1

Motorsport Aerodynamics

Jamie

Arner

BrianFeldman

16

2

Importance of Aerodynamics

2

12

3

Aerodynamic Design Methodology

• CFD– Develop parts and visualize flow over car and

devices before models are built

• Wind Tunnel– Scale model testing of devices– Full size testing of vehicle (rolling road)

• Track– Devices are assessed and approved for racing

4

Motorsports• Highly Developed

• Highly Secretive

• Heavily Regulated

•Extremely Competitive

5

5

Front Air Dam

• Limits the Amount of Air Passing Underneath the Car• Contributes to High Pressure In Front of the Car• Causes Low Pressure Underneath the Car

10

6

Front Splitter

Captures Some of the High Pressure Created In Front of the Air Dam, Causing Downforce

13

7

Side Skirts

• Side Skirts Prevent High Pressure Air Around the Car fromDisturbing the Low Pressure Underneath the Car

11

8

Canards (French for Duck)

• Captures a Small Amount of Downforce by RedirectingAirflow Upward.

• Main Purpose is the Creation of Large Vortices That Run the Length of the Vehicle

5

9

Canard Vortices

• Vortices Spin Inwards, Towards the Vehicle on Each Side

• Canard Vortices Work In Conjunction With Side Skirts to Limit the High Pressure Air From Mixing With the Low

Pressure Air Found Underneath the Car

5

10

Venturi Tunnels / Rear Diffusers

5

11

Ground Effects

• Ride Height– Downforce Increases

with Reduced Clearance h

– Diffuser Stall• Individual to Each Design

2

12

Underbody Pressure

• Suction Peak Occurs Near the Diffuser Entrance

• Can be used to Control the Vehicle Center of Pressure

12

13

Rear Diffuser

• Expands Air Back Down to Road Velocity

• Slows the Flow and Raises the Air Pressure

• Acts as a Pump Drawing More Air from the Undertray

• Rear Wing Drives the Diffuser Because of its Proximity to the Wing's Low Pressure Side

• Reduce the Overall Pressure Drag on the Vehicle by Introducing High Pressure Air into the Low Pressure Wake Region

13

14

Diffuser Vortices

• Two Vortices form at the Side Edges of the Diffuser– Flow Separates at the Sharp Leading Edge– Reattached by the Side Vortices – Separation Line is Dictated by the Leading Edge

• Reynolds Number Effect Insignificant

• Reasons for Loss of Downforce at Low Ground Clearance– Vortex Breakdown

2

9

15

Venturi Tunnels

• Venturi effect– constricts the flow

• creates low pressure• high velocity flow

• Less pitch sensitive than flat bottom

• Highly regulated to limit cornering speeds

16

Underbody Vortices

• Induced vortex speeds up flow over F-16 wing– Vortex Lift

• Vortex Generators speed up flow at the front – Strong stable vortex creates

suction loads along its trail– Bernoulli says air

pressure decreases

Strakes

12

17

Vortex Generators

•Trips the Boundary Layer, Causing the Laminar Air Traveling Over the Roof to Become Turbulent

• Typically the size of the boundary layer, 15mm - 30mm at the rear end of a vehicle roof

13

18

Effect of VG’s on Flow Separation

With VG Without VG

VG Separation Region Separation Region

3

19

Effect on Bluff Bodies

With VG Without VG

• Higher Velocities Closer to Car, Resulting in a Smaller Wake Behind Vehicle

• VG’s Cause Drag, but Reduce Pressure Drag by Delaying Flow Separation from Occurring Reducing the Magnitude of the Separation Region Increases the Static Pressure of Separation Region

3

20

Wing Endplates

• A Wing with Endplates is Equivalent to a Longer Wing Without Endplates

• Effective Aspect Ratio

Aspect Ratio (AR) = Span/Chord

AR_effective =

AR*(1+1.9*(Endplate Depth/Span)12

21

Gurney Flaps

Typically 1% to 4% of the Wing Chord Length

5

22

Effect of a Gurney Flap on Flow

• At High Angles of Attack, Flow Separates

• Addition of Gurney Flap Reattaches Flow

• Can Increase the Downforce of the Wing up to 30%

• Can Provide the Same Amount of Downforce with 3O less Angle of Attack

15

23

How a Gurney Flap Works

• Vortices Reattach Flow and Redirect it Slightly Upwards Relative to the Flow Over a “Clean” Wing, Implying An Increase in Circulation as the cause of increased Downforce

5

24

Gurney Flap Size Vs. Lift

6

25

Current Research on Gurney Flaps• Von Karman Vortex Street Forms Downstream of Gurney Flap

• Fluid of Negative Vorticity Becomes “Trapped” Upstream of Gurney Flap

• Trapped Fluid Escapes, Interfering Either Constructively or Destructively with Downstream Vortices

• Increases Circulation and Downforce

6

26

Additional Aerodynamic Considerations

9

5

27

Questions ?

14

28

References1. Jang et al, “Numerical investigation of an airfoil with a Gurney flap” 19982. Katz, Joseph. “Aerodynamics of Race Cars” 20063. Koike et al, “Research on Aerodynamic Drag Reduction by Vortex Generators” 2004 4. Nikolic, “Additional Aerodynamic Features of Wing-Gurney Flap Flows” 20065. Sport Compact Car Magazine “Automotive Aerodynamic: Part 2”6. Troolin et al, “Time Resolved PIV Analysis of Flow Over a NACA 0015 Airfoil with Gurney Flap” 20067. Troolin et al, “Time Resolved PIV Analysis of a Gurney flap on a NACA 0015 Airfoil” 20058. Troolin et al, “The Effect of Gurney Flap Height on Vortex Shedding Modes Behind Symmetric

Airfoils” 2006 9. Zhang, Xin et al. “Ground Effect Aerodynamics of Race Cars” 200610. www.army.mil/armyimages/armyimage.php?photo=680611. www.autocult.com.au/img/gallery/997gt3773.jpg12. www.mulsannescorner.com13. www.seriouswheels.com14. www.topgear.com15. www.insideracingtechnology.com/tech107bndrylayer.htm16. http://zedomax.com/blog/2007/04/19/rolling-wind-tunnel-that-goes-at-180mph/