Drag loading in a steady flow

Question 1

What is creeping flow?

Answer 1

A flow regime where Reynolds number is very small (Re ≪ 1), often idealised as potential flow.
u ≈ 1 mm/s

Question 2

In creeping flow through a Hele-Shaw cell, the Reynolds number is very ________.

Answer 2

small

Question 3

True or False: Flow separation begins at Reynolds number around 1.54.

Answer 3

False – symmetry is lost around Re = 1.54, but separation begins around Re ≈ 5.

Question 4

What forms when flow separates around a cylinder at Re ≈ 9.6?

Answer 4

Re-circulating eddies

Question 5

As Reynolds number increases, standing eddies become _________.

Answer 5

elongated

Question 6

What happens to the vortex cores as Re increases?

Answer 6

• Distance to cores increases linearly with Re
• Separation between cores increases as Re

Question 7

True or False: At Re = 30.2, flow becomes unsteady.

Answer 7

False – flow is still steady but the wake is more than a cylinder diameter long.

Question 8

Match Re with flow behavior:

Re = 41 →
Re = 105 →

Answer 8

• End of steady flow range (wake begins oscillating)
• Onset of Von Karman vortex shedding

Question 9

What is a Von Karman vortex street?

Answer 9

A repeating pattern of swirling vortices shed alternately from each side of a bluff body in unsteady flow.

Question 10

At Re = 2000, flow separates into a turbulent ________.

Answer 10

wake

Question 11

True or False: Increasing the incident velocity always changes the flow pattern significantly.

Answer 11

False – at high Re, doubling velocity may not significantly affect wake pattern.

Question 12

What physical forces are produced due to asymmetric wake and vortex shedding?

Answer 12

• Drag force (inline)
• Lift force (transverse)

Question 13

What is the definition of Drag Coefficient C_D?

Answer 13

C_D = F_D / (½ ρU²A)

Question 14

What is the definition of Lift Coefficient C_L?

Answer 14

C_L = F_L / (½ ρU²A)

Question 15

In the drag coefficient equation, the reference area A for a vertical cylinder is A = ___ ⋅length.

Answer 15

diameter

Question 16

True or False: Drag coefficient remains constant regardless of Reynolds number.

Answer 16

False – it varies with Reynolds number.

Question 17

How is drag force produced?

Answer 17

At large Re, the pressure differences between the front and the back of the cylinder produces a significant drag force

Question 18

What can the lift force be described as?

Answer 18

The force perp. to the inline velocities