# Lattice unit conversion

Hello house,

I think I have come to the point where I need a concrete help on how to inter-relate the lattice units and physical units,.especially when it has to do with pressure, density, viscosity conversions.
I would appreciate help in understanding ths issue.

Thanks

I’m the same, finding this difficult

Hello,

there are various threads in this forum where this issue is discussed. Also have a look at these resources.

Timm

Can’t believe I missed that, thanks!

Thanks Timm and was useful. I think I understand that, the problem is how density is taken as 1. Again, in flow boundary conditions, density is related to pressure and pressure changes along the flow path. so does it mean that the density of a fluid ( assumed incompressible) would also change along the flow path?

An incompressible fluid has a constant density, both in space and time. The pressure of course is not constant.
The lattice Boltzmann fluid on the other hand is (slightly) compressible, since its equation of state reads p = c[sub]s[/sub][sup]2[/sup] rho. Whenever you have a (desired) pressure gradient you also have an undesired density gradient; the density then should be fluctuating about its mean value (1 in most cases). If you want so simulate incompressible flows, you have to make sure that the density fluctuations are small. This can be controlled by the Mach number of the flow, see also this article. This clearly shows you that computer simulations always have some drawbacks.

Timm

Thanks Timm.

I now have a good grasp of that

Can anyone help with the unit conversion from lattice to physical units, especially for Grad P and absolute permeability.