Quote:
Originally Posted by twinair
Well this is the point: If you somehow managed to go so low to actually increase aerodynamic drag, the car would simply not be driveable anymore (not just from a riding point but more so from not being able to drive over bumps and such).
A movable ground isn't necessary to test a passenger car in a wind tunnel unless effects of lowering are to be measured: If lowering effects are to be measured a moving ground (road simulation) is imminent, since not doing so will increase the frictional drag underneath the car by a factor of 12 (compared to the 'real world') even if a boundary layer on the wind tunnel floor was missing (which it is not).
(And if frictional drag underneath the car wouldn't play a role, car manufacturers and hyper-milers wouldn't install smooth underbody panels).
Race cars with wing like structures on the underside of the surface wouldn't work in a wind tunnel without a moving floor as the wing like surface would simply stall (due to the low air speed underneath the vehicle provoked by the unnaturally high frictional drag including wind tunnel floor boundary layer).
Air is not being compressed at the speeds most cars and especially hyper-milers travel. At 120 km/h dynamic pressure (ram pressure) only reaches 0.6% of atmospheric pressure: Dynamic pressure - Wikipedia, the free encyclopedia
The pressure difference (pressure drag) between front and rear of the car is primarily created by the 'negative pressure region' behind the car (wake). (And the size of the wake is also affected by what is happening in front and underneath the car). |
*Public domain literature reports that any wheel/tire affect can be well represented without a moving-floor wind tunnel.General Motors investigated it as early as the 1950s and found it not worth the trouble.Simple trip strips attached to the tires can create 'moving-floor' flow phenomena as far as drag measurements are concerned.
*Wheel drag,as associated with rotation has been reported at Cdr 0.005 or less for passenger cars.
*The drag coefficient of the spinning tire IS different from a stationary counterpart,but spinning it is not necessary to predict the drag.
*Dr.Alberto Morelli reported from the Pininfarina tunnel that tire/wheel Cd does not vary as the wheel is retracted into the wheelhouse,only the frontal area is reduced.
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*The issue with the underbody of a car is turbulence.Eddies and turbulence are pure entropy.Their kinetic energy cannot be converted to static pressure.
*An airdam can shield underbody parts from source flow,which would otherwise be tripped into turbulence.If the airdam is too deep it can increase drag.Ask bondo.
*The belly pan prevents any turbulence from forming and the integrity of the flow allows for the use of the diffuser which acts synergistically with the pan.
*By shunting air down the sides of the car,less travels underneath,which is beneficial to lift issues,and minimizes the velocity of the undercar flow to near the road velocity,although the convention is to consider this flow at less than the road speed.
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*Race cars with wings are open-wheeled and the frontal area of their tires are enormous in relation to the actual body of the car,minus pods and teams typically will use moving-floor tunnels although 'flow-trippers' can be used in stationary-floor tunnels to good effect to measure accurate drag.
*And bear in mind that not too long ago,an Formula-1 car with Cd 1.2 was considered 'good.'