Free-slip boundary

Hello everybody;
I applied the periodic for my inlet and outlet (z direction), and free slip condition for four walls.
Box3D inlet(0,nx-1,0,ny-1,0,0), outlet(0,nx-1,0,ny-1,nz-1,nz-1);
Box3D wall1 (0,nx-1,0,0,0,nz-1) , wall2 (0,0,0,ny-1,0,nz-1) , wall3 (nx-1,nx-1,0,ny-1,0,nz-1), wall4 (0,nx-1,ny-1,ny-1,0,nz-1);
boundaryCondition.setVelocityConditionOnBlockBoundaries (
lattice, wall1, boundary::freeslip );
boundaryCondition.setVelocityConditionOnBlockBoundaries (
lattice, wall2, boundary::freeslip );
boundaryCondition.setVelocityConditionOnBlockBoundaries (
lattice, wall3, boundary::freeslip );
boundaryCondition.setVelocityConditionOnBlockBoundaries (
lattice, wall4, boundary::freeslip );
But by some reasons, the velocity of at wall 3 and wall 4 are much higher than wall 1 and 2. The picture below is the velocity from wall 1 (x= 0) to wall 3 (x= 0.0064)

I expected the velocity at x=0 and x = 0.0064 should be similar.
I looked at the example code in Palabos, for example, ExternalFlowArondObstacle, sometime they use ny-1, and sometime they use ny-2; so I am not sure how the thickness of the wall where you apply the boundary condition will affect the result ? My wall now is 0 thickness cell, should I change it to 1 thickness cell ?

Thank you

1 Like

I tried to change my boundary according to GeneralExternalFlow; I apply the rho at the inlet and rho-deltarho at the outlet. I apply the freeslip boundary for the walls. However, my velocity at one side is still higher than the opposite. I expected they should be the same. Anyone can suggest me how to solve this problem ?
Thank you

In the previous code, I only applied the periodic boundary in z direction. I tried to apply the periodic boundary in all direction (x,y,z). In the Palabos guideline it states: " all fluid nodes which are in contactwith an outer boundary of the multi-block can be periodic, if the corresponding node on the opposite wall is a fluidnode."
So the velocity at the opposite walls should be the same. But I get the same results, one is higher than another.
Can anyone give me any advice?
Thank you

hey Nina,
How long is this running for / are you just setting a specific run time ? it could just be that it hasn’t reach equilibrium yet.