Wind Tunnel Testing

Wind tunnel testing is another experiment, like CFD, to help validate aerodynamic changes.  The reason for writing this is to help clarify some of the misconceptions on wind tunnel testing. First, lets start with a little history of wind tunnels…because everybody loves history.

‘Whirling Arm’ apparatus was the first wind tunnel if it could be considered one. This was in the mid 1700’s to 1800’s. Francis Wenham was not happy with his experiences with the whirling arm, so he, with the British Aeronautical Society built the first wind tunnel in 1871. Osborne Reynolds (Reynolds number, ratio of inertial forces to viscous forces) showed a scale model could exhibit the same flow pattern as a full model in 1900. World War II led the rapid wind tunnel and aerodynamic advances. The United States had a 400 mph wind tunnel and Germany had three supersonic wind tunnels capable of Mach 4.4 by the end of the war. The rest is history…

One big misconceptions of wind tunnels are they reproduce what the car will see on the road or track. This is false because wind tunnels only simulates the conditions on the road or track. Simulations inherently deviate from reality and it is often are hard to quantify all the sources of the error.

What do wind tunnels idealize? 

-temperature change, wind boundary layer, wakes of other vehicle, ect.

Sources of wind tunnel inaccuracy:

1. Rolling road system

2. Wall Effects – tunnel sections

Different wall test sections

3. Tires – shape from deformation

4. Cornering Conditions – steer and yaw

Wind tunnel simulation vs. real world track situation

5. Sting interference – the sting holds the model in place (also can be on the wheels)

Sting located on center of car and on wheels

6. Wheel lift – wheel lift is not usually measured in a tunnel.

7. Model is not moving – aka relative motion isn’t the same

Wind tunnel data is precise but saddled with inaccuracy of simulation; road data is free from they inaccuracy but are not precisely measured. Anyone who will quote you extremely accurate numbers are most likely trying to sell you something. This is obvious by just reading above.

That is it for tonight on wind tunnels, but next will be information on scaled wind tunnel testing. That will get into Reynolds Number effects and Reynolds Scaling.

-Paul

 

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