# Full problem

Hi All,

Inspite of working on LBM for so many days, it is still a confusion on how to start with a given problem, especially with the Units. It will be very helpful for the forum members if somebody can post the Units conversion for the following parameters.

1. Pressure
2. Density
3. Temperature

It will be appreciated if an example is given. I request the owner of this forum to address this issue as it will benefit many upcoming researchers in the field.

Thanks,
Narender

Hi All,

I have found a link which gives the total problem description (from start to end). More exercises like this will be helpful.

Thanks,
Narender

Dear Narender
Thanks, it was so helpful. but I see in many references that in the case of fluid flow in porous media we should have low Mach number to increase time step and in order to compensate this, it was suggested to increase viscosity simultaneously(I don’t got it why we should do that). I see in some references kinematic viscosity is set to 0.1(!!!). I think it was so strange, whats your opinion? Please give me a hand in the area of porous media.

Kind Regards

Dear Narender,

Thanks alot for the useful link of LBM workshop. I’ve studied that power-point and a big question involved my mind. According to that power-point, the selection of dt (time-step) is strongly linked to kinematic viscosity and dx.

On the other hand, based on Dr. Latt’s article about unit conversion in LBM, “There is no straightforward intuitive way to choose dt.” and it’s better to have the relation of (dt ~ (dx)^2).

But that relationship between dt and dx hasn’t mentioned in the LBM workshop power-point. Now I am fully confused in selection of dt. Which one is correct? Is there a standard way for choosing dt as mentioned by Timm Kruger in LBM workshop? Or there is no straightforward way to choose dt as written by J.Latt?

Hesam

Dear Hesam,

First of all, the approach I have presented during the LBM workshop is correct. It is important to keep in mind that you have to set all simulation parameters in a consistent way. How you do this (i.e., with which parameters you start) is up to you.
What Jonas says about the scaling dt \propto dx^2 concerns the problem how to rescale the simulation parameters to increase resolution: Imagine you have a valid set of simulation parameters (S1), for example obtained as described in my presentation. Now, you want to refine your grid to improve the results. For this, you need another set S2 with higher resolution. The question Jonas wrote about is how to get from S1 to S2. The best way to do it is to rescale dt \propto dx^2. But this has nothing to do with the approach how to find S1 in the first place.
Is it clear what I mean?

Timm

Dear Timm,

I really appreciate your nice answer. I think I’ve got what you meant
Thanks alot…

Regards,
Hesam

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