An open-circuit wind tunnel differs from a closed-circuit one in that it
Answer: A
An open-circuit or Eiffel tunnel takes air from the room and returns it there, which makes it cheap and simple but noisy and sensitive to conditions in the building. A closed-circuit tunnel recirculates the same air, giving better flow quality and lower running power for the same test section.
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The main advantage of a closed-circuit wind tunnel is
Answer: B
Recirculating the air lets the tunnel recover much of the kinetic energy each pass, so the drive power for a given test section is far lower, and the flow can be conditioned consistently. The penalties are cost, floor space and the need to remove the heat the fan adds.
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The contraction section of a wind tunnel serves to
Answer: C
Accelerating the flow stretches the streamwise vortex lines and reduces the turbulence intensity relative to the mean velocity, so a large contraction ratio buys good flow quality. Ratios between six and ten to one are usual on subsonic tunnels.
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Honeycombs and screens are placed in the settling chamber to
Answer: D
The honeycomb removes swirl and lateral velocity components while successive fine screens break large eddies into smaller ones that decay quickly. Placing them where the speed is lowest keeps their pressure loss, and so the drive power, acceptable.
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Geometric, kinematic and dynamic similarity together require that model and full scale share
Answer: A
Geometric similarity fixes the shape, kinematic similarity follows from it if the flow patterns correspond, and dynamic similarity demands that the governing dimensionless force ratios match. In aerodynamics those ratios are principally the Reynolds and Mach numbers.
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The central difficulty of subsonic wind-tunnel testing is that
Answer: B
A small model needs a higher speed to match the Reynolds number, but raising the speed also raises the Mach number past the value being simulated. Escaping the conflict means changing the density or viscosity instead, which is why pressurised and cryogenic tunnels exist.
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A cryogenic wind tunnel raises the Reynolds number by
Answer: C
Cooling nitrogen to around 110 K increases the density and cuts the viscosity together, raising the Reynolds number several-fold at the same Mach number. The National Transonic Facility works this way, at the cost of a cryogenic plant and models that must survive the temperature.
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A pressurised wind tunnel raises the Reynolds number by
Answer: D
Reynolds number is proportional to density at a fixed speed and size, so running the circuit at several atmospheres buys a proportional increase without touching the Mach number. The penalties are a pressure shell, higher loads on the model and a slower turnaround between runs.
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A trip strip or turbulator is fitted to a wind-tunnel model in order to
Answer: A
At model Reynolds numbers the layer would stay laminar far further back than it does at full scale, so a small strip of roughness is added to force transition where it belongs. Placing and sizing it correctly is a matter of some art, and getting it wrong corrupts the drag data.
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A supersonic wind tunnel requires which feature that a subsonic one does not?
Answer: B
Reaching supersonic speed demands a throat followed by a divergent section, with the area ratio setting the test-section Mach number. A well-designed contour, usually produced by the method of characteristics, leaves uniform parallel flow at the model.
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