Laminar vs Turbulent Boundary Layers

Question 1

Describe the difference in velocity profiles between laminar and turbulent boundary layers.

Answer 1

The turbulent profiles are flatter, have a larger velocity gradient at the wall, and produce a thicker BL than do laminar profiles.

Question 2

Define Reynolds number.

Answer 2

Re = (density * velocity * distance)/ absolute viscosity
= ratio of inertial forces to viscal forces.

Question 3

Describe two different methods to calculate Re.

Answer 3

1) A function of distance in the general direction of the mean flow, from the LE of the surface
2) A function of the BL thickness

Question 4

Define Re for distance from LE.

Answer 4

Re_x = (rho * Vo * x) / mew, where x = distance from LE.

Question 5

Define Re for distance from boundary layer thickness.

Answer 5

Re_delta = (rho * Vo * delta) / mew, where delta = BL thickness.

Question 6

Define boundary layer thickness, and explain why it is so defined.

Answer 6

The distance from the solid boundary to the point where v = 0.99 * Vo = 99% of free stream velocity.
In reality there is no sharp demarcation between the boundary of the BL and the freestream velocity.

Question 7

Which type of boundary layer is more likely to separate and why?

Answer 7

Laminar, the velocity gradient normal to the surface is less steep. The steeper the gradient, the further the velocity is from zero and thus the less likely it is to separate.

Question 8

Define momentum thickness.

Answer 8

The distance the surface would have to be displaced in an ideal flow to account for the deficit in the total momentum flow.

Question 9

Define displacement thickness.

Answer 9

The distance the surface would have to be displaced in an ideal flow to account for the deficit in Q due to the BL.