My case is VOF model with two phases flow. Primary phase is air and secondary phase is water-liquid. Air is turbulence flow, 2.22m/s and water is laminar flow, 0.0094m/s. Therefore, i initialize velocity of air. My question is, do i need to use the patch function to simulate my case? If yes, what is the step to do so? Thank you!

You should go to Adapt->Region and give the coordiantes of the region you want to patch and press the Mark button. It would mark the cells that are to be patched. Then go to solve->Iterate->Patch and give the values for the variables you want to patch after selecting the patched region in the Register to Patch box. You can patch vol. frac, and the velocities of the two phases. It helps in quicker convergence. Raja

Dear Raja, In our case, we have bend with separated flows (air in the top and water in the bottom with different velocities). So, If we want to patch the whole system for more accuracy, how to specify the coordinates? Is it the area of the inlet, pipe or the outlet?

If your geometry is complex where patching is not possible, then you should use unsteady solver. VOF model would then determine the liquid/gas interface as the fluids flow through the geometry. Raja