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 idrama August 1, 2011 09:53

Boundary layer thickness

Hello Foamers,

I have ran a turbulent flow simulation. Now I need to know, how to compute the boundary layer thickness. Actually, I must know which formula is appropriated. Does anybody know?

Cheers

 lovecraft22 August 1, 2011 14:17

I would probably do a plot of the velocity near the wall and then compute the distance from the wall itself at which the velocity reaches 99% of the free stream velocity.

hi
i think , you must save the velocity on text file and solve integral (teta) with MATLAB
anyone have better idea?

 Captain Crossflow August 3, 2011 05:20

Quote:
 Originally Posted by idrama (Post 318322) Hello Foamers, I have ran a turbulent flow simulation. Now I need to know, how to compute the boundary layer thickness. Actually, I must know which formula is appropriated. Does anybody know? Cheers
What kind of geometry are you considering? An airfoil?
What exactly do you need the boundary layer thickness for? Visualization?

For just visualizing the boundary layer for the flow around an airfoil you could use the total pressure, which is constant in the free stream and decreases inside the boundary layer.

I only know of a 'direct' formula for calculating the boundary layer thickness for flat plate flow. For general flows you have to find the boundary layer edge by applying an appropriate criterion, e.g. reaching 99% of the velocity of the free stream.
However, the boundary layer thickness as a representative value can be quite inaccurate because of the small velocity gradients at the boundary layer edge. Applying 99% or 99.5% as edge criterion can already result in a quite large difference. Integral boundary layer values (displacement thickness / momentum loss thickness) are in my opinion a better choice.

 idrama August 3, 2011 05:49

The geometry is a S-Shaped pipe turned up-side down. Flow conditions are turbulent and two-phase where the water flow in only at the surface; the geometry center is occupied by air alone. The thing is, how do I proceed with ParaView or external tools to figure out. I don't really need detail information, just an idea or a stimulus; the rest I can make it up on my own.

How should I proceed to compute the 99% interval via e.g. ParaView.

Cheers

 Captain Crossflow August 3, 2011 06:28

I have to say, that I have little experience with internal flows and no experience with two-phase flows... this means, it is possible that my ideas may not be valid for your case.

Generally, to detect the boundary layer edge, you have to evaluate data along wall normal lines. I don't know if this is possible in an easy way in an visualization tool as e.g. ParaView.
For external flows it is possible to determine the velocity at the boundary layer edge from the surface pressure distribution by assuming constant pressure normal to the wall inside the boundary layer and applying the Bernoulli equation from farfield to boundary layer edge. Then you can check at what point your velocity reaches 99% of that value.
Criteria to find the boundary layer edge are based on the characteristics of the boundary layer flow. Besides that the velocity reaches a certain value, e.g. the shear stress at the boundary layer edge must become small and the total pressure loss must vanish.

I have the feeling, it is easiest to plot the total pressure (ptot = p + rho/2 * u^2) to identify the boundary layer.

A paper that deals with boundary layer edge detection:
Cliquet, Houdeville, Arnal: Application of laminar-turbulent transition criteria in Navier-Stokes computations. AIAA Journal, Vol. 46, No. 5, May 2008

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