Wind Tunnel Testing Questions and Answers

Answer: C

The strip exists to fix where transition occurs, because at model Reynolds numbers the layer would otherwise stay laminar unrepresentatively far aft. It does not by itself correct the drag; it makes the measured drag mean something, provided it is placed and sized with care.

Answer: D

Streamlining reduces the wake blockage term but does nothing about the solid term, which depends only on the volume displaced. Any model of finite size blocks the section to some degree, which is why frontal area is capped at a few per cent regardless of shape.

Answer: A

Schlieren responds to density gradients in the flow field and says nothing directly about surface pressure, for which taps or pressure-sensitive paint are needed. The other three all involve density gradients and are exactly what the technique is for.

Answer: B

The factor converts the tunnel Reynolds number to a higher effective free-air value, because free-stream turbulence promotes early transition. A value below one would mean the tunnel was quieter than still air, which is not physically meaningful; the best tunnels approach one from above.

Answer: C

The sting may lie downstream but its influence propagates forward through the subsonic base region and alters the near wake, so it cannot be ignored. The other three are all genuine approaches, magnetic suspension being the most elegant and the most expensive.

Answer: D

A reflected shock heats the test gas violently for a few milliseconds, which is ample for the flow to establish over a small model and be recorded by fast instrumentation. No continuous facility could supply hypersonic stagnation enthalpy at acceptable cost.

Answer: A

The paint works across the speed range and is used routinely at subsonic and transonic conditions. Its genuine limitations are the temperature sensitivity, the calibration effort and the need to see the painted surface, all of which are real constraints on where it can be applied.

Answer: B

The full load on a body has three force components and three moment components, and a six-component balance separates all of them. Keeping the channels from contaminating one another, so that a large lift does not corrupt a small yawing moment, is the central design problem.

Answer: C

Magnetic levitation removes support interference altogether, which is its great attraction. The systems are expensive, limited in model size and demanding to control, so they remain rare and are used mainly where base and wake measurements must be free of a sting.

Answer: D

If the stream is not exactly parallel to the balance axes, part of the lift is resolved into the drag channel, and since drag is much the smaller quantity the error is proportionally large. Calibrating the angularity and testing the model both upright and inverted are the standard remedies.