Aerodynamics - Laminar and Turbulent Flow

10 important questions on Aerodynamics - Laminar and Turbulent Flow

How would you describe a boundary layer?

A thin region of reduced flow speed. The friction 'eats up' a part of the kinetic energy of the particles close to the object --> that slows down the particles.

What can you say about the pressure through the boundary layer in a direction perpendicular to the surface?

Static pressure remains constant --> is not alternated due to friction

What can you do if you know that the pressure is constant through the boundary layer?

You can make a hole, and say something about the velocity of the air outside the boundary layer. --> bernoulli equation (not valid in the boudary layer, but because p=constant, you can say something about the velocity of the air near the edge)
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Why is this velocity profile excisting?

Total force = Total pressure force + total friction force  (there's no pressure force due to thin plate)

Friction force is shear stress

μ = absolute viscosity coefficient or viscosity (kg/ms)

The steaper the gradient of the profile close to the surface, the smaller the τw

What is the critical Reynoldsnumber?

It's a number specific to the flow at which transition occurs.

It is difficult to find --> Experimental data applicable for the given problem

But not every windtunnel has a perfect flow --> Flow will be different for every windtunnel, thus the reynoldsnumber.

What does it mean if the Reynolds number is high?

Very low turbulance in the windflow (due to windtunnel) or very low friction.

When is the shear stress on an airfoil high? And when low?

Laminar boundary layer --> low shear stress
Turbulent boundary layer --> high shear stress

What does the black region mean?

Here, due to the turbulent boundary layer, the shear stress is high. Thus, the high shear stress takes all the oil away.

What happends if the adverse pressure gradient becomes too high?

Particles start to detaches from the surfaces --> movement of air --> wave with a lot of drag

they move with the incomming flow

How can you see that the flow has seperated?

It makes a straight Cp --> Pressure does not change anymore --> direction of particles are in the direction of the incomming flow. 

Transition also moves forewards.

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