[cleoo]

how can rhoSimpleFoam be a compressible solver and a steady state solver at the same ime?

From what I know (please correct me if I am wrong)
If its a compressible solver it means it takes into account that density is changing.
if its a steady state you don't take time into account.

so there comes my confusion it seems like the two are contradicting each other.

[hunger]

The density can change in space, e.g. due to a certain temperature field present in your geometry. It can be stationary though, if in every single control volume, the density does not change over the time.

Another example: if you consider a steady-state solution for your velocity field, it doesn't mean that your velocity has to be constant in all your simulation domain. If the cross-section of a pipe gets smaller, the flow velocity will increase (assuming incompressible flow). This change in velocity is not connected with the time.

Best regards
Harald

[cleoo]

thanks, that makes sense and great name 'hunger'