Experimental AerodynamicsVehicle Aerodynamics
ExperimentalAerodynamicsVehicle AerodynamicsLecture 2:A history of car aerodynamicsG. DimitriadisExperimental Aerodynamics
What has aerodynamicsever done for us?Experimental Aerodynamics
It all started in Schaerbeek ! Camille Jenatzy, a Belgian race cardriver, was the first to design a carusing aerodynamic principles.La Jamais Contente byJenatzy. It broke the 100kphbarrier in 1899 and waselectrically powered.The body itself wasstreamlined but the driverand the wheels were not Experimental Aerodynamics
Alfa Romeo Ricotti ! Built in 1914 for Count Ricotti andnicknamed ‘La bomba’.Its geometry gave it atop speed of about135kph.It was a commercialfailure either due tothe war or due to itsweird appearance.Experimental Aerodynamics
Birth of aerodynamics ! Aerodynamics was born as a science atthe end of the 19th and beginning of the20th century. ! The main factors contributing to thedevelopment of aerodynamics were:–! Experimental work by Lillienthal, Langley andthe Wright brothers.–! Theoretical developments by Kutta,Joukowski, Prandtl and others.–! Heavier than air flight by the Wright Brothers,Santos-Dumont and others.Experimental Aerodynamics
A bit about drag ! Aerodynamic drag is the force applied by thewind on the car in the wind direction. ! It can be written in the form1D !V 2CD S2 ! where ! is the air density, V the airspeed, CD anon-dimensional drag coefficient and S thefrontal area of the car. ! The drag coefficient is assumed to be aconstant in a specific range of Reynoldsnumber values.Experimental Aerodynamics
Drag coefficient ! The drag coefficient is important butdoes not determine completely the dragof a car. ! A small drag coefficient can still giverise to high drag if the frontal area is bigand vice versa. ! The product CDS provides a morecomplete picture of the drag of a car.Experimental Aerodynamics
Car streamlining ! As aerodynamic principles becameavailable to car engineers, the concept ofstreamlining was developed. ! The shape of a falling drop of water wasconsidered to be aerodynamically perfect. ! Hence, several drop-shaped cars madetheir appearance, starting just after WWI.Experimental Aerodynamics
Rumpler Tropfenwagen ! Edmund Rumpler was a Viennese aeronauticalengineer who designed cars after the war. ! In fact, as Germany was forbidden from buildingaircraft after the war, many aero engineersconverted to car design with an ‘aeronautical’flavour.Experimental Aerodynamics 1922Tropfenwagen
First wind tunnel experiments ! Rumpler was also one of the first to carryout wind tunnel experiments on cars. ! In 1922 he measured the aerodynamicdrag of a scale model of theTropfenwagen. ! He found that the drag of his car wasabout 1/3 of the drag of contemporaryvehicles. ! Nevertheless, the car was a commercialfailure.Experimental Aerodynamics
Paul Jaray ! Paul Jaray was an Austro-Hungarian engineerwho designed Zeppelin airships. ! After the war he created many streamlined cardesigns. ! His cars also featured smooth body surfaces,integrated fenders and headlamps, camberedwindshield and other innovations. ! His designs were adopted or copied by severalcar manufacturers, such as Audi, BMW, VWDaimler-Benz and others.Experimental Aerodynamics
Jaray cars1933, Tatra V5701923, Audi Type K1933 Tatra 77Experimental Aerodynamics1933 Mercedes Benz 200
Minimum drag coefficient ! Jaray and Kemplerer carried out aseries of wind tunnel tests on halfbodies in 1922. ! The starting point was a body ofrevolution with a length-to-diameterratio of 5. ! Half-bodies based on this shapewere tested close to the ground. ! The minimum drag coefficient for ahalf-body with wheels was found tobe equal to 0.15. ! Subsequently, many vehicleaerodynamicists tried to createvehicles with CD 0.15.Experimental Aerodynamics
Other streamlined cars1923 Benz RH TropfenwagenExperimental Aerodynamics1932 AutoUnion Type C1940 Alfa Romeo 5121923 Bugatti Type 32
Comments on streamlined cars ! Streamlined cars are based on airfoils or halfairfoils. ! These shapes were shown to have very lowdrag in the wind tunnel. ! However, real cars based on these shapes hadmuch high drag coefficients of around 0.4 ! Real cars have windows, bumpers, sidemirrors, headlights, engine intakes, wheelwells, wheel fairings etc. ! All of these elements increase the dragcoefficient significantly.Experimental Aerodynamics
Success of streamlined cars ! Streamlined cars never became reallypopular. ! Airfoil shapes are not very practical orcomfortable. ! Streamlined cars remainedexperimental or limited to speed recordattempts. ! There some notable exceptions.Experimental Aerodynamics
Porsche 911The Porsche 911 is one of thevery few streamlined cars thatare still in production today.Experimental Aerodynamics
VW BeetleThe Beetle was a Porschedesign based on Jaray’scars, particularly the TatraV570.It was a pseudo-Jarayshape, because it had amuch too steep rear slope.Such cars are known as‘fastbacks’.Nearly 22 million beetleswere built from 1938 to2003.Experimental Aerodynamics
Kammback cars ! Another German aerodynamicist,Wunibald Kamm, imported an importantaerodynamic principle into car design. ! Airfoils with a truncated trailing edge haveonly slightly higher drag than completeairfoils. ! By consequence, streamlined cars withtruncated rear ends have slightlyincreased drag but are lighter and cheaperto build.Experimental Aerodynamics
Kammback cars1938 BMW KammCoupé1976 Porsche 9241974 Citroën CXExperimental Aerodynamics
Postwar era ! The immediate postwar era saw a moveaway from aerodynamics. ! Car design in the United Statesconcentrated on large, spacious andcomfortable cars with a ‘bathtub body’. ! In Europe cars remained smaller due tothe small European city roads and lack ofparking space. Nevertheless,aerodynamics was not fashionable.Experimental Aerodynamics
US cars of the 50s ! Large and bathtuby ! Nevertheless, thestreamlinedbumpers, headlightsand wheels dodecrease drag to acertain extent.Experimental Aerodynamics
European cars of the 50s1959 Fiat 5001950 Citroen 2CV1952 Mercedes Type 300Generally small and boxy.Experimental Aerodynamics
The petrol crisis ! Aerodynamics came back into cardesign in the 1970s after the end of thepostwar boom. ! The petrol crisis in particular turned fuelefficiency into an important issue again. ! Aerodynamic design started to improveagain and the drag coefficient of theaverage car started decreasing.Experimental Aerodynamics
European car CD valuesFord Probe VExperimental Aerodynamics
US drag historyChrysler’s car linea)! Cadillacb)! BuickGM carsExperimental AerodynamicsFord’s US car line salesweighted average dragcoefficient.
Detail optimization ! The drag improvements of the 70s wherebased on detail optimization. ! It’s based on carrying out numerous minorlocal modifications on order to obtainsignificant total drag reductions. ! Such details are the curvature of edgesand pillars, camber of panels, tapering,size and location of spoilers, side mirrorfairings etc.Experimental Aerodynamics
The devil is in the details1974 VW Scirocco. Designedby detail optimization. Its dragcoefficient was 0.411969 Opel GT. Designed tobe streamlined. Its dragcoefficient was also 0.42Experimental Aerodynamics
Shape optimization ! Detail optimization yielded impressive results but quicklyreached its limits. ! From the late 70s onwards, there was a re-evaluation ofthe work of 1930s aerodynamicists in order to obtainfurther drag reductions. ! Shape optimization was introduced. It starts from abasic, low drag shape, which is gradually modified insmall steps to yield a realistic car shape. ! The first car to be designed using this approach was theAudi 100. The 1983 C3 had a drag coefficient of 0.3. ! This car also pioneered flush windows, which helpedachieve this low drag value. Such windows becamestandard on all cars later on.Experimental Aerodynamics
80s shape optimization1983 Audi 100 C31984 Renault Espace1983 Ford probeIV. CD 0.15Experimental Aerodynamics
Current state of the art ! The current state of the art in aerodynamicdesign combines both shape and detailoptimization. ! A reasonable drag coefficient is around0.25-0.3 for a modern car. Cd values of under0.25 is a reasonable future target. ! Some common characteristics:–! Small radiator air intakes placed near the frontstagnation point, under the bumper.–! Higher inclination angles for both front and rearwindscreensExperimental Aerodynamics
More about drag ! Car drag can be split into two principalcomponents: ! External flow drag:–! Car body–! Protuberances: mirrors etc–! Wheels and wheel wells ! Internal flow drag:–! Engine cooling–! Heating and ventilation–! Components: brakes etcExperimental Aerodynamics
Physics of external drag ! For external car flow, there are threeprinciple mechanisms that cause drag:–! Pressure drag: drag caused by differencesin pressure between the front and the backof the car.–! Skin friction drag: drag due to the tendencyof the airflow to stick to the body’s surface.–! Interference drag: drag due to flowinterference between different componentsof the car.Experimental Aerodynamics
Pressure dragPressure drag is causedby the difference betweenattached flow at the frontof the car and detachedflow at the back.The absence of a rearstagnation point meansthat there is a significantdifference in pressurebetween front and back.Therefore, there is a netforce opposing themotion.Experimental Aerodynamics
More on pressure dragThis smoke flowvisualization photoshows clearly thedifferences in thenature of the flowbetween the front andback.The separated flow in the wake is air that is set in motionby the car. If the car is adding momentum to the air, then itmust lose momentum itself. This loss of momentum is adrag force.Experimental Aerodynamics
Pressure distributionExample of pressure distribution around car. The dragforce is given by the integral of the pressure distribution:d Experimental Aerodynamics! pdS
Vortices behind carTypical vorticalstructures behind acar. The size,strength and shapeof the vorticesdepends on the bodygeometry.Experimental Aerodynamics
Car wakes are 3D structures3D car wake fromsuperposition of two2D sections. SectionA1 shows separationin the longitudinaldirection. Section A2shows shed vortices.The result is a Ushaped vortex.Experimental Aerodynamics
Skin friction drag ! This type of drag is caused by the frictionbetween the air and the body’s surface. ! Skin friction is generated inside theboundary layer and depends on itsproperties.Skin friction dependson the wall shear"u!w µ"y y 0d Experimental Aerodynamicsl" ! ( x )dx0 w
More on skin friction ! Skin friction depends on whether theboundary layer is laminar or turbulent. ! Turbulent flow causes higher values ofwall shear. A smooth body surfaceensures that the boundary layer remainslaminar as much as possible. ! Skin friction, as defined here, is onlypossible when there is a boundary layer.When the flow is separated the definition isnot valid.Experimental Aerodynamics
Interference drag ! Interference drag is very difficult to quantify. ! As an example consider this: the drag of a carwith side mirrors is higher than the drag of thecar without the mirrors plus the drag of themirrors themselves. ! The additional drag is caused by aerodynamicinterference between the car body and themirrors. ! Other components causing interference:wheels, wheel wells, bumpers etcExperimental Aerodynamics
A history of car aerodynamics G. Dimitriadis Experimental AerodynamicsVehicle Aerodynamics. Experimental Aerodynamics What has aerodynamics . such as Audi, BMW, VW Daimler-Benz and others. Experimental Aer
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