Wind Tunnel Testing Questions and Answers

Answer: C

Decelerating the stream through a series of weak shocks rather than one strong normal shock saves a great deal of stagnation pressure and therefore power. Sizing it is delicate, because the same throat must also allow the tunnel to start.

Answer: D

Near Mach one a solid-walled tunnel chokes and the wall interference becomes intolerable, because small area changes have very large effects. Ventilated walls let the flow spill in or out and largely remove both problems, which is what made transonic testing practical from the 1950s.

Answer: A

Storing compressed air and discharging it through the circuit gives high Reynolds and Mach numbers for a modest capital outlay, but run times are measured in seconds or tens of seconds and conditions drift as the reservoir empties. Continuous tunnels cost far more but allow leisurely measurement.

Answer: B

Reproducing hypersonic flight means reproducing the enormous stagnation temperature, and no continuous facility can supply it affordably. A shock tunnel uses a reflected shock to heat the gas violently for a few milliseconds, which is long enough for the flow to establish and be measured.

Answer: C

Strain gauges on a carefully designed flexure inside the model resolve the aerodynamic load into its components. Keeping the components separate, so that a large normal force does not contaminate a small axial one, is the central design difficulty of such a balance.

Answer: D

The model occupies part of the test section, so continuity forces the surrounding air to speed up and the measured forces come out too high. The correction scales with the ratio of model volume to test-section area, which is why frontal area is usually kept below a few per cent of the section.

Answer: A

The wake carries slower air, and since the walls confine the total flow, the air outside the wake must speed up to compensate. The effect grows with the model's drag, so it matters most for bluff bodies and for anything tested past the stall.

Answer: B

The tunnel's own wall boundary layers thicken along the test section, contracting the effective area and producing a slight fall in pressure downstream. That gradient pushes the model backwards exactly as buoyancy would, and it must be subtracted from the measured drag. Slightly diverging walls are often used to remove it.

Answer: C

Free-stream turbulence promotes early transition, so a model in a turbulent tunnel behaves as though it were at a higher Reynolds number in still air. The factor, obtained by testing a sphere of known behaviour, converts between the two and lets results from different tunnels be compared.

Answer: D

A schlieren system deflects light in proportion to the density gradient and cuts part of the beam with a knife edge, so gradients appear as changes in brightness. Shadowgraph responds to the second derivative and interferometry to density itself, so the three are complementary rather than interchangeable.