# a question about Document' choice of unit in LBM' by J.Latt

Hi friends
in documement "choice of unit in LBM’ sec 2.4 mentioned that dt=210**(-4) and dx=1/100. in every text there is this formula: c=dx/dt. and we use c in equilbrium distribution: feq(i)=w(i)rho(1+3/(c**2)(e(i)*u)**2+… in this example : c=dx/dt=50!!! and we must set c=50 in program for feq(i)???
Thanks a lot.

Hello,

when implementing the LBM algorithm one usually takes dt=dx=1. With these “lb” units c=1/sqrt(3) (for the D2Q9 lattice).

People that are using dx/dt=c are just taking this 1/sqrt(3) are the reference velocity of the physical setup. In the case of the document you are mentioning the reference velocity is an arbitrary value U=0.02.

Thanks Malaspin
in other words we can say :notice the difference between dx,dt(physical) and dx,dt(lb) :c which is used in algorithm is c=dx(lb)/dt(lb) not in physical unit. and in lb unit always we use: dt=dx=1.
Is it right?

Hi guys,

So does this mean that in the algorithm and where c is used in calculation of Equilibrium functions, we should always use c=1 ??? I have seen a lot of literature that have adopted various values of c=dx/dt, some even as high as 500. I am still not sure to find the appropriate c=dx/dt that should be used in the feq formulas. Thanks in advance for your help.