basics of aero in motorsport
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Motorsport Aerodynamics
Jamie
Arner
Brian
Feldman
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Importance of Aerodynamics
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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
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Motorsports
• Highly Developed
• Highly Secretive
• Heavily Regulated
•Extremely Competitive
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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
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Front Splitter
Captures Some of the High Pressure Created In
Front of the Air Dam, Causing Downforce
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Side Skirts
• Side Skirts Prevent High Pressure Air Around the Car from
Disturbing the Low Pressure Underneath the Car
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Canards (French for Duck)
• Captures a Small Amount
of Downforce by Redirecting
Airflow Upward.
• Main Purpose is the
Creation of Large Vortices
That Run the Length of
the Vehicle
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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
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Venturi Tunnels / Rear Diffusers
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Ground Effects
• Ride Height – Downforce Increases
with Reduced Clearance h
– Diffuser Stall
• Individual to Each Design
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Underbody Pressure
• Suction Peak Occurs Near the Diffuser Entrance
• Can be used to Control the Vehicle Center of Pressure
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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 theDiffuser Because of its Proximity to the Wing's Low Pressure Side
• Reduce the Overall Pressure Drag on the Vehicle by Introducing HighPressure Air into the Low Pressure Wake Region
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Diffuser Vortices
• Two Vortices form at the Side Edgesof 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
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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
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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
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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
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Effect of VG’s on Flow Separation
With VG Without VG
VG Separation Region Separation Region
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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 RegionIncreases the Static Pressure of Separation Region
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Wing Endplates
• A Wing with Endplates is Equivalent toa Longer Wing Without Endplates
• Effective Aspect Ratio
Aspect Ratio (AR) = Span/Chord
AR_effective =
AR*(1+1.9*(Endplate Depth/Span)12
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Gurney Flaps
Typically 1% to 4% of the Wing Chord Length
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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 Attack15
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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
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Gurney Flap Size Vs. Lift
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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
Downforce6
Addi i l A d i
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Additional Aerodynamic
Considerations
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Questions ?
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References1. Jang et al, “Numerical investigation of an airfoil with a Gurney flap” 1998
2. Katz, Joseph. “Aerodynamics of Race Cars” 2006
3. Koike et al, “Research on Aerodynamic Drag Reduction by Vortex Generators” 2004
4. Nikolic, “Additional Aerodynamic Features of Wing-Gurney Flap Flows” 2006 5. 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” 2006
7. Troolin et al, “Time Resolved PIV Analysis of a Gurney flap on a NACA 0015 Airfoil” 2005
8. 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” 2006
10. www.army.mil/armyimages/armyimage.php?photo=680611. www.autocult.com.au/img/gallery/997gt3773.jpg
12. www.mulsannescorner.com
13. www.seriouswheels.com14. www.topgear.com
15. www.insideracingtechnology.com/tech107bndrylayer.htm
16. http://zedomax.com/blog/2007/04/19/rolling-wind-tunnel-that-goes-at-180mph/