Unsteady Flows

Question 1

When is a flow unsteady?

Answer 1

A flow is unsteady when the fluid properties at a spatial point change are time dependent.

Question 2

How do we characterise unsteadiness?

Answer 2

Strouhl number is one dimensionless parameter that characteriszes the unsteadyness.

Question 3

What are some sources of unsteadiness?

Answer 3

• Wake structres
• Motion of the object
• Velocites around an object
• Pressure around the object

Question 4

What are some examples of unsteady flows?

Answer 4

• A bird
• A fish
• A wind turbine

Question 5

What is dynamic stall?

Answer 5

Dynamic stall is when for example you have a helicopter blade with an angular velocity. Due to the angular veloctiy you will have a tip speed that is proportional to v=omega*R. When it is rotating in the oposite direction to the free stream it is okay, but on the other half of the rotation then the velocity is larger, thus stalling can occur.

Question 6

What are some characteristic features of a dynamic stall?

Answer 6

lift overshoot

• Hysteresis
• Dynamic stall vortex
• Stall delay