Boundary Layer and Drag Questions and Answers

Answer: C

Skin friction comes from the shear stress integrated over the surface and pressure drag from the pressure deficit left by separation. Which dominates depends on the body: a streamlined shape is nearly all friction, a bluff one nearly all pressure drag.

Answer: D

Streamlining tapers the rear so the pressure rise is gentle enough for the boundary layer to negotiate, keeping the flow attached and the wake narrow. The skin-friction drag actually rises because the wetted area is greater, but the saving in pressure drag far outweighs it.

Answer: A

The flow separates near the shoulder and leaves a broad wake at roughly base pressure, so the front of the cylinder is pushed much harder than the rear. That imbalance dwarfs the friction contribution, and it is why a cylinder's drag coefficient is an order of magnitude above a streamlined strut's.

Answer: B

Once the layer goes turbulent before separating, its extra near-wall momentum carries it further round the body, so the wake narrows sharply and the pressure drag collapses. The drag coefficient of a smooth sphere drops from about 0.5 to near 0.1 across the transition.

Answer: C

The dimples force transition at a lower Reynolds number than a smooth ball would manage, bringing on the drag crisis deliberately. The wake narrows, pressure drag falls, and the ball flies perhaps twice as far as a smooth one would. Friction rises, but it is a small price.

Answer: D

Each generator sheds a small streamwise vortex that stirs faster-moving air down towards the surface, re-energising the layer so it can survive a stronger adverse gradient. They are a cheap retrofit and are widely used to cure buffet or restore control authority.

Answer: A

Drawing off the slowest fluid through a porous or slotted surface leaves a fuller profile that can withstand a greater pressure rise. It is highly effective and is used for laminar flow control as well, but it demands ducting, pumping power and a surface that will not clog.

Answer: B

It compares the inertia of the fluid with its viscosity, so a high value means viscous effects are confined to thin layers and a low value means they pervade the flow. Matching it between model and full scale is the central difficulty of wind-tunnel testing.

Answer: C

Immediately at the wall the turbulent fluctuations are suppressed and momentum is carried by molecular viscosity alone. Whether a surface behaves as hydraulically smooth depends on whether its roughness elements are buried inside this sublayer or protrude through it.

Answer: D

Roughness buried inside the sublayer never meets the turbulent part of the flow and so adds no drag. Once it protrudes through, it generates its own wakes and the friction rises sharply, which is why the leading edges of transport wings are kept exceptionally clean.