup my model for the Buckeye Bullet and in general giving advice on vehicle design and analysis methods.

The Ohio Supercomputing Center has kind enough to allow us to use their facilities for running our simulations. They have supported the program from the beginning and continue to offer their services.

Venturi Automobiles has been the primary monetary sponsor for the program since the

BB2.5. They have had a strong input on the development of the new vehicle from the start.

They have graciously trusted our team to develop a new electric landspeed vehicle and worked with us to develop a driveline to power it.

Vita

August 2, 1985………………………………………………………. Born – Cleveland, OH. USA

2006-2007…………………………………………………………….. Underbody Design Coop Honda R&D of America

2005-2009…………………………………………………………….. Member, Buckeye Bullet 2 Team Center for Automotive Research

March 2009…...... B.S. Mechanical Engineering Ohio State University

2009-2010…………………………………………………………….. Integration Engineer Rolls-Royce Energy Systems

2010-2012…………………………………………………………….. Member, Buckeye Bullet 3 Team Center for Automotive Research

2010-2012…………………………………………………………….. CFD Engineer TotalSim LLC

Fields of Study

Major Field: Mechanical Engineering

Table of Contents

Abstract ...... ii

Dedication ...... iii

Acknowledgments ...... iv

Vita ...... vi

Fields of Study ...... vi

Table of Contents ...... vii

List of Figures ...... x

List of Tables ...... xiii

Chapter 1 Introduction to Electric Land Speed Racing ...... 1

1.1 Project Background ...... 1

1.1.1 The Buckeye Bullet ...... 2

1.1.2 The Buckeye Bullet 2 ...... 3

1.1.3 The Buckeye Bullet 2.5 ...... 5

1.1.4 The Buckeye Bullet 3 ...... 7

1.2 The Bonneville Salt Flats ...... 7

1.3 Thesis Motivation ...... 9

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Chapter 2 General Vehicle Information ...... 11

2.1 Project Goals ...... 11

2.2 Vehicle Overview ...... 13

2.2.1 Driveline ...... 13

2.2.2 Battery System ...... 22

2.2.3 Suspension and Steering ...... 26

2.2.4 Brake System ...... 28

Chapter 3 Analysis Methods ...... 33

3.1 CFD Model Setup ...... 33

3.1.1 Introduction to OpenFOAM ...... 34

3.1.2 snappyHexMesher ...... 34

3.1.3 Solver Information ...... 38

3.1.4 Computing Resources ...... 39

3.2 Correlating Results ...... 40

3.2.1 BB2 Wind Tunnel Testing ...... 40

3.2.2 Correlating Results with BB2 Suspension Data ...... 42

3.2.3 Limitations of Wind Tunnel Testing and CFD Testing ...... 51

3.3 Aerodynamic Performance Goals ...... 53

Chapter 4 BB3 Aerodynamic Design ...... 57

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4.1 Vehicle Packaging ...... 57

4.2 Yaw Stability ...... 67

4.3 Vehicle Ride Height ...... 73

4.4 Vehicle Design Features ...... 80

4.4.1 Wheel and Wind Deflectors ...... 80

4.4.2 Tail Geometry ...... 82

Chapter 5 Conclusion ...... 84

5.1 Summary of Results ...... 84

5.2 Future Work ...... 87

Bibliography ...... 89

List of Figures

Figure 1.1: The Buckeye Bullet ...... 2

Figure 1.2: The Buckeye Bullet 2 on the Bonneville Salt Flats in 2007 ...... 5

Figure 1.3: The Buckeye Bullet 2.5 on the Bonneville Salt Flats in 2009 ...... 6

Figure 1.4: Comparison of US and International Race Courses at the Bonneville Salt Flats ...... 8

Figure 2.1: Driving Force at Wheels (per axle) ...... 16

Figure 2.2: Potential Speed Record Based on Various Maximum Power Levels ...... 17

Figure 2.3: Assumed Motor Power/Torque Curve Shape ...... 18

Figure 2.4: Motor Torque Curve for Direct Acting Motor (without Gearbox) ...... 19

Figure 2.5: Motor Requirements for 1 Speed Gearbox for Various Gear Ratios ...... 20

Figure 2.6: Motor Torque and Speed Requirements for Various Gearbox Configurations ...... 22

Figure 2.7: A123 Prismatic Lithium Ion Battery Modules [4] ...... 23

Figure 2.8: Thermal Testing for Uninsulated Battery Module ...... 25

Figure 2.9: Thermal Testing for Insulated Battery Module...... 26

Figure 2.10: Close-up of Salt Roughness at the Start of Track ...... 27

Figure 2.11: BB3 Brake System Exploded View...... 29

Figure 2.12: BB3 Driveline Unit ...... 30

Figure 2.13: BB3 Braking Strategy ...... 31

Figure 3.1: Meshing with snappyHexMesh ...... 35

Figure 3.2: Use of snappyHexMesh Mesh Refinement for BB2 ...... 38

Figure 3.3: The Buckeye Bullet 2 Wind Tunnel Model ...... 41

Figure 3.4: Drag Distribution for BB2 ...... 42

Figure 3.5: Suspension Ride Height Data From 300+ MPH Run ...... 44

Figure 3.6: BB2 Aerodynamic Force Free Body Diagram ...... 45

Figure 3.7: Tire Growth Model ...... 48

Figure 3.8: Buckeye Bullet 2 at 300 mph ...... 49

Figure 3.9: Lift and Balance Comparison of CFD Results and Experimental Data ...... 50

Figure 3.10: On the Salt "Flow Visualization" ...... 52

Figure 3.11: Salt Build-up in Wheel Well ...... 53

Figure 3.12: Distribution of Vehicle Losses for BB3 ...... 54

Figure 4.1: Comparison of BB2 and BB3 CV Joint Placement ...... 60

Figure 4.2: Driver in Front vs. Driver in Middle Design ...... 61

Figure 4.3: Comparison of BB2 and Initially Proposed BB3 Frontal Area ...... 62

Figure 4.4: Revised Driver in Middle Design ...... 64

Figure 4.5: BB2 Yaw Stability ...... 68

Figure 4.6: BB2 Force X-ray Yawed at 1 degree (side view) ...... 69

Figure 4.7: Cp Slice for BB2 at 1 degree of Yaw (top view) ...... 69

Figure 4.8: BB3 Force X-ray at 1 degree of Yaw (side view) ...... 70

Figure 4.9: Cp Slice for BB3 at 1 degree of Yaw (top view) ...... 70

Figure 4.10: Front Axle Aerodynamic Lift Force and Suspension Force ...... 74

Figure 4.11: Rear Axle Aerodynamic Lift Force and Suspension Force ...... 75

Figure 4.12: Ride Height Determination at 400 mph ...... 76

Figure 4.13: Vehicle Ride Height as a Function of Velocity...... 78

Figure 4.14: Vehicle Lift as a Function of Velocity ...... 79

Figure 4.15: Vehicle Drag as a Function of Velocity ...... 79

Figure 4.16: Streamlines Around Wheel Inside of Wheel Well ...... 80

Figure 4.17: Under Floor Flow Comparison with and without Tire Wind Deflectors ...... 81

Figure 4.18: BB2 Tail Flow Separation ...... 82

Figure 4.19: (left) Vesco #444 Streamliner and (right) Burkland 411 Streamliner Parachute Doors

...... 83

Figure 5.1: BB3 Body Design ...... 87

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List of Tables

Table 2.1: List of Wheel Driven Land Speed Records [2] ...... 12

Table 3.1: Suspension Ride Height Data ...... 46

Table 3.2: Comparison of BB2 Wind Tunnel Results and Vehicle Data ...... 46

Table 3.3: Comparison of CFD Model and Wind Tunnel Model ...... 47

Table 4.1: Driver Position Comparison Summary ...... 66

Table 5.1: BB2 vs. BB3 Comparison ...... 86

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Chapter 1

Introduction to Electric Land Speed Racing

This chapter gives information on the history of electric land speed racing as it directly pertains to this thesis. The focus will be on Ohio State University’s efforts in the area of electric land speed racing. Ohio State has been at the forefront of electric racing for nearly two decades and holds several world records. This chapter also discusses the motivation for performing this research and its relevance to today’s automotive community.

1.1 Project Background

Almost immediately after the inception of the first automobiles, man has raced them. There is an inherent drive to push vehicles to their limits of performance and speed.

One of the purest forms of motorsport is the all-out pursuit of top speed. Known as land speed racing, this motorsport has been populated by big money teams and backyard mechanics alike. In the past decade, a team of engineering students from the Ohio State

University has joined the ranks of these speed-enthusiasts as part of the Buckeye Bullet racing program.

1.1.1 The Buckeye Bullet

The Buckeye Bullet program evolved from the Ohio State University Smokin’

Buckeye team which raced in the Formula Lightning racing series during the 1990s. This series was an intercollegiate series that raced open wheeled battery electric race cars. The series ended in the late 1990s, leaving a group of students with a significant amount of electric racing experience without a project on which to work. This group of students eventually started what has become the Buckeye Bullet Land Speed Racing Team. The original Buckeye Bullet (BB1) was a battery powered electric streamliner. A streamliner is a purpose-built car for land speed racing characterized by a long narrow body with enclosed wheels. The Buckeye Bullet is pictured in Figure 1.1.

Figure 1.1: The Buckeye Bullet

The Buckeye Bullet was initially powered by 12,000 sub-C cell batteries. In later seasons the batteries were upgraded to prismatic NiMH batteries equivalent to that of almost 16 Toyota Prius battery packs. In August 2004 the BB1 became the first electric car to travel faster than 300 mph. The BB1 currently holds the US record under SCTA sanctioning for the fastest electric car at 315.958 mph.

The Buckeye Bullet last raced in October 2004. At the end of the 2004 racing season it was determined that the Buckeye Bullet had achieved its top speed potential given the state of available battery technology at the time. Thus, a new generation of electric land speed vehicle was proposed.

1.1.2 The Buckeye Bullet 2

The Buckeye Bullet 2 (BB2) was the successor to the BB1. This vehicle was an entirely new design and a concept that had never been attempted previously. This was the world’s first hydrogen fuel cell powered land speed race car. Like the BB1, the BB2 was designed and built by students at The Ohio State University with the help of industry supporters. For the project, Ohio State partnered with Ford Motor Company and Ballard

Power Systems, the latter of which supplied the fuel cell modules that powered the vehicle.

Fuel cells were used in the BB2 as opposed to batteries for several reasons. Fuel cell systems were increasing in popularity at the time and were considered to be relevant to the automotive world. Fuel cells systems remain a strong candidate for widespread use in

3 transportation applications in the future. Working with fuel cells offered a new set of technical challenges for the Buckeye Bullet team to overcome. Comparatively, the fuel cell system in the BB2 was far more complex than the battery system used in the BB1.

Additionally, from a performance standpoint, fuel cells offer several advantages over batteries in land speed racing. A battery’s voltage and potential power output is dependent on the state of charge of the battery. During a run the battery voltage and subsequently the available power output decreases as the battery’s state of charge depletes, meaning that the vehicle can expend less power just when it needs it the most: at high speed. For a fuel cell system the power output is effectively constant for the duration of the run.

Additionally, a battery takes time to recharge. Due to the nature of the NiMH batteries used for the BB1, several hours of charging were necessary for a complete charge, whereas, the BB2 could be refueled within approximately 20 minutes. This an important consideration when competing for an international record as two consecutive runs need to be completed within an hour to be considered for a record under FIA guidelines.

The BB2 raced from 2006 to 2009 finally setting an international record for the fastest Hydrogen Fuel Cell powered vehicle in 2009 at 302.877 mph under FIA sanctioning.

An image of the BB2 on the salt flats is shown in Figure 1.2.

Figure 1.2: The Buckeye Bullet 2 on the Bonneville Salt Flats in 2007

1.1.3 The Buckeye Bullet 2.5

At the end of 2009 the Buckeye Bullet team decided to return to the use of batteries for the main power source for the vehicle. This decision was motivated by the rapid progression of battery technology within the decade. After reviewing the state of battery technology it was determined that a battery powered car could now reach much higher speeds than either the

BB1 or BB2. It was clear that an entirely new vehicle would need to be developed in order to reach these higher goals. However, developing an entirely new vehicle capable of reaching over

300 mph is a costly and time consuming effort. Rather than building an entirely new vehicle initially, the team decided to use the proven design of the BB2 as a test bed for potential new technologies that would be used on the future vehicle. In this way the team could gain experience with the new vehicle power system before designing a vehicle around it. The BB2.5

5 was powered by cylindrical cell A123 32113 Lithium ion batteries. Although the BB3 will not use these same batteries, the battery chemistry of the BB3 batteries is essentially the same as that of the 32113s. The primary difference between the batteries is the packaging. The BB2.5 used cylindrical cells, while the BB3 will use prismatic cells. Prismatic cells were favored for reduced weight and better volumetric efficiency. Other than the electrical power source the BB2 and the

BB2.5 were essentially the same (except for a new paint scheme, see Figure 1.3). The BB2.5 raced during the summer of 2009 and set an international record at 307.666 mph. It should be noted that this record is lower than that of the BB1; however, these two records were completed under different rules and governed by different sanctioning bodies. So, even though the BB1 actually achieved a higher speed the BB2.5 holds the record for the world’s fastest electric car.

Figure 1.3: The Buckeye Bullet 2.5 on the Bonneville Salt Flats in 2009

1.1.4 The Buckeye Bullet 3

The Buckeye Bullet 3 (BB3) is the future of electric land speed racing. Initial design began in the summer of 2010 with the intention to race for the first time in the summer of 2013.

The design will be discussed in further detail in the following chapter.

1.2 The Bonneville Salt Flats

The Bonneville Salt Flats in Utah are the mecca for land speed racing. The salt flats are a naturally occurring surface created by an ancient salt lake that evaporated thousands of years ago and left behind over 30,000 acres of perfectly flat terrain. For most of the year the salt flats are covered by shallow water, making it unsuitable for racing. However, during the late summer and early fall, the water dries and the salt forms a hard salt surface ideal for land speed racing. Before racing events, a strip of salt is compacted and groomed to form a smooth, straight race track.

The Southern California Timing Association (SCTA) is the governing body under which US land speed records are established. Under SCTA rules, the track consists of 7 total miles. The first two miles of the track are for acceleration, miles three through five are timed miles, and the last two miles are for deceleration. Because the record speed is actually established by a timed mile, it represents the average speed over the timed mile rather than an instantaneous speed. The speed record is then established by the average of two timed miles from consecutive record attempts completed in the same direction [1]. A team may have up to four hours to service the vehicle between record attempts.

International records are governed by the Federation Internationale de l'Automobile

(FIA). Under FIA rules the track may be any length; however, the same physical mile must be crossed within an hour in opposite directions to establish a record. Same as with the national record, the average of the two timed miles is used for the record speed. Although the track length is not limited by the rules, it is limited by the geography of the salt flats.

Therefore, the timed mile is typically the sixth mile in the course. Figure 1.4 shows a comparison between the two racing courses and the associated rules.

Figure 1.4: Comparison of US and International Race Courses at the Bonneville Salt Flats

1.3 Thesis Motivation

Environmental and national security concerns have motivated industry and the public to look for alternatives to petroleum based means of transportation and energy generation. With this trend has come a general push for electric vehicles. Electric vehicles have no direct emissions and can theoretically be powered by a host of different renewable sources. Almost every automotive manufacturer is working on developing electric vehicles and many large automotive manufacturers; including Nissan, General Motors and Ford; have electric vehicles on the market today. There are also many smaller manufacturers for which electric vehicles are their primary product.

The Buckeye Bullet 3 is more than just a student project. Although its main goal remains to provide experience and learning opportunities for students in Ohio State

University’s College of Engineering, it encompasses more than just education. The Buckeye

Bullet 3 is a showcase for electric vehicle technology. It is meant to show industry leaders and the general public what an electrical vehicle is capable of. Additionally, motorsports, in general, tend to improve technologies by competitively pushing the limits of what is possible. This project and other alternative fuel motorsports projects also help advance the state of technology for such areas and generate excitement about these green technologies.

Aerodynamics plays an important role in high speed vehicle applications. At 300 mph and faster, the air traveling over the surface of the vehicle body can impart tremendous forces on the vehicle. The main goals of this research are to improve the

9 aerodynamic performance of the Buckeye Bullet 3, and ensure vehicle stability and safety.

The performance can be measured primarily by reduction of drag. Vehicle stability is critical to ensure the safe operation of the vehicle at high speed. A very common crash scenario for land speed racing is a “spin-out” in which the vehicle loses directional stability and yaws out of control. For narrow streamliners this failure mode typically results in “penciling” whereby the vehicle yaws roughly 90 degrees and subsequently starts rolling repeatedly sideways. The best way to reduce the potential for this occurrence is to maintain the directional stability of the vehicle. The work done in this thesis will push the project closer to its goal of breaking 400 mph and advancing electric technology.

Chapter 2

General Vehicle Information

This chapter gives an overview of the Buckeye Bullet 3 and its systems. As with all modern vehicles, the design of all the systems is highly interrelated. The intent of this chapter is to allow the reader to gain an understanding of the design considerations of the vehicle and how these may affect the aerodynamic development.

2.1 Project Goals

After setting both US and international records for electric vehicles and fuel cell vehicles the Bullet team needed to determine its next objective. Given that the BB3 will be an entirely new vehicle, it is not limited to any of the technologies or components used on the previous vehicle (accept for the driver and the tires). This actually makes determining the vehicle goals rather challenging. Ideas were initially presented for a smaller and lighter vehicle that would compete in lower weight categories. However, the majority of the Buckeye Bullet team had different ideas. The general movement toward vehicle electrification has acted to accelerate the advancement of electric drive systems and batteries. After reviewing the state of electric vehicle technology available it became apparent that this new vehicle could not only break the previous electric vehicle record but could conceivably contend with the fastest internal

11 combustion engine vehicles in the world. Table 2.1 lists some of the important land speed records for the purpose of placing a speed goal in context. To break the ultimate wheel driven the vehicle would need to set a record above 458.444 mph. A wheel driven record is characterized by the vehicle being powered through the wheels as opposed to being powered by thrust as with a jet powered vehicle.

Table 2.1: List of Wheel Driven Land Speed Records [2]

Vehicle Record Type Power Source Record [MPH]

Buckeye Bullet 2.5 Battery Electric Lithium ion batteries 307.666 Spirit of America Naturally-aspirated IC engine 414.316 Piston Engine Record Burkland 411 Piston-Engine Record Supercharged IC 415.896 Streamliner engine Vesco Turbinator Wheel Driven Record Turboshaft IC engine 458.444 Thrust SSC Ultimate Land Speed Turbofan jet engine 763.035 Record

From the start, simulation has been a tool to determine potential vehicle performance.

A simulator was developed for the original Buckeye Bullet and has been refined and advanced significantly over the years [3].

It is important to note that the vehicle is designed around the Bonneville Salt Flats. This is important because, although the salt flats are a vast open area, the track still has a finite length. Consequently, for every previous Buckeye Bullet vehicle record, the vehicles were accelerating through the timed mile. In other words, the vehicles had not yet achieved terminal velocity, wherein the force exerted by the motor is matched by the frictional and aerodynamic drag on the vehicle. At terminal velocity the vehicle is unable to go faster without either

12 increasing power output or decreasing the resistive forces. Due to the track length limitations, vehicle acceleration is a key concern in the vehicle design.

Given that speeds in excess of the wheel driven record are conceivable for the new vehicle, the ultimate wheel driven record will be the ultimate goal of the vehicle. However, this is a significant jump in speed from the previous electric vehicle record. Therefore, it is justifiable to set a more manageable starting target of 400 mph. The Buckeye Bullet 1 and Buckeye Bullet

2 were the first electric and hydrogen powered cars to break 300 mph, respectively. Now the team attempts to be the first to break 400 mph with electric power.

2.2 Vehicle Overview

The BB3 will be an entirely new vehicle and will not share much similarity to any of the previous Buckeye Bullet vehicles. The only carry-overs from the previous vehicles will be the driver and the tires. Due to the operating conditions and unique constraints of the project, many of the components and systems used for the vehicle are completely custom. The brakes, suspensions, driveline and batteries systems have been either specified or designed by engineering students from The Ohio State University.

2.2.1 Driveline

Land speed racing offers unique conditions and challenges to be considered when compared to an industrial application or even other forms of motorsports. The duty cycle, size, weight, rotational speed, voltage and current requirements can differ greatly from other applications. Specifically, the motor for the BB3 will need to be compact and lightweight yet have very high torque and power.

In order to reach the specified speed goals, it was determined that a four-wheel-drive system would be necessary for the vehicle. Delivering power through all four wheels increases the amount of power that can be transmitted to the ground, and increases the maximum acceleration potential. Unlike a top fuel dragster or formula 1 car that uses only rear wheel drive, most land speed cars do not generate large amounts of downforce to increase traction.

This is because generating downforce comes at the price of increasing drag which would hinder the top speed of the vehicle. Also, the surface of the salt flats has a lower coefficient of friction compared to asphalt; therefore, traction is a primary concern and supports the use of a four- wheel-drive system. The BB3 will use two separate drivelines front and rear, consisting of an electric motor and transaxle. The drivetrains will be essentially identical to reduce the number of spare parts required.

The duty cycle for the drive motor consists of a short period of maximum output followed by a significantly longer period of inactivity. A run is approximately 90 seconds long with a period of no less than 45 minutes between each run. This means that the motor will be run at 100% output for 90 seconds followed by 45 minutes at rest followed by another 90 second period of 100% output. The Buckeye Bullet team used this to their advantage with the design of the BB1 motor, and was able to eliminate the need for a cooling system. This was possible because the BB1 motor had enough thermal mass that it could run for a short duration without overheating. The motor was simply allowed to increase in temperature over the duration of a race and then cooled during the rest period. This reduced the weight and complexity of the motor significantly. The increased power density of the BB3 motor, however,

14 necessitates the use of active cooling during the run. This is achieved by pumping ice-cooled water through the motor’s cooling jacket.

The design of the BB3 has been centered around the driveline. Because the vehicle will not use the same motor as the previous vehicles, it was necessary to determine suitable motor characteristics. For this study, we will start from the tires and work backwards through the driveline to determine the motor characteristics. The ultimate limit for vehicle acceleration is traction at the tire. Maximum acceleration occurs when the force at the wheel is maximized.

The maximum tractive force of the tires is dependent on the salt conditions, tire temperature, tire wear and the vertical force on the driven axial. The wheel and tire can only transmit a certain amount of torque to driving the vehicle forward. Applying more torque to the wheels beyond what they can transmit to the ground will result in uncontrolled wheel slip and reduced acceleration.

In addition to tire traction, power is another limiting factor for vehicle acceleration.

Power ( ) is a function of both force ( ) and velocity ( ):

(2.1)

With a constant power output, the force will decrease with increased velocity.

Therefore, above a certain speed with a finite amount of power, the vehicle will not be able to product enough torque to achieve the maximum tractive force of the tire. This is known as the power limited region of operation. Figure 2.1 shows the ideal driving force at the wheels for one possible BB3 configuration. The ideal driving force is composed of two regions. The sloped region represents the maximum force that the motor can generate for a given speed based on its maximum power output. The flat section is the maximum tractive force that the tires can

15 produce. The maximum tractive force in this case is based on a constant coefficient of friction with weight transfer. The motor for the front and rear axles will be identical. Therefore, the forces at the rear axle will dictate the desired motor parameters. The actual torque at the motor is obtained by multiplying the curve in Figure 2.1 by the tire radius and the effective gear ratios.

Traction Limited Region

Power Limited Region

Figure 2.1: Driving Force at Wheels (per axle)

The desired maximum power of the driveline is dependent on the vehicle weight, target speed and track length. The record mile for an American national record is set at the 5th mile under the SCTA rules. For an International FIA record the 6th mile has been used for previous

Buckeye Bullet records. However, this is limited by the salt conditions and available space and

16 not the FIA regulations. The possibility exists to run on a longer track for an international record.

Figure 2.2 shows various potential speed records based on different total vehicle power levels. The power indicated in the plot is for the total vehicle, i.e., for two motors combined.

The vehicle weight is scaled accordingly based on power. From the plot it can be seen that to reach the aforementioned goals the total vehicle power will need to be around 2 MW or about 1

MW per motor.

Figure 2.2: Potential Speed Record Based on Various Maximum Power Levels

The torque and speed curve for an electric motor can be modeled by a constant torque region followed by a constant power region. This simplified motor model is shown in Figure 2.3.

Conveniently, this simplified model matches the shape of the ideal driving force from Figure 2.1.

This allows the ideal motor parameters to be fit directly from the desired force at the wheels.

Constant Power Region Constant Torque Region

Figure 2.3: Assumed Motor Power/Torque Curve Shape

For a direct acting motor without a gearbox, ideally the constant torque of the motor

would exactly equal the maximum amount of torque the wheels can transmit to the ground

from Figure 1. Also, the transition from constant torque to constant power of the motor would

occur at the point when the vehicle transitions from traction limited to power limited. The

maximum motor speed is then determined by the desired maximum speed of the vehicle. This

is assumed this to be 500 mph. Figure 2.4 shows the torque speed curve for such a motor.

Figure 2.4: Motor Torque Curve for Direct Acting Motor (without Gearbox)

From Figure 2.4 it can be seen that the maximum motor torque for this case is approximately 3050 N*m and the maximum motor speed is about 6800 RPM. Compared to the current BB2 motor (max torque: 675 N*m; max speed: 10500 RPM), this is an enormous amount of torque and a rather low maximum motor speed. However, this curve can be shifted by introducing a gearbox to the driveline and varying the gear ratios while still keeping the ideal force at the wheels.

In order to have a direct acting motor without any sort of gearbox would require that the motor be packaged laterally within the vehicle between the driven wheels. The overall width of the vehicle is directly related to the width between the wheels or the vehicle track. Reducing

19 the vehicle track will reduce the frontal area of the car and reduce aerodynamic drag. Due to the length of the motor it is not possible to package it laterally in the vehicle between the wheels. The motor will be oriented longitudinally in the vehicle. Therefore, it is necessary to have a bevel gear arrangement in order to translate the rotation of the motor 90 degrees. The introduction of this gearbox allows a gear ratio to be used without increasing the complexity or weight of the system. However, it also introduces a new variable in determining the motor characteristics.

Figure 2.5: Motor Requirements for 1 Speed Gearbox for Various Gear Ratios

Changing the gear ratio will affect the required torque of the motor to achieve the maximum torque at the wheel and also affect the motor's maximum speed. Figure 2.5 shows the maximum torque and required maximum motor speed for a one speed gearbox with various

20 gear ratios. In this situation the gear ratio is fixed (the vehicle does not shift gears during a run).

It is important to note that for all these examples the vehicle performance is exactly that same.

The force at the wheel for each case is identical.

From this curve the analysis can be taken one step further by comparing the required motor parameters for a multispeed gearbox. Figure 2.6 shows a comparison between several multispeed configurations. For this analysis it is assumed that the motor operates only in the constant power region for every gear beyond first gear. It is geared such that the motor will use its entire constant power region through every gear change. In other words, when shifting into second gear the motor will reduce speed from its maximum speed to the transition speed from constant torque to constant power. The motor will then accelerate the vehicle through the constant power region and then shift again once it reaches the maximum motor speed. Again, in all the curves the vehicle performance is the same except in shift times. Time spent shifting between gears will negatively affect the overall record speed. However, the effect is rather small.

Although automotive sourced electric motor technology has improved significantly within the past decade, a signal motor does not exist that meets all of these requirements.

Therefore, each axle will have several motors coupled together to meet the torque and power requirements. Throughout the design process several different motor configurations have been proposed from two to four motors per axle with maximum speeds ranging from 5000 rpm to

14000 rpm. The final supplier and motor configuration have not been determined at the time of this publication.

Figure 2.6: Motor Torque and Speed Requirements for Various Gearbox Configurations

2.2.2 Battery System

Land speed racing offers a unique battery duty cycle. The races typically last less than

90 seconds and require very high power output for the duration of that time. In addition to the high power requirement, the battery weight is also a significant consideration. The batteries account for approximately 45% of the total weight of the vehicle. Any weight savings form the battery pack significantly affects the weight of the vehicle as well as the weight distribution.

The team tested several batteries before selecting A123 as the battery supplier for the

BB3. For the BB2.5 the team used A123 32113 cylindrical lithium ion batteries. For the BB3 the

A123 HEV prismatic batteries will be used. These cells have nearly identical chemistry to the

32113s but in prismatic packaging rather than cylindrical packaging. The prismatic packaging is

22 desirable for this application because it offers a much better volumetric efficiency and reduced weight compared to the cylindrical cells.

Figure 2.7: A123 Prismatic Lithium Ion Battery Modules [4]

The BB3 will be powered by a total of approximately 2000 battery cells broken down into 8 separate battery packs. Four battery packs will be used to power each axle, respectively.

The nominal battery pack voltage will be between 750 and 900 V. Each battery pack will have control modules and fuse circuitry to ensure the safe operation and charging of the batteries.

The battery system is capable of supplying the combined 2MW of power needed.

Thermal testing was performed to determine the cooling requirements of the batteries.

The batteries were discharged at a rate consistent with a race profile followed by periods of charging. This cycle was completed several times to simulated repeated races throughout the day. At a minimum the battery pack needs to be able to perform two races within a day in order to set an international record.

For this application the temperature limit for the battery cells is 60° C. This temperature is above the recommended industrial use levels as the cells will degrade faster at these temperatures. However, for a racing application this is acceptable. Figure 2.8 shows the results from a module in open ambient conditions. From the figure, it can be seen that the module heats up during the run and cools slightly in-between runs. During this operation five consecutive runs can be completed without exceeding the maximum cell temperature. Of course, the module will not be in open ambient conditions during operation. It is difficult to accurately simulate the thermal condition of the battery cells during a speed attempt without testing an entire battery pack. As a worst case scenario the pack would operate without any heat exchange to the surroundings. To test this scenario the battery modules were fully insulated and run through the same test. The results from this test are shown in Figure 2.9. For this case the module is able to complete three races without cooling. Although this limits the

24 number of runs that can be completed within a day it allows the battery pack to be designed without a liquid cooling system which reduces the system complexity, size and weight. This saves approximated 40 kg from the total battery weight.

Figure 2.8: Thermal Testing for Uninsulated Battery Module

Figure 2.9: Thermal Testing for Insulated Battery Module

2.2.3 Suspension and Steering

The Bonneville salt flats have been selected as the test site for the Buckeye Bullet 3 specifically for their vast expanse of flat land. Due to the natural flooding of the salt flats during the winter and spring months the surface is releveled every year. Additionally, the track is groomed to further improve the track condition. This makes it tempting to use fixed axles without a suspension system. Doing so would significantly simplify the design and potentially reduce the weight of the vehicle and reduce frontal area. However, the salt flats are still a natural granular surface and, as such, are not perfectly smooth. In order to maximize traction it is desirable to maintain wheel contact with the ground. The suspension system facilitates maintaining wheel contact with the ground and absorbs surface irregularities that would transfer significant shock load to the chassis and driver. Also, the Salt Flats are not a

26 homogeneous surface. The surface is noticeably rougher near the edges of the Salt Flats. This means that the track is rougher typically at the very start and end of the track. Figure 2.10 shows the salt surface at the far end of the track. Using a suspension system allows the vehicle to operate on a rougher surface and in some cases can allow the vehicle to start a speed attempt further from the timed mile. This effectively lengthens the track length and can increase the speed through the timed mile.

The suspension is primarily needed at low speed. Suspension ride height data indicates that the suspension oscillation amplitude reduces with increased speed. This is because at higher speeds the suspension is unable to react fast enough to road inputs due to the mass of the suspension system.

Figure 2.10: Close-up of Salt Roughness at the Start of Track

The BB3 suspension system is designed around the premise of high speed stability without compromise for turning dynamics as the vehicle does not perform high speed cornering.

The BB3 will have a fully independent double A-arm suspension similar to that of the BB2. The suspension has equal length A-arms top and bottom to eliminate camber gain. The suspension has no kingpin inclination to eliminate weight jacking and camber gain while turning. The upper and lower ball joints are positioned at the wheel centerline in order to eliminate scrub radius.

The CV joints are positioned within the wheel centerline to eliminate torque steer. The suspension system is mounted to the gearbox to form a complete drive unit. In order to improve serviceability and reduce the number of spare parts required to keep on hand, the suspension is identical front and rear with the exception of the steering linkages.

The vehicle primarily operates in a straight line. However, the vehicle must complete turns during reduced speed testing on the test track in Ohio. The front wheel has a maximum steer angle of 6°. The suspension has 38 mm of travel in both the droop and rebound directions.

Conventional CV joints are not suitable for operation at the rotational speeds required for the vehicle. Custom made CV joints will be used to cope with the speeds necessary to reach above

400 mph.

2.2.4 Brake System

Stopping the vehicle from 400+ mph is nearly as difficult as accelerating it to those speeds. The primary means for stopping the vehicle from high speed is parachutes. Although the vehicle carries multiple parachutes, parachute failures sometimes occur and can lead to a crash. Therefore the BB3 will have a redundant mechanical braking system capable of stopping the vehicle from full speed without parachutes. This could not be achieved with conventional

28 automotive brakes. A custom braking system was designed to accomplish this task utilizing carbon disks supplied by Goodrich Aerospace. The carbon disks used in the system are from an

Embraer 145 regional jet. The BB2 utilized a similar system; however, due to the increased vehicle speed an all new system had to be developed for the new vehicle. This system functions more like a clutch rather than a conventional brake disk. There are four rotors and five stators per each brake unit. The rotors and stators are stacked together in an alternating fashion and compressed by six hydraulic pistons. An exploded view of the system is shown in Figure 2.11.

Figure 2.11: BB3 Brake System Exploded View

Like the BB2, the BB3 will utilize an inboard mounted brake system, i.e. the brakes will be mounted to the gearbox rather than the wheel hubs. The BB3 will have one brake assembly per axle rather than per wheel as done for the BB2. The BB3 driveline brake assembly will be

29 mounted longitudinally within the vehicle and will be connected to the halfshafts via the gearbox. One brake assembly will provide the brake torque to two wheels. The gearbox does not contain a differential; the two halfshaft outputs on the gearbox are constrained to equal speed via a common shaft. Therefore it is impossible to lock only one tire under braking (both wheels would lock at the same time). A diagram of the BB3 driveline with the brake system is shown in Figure 2.12.

Figure 2.12: BB3 Driveline Unit

The friction braking system is for use primarily at low speeds under 100 MPH (44.7 m/s).

In the event of a complete failure of the parachute system the frictional braking system in conjunction with the vehicle drag and rolling resistance must be able to stop the vehicle within

5.5 miles. Although all safety systems are being designed for safe operation up to 500 MPH, it is impractical and unnecessary to size the friction braking system for this speed. In the event of a parachute failure at high speeds, the vehicle will be able to coast until the vehicle reaches 350

MPH and then apply the frictional brake system. In this way the amount of energy dissipated by the air resistance is maximized. Figure 2.13 shows a speed trace of the braking strategy described. Given this strategy the individual brake system must be able to absorb 170 MJ of power. Under this condition the bulk temperature of the carbon heat sink would reach close to

1000° C.

Figure 2.13: BB3 Braking Strategy

Extensive thermal modeling was performed to ensure that the surrounding components would be able to withstand these temperatures. Fortunately, the braking event lasts for less than 1 minute. During this short duration the temperature of the surrounding parts increases within acceptable levels. However, after the vehicle has stopped within 10 minutes the surrounding components will exceed acceptable levels and the system may sustain damage. In other words, the braking system is able to stop the vehicle from full speed safely, but doing so

31 will damage the system. It should be pointed out that this is an extreme scenario. The BB2 had a similar system and never deployed it from full speed. The mechanical braking system can be used repeatedly from low speeds under 200 mph.

Chapter 3

Analysis Methods

This chapter discusses the simulation methods used to analyze the BB3 aerodynamics.

A comparison of the numerical simulation and wind tunnel testing performed on the BB2 is presented. Additionally, the numerical simulations are correlated with data collected from the

BB2 during actual racing on the Bonneville Salt Flats.

3.1 CFD Model Setup

Computational Fluid Dynamics (CFD) is a method for analyzing complex fluid flow problems using numerical methods to solve the Navier Stokes governing equations. There are many CFD codes commercially available today. Ansys Fluent was used in the analysis of the two previous Buckeye Bullet vehicles. For the BB3, OpenFOAM has been used primarily for the analysis of the vehicle. This is in part due to the technical support and training offered by

TotalSim LLC which uses OpenFOAM and has developed several custom tools involved in the analysis of road vehicles. TotalSim LLC is an aerodynamic consulting company based in Dublin,

Ohio.

3.1.1 Introduction to OpenFOAM

OpenFOAM is a collection of open-sourced CFD tools including solvers and utilities.

OpenFOAM has an extensive range of solvers that can be used to solve many different problems in continuum mechanics from complex fluid flows involving chemical reactions, turbulence and heat transfer, to solid dynamics and electromagnetics [5]. Open-source software is free for anyone to use and the source code is also available for download. This allows the software to be customized and modified for specific tasks. New solvers or modifications to solvers can be implemented by users. Also, this allows students to gain a better understanding of how the code is actually working.

OpenFOAM is primarily utilized in the Linux environment at the command line or script level. This can make it difficult for a beginner to learn how to use the software as compared to other commercial CFD codes with graphical user interfaces (GUIs). However, this aspect also makes OpenFOAM highly powerful in that all operations can be performed in scripts. This makes using OpenFOAM very powerful for doing iterative designs. For an application like vehicle development where the model setup is not changing and the basic operating conditions are similar, new geometry can be introduced very quickly and simulated.

3.1.2 snappyHexMesher

OpenFOAM has a variety of tools that allow a user to import meshes from other programs or create a mesh using the built-in meshing tools. These meshing tools include snappyHexMesher which is a fully featured meshing program that can create high quality meshes with complex geometry. The snappyHexMesher utility works by partitioning an existing grid by the model geometry and removing the unwanted grid. Refer to Figure 3.1 for a visual

34 representation of the snappyHexMesh process (note that this figure does not accurately represent the mesh used for simulation it is simply a pictorial representation). First the domain is coarsely meshed using the blockMesh utility. This builds a hexagonal mesh that defines the outlying domain. The geometry to be meshed is input as a sterolithography (STL) surface. An

STL file is a mesh file in itself; however, the STL mesh is only used as a representation of the geometry and does not have an effect on the surface mesh quality.

Figure 3.1: Meshing with snappyHexMesh

After the blockMesh has been constructed, snappyHexMesh then refines the mesh at every point where the mesh intersects the geometry STL. The eventual size of a cell in a particular area is determined by the number of refinement iterations performed. For each refinement iteration the length of the cell is divided by two, i.e. each cell is divided into eight smaller cells. The level of refinement for specific parts can be controlled as well the area within

35 a certain proximity to specified parts. This allows for very precise control of the mesh refinement in different areas of the domain.

Once the mesh has been adequately refined the cell vertexes close to the STL surface

“snap” to the surface. This feature is how snappyHexMesh has gained its namesake. This snapped mesh forms the surface mesh for the geometry. Surface layers, which are necessary to accurately model the boundary layer near a wall, are extruded from this surface into the domain. The mesh within the domain is pushed back to accommodate the surface layering. The mesh that remains unconnected from the simulation domain, i.e. the mesh interior to the vehicle body, is deleted, leaving a fully meshed model.

This style of meshing is quite different from the meshing algorithms that have been used for the previous Buckeye Bullet CFD simulations. Typically these softwares generate surface meshes first and then grow the volume mesh from the surfaces. SnappyHexMesh is just the opposite, whereby, the volume mesh is created first and the surface mesh is generated from the volume mesh.

This style of meshing offers some distinct advantages over the previous methods. The most important advantage is that the entire process is automated. Building meshes manually can take on the order of several minutes to several hours. During this time the user is engaged for the majority of the process. To simulate several different geometries requires that the user manually create several different meshes which can be very time consuming. SnappyHexMesh allows this process to be automated which means that one or one hundred different geometries can be queued into the meshing process within only a few minutes. Of course, it still may take hours to mesh, but user input is not required during this time. SnappyHexMesh can be run in

36 parallel on a supercomputing cluster. Also, snappyHexMesh is very tolerant of messy geometry allowing it to mesh almost anything without failing. However, this can also be an issue as it may mesh unwanted areas inside the model if the model is not entirely “water tight”.

There are some downsides to using snappyHexMesh. The mesh is limited to primarily hexahedral cells so there is no flexibility in cell type or shape. Also, because the utility is entirely automated it means the user can’t readily view the mesh once it’s completed. The mesh can be viewed by importing it into a visualization software such as Paraview, however, this takes extra steps compared to being able to instantly view the mesh as in another meshing software.

The mesh used for analyzing the BB2 and BB3 is approximately 20 million cells ranging in size from 1.6 m near the edge of the domain, to 3.125 mm near the leading and trailing edge of the vehicle. Proximity refinement was used to generate a gradual transition from coarse to fine elements and to control refinement in sensitive areas of the vehicle. Figure 3.2 shows an example of how this proximity refinement was used and gives a general idea of the scale of the mesh elements. The simulation domain extends approximately 5 car lengths in front of the vehicle and 10 car lengths rear of the vehicle. A wake refinement region also extends rearward from the vehicle to help capture turbulence in the wake of the vehicle.

Figure 3.2: Use of snappyHexMesh Mesh Refinement for BB2

3.1.3 Solver Information

OpenFOAM has a variety of solvers for a variety of applications. Many of the codes used for this project are custom adaptations of OpenFOAM codes developed by TotalSim LLC. Most of the functionality remains the same as the publicly available versions of OpenFOAM with the addition of some tools to automate case setup. In general, OpenFOAM requires boundary conditions and meshing criteria to be explicitly stated for every patch in the model. Generating these files can be laborious and time consuming and consequently, the TotalSim version of

OpenFOAM automates much of this process. In addition, the TotalSim version of OpenFOAM has some added functionality for implementing porous zones, rotating reference frames, outputting forces and post-processing data.

The high speed application for this vehicle necessitates the use of a compressible fluid solver. At the time of the start of this project, OpenFOAM only had one choice for a steady- state compressible flow solver: rhoSimpleFoam [5]. This code is based on the SIMPLE algorithm.

OpenFOAM has since released a SIMPLEC solver as well for steady-state applications. Initial testing with the rhoSimpleFoam solver proved unsuccessful in being able to solve the full size mesh at full speed. The solver could be used on a coarse mesh of ~1 million cells. However, when the solver was applied to the 20 million cell mesh the solution diverged.

Due to this instability for the steady-state solver, a transient solver was used. The transient solver is more stable because the solution matrix is better conditioned.

TSrhoPimpleFoam (a TotalSim implementation of rhoPimpleFoam) was used in this case. This solver is a combination of SIMPLE and PISO algorithms for compressible flows. The flow was initialized using an adaptation of the incompressible simpleFoam solver (TSfoam). The solution is run for 2000 iterations using this incompressible solver to initialize the flow field for the compressible solver. This method improved convergence of the compressible solver and, in general, reduced the computation time by reducing the number of transient time steps needed to reach steady-state. The solver is run until force and residual convergence is achieved. The simulation time depends on many factors include the mesh size, inlet velocity and geometry.

However, the solution generally converges within 0.1 – 0.2 seconds of simulation time which takes less than 50 hours of computing time on 48 processors.

3.1.4 Computing Resources

Running a large CFD model such as is used for the BB3 development requires high powered computers with large amounts of memory and many processors. The majority of the simulations for this project were completed at the Ohio Supercomputing Center (OSC), an avid supporter of the Buckeye Bullet program for a number of years. OSC has more than 10,000 CPU

39 cores operating AMD Opterons and Intel Xeon processors. Additionally, some models were run using TotalSim LLC’s in-house computer cluster.

3.2 Correlating Results

Complex fluid flow problems can be analyzed in two basic techniques: computationally and experimentally. Often these two methods are used in conjunction with one another. Both the BB1 and BB2 body shapes were developed using a combination of CFD and wind tunnel testing. The body shapes were initially developed and analyzed using CFD and then validated using wind tunnel testing.

3.2.1 BB2 Wind Tunnel Testing

The BB2 analysis methodology, as well as body shape, provided a starting point for the

BB3 development. The goal from the start of the BB3 project was not only to develop a new shape but also evaluate the analytical methods previously used. For the BB2, wind tunnel testing was used in conjunction with CFD modeling in order to: verify the CFD results, determine the stabilizer fin sizing and confirm a lack of flow separation at the rear of the vehicle. The main objective of the BB2 wind tunnel testing was to aid in the selection of several key body designs, including several different nose configurations and tail fin sizes. The BB2 wind tunnel model was run at the Penske Technology Group (PTG) wind tunnel in Mooresville, North Carolina. The PTG wind tunnel is an open jet wind tunnel that features a rolling road and boundary layer control.

The BB2 wind tunnel model was 1/3 scale. It was constructed of high density foam that was CNC machined to accurately represent the BB2 CAD models. The vehicle featured a smooth

40 under floor without wheels or wheel wells. A picture of the model in the tunnel is shown in

Figure 3.3.

Figure 3.3: The Buckeye Bullet 2 Wind Tunnel Model

Although the body was accurately machined to match the vehicle body, the model still exhibits some approximations. The model has no wheel geometry or wheel wells. Seemingly this would be an acceptable approximation as the wheels are almost entirely enclosed within the vehicle bodywork. They only extend from the underside of the vehicle by a few centimeters.

In fact, it’s difficult to even see the wheels in pictures of the vehicle. However, with more careful thought, it can be easily be seen why this may not be a good approximation. If we think of the underside of the vehicle as a narrow channel with flow traveling lengthwise through it, the wheels pose a significant blockage to the flow. The wheels block approximately 35% of the underside of the vehicle in the frontal direction. This not only decreases the airflow under the vehicle, but also affects the flow structure and the pressure distribution under the vehicle, ultimately affecting the drag and downforce. Additionally, CFD simulations indicate that the wheels themselves account for approximately 20% of the total vehicle drag. This is due to the

41 large stagnation region directly in front of the contact patch. This area actually sees higher pressure than the tip of the nose. The impact of these conditions on the wind tunnel results and their correlation with the actual vehicle will be discussed in the next section.

Figure 3.4: Drag Distribution for BB2

3.2.2 Correlating Results with BB2 Suspension Data

Even the most accurate wind tunnel testing incurs some approximations. For the most accurate representation of the vehicle it is desirable to correlate the modeled data with the actual vehicle under race conditions. Due to the nature of operating at such speeds, this can only be achieved at the Bonneville Salt Flats. Unfortunately, the BB2/BB2.5 body was not instrumented for aerodynamic development, but there is still some data from this time that can be used to determine the aerodynamic forces on the car.

In general, it is possible to compute the drag force for a vehicle by monitoring the vehicle deceleration while coasting. While a vehicle is coasting the two primary forces acting against the forward momentum of the vehicle are rolling resistance and aerodynamic drag. If an

42 accurate estimate of rolling resistance is available, the drag force can be easily determined by subtracting the rolling losses from the total vehicle losses. Unfortunately, the team does not have a good estimate of the rolling resistance of the tires at high speed on the salt flats. The tires for the BB2 are specialized land speed racing tires and information is not available about their rolling resistance properties. Therefore, only a lumped drag including both rolling resistance and aerodynamic drag force can be obtained from a coast down test. This lumped drag number is effectively useless for verifying CFD results.

Of course, there are other components for the aerodynamic forces acting on the vehicle besides drag. Downforce can be obtained by measuring the change in normal loads on the tires.

This information can be calculated from the suspension displacement data from the vehicle.

The BB2 was equipped with displacement transducers on all four suspension springs. A sample of the data collected from these sensors is shown in Figure 3.5. At low speeds the suspension oscillates heavily and it is difficult to separate displacement due to aerodynamic forces from displacement caused by road disturbances. However, at higher speeds the signal smoothes out considerably. This is because the suspension is unable to react to road disturbances at higher speeds due to the mass of the wheel hub assembly. Much in the same way a boat will trim out and skip over waves at higher speeds, the suspension oscillations will diminish at higher speeds.

This is fortunate for this analysis as it gives reliable data for the downforce on the vehicle and distribution of that force front to rear.

15 Front Left 10 Front Right Rear Left 5 Rear Right

-5

Suspension Suspension Displacement [mm] -10 150 200 250 300 350 400 time

400

300

200

100 Vehicle Speed Vehicle [mph] Speed

0 150 200 250 300 350 400

Figure 3.5: Suspension Ride Height Data From 300+ MPH Run

A total of five high speed runs where used for this analysis. The data was isolated above 290 mph to eliminate the oscillations at low speeds.

The force in each spring can be determined by multiplying the spring displacement by the spring rate. This force can then be related to the force at the wheel by multiplying by the appropriate suspension geometric relationships. It should be noted that this is not the true force on the wheels; rather it is the force acting only on the body and chassis. Any aerodynamic forces acting upon the wheels will not be reflected in spring forces. Ideally these forces would

44 be calculated for a period of time when the car was coasting, however, the driver often applied the brakes or deployed the parachutes directly following the timed mile. Therefore, the forces will need to be calculated for a time when the vehicle is under power. Because of this, the moment due to the drive torque at the half shafts as well as the acceleration forces need to be accounted for. A free body diagram of the vehicle for this scenario is shown in Figure 3.6.

T ma

F ΔFr aero,f ΔFr Faero,r

Figure 3.6: BB2 Aerodynamic Force Free Body Diagram

In this diagram, and are the change in the front and rear forces on the body, respectively. These values are directly calculated from the suspension displacement. is the motor torque at the halfshaft; is the vehicle acceleration and is the vehicle weight. and are the aerodynamic loads resolved at the contact patches on the body. The resulting equations from this diagram are shown below where is the height of the center of gravity and is the wheelbase.

(3.1)

(3.2)

The results from these equations are shown in Table 3.1. In this case positive wheel travel indicates upward movement of the wheel with respect to the body, i.e., positive wheel travel

45 indicates downforce. From the table it can be seen that the vehicle is pitched slightly nose up and exhibits significant lift on the front axle with a relatively neutral rear axle.

Table 3.1: Suspension Ride Height Data

Standard Value Deviation Unit Total Lift 1483 118 [N] Front Lift 1555 201 [N] Rear Lift -72 216 [N] Front wheel travel -8.62 0.548 [mm] Rear wheel Travel 1.34 0.816 [mm] Pitch 0.097 1.70E-04 [deg]

These same results are included with the BB2 wind tunnel results in Table 3.2. From this table it clear that the wind tunnel model and the real vehicle differ significantly in terms of lift.

It should be noted that the vehicle was not at the same ride height and pitch angle as the wind tunnel model. This brings up an important point about the vehicle aerodynamics: performance at the design ride height and pitch may differ significantly from ride height and pitch at speed due to suspension compliance.

Table 3.2: Comparison of BB2 Wind Tunnel Results and Vehicle Data

Total Lift Front Lift Rear Lift Balance [N] [N] [N] [% Front] Windtunnel Model at Baseline Rideheight -1484 -1082 -402 72.9% Forces from Suspension Ride Height Data 1483 1555 -72 104.9%

There are further geometric differences between the actual vehicle and the wind tunnel model. Most importantly, the wheels have a tremendous effect on the airflow underneath the

46 vehicle, as previously discussed. To illustrate the importance of wheel well and wheel geometry on the aerodynamics two cases were simulated using CFD. The first case was the full vehicle at the design ride height with wheels and wheel wells. The second case was a vehicle without wheels or wheel wells. This is effectively a CFD model of the wind tunnel model. The results are shown in Table 3.3. The results from the second case, the CFD of the wind tunnel model, closely match with the actual wind tunnel results. The CFD of the full vehicle, however, differs significantly from the body without wheels. The total down force is reduced by more than 1100

Table 3.3: Comparison of CFD Model and Wind Tunnel Model

Total Lift Front Lift Rear Lift Balance [N] [N] [N] [% Front] Wind Tunnel Model at Baseline Ride Height -1484 -1082 -402 72.9% CFD of Wind Tunnel Model at Baseline Ride Height -1523 -928 -595 60.9% CFD of Full Vehicle at Baseline Ride Height -336 -307 -28 91.5%

It is important to note that the CFD of the full vehicle at the baseline ride height still does not match the experimental results from the suspension ride height data. This is due to the difference in the vehicle attitude between the two cases. At 300 mph, the experimental data indicates that the vehicle is pitched approximately 0.1 degrees. This can be established with relative certainty, assuming the vehicle was initially setup with a static pitch of 0 degrees.

The vehicle ride height at speed, on the other hand, is slightly more difficult to establish. This difficulty stems from the fact that the tires change as the speed increases. It can be determined from wheel speed sensors and GPS data that the effective radius of the tire changes with speed.

The relationship between the effective tire diameter and vehicle speed is shown in Figure 3.7. It

47 has also been observed that a free spinning Mickeye Thompson Bonneville tire not in contact with the ground will grow in diameter.

326

324

322

320

318

316

314

312 Effective [mm]Diamter Wheel 310

308

306 0 50 100 150 200 250 300 350 Vehicle Speed [mph]

Figure 3.7: Tire Growth Model

It is not clear whether this change in effective wheel radius correlates directly to the same amount of change in the geometric radius. In other words, it is not known whether a change in the effective radius of the tire of 10 mm will increase the ride height 10 mm.

Additionally, the static vehicle ride height was not recorded during the racing events. Therefore, the actual vehicle ride height might vary significantly. Assuming the vehicle was statically set at

38mm of front and rear ride height and accounting for tire growth and suspension displacement the vehicle ride height would be 65 mm in the front and 55 mm in the rear. If we ignore the tire

48 growth and assume the vehicle static ride height was 15 mm, the ride height at 300 mph would be 24 mm front and 14 mm rear.

Deflection in the chassis can also affect the body shape and aerodynamic loads. From

Figure 3.8, which shows an image of the BB2 at high speed, it can be seen that the nose of the vehicle is actually deformed upward. This could be flex in the body panels or a result of chassis adjustment. The chassis of the vehicle has several removable sections that are adjustable. It is possible that while these braces were being installed, the chassis was flexed upward (a crane was typically used to tension the chassis while installing the cross-bracing). In either case, the nose in the body geometry was deformed similarly in the CFD model for purposes of correlating the data to experimental results.

Figure 3.8: Buckeye Bullet 2 at 300 mph

Figure 3.9 shows a ride height sweep of the CFD model compared to the experimental data. In all the cases the deformed body shape was used with a vehicle pitch of 0.1 degree.

From the results it is clear that the model is fairly sensitive to ride height, and at the lower ride height the model correlates very well with the experimental data. This would suggest that the actual vehicle ride height was on the lower end of the estimates, which would also agree with the image shown in Figure 3.8.

2.5 CFD Lift CFD Balance Measured Value 2

1.5

1500

1 Lift [N]

1000 Balance [% Front]

500

0 24/14 29/19 34/24 65/55 Front/Rear Ride Height [mm]

Figure 3.9: Lift and Balance Comparison of CFD Results and Experimental Data

Several key points can be made from this analysis; the first point being that the CFD model provides accurate results when the model geometry accurately represents the real world geometry. The second is that it can be fairly difficult to make the geometry match the real vehicle or conversely it may be difficult to make the actual vehicle match the CAD generated design. The model is very sensitive to minor geometry changes. Pitching the vehicle 0.1 degree and changing the ride height 10 mm drastically affects the vehicle forces. This is an important point, not only for simulating the vehicle, but also for building the new vehicle. In general, the ride height should be accurately set and monitored during the run (or actively controlled during

50 the run). Body panels should be rigidly fixed to the chassis and constrained such that they do not deform under load.

3.2.3 Limitations of Wind Tunnel Testing and CFD Testing

As was pointed out in the previous section the wind tunnel testing performed during the

BB2 development had some shortcomings. Any analysis will have some approximations that need to be made. Both CFD and wind tunnel testing alike have advantages and disadvantages.

Using accurate geometry seems to be an obvious requirement, and one that would seemingly be easy to accomplish. The CFD model presented previously does provide good results that match experimental data; however, it is still a simplification of the total vehicle. The physical body of the vehicle has seams and openings that are not represented in the CFD model. For example the back of the vehicle where the parachutes extend from the body is open to the vehicle interior, and gaps are present in and around the wheel wells. The resulting interior flows from these body cavities are not modeled in the current CFD model.

Figure 3.10 shows a particularly interesting example of how a flow through one of the body seams can affect the external airflow. This image is of the body seam between the tail and the fin. In this case wet salt has been blow by the air during a run and left streak lines along the tail indicating the direction of flow near the body. In this way, it has created an ad hoc flow visualization. From the image it can be seen that flow clearly exits the body seam between the tail and fin, and travels vertically along the fin before transitioning to longitudinal motion. This is likely a result of a vortex being generated as the low pressure outside the vehicle sucks air from inside the body cavity. This flow feature is not captured in either the CFD model or the wind tunnel model due to the fact that this flow passage does not exist in either. Many of the

51 vehicle seams are taped during the run; in fact, this seam in question usually is as well, so it is safe to ignore this case. However, the point stands that accurate geometry is required to obtain accurate results.

Figure 3.10: On the Salt "Flow Visualization"

Another factor that may affect the air flow is salt, both in its modification of the vehicle geometry and the affect it has on the fluid properties. The salt conditions can vary greatly from year to year, and even day to day. In some instances the salt is dry and powdery, and in other situations it is wet and sticky. It can build-up in the wheel wells and on the underside of the vehicle which can affect the airflow. In addition, the salt particles can mix with the airflow on the underside of the vehicle, forming a non-homogenous mixture with different properties than air by itself.

Figure 3.11: Salt Build-up in Wheel Well

3.3 Aerodynamic Performance Goals

Safety is the primary criteria for all the design decisions for the vehicle. Accordingly, the primary goal of the aerodynamic analysis is to achieve a stable vehicle. The vehicle must be able to maintain its heading at high speed, and should respond in a controlled and predictable manner to the driver inputs. Aerodynamically the vehicle is considered stable if it returns to a no yaw condition when perturbed. In other words the yawing moment on the vehicle should be in the direction counter to the yawing direction when perturbed from equilibrium.

Aerodynamic forces, in general, increase with the square of velocity. This means that the drag, downforce and sideforce at 425 mph are two times higher than at 300 mph. Figure

3.12 demonstrates the impact of this effect on the vehicle losses. From this figure it can be seen

53 that the aerodynamic drag drastically dominates the vehicle losses at high speed. Thus, the aerodynamic drag accounts for a significant portion of the energy used in accelerating the vehicle to high speed. Aerodynamic drag also largely determines the absolute maximum speed of a vehicle, or terminal velocity. This is the velocity at which the driving force equals the resistive forces exerted on the car by aerodynamic drag, rolling resistance and driveline losses.

Due to the weight, power and limited track length; the vehicle will never reach its terminal velocity while racing on the Bonneville Salt Flats. Reducing the aerodynamic drag, however, does allow the vehicle to accelerate faster and achieve a higher velocity in the timed mile.

Therefore, reducing the aerodynamic drag is a high priority in the vehicle development.

900 Rolling Losses 800 Driveline Losses Aero Losses 700

600

500

400 Power Power [kW]

300

200

100

0 0 50 100 150 200 250 Vehicle Speed [m/s]

Figure 3.12: Distribution of Vehicle Losses for BB3

In terms of the vertical component of the aerodynamic forces, downforce is desirable for landspeed racing, as it increases the normal force on the wheels which in turn improves tire grip. This increases the tractive forces of the tires, as well as increases the tire lateral force capacity. This improves the performance and stability of the vehicle. In terms of tractive forces, the requirement for high downforce is greatest at low speed during the traction limited portion of the run. At higher speeds when the vehicle is power limited, additional downforce does not improve acceleration as the tires are not at the traction limits. In fact, higher downforce at high speed acts to increase the rolling resistance of the tires and can actually reduce acceleration.

Unfortunately, the aerodynamic forces are highest at high speed and lower at low speed which is exactly opposite of the ideal case for vehicle traction. Additionally, producing high downforce during the traction limited region of acceleration may produce a downforce that would exceed the load rating of the tires or wheel bearings at higher speeds. This problem suggests that use of a system whereby the aerodynamic forces can be controlled would be of huge benefit. This could be achieved by developing movable aerodynamic elements on the vehicle. The SCTA and

FIA have no rules outlawing the use of such a system for the class that the Buckeye Bullet runs in.

As the development process for this vehicle progressed, it became apparent that the motor will not be able to produce enough torque to exceed the traction limits of the wheels at any speed. Therefore, developing large amounts of downforce is not a primary goal. However, lift at higher speeds is still a concern as it reduces the tire loads and thus reduces the vehicle stability. In extreme cases enough lift can be generated to lift the vehicle off of the ground and cause a crash. Therefore, in this analysis the goal will be to produce a vehicle design with a

55 small amount a downforce over a large range of vehicle attitudes. In this way we can ensure that the vehicle does not produce any lift at speed.

Another consideration is the balance of downforce on the vehicle. For stability, it is important to have a vehicle that understeers. This can be attained by ensuring that the front tires are always closer to saturation in comparison to the rear tires. It is desirable to have a rear biased downforce, however, creating too much of a rear balance would create a positive pitching moment that could lead to front end lift as with the BB2.

Chapter 4

BB3 Aerodynamic Design

This chapter discusses the aerodynamic development of the Buckeye Bullet 3. The positioning of vehicle components and the resulting effect on the aerodynamics is presented.

The selection of driver location within the vehicle is discussed and two different options are compared. The aerodynamic yaw stability of the vehicle is analyzed and the sizing for the stabilizer fin is determined. The vehicle pitch stability is analyzed and a method for determining the vehicle attitude at speed is presented. Other aerodynamic features of the vehicle are also discussed.

4.1 Vehicle Packaging

As with all forms of higher motorsports, aerodynamics increasingly dictates placement of certain components within the vehicle to improve external airflow. Designing a streamliner for land speed racing is unique among these other forms of motorsports in that there are no regulations on the size, shape and layout of the vehicle. To be considered a car (under FIA and

SCTA rules) a vehicle must have 4 wheels. However, there is no stipulation as to where the wheels are located. Some teams have two wheels mounted longitudinally inline in the front or back of the vehicle, while other teams choose to offset and stagger the wheels. Both of these

57 tactics are efforts to give the vehicle a more streamlined shape resulting in reduced drag. The

BB3 will maintain a more conventional wheel layout with the wheels positioned symmetrically left to right, and with equal track width front and rear. This layout was selected to allow the use of a common drivetrain and suspension front to rear.

The positioning of the driver is also an area where some teams tend to experiment in order to improve performance. There are a variety of car configurations in which the driver is placed in the center of the vehicle, behind the rear axle or in front of the front axle. The seating position of the driver also varies significantly from almost completely reclined to more upright.

In some rare cases the driver is not technically “seated”, but rather laying in a prone head-first position.

These tactics are employed not only to make the resultant body shape more streamline, but also to reduce the frontal area of the vehicle. Consider the generic equation for aerodynamic drag [6]:

(4.1)

Where: = drag coefficient = fluid density = vehicle velocity = frontal area = drag force

The drag on the vehicle is directly related to the frontal area. Given that most streamliners already have very efficient streamlined shapes, it is difficult to drastically improve the drag performance by refining the shape alone. Reducing the frontal area is often a more effective way to reduce the vehicle drag. From the start of the BB3 program, this concept was

58 kept in mind, and reducing the frontal area of the vehicle was a primary goal. For the BB2 there were several limitations to minimizing the frontal area: the width of the vehicle was limited by the driveline and suspension, and the height of the vehicle was limited by the driver’s roll cage.

The BB2 had a track width of 762 mm, a limit set by the transmission and half shafts. To eliminate torque steer it is desirable to place the CV joint within the centerline of the wheel.

The other end of the CV joint is mounted to the transmission. The length of the halfshaft is then determined by the required amount of suspension travel and the allowable misalignment of the joint. The combination of these constraints effectively determines the minimum width for the vehicle track which then dictates the overall vehicle width.

The only way to narrow the track width is to move the transmission output shafts inboard. This is difficult to do as most racing transmissions are already designed to be very compact. Therefore, for the BB3 a custom gearbox was designed that housed the inboard CV joints inside of the gearbox through shaft. This is shown in Figure 4.1. This allows the center-to- center distance of the in-board CV joints to be reduced to only 61 mm, subsequently reducing the track width to 600 mm. This greatly reduces the frontal area of the vehicle and significantly reduces the drag.

Wheel Gearbox Halfshaft

BB2

CV Joints

Gearbox Wheel

Halfshaft

BB3

CV Joints

Figure 4.1: Comparison of BB2 and BB3 CV Joint Placement

The height of the BB2 was primarily limited by the driver’s head position. To reduce the overall vehicle height would have required lowering the driver’s head and placing him in a more reclined position. With the driver positioned in the middle of the vehicle, the ability to reduce the driver’s head height is limited by the driver’s line of sight. For the BB2, the driver’s visibility was limited by the gearbox and moving the driver’s head any lower would sacrifice his field of vision. To avoid this scenario with the BB3, it was initially proposed to move the driver in front of the front axle, eliminating the obstructions from his field of view, and allowing a lower driver head height. In this scenario, the driver position is only limited by the human body. See Figure

4.2.

Front Driveline

Driver Batteries Rear Driveline

Front Driveline

Figure 4.2: Driver in Front vs. Driver in Middle Design

The combination of the reduced height and width limitations on the vehicle significantly reduced the frontal area of the vehicle. Figure 4.3 shows a comparison of the BB2 frontal area and the frontal area of this proposed new shape. The resulting driver position, however, places the driver in a severely reclined position, which forces the driver to severely bend and strain his neck. This greatly compromises the driver safety in the event of a crash. Thus, it was determined that a more upright position would result in an overall safer vehicle. The BB2 utilized a custom made composite crash structure in addition to a steel roll cage. Initially, the

BB3 was designed with a crash structure similar to the BB2, but it was determined that using an existing Indy Racing League (IRL) driver tub would be cheaper than fabricating a bespoke tub.

Additionally, the IRL tub design has been scrutinized by industry safety experts for crash worthiness and undergone crash testing, giving it the potential to be safer than a one-off tub produced by the team.

BB2 Frontal Area = 0.85 m2

Proposed BB3 Frontal Area = 0.71 m2

Figure 4.3: Comparison of BB2 and Initially Proposed BB3 Frontal Area

The use of the IRL tub increases the driver head height marginally higher than that of

the BB2. However, the driver-in-front (DIF) vehicle configuration was initially kept for several

reasons. For the same frontal area, the DIF configuration results in a lower drag compared to

the driver-in-middle (DIM) configuration. Initially, the DIF design achieved an 8% drag reduction

compared to a comparable DIM design. The driver also has better visibility with the DIF

configuration, as he has no obstructions to his line of sight. Visibility had previously been a

concern with the BB2 as the drivers vision of the track was limited by the front wheel bulges in

the bodywork. The DIF configuration also results in an overall shorter vehicle because the

packaging is more volumetrically efficient in this configuration. Most importantly, this

configuration initially led to a more favorable weight distribution in the vehicle.

For stability it is desirable to have a neutral or understeering vehicle. Understeer is characterized by a reduction in yaw rate as the vehicle approaches the traction limits of the tires. In this way, the front tires will exceed traction limits before the rear tires. This reduces the likelihood of the vehicle to lose control. There are many factors that affect the tendency of the vehicle to under or oversteer. However, since the BB3 will have the same suspension setup and the same tires front to rear, the balance of the vehicle is largely dependent upon the weight distribution. The following equation can be used to determine the steady-state stability factor which can then be used to determine the vehicle’s balance.

(4.2) ( )

Where: = vehicle mass = vehicle wheel base = distance from the center of mass to the front axle = distance from the center of mass to the rear axle = front and rear cornering stiffnesses, respectively = stability factor

From this equation the balance of the vehicle can be determined based on the value of

K. The relationship is as follows:

Understeer Neutral Steer Oversteer

Given that the front and rear cornering stiffnesses are identical for the BB3, it is desirable to have a forward weight balance. The more forward the weight is in relation to the tires, the more the vehicle will understeer. A forward weight balance also improves the aerodynamic stability as will be discussed in the next section.

Initially, the DIF design resulted in a weight distribution of 52% front while the DIM design produced a weight distribution of 46% front. Therefore, the DIF design was adopted for stability performance. However, as the design of the vehicle progressed, and various components changed, the weight distribution also changed. At this time, with the current vehicle weight estimations, the DIF design has a 49% front weight distribution which is not unfavorable.

Until this point the DIM design positioned the driver in front of the battery packs. This was required for driver visibility. However, as the driver head position increased in height with the IRL tub integration it is now possible to position the driver between the battery packs or rearward of the battery packs. The former of which is shown in Figure 4.4. This produces a 50% front weight balance which is more favorable. In either the DIF or DIM configurations the weight distribution can be adjusted by moving the rear axle more rearwards. However, this is not ideal as it increases the vehicle weight, length and subsequently the vehicle drag, as well as making it more difficult to transport.

Driver Batteries Front Driveline Batteries Rear Driveline

Figure 4.4: Revised Driver in Middle Design

From this point, the DIF loses its weight distribution advantage as well as most of its aerodynamic drag advantage over the DIM configuration. With the higher head position the

DIM body without a fin has a 4% higher drag coefficient than the DIF body without a fin.

However, the DIF body is less stable in yaw than the DIM body and thus requires a larger fin which increases the drag. With an appropriately sized fin the DIM body has only 2.4% higher drag compared to the DIF with a fin sized for the same level of stability. A difference of 2.4% equates to a difference of approximately 1 mph average over the timed mile for this vehicle at its current weight and power levels.

The DIF configuration results in a much more complicated steering system, as the steering shaft needs to be routed around the driver to reach the front axle. The proposed system for this steering involves the use of several bevel gear sets and linkages. This would likely reduce the steering feel and result in a more compliant steering system which is not desirable.

Safety is also a primary concern when considering the vehicle configuration. It is difficult to quantify the advantages with regard to safety without performing crash testing or simulation. Even with those tools it would be hard to predict the performance of either configuration in all conceivable crash scenarios. For a road vehicle the DIM design would unequivocally be safer in a frontend collision. However, crash modes in the landspeed racing are significantly different than that of a road vehicle or even other motorsports. The salt flats are an expansive area of flat terrain with essentially no obstacles to hit head-on. The vehicle does perform testing at the Transportation Research Center in Ohio, where there is a chance of crashing into a guardrail; however, this would be at much lower speeds and is much less likely.

The most likely crash scenario on the salt flats would be a loss of control and a subsequent vehicle roll. In this scenario the vehicle can be impacted from almost any angle by the ground.

Placing the driver in the center of the vehicle closer to the center of mass can reduce the acceleration forces felt by the driver in the event of such a crash. In general, in the center of the vehicle the driver will have more of a crush structure surrounding him compared to the DIF configuration. For these reasons it is considered that the DIM configuration is the safer alternative.

A summary of the tradeoffs between the DIF and DIM configuration is shown in the following table:

Table 4.1: Driver Position Comparison Summary Driver in Front Driver in Middle

Vehicle Length 11.1 m 11.2 m Wheel base 5.2 m 6.5 m Weight Distribution 49% Front 51% Front Stability Stability factor slightly less Better mechanical stability than zero due to rear weight due to longer wheelbase balance resulting in mild oversteer 2 2 CDA 0.0936 m 0.0973 m 2 2 CLA -0.0498 m -0.0624 m Downforce Balance 15% Front 53% Front Driver Safety Driver completely separated Better overall crash safety from battery area Other Design Considerations Better driver visibility Simplifies steering system

Due to the changing vehicle parameters, namely the component weights and driver seating position, the DIF configuration loses most of its advantages over the DIM configuration.

For this reason the DIM configuration has been selected for the BB3.

4.2 Yaw Stability

Yaw stability is the most critical aspect of the vehicle aerodynamic design as it has a direct impact on the vehicle safety. Yaw stability is defined as the tendency of the vehicle to maintain course and heading given a disturbance. This is accomplished by having a body shape which generates a restoring moment that acts in opposition to the direction of yaw.

For a land vehicle the yaw moment coefficient can be defined as follows [7]:

(4.3)

Where:

= yaw moment = dynamic pressure = reference area = wheelbase

In order to have a stable vehicle the yaw moment coefficient must have an opposite sign from that of the vehicle yaw, . Therefore, the stability criteria can be summarized as follows

[7]:

unstable

stable

Figure 4.5 shows the yaw moment coefficient for the BB2 as a function of vehicle yaw.

In this figure the overall vehicle yaw moment coefficient is shown, as well as the separated components from the body and fin. From the figure it can be seen that the BB2 design satisfied this condition and achieved a stable body. However, it can also be noted that the body of the vehicle without the fin is actually unstable. Therefore, a fin was required to achieve an overall stable vehicle.

0.2

0.1

0 N

-0.1

-0.2

-0.3

Yaw Yaw Momemnt Coefficient C -0.4

Vehicle -0.5 Body w/o Fin Fin -0.6 0 1 2 3 4 5 6 7 8 9 10 Yaw Angle 

Figure 4.5: BB2 Yaw Stability

In order to understand why the base body was unstable it is helpful to look at the force

X-ray plot of the vehicle. A force X-ray plot shows the force per unit area normal to a reference plane projected onto the reference plane and summed. Figure 4.6 shows the force X-ray in the y direction for the BB2 yawed at 1 degree. In this figure the y axis is into the page meaning that forces into the page are considered positive. In this case the vehicle is yawed to the vehicle left meaning that a positive force at the rear of the vehicle represents a destabilizing effect and a negative force at the front of the vehicle represents a destabilizing effect.

Figure 4.6: BB2 Force X-ray Yawed at 1 degree (side view)

From the Figure 4.6, it can be seen that there is a large destabilizing force on the tail of the vehicle represented by the large red region. Figure 4.7 shows a Cp slice in the z direction for the

BB2 at 1 degree of yaw. From this figure it can be seen that the transition in the body from the constant width section to the tapered tail section creates a large low pressure region on the side of the vehicle yawed into the wind. The acceleration of the flow around the abrupt tail transition creates this low pressure region that results in a larger destabilizing moment.

Figure 4.7: Cp Slice for BB2 at 1 degree of Yaw (top view)

Smoothing this transition can reduce this destabilizing moment and improve the baseline body stability. Figure 4.8 and Figure 4.9 show the force X-ray and top view Cp slice for the BB3, respectively. From these figures, it can be seen that smoothing out the tail transition can reduce the destabilizing effect at the rear of the vehicle resulting in an overall better stability.

However, even with this change and other modifications the baseline body without a fin still remains unstable. Therefore, the BB3 still requires a stabilizer fin in order to achieve stability. It is important however, to minimize the size of the stabilizer fin as much as possible to reduce the vehicle drag.

Figure 4.8: BB3 Force X-ray at 2 degree of Yaw (side view)

Figure 4.9: Cp Slice for BB3 at 2 degree of Yaw (top view)

The design process for the stabilizer fin is very similar to that of an airplane wing. The fin sizing for the BB2 was performed using several iterations in a wind tunnel. It is possible to run several different sized fins using CFD. However, this would be unnecessarily computationally expensive and would have to be repeated for every new body design. The yaw coefficient for the body can be determined from the CFD model. Therefore, it is possible to calculate the yaw coefficient of the stabilizer fin required to achieve vehicle stability. This can be expressed with the following equation:

(4.4) ( ) ( ) ( )

The yaw moment developed by the stabilizer fin can be determined by simply multiplying the “lift” force generated by the wing at a given angle of attack by the distance from the fin center of pressure to the center of gravity of the vehicle. In this instance the lift is actually a side force as the fin is oriented vertically and the angle of attack is simply the vehicle yaw angle. In order to maintain congruity with wing design, the side force of the wing will be left in terms of the wing coefficient of lift, CL.

The expression for the stabilizer fin restoring moment is shown below:

(4.5)

Where:

= distance from the stabilizer fin center of pressure to the vehicle center of gravity = side force generated by the stabilizer fin = fluid density = vehicle velocity = stabilizer fin planform area = stabilizer fin coefficient of lift

From this equation the partial derivative of with respect to can be determined as follows:

(4.6) ( )

In this equation, represents the slope of the lift vs. angle of attack line for the finite

wing section of the stabilizer fin. For simplicity we will call this value . There are numerous methods in the literature for finding the lift of a finite wing. The following equation can be used to determine the lift of a finite swept wing for a compressible flow [6]:

(4.7)

√ [ ]

Where:

= or the change in lift coefficient with angle of attack of the airfoil section

= angle of wing sweep = wing aspect ratio = freestream mach number

Once has been determined for a given fin shape and airfoil section the size of the fin can be determined by specifying the wing planform area. The combination of equations 4.4 and 4.6 yield the following equation for the planform area of the stabilizer fin as a function of the baseline body yaw moment and the desired vehicle yaw moment.

(4.8) (( ) ( ) )

For the BB3 body the value for ( ) is equal to 3340 N/deg at 400 mph. The

desired value for ( ) needs to be negative to ensure stability of the vehicle. Therefore,

72 the minimum planform area for the BB3 fin is 0.7044 m2. This results in a fin that is approximately 0.84 m high. To achieve the same level of stability as the BB2 would require a fin with a planform area of 1.096 m2.

4.3 Vehicle Ride Height

The downforce and balance of the BB2 was very sensitive to pitch and ride height. Also from the study of the BB2, it should be recalled that the aerodynamic forces will affect the vehicle attitude at high speed. For the development of the BB3, this makes determining the aerodynamic loads on the vehicle much more complicated since it is not known a priori what attitude the vehicle will assume at high speed. Additionally, in the case of the BB2, the aerodynamic characteristics and suspension compliance resulted in a vehicle position whereby the vehicle produced lift at high speed which is undesirable. It is the goal of the BB3 development that the vehicle produces moderate downforce at high speed. Therefore, in order to determine the level of downforce the vehicle produces the vehicle ride height and pitch will need to be determined.

In order to determine this, the vehicle was first simulated across of variety of front and rear ride heights in order to obtain the aerodynamic response to different vehicle attitudes. It is important to note that the front lift is a function of both the front and rear ride heights, and similarly for the rear. From this a surface can be constructed of the front and rear aerodynamic forces in terms of the front and rear ride heights. Similarly, the suspension forces can be determined as a function of the ride height given the suspension geometry and spring rates.

The springs rates for the BB3 are assumed in this case to be 25 % stiffer than the BB2 spring rates because the vehicle is 25% heavier. The front suspension force is assumed to be

73 dependent only upon the front ride height and the rear suspension force is only of function of the rear ride height; however, both can be plotted as a surface in terms of the front and rear ride heights.

These aerodynamic and suspension responses are plotted in Figure 4.10 and Figure 4.11.

Where the two surfaces intersect represents the line at which the aerodynamic force equals the suspension force due to suspension displacement.

Suspension Force Response Surface

Aerodynamic Force Response Surface

Figure 4.10: Front Axle Aerodynamic Lift Force and Suspension Force

Aerodynamic Force Response Surface

Suspension Force Response Surface

Figure 4.11: Rear Axle Aerodynamic Lift Force and Suspension Force

We now have a set of equilibrium positions for the front and rear separately; however, these must be combined in order to determine the equilibrium position for the entire vehicle.

To determine the equilibrium position for the entire vehicle the lines generated in Figure 4.10 and Figure 4.11 are projected onto the front ride and rear ride height plane as shown in Figure

4.12. The intersection of these two lines represents the total vehicle equilibrium position.

50 Front Axle Equilibrium Line 45 Rear Axle Equilibrium Line

35 Equilibrium Position 30

25 Rear Ride Height Ride Rear

10 10 15 20 25 30 35 40 45 50 Front Ride Height

Figure 4.12: Ride Height Determination at 400 mph

From Figure 4.12 it can be seen that the vehicle equilibrium position at 400 mph is 23.8 mm in the front and 38.9 mm in the rear. In this case it was assumed that the static ride height was

38 mm. If the static ride height was set at a different level or different spring rates were used, the ride height at speed would also change. Also, this method does not currently include tire growth as it is currently indeterminate. Future testing is planned to accurately measure tire growth which can then be incorporated into this method.

It is important to note that Figure 4.12 also indicates that only one equilibrium position exists within this set of possible ride heights. It is possible that more than one equilibrium position could exist based on a differently shaped aerodynamic response surface. It is also important to notice from Figure 4.10 and Figure 4.11 that the aerodynamic and suspension response surfaces do in fact intersect. If this were not the case there are two possible scenarios.

If the aerodynamic response surface was always below the suspension response surface this would indicate that the aerodynamic force was producing enough downforce to exceed the force capacity of the suspension and would result in the body being pushing into the ground.

This could damage the body work or, worse, cause part of the body or chassis to dig into the ground and destabilize the vehicle potentially causing a crash. If the aerodynamic response surface was always above the suspension response surface this would indicate an unstable condition whereby the body produces enough force to lift the vehicle from the ground, which would result in a vehicle crash. It should be noted that the previous figures do not show the entire suspension and aerodynamic response surfaces, but rather a region about which the vehicle is intended to operate. Non-intersection locally in this region may not indicate a vehicle that fully lifts from the ground, but would indicate a significant deviation from the intended vehicle attitude and ride height which should be avoided.

Up to this point the ride height at only one speed has been considered. As seen from the previous plots, the vehicle attitude will change with speed and subsequently the vehicle drag and lift forces. It is desirable for more accurate simulation to determine these forces throughout the entire run. For this speed range, the drag and lift coefficients can be considered to be linear, i.e. that the vehicle forces scale with the square of velocity. In this way the same analysis can be applied to varying speeds by scaling the aerodynamic forces accordingly without completing additional CFD simulations. It should be noted that the suspension response surfaces remain unchanged with speed. The result of this analysis is shown in Figure 4.13. From this plot it can be seen that the front ride height reduces with increasing speed and the rear ride

77 height increases slightly. Furthermore, the aerodynamic forces at these revised ride heights can be similarly calculated as shown in Figure 4.14 and Figure 4.15.

20 Ride Height [mm]Height Ride

10 Front Rear 5 0 100 200 300 400 500 600 Velocity [mph]

Figure 4.13: Vehicle Ride Height as a Function of Velocity

From Figure 4.14 it can be seen that the vehicle produces significant downforce at high speed with moderate rear lift. This is not ideal; it would be more desirable to maintain perfectly balanced moderate downforce front and rear. However, maintaining front downforce over the entire speed range does ensure a pitched down position which reduces drag and reduces the likelihood of reaching a high lift vehicle attitude as with the BB2. Figure 4.15 shows the vehicle drag as a function of speed which can be used for vehicle simulation.

1000

-1000

-2000

-3000 Lift [N]

-4000

-5000

-6000 Front Rear -7000 0 100 200 300 400 500 600 Velocity [mph]

Figure 4.14: Vehicle Lift as a Function of Velocity

3000

2500

2000

1500 Drag [N] Drag

1000

500

0 0 100 200 300 400 500 600 Velocity [mph]

Figure 4.15: Vehicle Drag as a Function of Velocity

4.4 Vehicle Design Features

The majority of drag reduction between the BB2 and BB3 is due to the reduced frontal area and improved overall shape. However, several small incremental changes have been incorporated into the BB3 design to improve the drag performance.

4.4.1 Wheel and Wind Deflectors

All Bonneville streamliners feature fully enclosed wheels to reduce aerodynamic drag as much as possible. However, even though the wheels are enclosed and only protrude from the body a few centimeters on the underside of the car, they still affect the aerodynamics of the vehicle significantly. As discussed in section 3.2.1, the wheels themselves account for approximately 20% of the vehicle drag. The majority of this drag is produce by the large stagnation region directly in front of the tire contact patch. Figure 4.16 shows a visualization of flow around the BB2 wheel enclosed inside of the wheel well.

Figure 4.16: Streamlines Around Wheel Inside of Wheel Well

From Figure 4.16 it can be seen that a high pressure region exists at the contact patch of the tire. Additionally, a large amount of air is forced into the wheel well at the front of the tire contact patch. Minimizing the wheel well opening reduces the airflow into the wheel well and reduces drag. Additionally, placing wind deflectors in front of the wheels can reduce the total vehicle drag by as much as 12%. Figure 4.17 shows the velocity contour of the flow under the vehicle with and without wind deflectors around the tires. From the figure it can be seen that the tire wake is significantly smaller with the wind deflectors and results in a much lower drag.

Figure 4.17: Under Floor Flow Comparison with and without Tire Wind Deflectors

4.4.2 Tail Geometry

The BB2 featured a vehicle tail geometry that essentially truncated where the parachutes exit the body. This creates a large separation region behind the vehicle as shown in

Figure 4.18 which increases the vehicle drag.

Figure 4.18: BB2 Tail Flow Separation

This flow separation region can be eliminated by extending the vehicle body work further and eliminating the tail truncation. This necessitates the use of movable body panels that open to allow for parachute deployment. There are several other Bonneville streamliners that feature such a system, some examples of which are shown in Figure 4.19. Using parachute doors such as these and extending the body panels to a thin edge reduces the vehicle drag by approximately 5%.

Figure 4.19: (left) Vesco #444 Streamliner and (right) Burkland 411 Streamliner Parachute Doors

Chapter 5

Conclusion

This chapter gives a general summary of the work presented in this thesis. The focus of the discussion centers around the driver layout, vehicle safety and performance improvement.

Topics for future work in this area are also discussed.

5.1 Summary of Results

Both the BB1 and BB2 vehicles were developed using a combination of CFD modeling and wind tunnel testing. For the development of the BB3, an alternate aerodynamic development method using only CFD was used for several reasons. Firstly, wind tunnel testing is costly and time consuming. In order to test a new body shape and geometry features, construction of a new wind tunnel model is required. Although the wind tunnel testing time is donated to the team, the access to these facilities remains limited to the availability of open testing time. This significantly reduces the effectiveness of using the wind tunnel testing for developing the new vehicle.

For these reasons, wind tunnel testing has been used by the team primarily as a means to correlate previous CFD data. However, the lack of wheel geometry significantly compromises the accuracy of the wind tunnel model in predicting the aerodynamic loads. Including wheel

84 geometry in the wind tunnel model could correct this discrepancy; however, this would also increase the cost and complexity of the model. Additionally, at 400+mph, the vehicle is operating in the compressible flow regime. The author is not aware of any wind tunnel in the world that is capable of producing speeds necessary to model a compressible flow and features the rolling road surface necessary for modeling a moving ground plane.

The downforce and balance obtained from CFD models has been shown to have good correlation with experimental results obtained from the suspension ride height data. Achieving good correlation between the two, however, was not a straightforward process. The model is very sensitive to pitch and ride height, making it necessary to run the model at the appropriate attitude to achieve good correlation. There are many other geometric variables that can affect the eventual aerodynamic forces. These variables should be constrained as much as possible in the vehicle design and construction in order to obtain predictable aerodynamic performance.

Initially the BB3 design was centered around the driver-in-front (DIF) vehicle configuration for its potential to significantly reduce the vehicle frontal area. This reduced frontal area would be achieved by allowing the driver to be placed at a more severe incline. The resulting driver body position was determined to be less safe than a more upright position and, therefore, the driver’s seating position was changed. This decreases the performance advantage that a DIF configuration has over a driver-in-middle (DIM) configuration. Although, the DIF still maintains a slight performance improvement compared to the DIM, the latter was selected as the design direction due to its increased safety and stability performance.

A method has been presented for accurately and quickly determining the vehicle fin sizing based on the vehicle center of mass and aerodynamic forces. Although, the yawing

85 moment of the BB3 has been improved from that of the BB2, the BB3 still requires a large stabilizer fin to achieve aerodynamic stability.

It was shown in section 3.2.2 that the BB2 design resulted in a slightly positive pitch at

300 mph that caused the overall vehicle to have lift on the front axle. The BB3 design achieves a more stable design; even at high speed the vehicle maintains negative pitch resulting in downforce over the entire speed range. This ensures that the normal loads on the tires are not reduced which improves traction and increasing the yaw stability. The analysis used in section

4.3 can be used to determine the effect of changing the static ride height or spring rates on the vehicle aerodynamics at various speeds.

Overall, the BB3 design achieves its primary goals of safety and stability, with improved performance compared to the previous vehicle. Through a combination of reduced frontal area and drag reducing features, the BB3 design has achieved a 17% reduction in CDA compared to the previous vehicle. A comparison between the BB2 and BB3 body designs are shown in Table

5.1.

Table 5.1: BB2 vs. BB3 Comparison BB2 BB3 Length 10.7 m 11.2 m Width 1092 mm 900 mm Track 762 mm 600 mm Wheelbase 5.881 m 6.5 m 2 2 CDA (static ride height) 0.113 m 0.094 m 2 2 CDA (dynamic ride height) 0.119 m 0.1029 m 2 2 CLA (static ride height) -0.059 m -0.0624 m 2 2 CLA (dynamic ride height) 0.100 m -0.1398 m Balance (static ride heigt) 59 % Front 53 % Front Balance (dynamic ride height) 105% Front 107 % Front

Figure 5.1 shows the final body design for the BB3.

Figure 5.1: BB3 Body Design

5.2 Future Work

One short coming of the correlation analysis performed in section 3.2.2 is that the exact ride height of the vehicle is not known for the Bonneville speed runs in 2009. This ambiguity arises primarily from tire behavior at higher speeds. It is known that the tire’s effective radius changes with speed; however, it has not been determined whether this directly applies to a change in ride height. Tire testing is necessary to determine the actual tire growth at high speed.

At the time of this writing, the BB3 team is designing a tire testing setup that can simulate speeds close to 500 mph. The apparatus loads two tires against each other and spins them using an engine dynamometer. The tires are loaded via a hydraulic ram that can apply a controlled force and still allow the tires to grow. The testing of the tires is necessary in order to determine the relationship between vehicle speed and the change in the effective radius of the tire, as well as the change in the geometrical radius of the tire. The effective radius change in the tire is needed for determining the appropriate vehicle gear ratio, while the geometrical tire radius is needed for determining vehicle ride height at speed.

Given the fact that the BB2 was never intentionally instrumented for aerodynamic development, it has still yielded valuable data that has been used to study the aerodynamic forces on the vehicle. The BB3 will be intentionally instrumented to allow for further correlation between future CFD models and the vehicle itself. Placement of pressure transducers at key locations along the body can provide data with which to correlate. Particularly the underside of the vehicle should have several pressure transducers running the length of the vehicle as well as pressure transducers along body curves where high pressure gradients are expected.

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