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 Ralf Schmidt May 31, 2006 07:30

boundary layer thickness

Hi!

I want to simulate a very simple case: turbulent flow over a plate. Now, my problem is, how to difine the Boundary Layer thickness.

Ok, it is 99% of the free stream vel. BUT, what free stream vel. should I use?

I do a 2D, incompressible calculation (air), with

- pressure outlet BC at the top of my domain and at the right side, and a

- vel. inlet BC at the left side (the plate is at the bottom).

Now (because of displacement due to the B-L. ) the vel. above the plate increases to a value about 5% higher than the inlet vel.

What value should I take as free stream vel.???

The top side of the domain has a distance of about 35 times the BL-thickness from the plate. Is that distance to small?? Or are the BC wrong?

Any idea?

Ralf

 Andrew Hayes May 31, 2006 08:47

Re: boundary layer thickness

the turbulent boundary layer is quite different from the laminar boundary layer.

But, that being said, I think you need to solve for a velocity profile and then use that profile to find the boundary layer displacement thickness. That would be the case with laminar flow. You could look into the Prandtl/Blasius solution for some correlation.

 ganesh May 31, 2006 10:20

Re: boundary layer thickness

Dear Ralf,

The boundary conditions look right, though you could also try out with a symmetry/inviscid boundary condition on the top. A distance of 35 times b.l.t. is good enough. As far as the b.l. thickness is concerned, the freestream velocity would be the velocity close to the upper boundary which would be unaffected by the wall and no-slip condition, and consequently free from b.l. effects. This would then help you to roughly estimate from corelations, the turbulent boundary layer thickness as a function of distance on the plate, from say the leading edge. Also, it would be a nice idea to allow for a slip b.c. on the lower surface for a length of 20% of the plate, before the plate on which the no-slip condition is applied. This means that if your plate is 1 unit long, the lower surface would be 1.2 units long, the first 0.2 being a slip surface, beyond which the plate starts.

Hope this helps

Regards,

Ganesh

 Ralf Schmidt June 1, 2006 02:59

Re: boundary layer thickness

Hi Ganesh,

I did a geometry like you suggested (in Fluent) it can be seen in:

As you can see, if I would follow your definition ("the freestream velocity would be the velocity close to the upper boundary which would be unaffected by the wall and no-slip condition, and consequently free from b.l. effects"), the BL would be VERY thick.

Any further ideas?

Ralf

 ganesh June 1, 2006 05:12

Re: boundary layer thickness

Dear Ralf,

The B.L. thickness is a quantity quite subjective by definition, as compared to displacement or mometum thickness. Another idea to get the B.L. thickness would be to look into the velocity profiles at every section and check where it becomes 99% of the freestream velocity, which as defined before. However, I believe this should corroborate relatively well with the empirical correlations( Please do check if you did use the correlation for the turbulent b.l. rathe than a laminar b.l., by mistake). Further, the B.L. thickness is computed from the B.L. that comes out of the simulation, so we do not have any control on it (Once the freestream vlaues inc. Re are fixed). If you would also like to see if the simulation is right, you just need to compare also the velocity variation ,(follows nearly the log-law everywhere) as also the Cf distribution that can be compared iwth the Schilz-Grunow law. You could also try out finding the momentum and siplacement thickness, by simple integration of the velocity profile in the section, and looking at the shape factor too.

However, as far as the BLT is concerned, its definition is subjective, and I believe the freestream velocity should be the velocity of the undisturbed flow, taking us back to the definiton already earlier. This would also be the initial flow velocity with which the flow is entering the domain, particularly easy to see if a symmetry wall condition is given on the upper wall. (This is also the velocity with which it would slip at the first 20% on the lower surface).

Hope this helps

Regards,

Ganesh

 nvtrieu April 24, 2010 11:50

Hello everyone,

I'm doing simulation on Flat Plate with Star-CCM+. I used the same BCs with Ralf Schmidt's BCs. For the first case, I want to check the Laminar boundary layer with Re1E4. Further for Turbulent B.L. But I've still gotten the unexpected results with the velocity profile. From the plate, the velocity increases util larger than the inlet velocity, then decreases at the far field. It is quite straight, right? That's why I don't know how to estimate where the velocity ~99% mean velocity, in order to calculate the BL thickness.
Is there anyone can tell me how to correct this problem?
Thanks to all!

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