On the Influence of Ride Height Changes on the Aerodynamic
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Bastian Schnepf et al./International Journal of Automotive Engineering 6 (2015) 23-29 Technical Paper 20154055 On the Influence of Ride Height Changes on the Aerodynamic Performance of Wheel Designs 1) 2) 3) Bastian Schnepf Gregor Tesch Thomas Indinger 1),3) Technical University of Munich, Institute of Aerodynamics and Fluid Mechanics Boltzmannstr. 15, 85748 Garching bei Muenchen, Germany (E-mail: [email protected]) 2) BMW Group, Aerodynamics, 80788 Muenchen Received on June 7, 2014 Presented at the JSAE Annual Congress on May 21, 2014 ABSTRACT: On the road a passenger car's ride height is elevated both by the radial expansion of the tires due to centrifugal forces and aerodynamic lift. Wheel size and design influence these forces and therefore may affect aerodynamic drag more than predicted using fixed-ride-height tools. In this study, on-road ride height and surface pressure measurements for different wheel designs on a BMW 3 Series sedan are compared to wind tunnel tests and numerical simulations. Compared to still conditions, the vehicle is elevated by 5 to 7 mm when driving at 140 kph. This ride height change increases drag by 4 counts in the wind tunnel. However, the drag differences between the specific wheel designs are only altered marginally. Using CFD, areas sensitive to wheel designs are identified and analyzed. Furthermore, lift differences between the wheels are explained by the vehicle’s pressure distribution. KEY WORDS: heat・fluid, aerodynamic performance, computational fluid dynamics, wind tunnel test, wheel, ride height [D1] 1. Introduction 2. Experimental Setup Wheels and wheel houses are responsible for approximately 2.1. Test vehicle 25 percent of a passenger car’s total aerodynamic drag.(1) Given upcoming regulations on CO2 emissions and the need for long- A 2012 BMW 3 Series sedan was equipped with three laser range electric vehicle concepts, this area has to be optimized sensors measuring the distances to the road from a) front left aerodynamically in more detail than in the past. Furthermore, wheel axle, b) front left underbody (body-fixed), and c) rear left aerodynamic front and rear axle lift, which influence the driving underbody (body-fixed). In this way, ride height and pitch of the stability and maneuverability, also depend on the wheel design. vehicle body, as well as the elevation of the wheel center from the Wäschle(2), Modlinger(3), Landström(4) and others have pursued ground could be captured. Moreover, out of this data the relative research in this area over the last years but still have not covered motion between wheel axle and body could be calculated. This all aspects of this topic. data then was used to position the vehicle body and the wheels Ride height is a crucial parameter for the aerodynamic correctly in the WT and in the CFD setup. performance of a vehicle. An increase in ride height leads to In order to record the differences in surface pressure higher drag. On the road, ride height is, however, not only distribution between the wheel designs, holes of 1 mm diameter influenced by the static load but also by velocity dependent forces. were drilled in the vehicle’s skin and then tubed to a 64-port In fact, a vehicle is elevated both by the radial expansion of the pressure scanner. The holes are placed in regions sensitive to tires due to centrifugal forces and by aerodynamic lift forces. changes in wheel geometry that were identified through Wheel size and design influence these forces. If the wheels preliminary CFD investigations. All of them are located on the consequently affect the dynamic elevation of the vehicle, they left side of the vehicle in order to achieve the best local resolution may have a larger effect on the aerodynamic performance than out of 64 available channels. Mounted in the middle of the license standard analysis tools would predict. Neither numerical simu- plate, a Pitot tube is able to measure the total pressure of the free- lations nor wind tunnel (WT) setups in which the model’s ride stream for the calculation of dimensionless pressure coefficients height is fixed throughout the measurement will reproduce the (CP). In Figure 1, the distribution of measurement locations is interaction of ride height variations with the flow around a vehicle displayed. Most of them are positioned in the vicinity of the left and the resulting aerodynamic drag. front wheel, but also at the rear wheel house, on the underbody The purpose of this study is to clarify the influence of wheel- and on the rear base. induced ride height changes on aerodynamic force deltas between In Figure 2 the three wheel designs investigated in this study wheel designs. Consequently, the authors investigated on-road are shown. They cover a wide range of drag and lift values, thus, ride height changes for three wheel designs on the proving ground allowing for representative statements regarding a typical wheel and fed them back into both numerical simulations and WT tests. portfolio. All of them were tested on 225/45 R18 Pirelli P7 Furthermore, surface pressure distributions recorded on the test Cinturato run-flat tires in order to eliminate tire influences in this track at the identical vehicle are used for validating WT and CFD part of the study. In general, however, tire influence is not results. negligible since details like the design of a rim protection edge Copyright 2015 Society of Automotive Engineers of Japan, Inc. All rights reserved 23 Bastian Schnepf et al./International Journal of Automotive Engineering 6 (2015) 23-29 can affect the differences in aerodynamic performance between increase drag by 0.002 but do not have a measurable effect on two wheels. Wittmeier(5) has studied the aerodynamic properties relevant force differences. of tires and their interaction with wheels extensively. It is important to note that the vertical position of the vehicle body is fixed during the WT measurements. Hence, aerodynamic lift forces or tire expansion cannot elevate the body. The wheels are, however, mounted on the wheel axle in the common way, therefore being able to move vertically. As a result the distance between the wheel house shells and the tire decreases with increasing velocity. Fig. 3 Test vehicle mounted on the five-belt system in the full-scale WT. 2.3. On-road tests Fig. 1 Surface pressure measurement locations. On the fast course of the BMW testing site Aschheim (Fig. 4), the different wheel designs were evaluated at a velocity of 140 kph according to GPS measurements. There are two straights in opposite directions on which laser and pressure signals could be recorded and averaged over a period of 30 s. The average of three laps (six straights) was used as the reference for the comparison with WT and CFD results. Only when windsocks at the test track signaled calm conditions, measurements were accepted. The vehicle’s elevation at 140 kph compared to still condition is of major interest in this study. Therefore, before and Fig. 2 Investigated wheel designs. after each lap the car was driven at a low speed of 10 kph in order to record a “zero” level for the ride height sensors. At a complete 2.2. Wind tunnel measurements stop imperfections on the road’s surface like small stones would have distorted the laser measurements leading to unrealistic The WT tests were conducted at the BMW Group Aero- values. Rolling at 10 kph, however, an average ride height could dynamisches Versuchszentrum in the full-scale tunnel. It is a be determined without significantly altering the values in Goettingen type WT featuring a three-quarter open test section. At comparison to still conditions. On the straights the unsteady laser a nozzle area of 25 m² the blockage caused by the vehicle is signals showed a standard deviation of 0.2 mm at the front wheel, approximately 9 percent. Using a five-belt rolling road system the 0.5 mm at the front body and 1.5 mm at the rear body when wheels rotate on the four wheel drive units (WDUs) during the driving at 140 kph. measurements while the center belt simulates the relative motion of the road to the vehicle’s underbody (Figure 3). The ground simulation is completed by a boundary layer control system including a scoop, flow suction and tangential blowing. More details of the WT are given by Duell et al. (6) During all tests, both wind and belt velocities were set to 140 kph. In the WT, the wires connecting the laser sensors and pressure sensors from the vehicle to the power supply and the measurement computer outside are laid near the rear door. The cables actually influence the flow around the rear wheel and Fig. 4 Aschheim proving ground. Copyright 2015 Society of Automotive Engineers of Japan, Inc. All rights reserved 24 Bastian Schnepf et al./International Journal of Automotive Engineering 6 (2015) 23-29 Table 1 Vehicle loading. discretizes the particles’ motion both in velocity and direction on an equidistant, cubic lattice.(7) Driver Passenger Back Trunk Tank Since the spectrum of turbulent time and length scales is very front seat large in the case of automotive aerodynamics a direct numerical Standard 68kg 68kg 68kg 21kg 1/1 simulation (i.e. resolving all turbulent scales) is not possible with today’s computational resources. Instead, the smallest turbulent Exp. 85kg 35kg 20kg 60kg 7/8 - 1/1 scales are modeled using an enhanced two-equation RNG k- epsilon turbulence model. The boundary layer is modeled Regarding the loading of the vehicle, deviations from the according to the logarithmic law of the wall, additionally taking loading standard (DIN 70020 / 1) had to be accepted as listed in into account the effect of pressure gradients on flow separation.(7) Table 1.