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 is less steep.

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.