Tài liệu RACE CAR AERODYNAMICS: KTH - ROYAL INSTITUTE OF TECHNOLOGY pdf

Race Car Aerodynamics - May 21*, 2010 Historic overview Race car categories Aerodynamic and performance Aerodynamic tools Validation: CFD, Wind Tunnel, Track Test... Historic overview

Race Car Aerodynamics

KTH - Royal Institute of Technology

Stockholm - May 21*, 2010

Corrado CASIRAGHI

Tatuus Racing

Race Car Aerodynamics - May 21*, 2010

Historic overview Race car categories

Aerodynamic and performance Aerodynamic tools

Validation: CFD, Wind Tunnel, Track Test

Historic overview

Race car evolution

Downforce research: tire and engine technology are improved

Race Car Aerodynamics - May 21%, 2010

se Race Car Evolution

e Extreme solution: adjustable wings, suction fans

Race Car Aerodynamics - May 21*, 2010

Historic overview

e Race Car Evolution

e Wing cars: reversed wing underbody and sealing skirts

| | i

Race Car Aerodynamics - May 21*, 2010

se Race Car Evolution

e Modern era: flat and “stepped” underbody

1983: McLaren MP4-1C 2004: Jordan “stepped” underfloor

Race Car Aerodynamics - May 21*, 2010

odynamics - May 21*, 2010

Race car categories

e Sedan-based race cars

e WICC, Rally, Nascar,etc

e Enhancements in stiffness and safety (roll-cage), minimum aerodynamic modifications

Race Car Aerodynamics - May 21*, 2010

Aerodynamic and performance

Race Car Aerodynamics - May 21*, 2010

Aerodynamic and performance

Aerodynamic and performance

e Downforce

e Vehicle stability and handling are primarily dictated by tyre performance, but _ this performance is considerably’ related to aerodynamic loads, i.e optimal loading of the tyres by the control of front and rear downforce can lead to:

e Improved braking performance

e Increased cornering speed

e Stability (necessary to achieve cornering speed)

Race Car Aerodynamics - May 21*, 2010

e Downforce and grip

The tyre can transfer a force through its contact that is a function of the vertical load (linear)

In the normal range of use it can be assumed:

May 21°, 2010

500 600

Race Car Aerodynamics -

Aerodynamic and performance

e Braking performance

e Increased downforce reduces braking space

Aerodynamic and performance

e Cornering Speed

e Steady-state turning leads to forces on the tyres which increase with downforce and to centrifugal forces which increase with cornering speed

Cornering speed

e Any lateral irregularity (bump, wind gust) will cause an

ee w initial side slip that tends to generate an aerodynamic side

force that tend to increase the side slip, i.e unstable without driver correction

e B: CP behind CG

c.p is aft of c.g CG in order to have a good lateral stability at high speeds

where aerodynamic forces are significant

Race Car Aerodynamics -

e A: Low-speed (negligible lift) vehicle with side slip angle 6 due to lateral force (wind or centrifugal)

e The side force created by tyres is proportional to the normal load, i.e proportional to the weight on the front (W.) and rear (W ) axles

e If the moment about the CG created by the rear tyres exceeds that created by the front tyres, such that the net

moment tends to rotate the car in the direction of slip,

then there is understeer (Stable)

e B: High-speed (significant lift) vehicle with side Slip angle B

e Here the downforce is generated at the front and there is some rear positive lift (typical of some production cars)

se If the moment about the CG created by the front tyres exceeds the rear tyre moment, such that the net moment

tends to turn the car away from the side slip direction, then

there is oversteer and possible vehicle spin (Unstable).

Race Car Aerodynamics -

Aerodynamic and performance

e Lap-time

e In racing top speed is often not relevant and each track requires different aerodynamic settings:

e High speed track with serious accelerations and sharp

corners (i.e Monza) requires low drag/low downforce setting

e High speed track with fast corners (i.e Barcelona, Spa) requires high downforce setting

e The overall lap-time is a result of corner, braking and top speed:

e Due to the modern circuit layout most of the lap-time is

spent ¡in acceleration, deceleration, cornering, so downforce plays a greatest part than pure efficiency

May 21°, 2010

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underfloor

Race Car Aerodynamics - May 21*, 2010

e Wings are the most efficient aerodynamic device

se Open wheeled rear wings have a very small aspect ratio

se Wings are installed far-forward far-after to enhance their balancing effect

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e Wings

e Race car wings are designed to heavily interact with the surrounding bodies: e.g the rear bottom wing works in symbiosis with the underfloor diffuser to pump air from the venturi

e Wings

e Endplates are important for lateral stability and to separate the wing from the turbulent wheel flow, big endplates are helping to restore a 2D flow

e Front wings operate in extreme ground effect and are affected by vehicle pitch

Race Car Aerodynamics - May 21°, 2010

Barge boards and side boards

e Bargeboard is a_ vertical panel situated longitudinally, between the front wheels and the sidepods

e Bargeboards act primarily as flow conditioners, smoothing and redirecting the turbulent (or dirty) air in the wake of the front wing and the rotating front wheels

Aerodynamic Tools

e Barge boards and side boards

e Bargeboards act as vortex generators, redirecting and energizing airflow: the upper, downward sloping edge shed a large vortex downstream around the sidepods, where it aid in sealing the low pressure underbody flow from the ambient stream The bottom edge of the bargeboard shed

Static pressure Changes to the

ueddirbelén catesed bey vemool of the vortices that energize the airflow to the

bargeboards, aes underbody, which can help delay flow separation

and allow the use of more aggressive diffuser profiles |

e Spoilers and splitters

e Spoilers on the front of a vehicle are often called air dams, because in addition to directing air flow they also reduce the amount of air flowing underneath the vehicle which — reduces aerodynamic lift

e The splitter is an horizontal lip that brought the airflow to stagnation above the surface, causing

an area of high pressure Below the splitter the air

is accelerated, causing the pressure to drop This, combined with the high pressure over the splitter creates downforce

Race Car Aerodynamics - May 21*, 2010

Race Car Aerodynamics - May 21°, 2010

e CFD process iterates during the design of the vehicle

e Neutral file are saved for the CFD analysis

e Geometry clean-up is needed to fix CAD model geometry (holes, overlapping )

e Meshing is required to solve the case

Race Car Aerodynamics - May 21°, 2010

e CFD solving requires high power computing (HPC) to get a good and reliable result

e Actual model size is about 40-60 Million volumes

e Cluster computing allows parallel solving of the model

e Solving time requires 8-24 hours

e Typical cluster sizing is:

e 32 cores CPU

s 64 Gb RAM

e High speed intranet

e CFD post processing is the key to iterate

the CAD design process

e Typical visualization can show

e CFD post processing is the key to iterate

the CAD design process

e Typical visualization can show

FLUENT 6.2 (2d, segregated, rke)

Race Car Aerodynamics - May 21*, 2010 - ~

Race Car Aerodynamics - May 21°, 2010

e Fluid-structure interaction (FSI) is still ongoing but the maturity of these fields enables numerical simulation

investigated

Race Car Aerodynamics -

e The largest test section would be desirable to reduce blockage and better simulate real condition, but operational cost of a full scale tunnel is huge

e Wind Tunnel: Scaled Model

e Most of the wind tunnels use scaled models

se The aerodynamic similitude is respected if

coefficients are the same for scaled and real model:

e Viscous similitude: Reynolds = pvl/u

e Compressible similitude: Mach=v/a

e Gravitational similitude: Froude = (v7/lg)"’”

When the model is steady and air is flowing Froude is neglected and to respect the dynamic similitude Reynolds and Mach numbers should be the same than full scale

In low speed tunnels Mach number is neglected and Reynolds remains the only coefficient to be targeted, in reality it cannot be matched because the air speed cannot be scaled up sufficiently

Race Car Aerodynamics - May 21*, 2010 (cost and transonic speed)