# some simple question for bigginers

hi dears
i have a simple question .assume that you have two diffrent flows with same nu{ph} and same geometries but velocity of first flow is twice the second u1{ph}=2u2{ph} and therefor Re1=2Re2.when we want to convert these parameters to the LB units we have Re1 and Re2 and we have same delta x and delta y and therefor u1{lb}=u2{lb} and nu1{lb}=2*nu2{lb}???is it correct???
can i put same delta x{lb} and delta y{lb} for different physical velocities???

Hello,

any dimensionless relation which is true in the physical system must also be true in dimensionless (lattice) units, for example the Reynolds numbers. In particular, dimensionless variables are the same in physical and lattice units.
This means that Re_1 and Re_2 are fixed in lattice units. If you additionally fix the lattice resolution in such a way that delta_x = delta_y (I assume that delta_x is for the first and delta_y for the second flow) you have two choices:

1. Choose u_1 = u_2 (in lattice units) and therefore nu_2 = 2 * nu_1
2. Choose nu_1 = nu_2 and therefore u_1 = 2 * u_2
Of course you can also choose another ratio of u_1 and u_2 and therefore another ratio of nu_1 and nu_2. Just keep in mind that
Re_1 = delta_x * u_1 / nu_1
and
Re_2 = delta_y * u_2 / nu_2
hold. From Re_1 = 2 * Re_2. the two above relations 1. and 2. follow.

Timm