Module 3 - Bluff Bodies

Question 1

What are the defining characteristics of a bluff body?

Answer 1

• High pressure drag (95% compared to 5% skin-friction)
• Non-streamlined body
• large region of separated flow (a large wake)

Question 2

What is the dominant drag in a bluff body?

Answer 2

Pressure drag

Question 3

What is flow-induced vibration?

Answer 3

Vortex shedding frequency is similar to natural frequency of material, causing resonance causing the cylinder to vibrate

Question 4

What is the stagnation point in flow over a bluff body?

Answer 4

Point where velocity is zero and pressure is highest (in incompressible flow)

Question 5

What causes flow to separate from a cylinder?

Answer 5

Pressure gradient along surface shifts from favourable to adverse

Question 6

What is the shear layer in flow over a bluff body?

Answer 6

Later with high velocity on one side, low velocity on the other (rolls up into karman vortex)

Question 7

What is the Karman Vortex Street?

Answer 7

Region in wake of cylinders which includes periodically shed vortices

Question 8

What are the defining features of flow over a cylinder in the subcritical regime?

Answer 8

40<Re<2x10^5
Cd~1.2
St independent of Re

Question 9

What are the defining features of flow around a cylinder in the critical (transitional) regime?

Answer 9

Re~3x10^5
Cd~0.4
Highly intermittent and irregular vortex shedding

Question 10

What are the defining features of flow in the supercritical regime?

Answer 10

Re>5x10^5
Cd~0.7
St sensitive to changes in Re

Question 11

Why do sharp edged bluff bodies tend to have drag coefficients insensitive to Re?

Answer 11

Flow will always separate at the sharp edges meaning the wake will always have a similar size

Question 12

What are some unique flow features around a 3D cylinder?

Answer 12

• Tip vortices
• Base vortices
• Horseshoe vortex
• Downwash region