Real Flows Flashcards
When and where are viscous effects important?
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What kind of different types of drag exist?
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.
What is the influence of Re on the drag?
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What is the influence of shape on the drag?
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What are boundary layers?
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What type of different boundary layers and regions exist?
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What are the differences between a laminar and turbulent boundary layer?
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What is wall shear stress and why is it important?
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Boundary layer seperation, why and consequences?
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What drag dominates a bluff/blunt body?
Pressure drag.
What is a bluff/blunt body?
plate perpendicular to flow
What is a slender body?
Plate aligned with flow
What drag dominaties a slender body?
Skin friction drag.
What is the influence of form on the drag?
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What can be said about the wake behind a body?
- The wake widens
2. The velocity deficit decreases.
What can be said about the effects of a body in a flow at different Re?
At lower Re the body can be sensed earlier in the flow.
At lower Re the boundary layer is also larger.
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)
c)
The velocity gradient for the turbulent boundary layer is larger than the laminar boundary layer
tau=mu*partial(u)/partial(y)
What is a condition on the pressure for seperation?
Seperation can only accure in region with an adverse pressure gradient.
What is an adverse pressure gradient?
dp/dx>0
The pressure is increasing in the flow direction
What can be said about the velocity gradient at a seperation point?
du/dy (@y=0)=0
Which boundary layer seperates around a cylinder first?
The laminar boundary layers seperation point occurs earlier than the turbulent boundary layer.
Which wake is wider behind a cylinder? A laminar boundary layer or a laminar boundary layer?
The wake from a laminar boundary layer is wider.
Recall the five seperation flows around a cylinder at different Re.
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What happens with the seperation point for a boundary layer for a cylinder with increasing Re.
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)
What happens with the seperation point around an airfoil for increasing alpha?
It moves upstream.
What happens with the stall angle with increasing Re
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Explain the drag crisis.
It is due to the transition from a laminar to a turbulent BL, leaving a narrower turbulent wake.
