Real Flows

Question 1

When and where are viscous effects important?

Answer 1

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Question 2

What kind of different types of drag exist?

Answer 2

There exists two types of drag
-pressure drag: Due to the pressure distribution in the drag direction.

-skin friction drag: Due to the skin friction distribution along the boundary.

Question 3

What is the influence of Re on the drag?

Answer 3

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Question 4

What is the influence of shape on the drag?

Answer 4

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Question 5

What are boundary layers?

Answer 5

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Question 6

What type of different boundary layers and regions exist?

Answer 6

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Question 7

What are the differences between a laminar and turbulent boundary layer?

Answer 7

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Question 8

What is wall shear stress and why is it important?

Answer 8

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Question 9

Boundary layer seperation, why and consequences?

Answer 9

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Question 10

What drag dominates a bluff/blunt body?

Answer 10

Pressure drag.

Question 11

What is a bluff/blunt body?

Answer 11

plate perpendicular to flow

Question 12

What is a slender body?

Answer 12

Plate aligned with flow

Question 13

What drag dominaties a slender body?

Answer 13

Skin friction drag.

Question 14

What is the influence of form on the drag?

Answer 14

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Question 15

What can be said about the wake behind a body?

Answer 15

1. The wake widens

2. The velocity deficit decreases.

Question 16

What can be said about the effects of a body in a flow at different Re?

Answer 16

At lower Re the body can be sensed earlier in the flow.

At lower Re the boundary layer is also larger.

Question 17

Consider a laminar and turbulent boundary layer with the same boundary layer height:

a) tau_turb(y=0) < tau_lam(y=0)
b) tau_turb(y=0) = tau_lam(y=0)
c) tau_turb(y=0) > tau_lam(y=0)

Answer 17

c)

The velocity gradient for the turbulent boundary layer is larger than the laminar boundary layer

tau=mu*partial(u)/partial(y)

Question 18

What is a condition on the pressure for seperation?

Answer 18

Seperation can only accure in region with an adverse pressure gradient.

Question 19

What is an adverse pressure gradient?

Answer 19

dp/dx>0

The pressure is increasing in the flow direction

Question 20

What can be said about the velocity gradient at a seperation point?

Answer 20

du/dy (@y=0)=0

Question 21

Which boundary layer seperates around a cylinder first?

Answer 21

The laminar boundary layers seperation point occurs earlier than the turbulent boundary layer.

Question 22

Which wake is wider behind a cylinder? A laminar boundary layer or a laminar boundary layer?

Answer 22

The wake from a laminar boundary layer is wider.

Question 23

Recall the five seperation flows around a cylinder at different Re.

Answer 23

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Question 24

What happens with the seperation point for a boundary layer for a cylinder with increasing Re.

Answer 24

A. Re~1 No seperation.
B. Re~10 Seperation at theta=pi/2
C. Re~100 Turbulent BL, seperation point moves down stream
D. Re~10000 Turbulent BL, seperation point moves upstream.
E. Re~500000 Drag crisis. (drag minimum)

Question 25

What happens with the seperation point around an airfoil for increasing alpha?

Answer 25

It moves upstream.

Question 26

What happens with the stall angle with increasing Re

Answer 26

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Question 27

Explain the drag crisis.

Answer 27

It is due to the transition from a laminar to a turbulent BL, leaving a narrower turbulent wake.