Dear all,
Hi. I’ m troubled with some unphysical phenomena.
I use the multiphase program based on Graunau two color model.
Without external force and body force, the droplet is moving to the nearest wall.
I think the interfacial surface tension code may include the error.
But I have not gotten the solution.
Have anyone had the similar phenomena?
Please give me a advise.
Thank you.
YY
sounds like a bug to me. can you create a geometry and an initial condition which are exactly symmetric? if the droplet still moves you are sure to have a bug because your code has a preferred direction.
Hi adam
Thank you for your reply.
If the droplet is put in the center of the cube, it doesn’t move.
but, if the position is not the center, the droplet moves to the nearest wall.
My program may include the bugs.
I’ll do continue checking.
Thank you.
YY
Have you considered finite size effects of the box?
Hi Timm
Thank you for your reply.
The answer is No.
I’m not unfamiliar with the idea of the finite size effect.
Is that important for the LBM?
Thank you.
YY
Especially for the droplet, the finite size effect is very important. A member of my group has investigated its influence. The point is that in reality there is no finite size box, but a nearly infinite size box around the droplet. This causes artifact forces in the simulations which are, however, well understood. I do not know any references right now, but I will ask my colleague and post you more information later. Maybe this solves your problem.
Timm
I have talked to my colleague. He has observed a similar effect, but only if the droplet is very close to a wall (about 3 lattice nodes). This is an effect caused by the finite size of the droplet interface. He does not know any further literature.
How large is your computational grid? Does your droplet also move, if you only put it 1 lattice node apart from the center? If this is the case, I believe that you have a flaw in your code.
Timm
Hi Timm
Thank you for your help.
My Box size is 100100100. For example, if I put the droplet which radius is 20 pixel near the wall(10 pixels), it moves.
In some case(it depends on the droplet size), if only I put it a few nodes apart from the center, it moves.
I’ll continue to find some bugs and at the same time research the finite effect.
Anyway, thank you for your kindness.
YY
Hi YY,
Once I had the same problem when I made a mistake in my code. If you have the periodic boundary conditions and implement your force, check for your boundary.
For example, Shan-Chen yields the following:
F=G\psi_0 \sum_a \psi_a weights_a, where \psi_a = 1 - exp(-rho(icoor,jcoor)), where icoor =(i+c_ax)%NX, jcoor=(j+c_ay)%NY - once I made a mistake here and that involved the anisotropy of force.
Also, as Timm noticed, if you are close to the boundary that can force some additional condensation or anisotropy in your force.
Hopefully that will help,
Alex