Effect of Pressure Gradient on Boundary Layers

Question 1

How does a negative pressure gradient affect air flow?

Answer 1

It creates a net pressure force on an element of fluid in the boundary layer in the direction of flow.

Question 2

How does a positive pressure gradient affect air flow?

Answer 2

It creates a net pressure force on an element of fluid in the boundary layer against the direction of flow.

Question 3

How does the net pressure force on an element in the boundary layer affect the velocity gradient?

Answer 3

If the pressure force increases velocity, the gradient increases, and vice versa.
If the pressure force brings the fluid to rest, the velocity gradient = 0.

Question 4

Why does fluid coming to rest in the boundary layer cause the velocity gradient to equal zero?

Answer 4

No-slip condition implies that fluid adjacent to boundary is at rest, so if the fluid further away from the boundary is also at rest then there is no velocity gradient.

Question 5

Velocity gradient is directly or indirectly proportional to pressure gradient?

Answer 5

Indirectly.

Question 6

Define the separation point.

Answer 6

When the velocity gradient = pressure gradient = 0.

Question 7

When du/dy < 0 in a boundary layer, what physical feature is created?

Answer 7

An eddy forms due to the reversal of flow. For flow moving in the net direction of left to right, the eddy rotates clockwise.

Question 8

Is a laminar boundary layer more or less likely to separate than a turbulent boundary layer?

Answer 8

More likely to separate because the increase of velocity with distance from the surface is less rapid/the velocity gradient is less steep; which is due to less interchange of momentum, causing lower energy flow, causing separation.