About ConvectionBC

[steed188]

Hi,
In my simulation domain, the Inlet BC is a profile of power law of height, I tried the ConvectionBC for outlet BC as:

Code:

bc.addConvectionBoundary(superGeometry, 4, omega, uAv);

where uAv is the average outflow velocity calculated automatically as Mr. Mathias and albert.min-k told before. But I found that the profile velocity was forced to a uniform average velocity when it went to the outflow BC, rather than a profile outflow.
The pressure BC has no this kind of problems, though the pressureBC is not stable in my high Reynolds cases.
Is this right?

Another question is that is there any wall functions provided in OpenLB to make the velocity gradient adjacent walls more accurate just like in other CFD software?

best wishes.
steed188

[Marc]

Hi steed188,

first we several pressure boundary condition approaches local and interpolation. I don’t know which one you use for your case. I prefer interpolation because the non equilibrium bounce back approach will be instable by high Reynolds numbers. If your geometry has an inlet and outlet of the same size, you should use periodic boundaries. I actually working on wall functions. OpenLB will get this feature in the future. If you want to learn more about simulating turbulent flows with LBM, I highly recommend you our spring school in March.

Best,
Marc

[steed188]

Dear Marc,
I used Interpolation pressure BC but it still was not stable in my high Reynolds case so that I tried the convection BC.
Do you mean that the convection BC DOES make the velocity uniform at the outflow?

And I tried Periodic BC. I defined a velocity at the inlet when iT=0, and let the inlet and outlet became periodic at x-direction. But seemed didn’t work. It is like below

Code:

cuboidGeometry.setPeriodicity( true, false, false );
…
sLattice.defineDynamics(superGeometry, 3, &bulkDynamics); //Inlet
sLattice.defineDynamics( superGeometry, 4, &bulkDynamics ); //Outlet
……
sLattice.iniEquilibrium(superGeometry, 3, rhoF, uF);
sLattice.iniEquilibrium(superGeometry, 4, rhoF, uF);
sLattice.defineRhoU( superGeometry, 3, rhoF, uF );
sLattice.defineRhoU( superGeometry, 4, rhoF, uF );
….
sLattice.defineU(superGeometry, 3, uSol); //Define inlet velocity

Best,
steed188

[Marc]

Dear steed188,

I think it depends, if you pass uAv to the convection boundary constructor.
For periodic boundaries you have to use forced dynamics and you don’t nedd additional material number for inflow and outflow. You can have a look at the forced poiseuille flow 2d example.

Best Marc

[steed188]

Dear Marc,

Yeah, I passed uAv to the convection boundary constructor. But If I did not pass it just like below, the simulation will divergence immediately.

Code:

bc.addConvectionBoundary(superGeometry, 4, omega);

Is there a method that I can use convection without passing uAv in high Reynolds flow?

best,
steed188

[Marc]

Dear steed188,

I don’t think so. You should deal with periodic boundaries to stabilize your simulation at high Reynolds numbers.

Best,
Marc

[steed188]

Dear Marc,

In my experience of other CFD codes, gradient zero BC usually means only the gradient in the mainstream direction is zero, while other directions are not . If I pointed uAv the convection BC will make the velocity uniform, is it still a gradient zero BC? But if I did not point the uAv, the simulation could not go on.

And the periodic BC is not fitting for my case because the inlet of my case is fixed.

Is there any other method to solve the problem?

with best wishes,
steed188

[Marc]

Dear steed 188,

I still believe that you want a “normal” pressure outlet condition (Dirichlet pressure=0)? It is possible to set a periodic boundary the only condition is that your outlet has an equal size like your outflow. You have to apply a forcing area instead of the inlet and force your massflow concerning a mean massflow. This is stable for high Re number flows. If you come to the spring School I will show you how to stabilize your case.

Best,
Marc

[steed188]

Dear Marc,
Thank you for your advice. I will try to use the periodic BC, and I’m still waiting for my prof.’s permission for the spring school. 🙂

best
steed188

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