Boundary Layers and Wakes

Question 1

What two fundamental characteristics does a flow of a real fluid have (except at extremely low pressures)?

Answer 1

• There’s no discontinuity of velocity.

- At a solid surface, the velocity of the fluid relative to the surface is zero.

Question 2

What is a boundary layer?

Answer 2

A thin layer of fluid next to the surface in which the flow is retarded due to viscous action.

The transitional regions between surfaces and main flow.

Question 3

Distinguish between what happens 1.) in the boundary layer and 2.) beyond the boundary layer.

Answer 3

1. ) In the boundary layer - where the shear stresses are of prime importance.
2. ) Beyond the boundary layer - Velocity gradients are small and so the effect of viscosity is negligible.

Flow is an inviscid or ideal fluid.

Question 4

Explain what the velocity gradient looks like for laminar and turbulent flow respectively?

Answer 4

The laminar layer has a greater velocity gradient at the surface.

In the turbulent boundary layer, there is much more chaotic intermingling of particles. This causes a more uniform velocity.

Question 5

Fill in the blank: The boundary layer ___ with distance from the leading edge.

Answer 5

thickens

Question 6

What is a dominant factor into considering the point at which a laminar boundary layer becomes unstable?

Answer 6

Reynolds Number of the flow relative to the surface.

Question 7

In regards to the boundary layer, what does it mean when the magnitude of the Reynolds Number is:

1. ) Re < 10^5
2. ) Re > 2x10^6

Answer 7

1.) Re < 10^5

The boundary layer is laminar and very stable.

2.) Re > 2x10^6

Virtually impossible to prevent transition in the boundary layer.

Question 8

Fill in the blank: The velocity within the boundary layer increases from zero to the velocity of the ___ asymptotically.

Answer 8

mainstream

Question 9

We’ve established that a definition for the boundary flow can be: A thin layer of fluid next to the surface in which the flow is retarded due to viscous action.

Give me another definition of the boundary layer in terms of the mainstream velocity

Answer 9

The distance measured from the solid surface to where the velocity reaches 99% of the mainstream velocity.

Question 10

The definition of displacement thickness is as follows:

The distance that a streamline just outside the boundary layer is deflected away from the wall due to the effect of the boundary layer.

Why does the streamline get deflected?

Answer 10

To satisfy the conservation of mass.

Question 11

What do we need to reduce the volumetric flow rate of an ideal, inviscid fluid with uniform velocity profile?

Answer 11

To displace the surface by some distance (delta star).

Question 12

What’s the sum of the skin friction drag and the normal pressure drag?

Answer 12

The profile drag.

Question 13

What is ‘a wake’?

Answer 13

When wind blows over the surface of an object (building), a region of disturbed fluid is created downstream of the body.

Question 14

What is a ‘bluff body’?

Answer 14

If the shape of the body:

• causes extensive flow separation.
• the wake is large, possibly with large scale eddies.
• pressure drag is much greater than the skin friction drag.

Question 15

Describe the difference between the bluff body and streamlined body in relation to pressure and skin friction drag.

Answer 15

Bluff body

Pressure drag&raquo_space; Skin friction drag

Streamlined body

Pressure drag &laquo_space;Skin friction drag

Question 16

What does the flow pattern in the wake depend on?

Answer 16

Reynolds number of the flow