Answer: There are different pressure discretization schemes used for porous zones and free fluid zones. If you set the resistance to zero the solutions as you describe will be different when default pressure schemes are used. To get similar solution you need to use PRESTO scheme which is applied in both porous and free fluid zones. Remark: some users may ask why is it necessary to activate the porous zone at all if the resistance coefficients are 0 and the porosity is 1. A possible scenario where this can have an impact would be a model where the user wants to define time or position dependent resistance/porosity, which can become zero/one at certain locations or time points. In such cases, the user will want to make sure porous cells with resistance=0 and porosity=1 will behave in the same way as free fluid cells.