04 Fast cars

Experimental Aerodynamics

Lecture 4: Fast cars

G. Dimitriadis

Experimental Aerodynamics Vehicle Aerodynamics


About fast cars

•! In order to discuss fast cars, some of their aerodynamic particularities must be introduced.

•! Such particularities include uncovered wheels in Formula 1, Indy and other race cars.

•! They also include the generation of downforce by wings or other devices.


Wheel aerodynamics

•! Wheels can have important contributions to car aerodynamics.

•! On some racing cars the wheels are outside the body and have no covers.

•! Wheels are not designed for aerodynamics.

•! However, some small detailing can improve their aerodynamic characteristics.


Flow around cylinders Cylinder in a free stream

Cylinder near the ground

Re=2e5


Discussion •! A cylinder in a free stream creates a periodic

wake behind it, shedding vortices from the top and bottom alternatively.

•! When the cylinder is on the ground, the periodic wake shedding is reduced but there is still a large area of separation behind the cylinder.

•! Furthermore, there is a zero speed condition on the front of the cylinder near the ground.

•! This stagnation zone causes a high pressure near the bottom of the cylinder, which creates lift.


Flow over wheels

Rotating wheel: The separation point lies in front of the highest point of the wheel

Stationary wheel: The separation point lies significantly behind the highest point of the wheel.

Re=5.3e5


Pressure distribution In both cases, the pressure is nearly constant downstream of the separation point.

The earlier separation on the rotating wheel results in lower lift and drag values than those of the stationary wheel.


Lift and drag •! Lift and drag coefficients of different geometries. •! The numbers in parentheses are for stationary

wheels. •! The coefficients are based on the wheel frontal area. •! In all cases the lift coefficient is higher than the drag

coefficient.


Enclosed wheels

•! The flow around wheels in wheel wells is different.

Separation areas are much smaller. Drag and lift values are also lower.

There are significant aerodynamic advantages in enclosing wheels.


Rim covers •! Wheel rim covers can decrease significantly

wheel drag. •! On average, the drag of a passenger car wheel

with CD=0.6 can be reduced to CD=0.5 using a smooth rim cover.

•! Rim covers can also reduce the drag of uncovered wheels. Indy cars use rim covers on high speed racetracks.

•! They also look cool!


Racing Cars Many racing cars have uncovered wheels. Alternative strategies are used in order to decrease wheel drag and wheel-induced drag. Turning plates can be installed to constrain the wake and to direct it away from the car body.


Four uncovered wheels The flow field around the wheels is quite horrible. Very little clean air is available. Nevertheless, Formula 1 cars must fit between the four wheels. Designers must be very clever in trying to force clean air of the car body and wings.


Flow field over uncovered wheel car


Discussion of uncovered wheels

•! Despite all the best efforts of Formula 1, Indy car etc designers, the drag of such cars is very high.

•! Typical Formula 1 car drag coefficients are between 0.7 and 1.1, depending on the circuit the car is set up for.

•! A lot of this drag is due to the wings but a lot of it is due to the uncovered wheels.

•! Thankfully such cars also have very powerful engines


Wings Wing in uniform flow V. As the wing section is cambered, the lower surface experiences high pressure (pressure side) and the upper surface experiences low pressure (suction side).

Cross-section of wing. The air pressure is high on the lower surface, medium at Y or Y’ and low on the upper surface. Therefore, as we move outwards on the lower surface, the pressure drops. As we move outwards on the upper surface, the pressure increases.


Flow streamlines The air always moves in the direction of decreasing pressure. Therefore, the streamlines on the lower surface (pressure side) expand while the streamlines on the upper surface (suction side) contract.

This causes the flow to wrap upwards at the wingtips.


Wing tip vortices Two photos of the wingtip vortices of a utility aircraft. It is clear that red smoke is sucked upwards and is caught in a vortex sitting just behind the wingtip.


3D Flowfield


Induced drag

V!"

#!"w" #i"

L"Lcos#i"

Lsin#i"V"

The lift component perpendicular to the free stream is approximately equal to L for small downwash angles.

The lift component parallel to the free stream is the induced drag, given by

d = l! i


End plates

•! The induced drag is proportional to the square of the lift. Therefore, it can take substantial values when the lift is high.

•! The induced drag can be reduced by placing end plates at the wing tips.

•! These tend to make the trailing vortices less powerful. Therefore the downwash velocity is smaller and the induced angle of attack is reduced.


Race car endplates

Rear endplates

Front endplates


Underbody tunnels

•! Underbody tunnels are another method for creating downforce

Indy car Formula 1 car


Tunnel downforce •! Underbody tunnels drop the pressure under the car by a

Venturi effect. •! Inside the tunnels, the entire underbody of the car is

shaped like an inverted airfoil. •! Inverted airfoils produce downforce, especially when they

are close to the ground. •! Formula 1 cars are not allowed tunnels because the

bottom of the car must be flat. •! Such cars can create a limited Venturi effect at the trailing

edge of the underbody which is not covered by the regulations.


Track speed records •! Despite the aerodynamic difficulties induced by

uncovered wheels and other restrictive regulations, race cars are getting faster all the time.

Variation of the one-lap speed record at Indianapolis


Land speed records •! Aerodynamics plays an even greater roll on

cars that attempt to break absolute land speed records.

•! This fact was recognised from 1899 onwards when the La Jamais Contente broke the 100kph barrier.

•! Since then, a lot of effort has been made in order to improve the aerodynamics of really high speed cars.

•! As speed records kept increasing, different aerodynamic challenges had to be overcome.


A bit of history •! The first recognized land speed record was set by

Gaston de Chasseloup-Laubat in 1898. The speed was an average of 63kph over 1km.

•! Over the next few months he duelled with Camille Jenatzy for new records until Jenatzy reached 105kph in 1899.

•! Both Jenatzy’s and de Chasseloup-Laubat’s cars were electrically powered.

•! The first record with a non-electrically powered car was set by Leon Serpollet in 1902, reaching 121kph in his steam powered car.

•! Henry Ford himself drove his model 999 on an iced lake to reach 147kph in 1904.


Steam records •! At the time, cars were

still slower than trains. The first car speed record faster than train records was achieved in 1906 (206kph).

•! The car was steam-powered and driven by Fred Marriott.

•! The steam record stood until 2009!


Stabilization •! As speeds increased, directional stability started

to become a serious problems. Several record-attempt cars started to feature vertical fins for stabilization.

•! The first example is probably Henry Seagrave’s Golden Arrow, which reached 372kph in 1929.


Rocket cars •! 1928 saw the start of a long and

venerable history of sticking rockets on the backs of cars.

•! Opel built its first rocket cars, the RAK 1 and RAK 2 in 1928.

•! The RAK 1 reached the astronomic speed of 75 kph.

•! The RAK 2 did considerably better: 230 kph

•! The RAK feature large wings for stabilization, placed just after the front wheel wells.


Turboshaft cars •! By 1939 the land speed record

stood at an impressive 595kph. •! In 1947, it was raised to 634kph

but no further record could be obtained for a long time.

•! The record was finally broken in 1964 by the Bluebird CN7, reaching 649kph.

•! The Bluebird CN7 was powered by a Proteus turboshaft engine. The power was routed from the shaft to the wheels so it still qualified as a wheel-driven car.


Turbojets •! Between 1964 and 1965, the land

speed record was broken several times by two turbojet-driven cars, the Spirit of America and the Green Monster.

•! The Spirit of America featured a turbojet engine from a F-4 Phantom.

•! The Green Monster featured a turbojet engine from a F-104 Starfighter.

•! In 1965 the Spirit of America reached 967kph. The record stood until 1970.

Spirit of America

Green Monster


1000kph

•! The 1000kph barrier was broken by a rocket-powered car, the Blue Flame in 1970. It reached 1014kph.

•! The Blue flame featured an innovative inverse triangle cross-section to overcome compressibility problems


Things that go boom in the desert

•! The first car to claim breaking the speed of sound is the Budweiser Rocket, which reached 1190kph in 1979.

•! This record was never officially recognised.

•! The first car to officially break the speed of sound was the Thrust SSC in 1997, setting a record of 1229kph.

•! The Thrust SSC was powered by two Rolls-Royce Spey turbofans (from and RAF F-4).

Budweiser Rocket

Thrust SSC


Land speed records

Budweiser Rocket

Jet engines

Electric engines

Steam engines

Standard speed of sound


Transonic conditions •! Aerodynamic design for very high speed cars

is significantly different. •! The behaviour of air when approaching the

speed of sound change significantly. •! Even when the land speed is lower than that of

the speed of sound, some areas of flow over the body can be supersonic.

•! The flow needs to decelerate from supersonic conditions to reach free stream conditions behind the car.

•! Subsonic flows with supersonic areas are known as transonic flows.


Transonic flow

M<1 M>1

M<1

Shock Mach number, M=V/a, a being the speed of sound

Supersonic flow decelerates by means of a shock wave, which causes an increase in pressure.

In this case, the shock wave is normal to the surface.

M<1


Normal shock waves •! The pressure ratio across a shock wave is

given by:

•! Where M1 is the Mach number before the shock and $ is the heat capacity ratio, equal to 1.4 for air.

•! It can be seen that, if M1>1, then p2>p1. •! Therefore, behind a shock there is a

significant rise in pressure.

p2p1

=2!! +1

M12 "

! "1! +1


Effect on drag •! The effect of the normal shock on drag is

ambivalent: –! The higher pressure near the back of the body can

decrease drag. –! Shocks cause turbulent transition, thus increasing

drag. –! Shocks can also cause boundary layer separation, thus

significantly increasing drag. •! Therefore, a body’s drag can decrease or increase

in transonic flow. •! Even if the drag does initially decrease, if the

speed is further increased towards sonic conditions, drag increases enormously.


Drag divergence

As the Mach number approaches 1, the drag of a wing section (NACA 2312) increases enormously. This phenomenon is known as drag divergence and affects many other streamlined bodies.


Consequences •! As a car approaches the speed of sound, its

drag increases very rapidly. •! Therefore, the car must have a very powerful

engine (or two) in order to keep accelerating. •! This is why only jet or rocket propelled cars

have ever flown close to or past the sound barrier.

•! The Thrust SSC engines provided a power output of 82MW and burned 18l of fuel per second (equivalent to a fuel consumption of 5,500l/100km).


Lift •! Compressibility also causes significantly increased lift

due to ground proximity. •! At low subsonic speeds, ground proximity can

accelerate the flow beneath the car, thus causing downforce.

•! A high subsonic airspeeds, if the flow is accelerated, it can reach sonic or supersonic conditions.

•! Then, it will decelerate through a normal shock wave, causing a significant increase in pressure under the car.

•! Consequently, a lot of lift is created, which destabilizes the car.


Ground effect M<<1" The streamlines are

compressed near the ground. Therefore, the flow is faster and the pressure lower than on the top side. A downforce is generated.

M<1"

High pressure

M>1"

L" The flow on the underside is again accelerated but it reaches supersonic conditions. A shock wave is formed and the pressure behind it is high. A lift force is generated.


Dealing with lift The nose stagnation point must be as low as possible.

The underside of the car must be shaped in such a way as not to accelerate the flow.


Supersonic flow •! In supersonic flow, the shock waves move to the

nose •! The shock waves become oblique •! The flow behind the shock may still be supersonic. •! Supersonic flow turns by means of expansion

waves.

M>1" Shock Expansion wave


Wave drag •! Shock waves increase pressure;

expansion waves decrease it. •! The pressure just behind the front shock

wave is high but it keeps decreasing downstream, until the rear shock wave is reached.

•! Therefore, there is high pressure at the front and low pressure at the rear.

•! This phenomenon causes a significant amount of drag, known as wave drag.


Shock waves in ground effect

•! The oblique shock waves from the bottom of the body will hit the ground and get reflected.

•! The reflected shocks will hit the body and get reflected etc.

M>1"

•! There is a very complicated shock pattern under the body that most likely increases drag quite significantly. It probably also causes lift.


Outlook

•! The aerodynamic problems increase significantly when the speed exceeds Mach 1.

•! It is not yet clear how they can be solved. •! The F-4 with two Speys can reach Mach 1.2 at

sea level, i.e. 1450kph. This is slightly better than the Thrust SSC, even though the F-4 does not suffer from ground proximity problems.

•! Further land speed increases may require significantly higher engine power and better aerodynamic design.

04 Fast cars

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