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Introduction to Aerodynamics

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Introduction to Aerodynamics Introduction to Aerodynamics Aerodynamic Vehicle Design and Analysis Table of Contents Introduction Page 3 Aerodynamic Basics Page 3 BMW Z4 Page 4 Concept Modelling Page 5 Initial Testing Page 6 Addition of Front Splitter, Side Skirts & Rear Diffuser Page 8 Addition of Rear Wing Page 11 Final Concept Page 13 Vehicle Dynamics Page 14 Mathematical Validation Page 15 Conclusion Page 16 Future Work Recommendations Page 17 References Page 18 Appendices Page 19 Introduction In this project it necessary to build a 3D surface model of a BMW Z4 concept car using industry standard aerodynamic CAE (Computer Aided Engineering) software then import the model CFD (Computational fluid dynamics) software. Using the CFD software an appropriate simulation will be created for accurate post processing. Using the results gathered from the CFD simulation a critical analysis will be made on the vehicles drag, lift and flow characteristics under, over and around the vehicle. After analysing the data from the CFD simulations, improvements will be made to the model to improve downforce while minimise the increase in drag. Basics of Aerodynamics Aerodynamics is the study of the interaction between moving bodies and the fluids around, under and though them. Aerodynamics first came from aeronautical engineers studying aircraft wing design for aircrafts that fly within the earth’s atmosphere. Aerodynamics are used in designing many different things including building design, bridge design and motorsports/automotive vehicle design and many more. (Jim Lucas, 2014) Drag Aerodynamic drag is the force opposing the vehicles direction of movement. (See figure 1) The main contributor to vehicle drag is the high pressure acting on the front of the vehicle, surface friction and the relatively negative pressure left behind the vehicle. (Jim Lucas, 2014) The formula to mathematically calculate aerodynamic drag is as followed. 1 퐷=푐퐷 ⁄2휌푣2퐴 D = Drag Force N CD = Drag Coefficient ½ = Mathematical Constants ρ = Air Density Kg/m3 V2 = Speed m/s A = Frontal Area m2 Figure 1 Lift & Drag Direction (NASA, 2014) (Joseph Katz, 2006) Lift Lift is force acting perpendicular to the motion of the vehicle. It is essential for aircrafts to create positive lift to fly, this is unwanted in motorsports. In motorsports negative lift it sought after to force the vehicle into the ground, this force acting on the vehicle helps increase vehicle grip which leads to faster Page 2 of 25 cornering speed. The formula to mathematically calculate aerodynamic lift is similar to the drag formula 1 although the drag coefficient is replaced with a lift coefficient and is as followed. 퐿=푐퐿 ⁄2휌푣2퐴 L = Lift N CL = Lift Coefficient A = Frontal Area (Vehicle) Plan Area (Aerofoil) m2 (NASA, 2016) (Joseph Katz, 2006) Downforce Downforce is a motorsports/automotive term that applies to negative lift. This is the force that pushes a vehicle into the ground this is a sought after affect in the motorsport industry to increase cornering speeds. Lift and drag coefficients Lift and drag coefficients are a number given to a model and is affected by from vehicle shape, surface friction (drag) and angle of attack (lift). 퐷푟푎푔 퐷푟푎푔 퐶표푒푓푓푐푒푛푡 = 1⁄2 휌푣2퐴 퐿푓푡 퐿푓푡 퐶표푒푓푓푐푒푛푡 = 1⁄2 휌푣2퐴 (Joseph Katz, 2006) BMW Z4 The BMW Z4 is a rear wheel drive sports car, with a 3 litre straight six engine mounted at the front. It has a wheel base of 2497mm and an overall length of 4091mm, width of 1781mm and a height of 1268mm. The Z4 weighs 1395Kg and has a 50:50 weight distribution. The quoted drag coefficient of the Z4 is 0.34 and has a frontal area of 1.91m2 given it a drag coefficient/area ratio of 0.65. (Car Foilo, 2006) Figure 2 BMW Z4 (BMW, 2016) Concept Modelling A concept model was designed using the basic dimension and canvas images of the BMW Z4. The software chosen for this was Autodesk Alias SpeedForm 2016. Autodesk Alias is an industry standard Page 3 of 25 surface modelling package, Alias is a great visualization tool for automotive design and has powerful rendering features. (Autodesk, 2016) See figure 3 below for an image of the Z4 concept during the modelling stage. During this design process a symmetry plane was used (Green line though the vehicle in figure 3), this was used to help keep the vehicle symmetrical, any modifications made to one side would automatically update on the other side. See appendix 1 for more images of the Z4 concept during the design process. Figure 3 Z4 Concept Design Stage See figure 4 below for a rendered image of the BMW Z4 concept design rendered in Alias SpeedForm with added wheels designed in Autodesk Inventor. See appendix 2 for more rendered images of the conecept. Figure 4 Z4 Concept Render image Initial Testing Initial testing was carried out on the Z4 concept vehicle using CD-Adapco’s Star CCM+ CFD (Computational Fluid Dynamics) Software. Star CCM+ is a powerful CFD solver that can handle multiple physics and complex geometries, as well as being a CFD solver Star CCM+ solve for heat transfer and stress. (CD-Adapco, 2016) Page 4 of 25 All tests were carried out at 120kph, and in a wind tunnel measuring 65m in length, 10m wide and 10m high. The air density used for testing is 1.2255kg/m3 which is the air density at sea level at 10°C. (Richard Shelquist, 2016) Analysing Results The result from initial testing are as followed, 퐹표푟푐푒 푊푒푔ℎ푡 퐹표푟푐푒 (퐾푔) = 9.81 (퐺푟푎푣푡푦) Drag 506.33 N (51.61Kg) Lift 314.48 N (32.06Kg) Frontal Area 1.74 m Drag Coefficient 0.43 Lift Coefficient 0.27 Drag/Lift Ratio 1.61 Lift/Drag Ratio 0.62 After analysing the results from initial testing its shown that the Z4 concept is actually creating lift, this is unwanted in a sports type vehicle, this could lead to undesirable handling characteristics e.g. under/over steer. When analysing the vector scene (see figure 5) it is clearly visable what the main contributers to the vehices lift are. The first being the large amount slow velocity air traveling under the vehicle, this low velocity will be creating a high pressure area which will try to force the vehicle upwards. The second biggest area creating lift is the high velocity air passing over the roof of the vehicle, this high velocity creates a low pressure passing over the vehicle, the low pressure area pulls the car upwards creating lift. Flow Separation Figure 5 Initial Testing Vector Scene Page 5 of 25 Figure 6 Initial Testing Resampled Volume Scene After further post processing its clear what the main areas that are creating drag are. The biggest contributor to the vehicles drag is the wake, a large, slow speed turbulent area of air behind the vehicle trying to pull the vehicle back this can be seen in the vector scene and the resampled volume scene. (See figure 5 & 6) Another area creating vehicle drag is the high pressure zone on the front of the vehicle this can be seen in the scalar scene (see figure 7). The pressure acting on the front of the vehicle tries to push the vehicle in the opposing direction to the vehicles movement. Figure 7 Initial Testing Scalar Scene Another part creating drag is the flow separation areas at the bottom of the windscreen and over the rear window these are highlighted in the vector scene. (See figure 5) The flow separation areas create turbulence on the surface of the vehicle, also within these separation areas the velocity of the air almost becomes zero. The final main area to cause drag are the vehicles wheel arches, the wheel arches reduce the velocity of the air creating high pressure zone. These high pressure zones try pushing the vehicle in the direction opposing its motion in the same way it occurs on the front of the vehicle. This can be seen in the resampled volume scene. (See figure 6) For more images from initial testing see appendix 3. Addition of Front Splitter, Side Skirts & Rear Diffuser To create more downforce from the underbody design of the vehicle the amout of air allowed to travel under the vehicle must be reduced and the velocity of the air that is allowed to travel under the vehicle needs to be increased which will create a low pressure area which will pull the car into the ground. In Page 6 of 25 an attempt to increase vehicle downforce a front splitter, side skirts and rear diffuser where designed and added to the Z4 concept. Front Splitter A front splitter consists of a parallel extension attached at the bottom of the front bumper. The way the front splitter creates downforce is by creating a high pressure area above the splitter and low pressure below, this high pressure is drawn to the low pressure forcing the front end of the vehicle into the ground. (Formula 1, 2016) Side Skirts Side skirts are used to minimise the clearance between the vehicle body and the ground at the side of the vehicle. The reason for side skirts is to help aerodynamic downforce this is done by reducing the amount of high pressure from around the vehicle being drawn to the low pressure area under the vehicle. The effectiveness of the side skirts depend on the clearance from the ground, 2cm or less is best practice anything above this any bigger ground clearance diminishes quickly. (Formula 1, 2016) Diffusers Diffusers are designed to increase the volume at the rear of the under body of the vehicle, this increase in volume creates a void which needs to be filled this increases the velocity of the air traveling at the rear of the vehicle.
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