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MD and LB uints

From: Ahmad Reza Motezakker
Subject: MD and LB uints
Date: Wed, 19 Jan 2022 13:58:37 +0000

Dear All, 

In the first LB tutorial it is mentioned that :  


Important note: all commands of the LB interface use MD units. This is convenient, as e.g. a particular viscosity can be set and the LB time step can be changed without altering the viscosity. On the other hand this is a source of a plethora of mistakes: the LBM is only reliable in a certain range of parameters (in LB units) and the unit conversion may take some of them far out of this range. So remember to always make sure you are not messing with that!

One brief example: a certain velocity may be 10 in MD units. If the LB time step is 0.1 in MD units, and the lattice constant is 1, then it corresponds to a velocity of 1 aτ in LB units. This is the maximum velocity of the discrete velocity set and therefore causes numerical instabilities like negative populations.


This part is a little bit vague to me. From the first part I come up with this that all the inputs and outputs of Espresso will be in MD units. so for the unit conversion (Espresso to SI or vice versa) , we use MD units.  Am I correct?

About the example that it is mentioned, Just to check, I found the maximum velocity that I have got from the simulation (5.79 in  [x]/[t] ,MD units). When I convert it to LB units (Lattice spacing = 5*[x], fluid tau = 5*[t]), I come up with 5.79 itself in LB units. Is it going to be wrong since it is bigger than 1? If yes, How should I set the LB parameters before hand to prevent any of these? I am really confused since I am analyzing my results.

Best regards,

Ahmad Reza

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