Aerodynamics of Cars

8/8/2019 Aerodynamics of Cars

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Aerodynamics of carsDrag reduction

Alessandro Talamelli

Johan Westin

Mekanik/KTH

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Outline

General remarks on drag of cars

• How to analyse dragLocal origins of drag

• Individual details and their contribution to dragEx. Optimisation of Opel Calibra


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Flow around a car

• Car=relatively bluff body(cD=0.25-0.45)

• Two types of separation1) Quasi 2D wakes2) Longitudinal vortices

• Rear end determines thewake structure

1) Square back

2) Fast back3) Notch back

• Underbody flow and wheels• Interactions – ground effect

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Drag and Lift

• Drag and lift normally related (lift generates drag)• Wing theory: Drag = profile drag (form drag + friction

drag) + induced drag (induced drag from wingtipvortices)

• Cars: low aspect ratio (Λ0.4)• Interaction between tip vortices and the central flow

c Di = k c L

2

ΛΛ =

b2

A plan


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Approaches to analyse drag I

Examine the physical mechanisms

• Identify separation regions• Measure pressure and wall-shear stress• Drag obtained from surface integral

• Lot of experimental data needed (unrealistic)• It is possible to find the local origins of drag

D = psinϕ dS + τ w cosϕ dS

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•Usually possible forsimple bodies (seefigure)

•Problem: in real carsdifferent components

interact!

Approaches to analyse drag II


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Wake analysis

• Control volume approach + momentum theorem• Energy assessment!! Stationary wall: must subtract contribution fromwall boundary layer

• Extensive measurements needed (costly)• Need of traversing mechanism

c D A = (1− c ptot )dS − 1− u

V

2

S

S

dS + v

V

2

+ w

V

2

S

dS

Approaches to analyse drag III

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Local origins of drag - Front end I

•Localseparation lesspronouncedsuction peak -increased drag

•Small edgeradius enoughto reduce localdrag


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• Optimization of the frontof Golf I

• Small radii can givesignificant drag reduction

Local origins of drag - Front end II

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• Drag reduction due to hoodangle (α)saturatesBUT: Combination effect of

hood angle & front radius!

• Increased angle of windshield (

δ) can reduce drag

• δ60° => visibility andtemperature problems

• indirect influence on drag: – Influence flow around A-pillar

– Smaller suction peak at the junction to the roof

Local origins of drag - Angle of hood andwind shield I


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• 3D-separation (vortex)• Wind noise• Water and dirt

deposition

Local origins of drag - A-pillar

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• Mainly friction drag (flow isgenerally attached)

• Increased camber givelarger radii =>reduced

suction peaks

• Negative angle of roof =>reduced wake

• Problems: large front areaand/or smaller internalspace

Local origins of drag – Roof and Sides


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Local origins of dragUnderbody flows

• Complex flow• Flow angles important• Avoid obstacles (stagnation)• Return of cooling air can

influence

• Large improvement by rearpanels

• Also effect on lift

Ahmed (1999)

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Local origins of drag - Rear End

Boat tailing

Mair: axisymmetric body

Mercedes 190Angle ca 10°

•Increase base pressure•Reduce base area•Minor improvements byfurther extension of thebody (x/d > 5)

•Squareback vehicles:lower the roof

•Flow devices (Air intakes,wings)


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Boat-tailed underbody

Local origins of drag - Rear End II

•Requires smoothunderbody

•Decreased drag formoderate diffuser angles

•Reduction in lift

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Fastback/squareback

• Basic experiments =>understanding of rear end flow• Drag due to strong side vortices• Vortex break-up above critical slant angle

Morel (1976) Bearman (1979)

Local origins of drag - Rear End III


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• Prismatic body near ground (qualitatively similar results)• Critical slant angle ca 30°• Drag minimum at ϕ15º (coupé)

Bearman (1982)

Morel (1976)

Fastback/squareback

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• Bi-stable separation aroundcritical angle (ϕ30º)

• ϕ>30º: reduced drag andflow conditions similar to asquare back

• ϕ>30º: Vortices are weakerand with opposite rotational

direction than ϕ


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Rounded rear edges

• Previous findings based on bodies with fairlysharp edges

• Rounded side edges => no fixed separationpoint

• Rounded rear => optimum base height morerelevant than optimum slant angle

• Also: the base height influence by sloping sideedges

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Notchback

• Interaction – quasi-2D separation

– 3D vortices

• Several geometricalparameters

• Also influenced by – Radius roof-window

– Shape of C-pillar

– Rear end of trunk


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Flow in the “dead water region”

• Counter rotating vortices(try to identify during PIV-

lab)

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Sensitivity to side wind

• Wake-analysis behind anotchback (Cogotti 1986)

• Total-pressure distributionshow strong influence of

small yaw angles (β=0,

0.5°& 1°)• Bi-stable flow at β= 0.5°


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• Attempt to explainasymmetric wake

– A-pillar vortices interact withrear vortices

– Very small yaw angleschange the relative strengthof A-pillar vortices

• Symmetric flow patternunlikely on three-box config.

Sensitivity to side wind II

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Mechanism of a 2D-diffuser

• Pressure increases as longas flow not separated

• Max diffuser length longerfor small diffuser angles

• Is the analogy with a 2Ddiffuser really correct?

• (Can explain reduced drag,but not reduced lift)

2θ


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Aspects of underbody diffuser

Rear end underbody diffuser brings up thevelocity below the car (normally reduces lift)

Higher velocity below the car changes flowangles around the wheels

• Reduced drag due to the wheels• Requires a smooth underbody to avoid drag from

obstacles

Underbody diffuser reduces the base area of thevehicle (can reduce drag)

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Underbody shaped for downforce I

Ferrari 360 Modena

• “Venturi-tunnel” formax downforce

• Smooth underbody

• No spoilers• 5400 hours in wind-tunnel (source:

Teknikens Värld 11/99)


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Underbody shaped for downforce I1994 1999

F355360 Modena

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Wheels

• Up to 50% of the drag of astreamlined car

• Wheels are not streamlined – 3 vortex pairs

– Influenced by ground androtation

• Local flow is yawed (15°) – Separation on the outer side

– Water drops sucked out


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Wheels (contd)

• Force on a rotating wheelchanges sign when contact with

ground

• Lift force due to wheel rotationfor a free-standing wheel

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Wheels (contd)

• How does the flow in thewheel-housings look like?

• Wheel-housings

– Smaller=better – Both lift and drag reduced

– Largest effect on lift (see e.g.Cogotti 1983)


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Influence of wheels on Audi A3

• 30-35% of drag due to wheels + wheel arches• Ca 25% only due to wheels From Pfadenhauer,Wickern & Zwicker (1996)

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Spoilers

Front spoiler

Hucho (1998)

+Reduced drag

+Reduced front axle lift

+Improved cooling air flow

•Reduced flow rate underthe car

•Low pressure regionbehind the spoiler

•Optimization needed


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Rear spoiler+ Reduced drag

(sometimes)

+ Reduce rear axle lift

• Higher cP in front of thespoiler

• Increased spoiler heightincreases the lift, but

also drag

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Miscellaneous

• Cooling air flow:cD0.02-0.06

• Side mirrors: cD0.01

• Antenna: cD0.001• Roof racks: up to 30-

40% increase in cD

• Ski box: Why are theyshaped in this way??


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Potential fields for drag reduction

More focus on underbody and wheels

Active reduction of the dead-water region

• Base bleedSeparation control

• Boundary layer suction?

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Discrepancies in cD

Different cD depending on equipment

• Tire width• Engine type (cooling air flow)• Ground clearance (load dependent)

• Angle of attack (load dependent)• Additional spoilers etc.

Official cD values “corrected”


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Aerodynamic optimization of Opel Calibra

• Front spoiler heightoptimization

– Determined by minimumground clearance

From Emmelmann et al (1990)

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• Rear end optimization(1:5 scale model)

• Rear end tapering• Decklid height optimization

– Interdependent effect of

decklif height and rear endtapering

– Large number ofparameter tests needed


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• cD0.28 at this time• Wake analysis (total

pressure) and flow vis.

– No noticeable tip vortices

– “Ears” due to A-pillars

– Wide wake close to ground

– 30° flow angle at frontwheels

• Drag reduction by – Reduced spoiler height in

the centre

– Increased spoiler height infront of wheels

– Lower the door sills

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• Further aerodynamical development – Anti-contamination lips

– Cooling air inlets (cD=0.014 for air passing throughfront end)

Before (cD0.28) After (cD0.26)

Aerodynamics of Cars

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