F4

Question 1

What is Prandl’s boundary layer concept

Answer 1

External flows can be analysed by dividing the flow into two regions
(1) ‘boundary layer’ close to the surface of a body - viscosity effect is important
(2) rest of the flow field - viscosity effect is negligible

Question 2

What is the boundary layer thickness δ

Answer 2

It is arbitrarily definite as the distance away from the surface, when the boundary layer velocity reaches, 99% of the upstream velocity.

Question 3

What is Drag force proportional to?
What is resulting power required proportional to?

Answer 3

Drag force is proportional to U²

resulting power required is proportional to U cubed.

Question 4

What happens WITHOUT a pressure gradient in the direction of flow in terms of Boundary Layer separation

Answer 4

For flow along a flat plate, without a pressure gradient, the Boundary layer remains attached to the surface, and the thickness increases continuously from zero at the leading edge

Question 5

What happens WITH a pressure gradient in the direction of flow in terms of Boundary Layer separation

Answer 5

If a pressure gradient exists along the length of the plate, the flow in the Boundary Layer is modified.

Question 6

What is a bluff body

Answer 6

Object where the flow can separate over much of the surface as opposed to a streamlined body where separation always occurs well towards the rear .

The nature of the flow is heavily dependant on the Reynolds number

E.g circular cylinder