Wind Tunnel Testing Questions and Answers

41.

Assertion (A): Reynolds and Mach numbers usually cannot both be matched on a subsonic model.

Reason (R): A smaller model needs a higher speed to reach the same Reynolds number.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

Answer: A

Both are true and the second explains the first. Raising the speed to recover the Reynolds number also raises the Mach number past the value being simulated. Escaping the conflict means changing the density or the viscosity instead, which is what pressurised and cryogenic tunnels do.

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42.

Assertion (A): Cryogenic tunnels raise the Reynolds number without altering the Mach number.

Reason (R): Cooling the gas raises its density and lowers its viscosity together.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: B

Both statements are true and the second is the reason for the first. Both effects push the Reynolds number the same way, giving roughly a sixfold gain at 110 K, while the Mach number depends on speed relative to the local speed of sound and is set independently.

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43.

Assertion (A): A trip strip is fitted to a wind-tunnel model.

Reason (R): At model Reynolds numbers the boundary layer would otherwise stay laminar much further aft than at full scale.

Select the correct answer from the options below.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

Answer: C

Both are true and the second explains the first. Without the strip the model would carry an unrepresentatively long laminar run, understating friction drag and misplacing separation. Sizing and siting the strip correctly is delicate, and a badly chosen one corrupts the data it was meant to fix.

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44.

Assertion (A): Transonic tunnels use ventilated test-section walls.

Reason (R): Near Mach one a solid-walled tunnel chokes and wall interference becomes severe.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: D

Both statements are true and the second is the reason for the first. Slots or perforations let the flow spill in or out, relieving the blockage that would otherwise choke the section and cutting the interference. The innovation is what made routine transonic testing possible in the 1950s.

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45.

Assertion (A): Horizontal buoyancy must be subtracted from a measured drag.

Reason (R): The tunnel's own wall boundary layers thicken along the test section.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

Answer: A

Both are true and the second explains the first. The thickening walls contract the effective area, so the pressure falls downstream and pushes the model rearward exactly as a buoyancy force would. Slightly diverging walls remove the gradient at source.

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46.

Assertion (A): A closed-circuit tunnel needs less drive power than an open one of the same test section.

Reason (R): The closed circuit recovers much of the flow's kinetic energy on each pass.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: B

Both statements are true and the second is the reason for the first. Only the circuit losses have to be made up each lap rather than the whole kinetic energy, which is why energy ratios well above one are normal. The price is capital cost, floor area and the need to remove the heat the fan adds.

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47.

Assertion (A): Schlieren and shadowgraph give the same information about a flow.

Reason (R): Both techniques respond to variations in the density of the gas.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: C

The assertion is false and the reason is true. Both do respond to density variation, but schlieren responds to the first derivative and shadowgraph to the second, so a shock appears as a line in one and as a light-dark pair in the other. They are complementary rather than equivalent.

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48.

Assertion (A): Blockage corrections grow rapidly as the model is made larger relative to the test section.

Reason (R): A larger model leaves less area for the flow, forcing the surrounding air to accelerate.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: D

Both are true and the second explains the first. Continuity in a confined section means the measured forces come out too high. Keeping frontal area below about five per cent of the section keeps the correction a modest adjustment rather than a dominant one.

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49.

Assertion (A): Shock tunnels are used for hypersonic testing despite run times of only milliseconds.

Reason (R): No continuous facility can affordably supply the stagnation enthalpy hypersonic flight requires.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

Answer: A

Both statements are true and the second is the reason for the first. Reproducing the enormous stagnation temperature continuously would demand impossible power, whereas a reflected shock delivers it briefly and cheaply. Milliseconds is long enough for the flow to establish and be measured.

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50.

Assertion (A): A large contraction ratio improves the flow quality in the test section.

Reason (R): Accelerating the flow reduces the turbulence intensity relative to the mean velocity.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: B

Both are true and the second explains the first. The mean velocity is multiplied by the area ratio while the velocity fluctuations grow far less, so the ratio between them falls. Ratios of six to ten to one are usual, limited by the length and cost of the contraction.

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Wind Tunnel Testing Questions and Answers

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