Boundary Layer theory (Week 3 and 4)

Question 1

Explain the three key features of boundary layers

Answer 1

1. Velocity along the wall changes rapidly with increasing distance from the wall
2. Strong gradients occur in the wall normal direction
3. Close to the wall, the velocity is substantially lower than “outside” the boundary layer

Question 2

What are the two different types of pressure gradient in a boundary layer?

Answer 2

1. Negative (favourable) pressure gradient in the direction of the mean flow (beneficial to boundary layer stability)
2. Positive (adverse) pressure gradient has an “adverse effect on boundary layer stability, results in decelerating flow and separation (at V=0).

Question 3

What does displacement thickness represent?

Answer 3

1. The thickness of the region carrying the “lost” mass-flow rate at the free-stream velocity
2. The distance the flow is effectively “displaced” from the wall

Question 4

What does momentum thickness represent?

Answer 4

The reduced velocity close to the wall in a boundary layer profile also corresponds to a “loss” in momentum compared to the undisturbed ‘free stream’ flow