Boundary Layer and Drag Questions and Answers

Answer: B

The three integral quantities each answer a different question -- how much the body is effectively thickened, how much drag has been generated, and how close the profile is to separating -- while the wall shear stress is the local quantity from which the friction drag is built up.

Answer: C

All four attack separation, but by different routes: two add momentum near the wall, one removes the fluid that has lost it, and streamlining reduces the demand instead by easing the pressure gradient the layer must survive.

Answer: D

The sequence traces the whole range from a body that never separates to one designed to separate as violently as possible. Where a given shape falls on that scale is largely a question of how gently its rear allows the pressure to recover.

Answer: A

Each coefficient multiplies the distance from the leading edge divided by the square root of the local Reynolds number. The shape factor is the ratio of the second to the third, and 1.721 divided by 0.664 gives 2.59 as it should.

Answer: B

The first pair shows that roughness matters only in relation to the sublayer's own thickness, not in absolute terms. The second pair shows the pressure gradient acting as the other main lever, holding the layer on in one direction and tearing it off in the other.

Answer: C

A steeper adverse gradient is the very thing that causes separation, so it can only hasten it. The other three all give the near-wall fluid more momentum, or remove the fluid that has lost it, and so postpone separation.

Answer: D

Skin friction rises, not falls, because the fuller profile gives a steeper gradient at the wall. The other three are all genuine consequences, and the first two together are why the choice between laminar and turbulent is never obvious.

Answer: A

Skin friction rises through the crisis. What collapses is the pressure drag, because the separation point moves aft and the wake narrows, and for a bluff body that term is so much larger that the total still falls sharply.

Answer: B

Below the critical Reynolds number a polished surface keeps the layer laminar, so it separates early and the wake stays broad. Roughening it, counter-intuitively, reduces the drag. The other options all narrow the wake by easing or delaying separation.

Answer: C

On a streamlined body where the flow stays attached, laminar flow is indeed worth having. On a bluff body, or anywhere a strong adverse gradient exists, keeping the layer laminar means early separation, a broad wake and far more total drag. Which is preferable depends entirely on the shape.