Boundary Layer and Drag Questions and Answers

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.

Answer: D

No-slip holds in any continuum viscous flow, laminar or turbulent, at any Reynolds number. It is the boundary condition that creates the velocity gradient at the wall and hence the shear stress, and everything about the boundary layer follows from reconciling it with the free stream.

Answer: A

The shape factor rises towards separation, not falls, because the profile becomes less full as near-wall momentum is drained away. The other three statements are correct, and the Blasius value of 2.59 against a turbulent figure near 1.4 illustrates the second.

Answer: B

Suction is used precisely to postpone transition by removing the slowest fluid and stabilising the profile; it is the basis of laminar flow control. The other three all introduce or amplify the disturbances that trip the layer, and all three limit laminar-flow aerofoils in service.

Answer: C

Streamlining works on the pressure term by keeping the flow attached, and it typically raises the wetted area and therefore the friction. So long as the fluid is viscous and the surface has area, skin friction cannot be removed; it can only be traded against something larger.

Answer: D

The turbulent profile is much fuller, rising steeply from the wall and then flattening, because turbulent mixing transports momentum across the layer far more effectively than molecular viscosity can. That fullness is the origin of both its higher friction and its greater resistance to separation.

Answer: A

Geometric similarity is a separate requirement and is comparatively easy to satisfy; it is not what makes the Reynolds number hard to match. The difficulty comes from the small model size and from the cost of the remedies, whether that is more speed, more density or a cryogenic tunnel.

Answer: B

Reynolds number is proportional to the characteristic length at a fixed speed and fluid, so the model runs an order of magnitude low. Transition sits proportionally further back on the model, so the measured drag and stall behaviour will not represent full scale without correction or a trip strip.

Answer: C

The integral form collapses the boundary-layer equations into a single ordinary differential equation for the momentum thickness, driven by the wall shear and the pressure gradient. It is the basis of every quick engineering boundary-layer estimate and needs no assumption about the profile shape until one is chosen.

Answer: D

Assuming a quartic profile whose shape is governed by a single parameter reduces the problem to an ordinary differential equation that can be integrated along the surface. The result is quick and gives a usable separation prediction, though it is less accurate than a full differential calculation.