Hello,
I am trying to simulate unidirectional flow in a CT image using the cavity3d example as template. My initial condition is zero velocity everywhere. The flow is driven by a pressure difference (density difference) between the inlet and outlet. The LBunits converter needs to have the Reynolds number to compute omega and the others. However, I do not know the Reynolds number at the beginning of the iteration as that is what I am interested in obtaining for a given pressure difference.
I would appreciate if somebody helps me with information on how to go about this.

My problem is not lattice conversion as I am pretty comfortable with it, It is just what would be the Reynolds number argument in the lBUnits converter since my fluid is intially at rest and the LBUnits converter needs a Reynolds number value to calculate omega.

The Reynolds number is defined with respect to a characteristic velocity. If you impose a pressure gradient, you should use your physical understanding of the problem (such as, an analytical solution for a simpler yet similar problem) to relate the pressure gradient to a characteristic velocity. If you can’t do this, your best bet is probably an a-posteriori estimate of the Reynolds number, by measuring the velocity at the end of the simulation.

But I insist: this type of arguments have already been discussed in the forum, and I’m sure it’ll be helpful for you to run a search.

Thanks Jonas for your explanation. I have read the comments in the previous threads and they were really helpful.

The first method you suggested is going to be kind of difficult because this is a 3D problem in cubic geometry.
I do want to start my simulation from zero initial velocity everywhere. What I ended up doing is to still use the LBUnits converter, but I manually calculated my omega while the Reynolds number argument in the converter is going to be zero (if it does not work, I will leave it blank in the converter). The velocity at the end of the simulation would then be used to calculate the Reynolds number (though the important parameter I need is the velocity for a particular pressure difference)