[kflora]

Hi Foamers,

I am wanting to model open channel flow for a river and am unsure of the best means to define the inlet and outlet boundary conditions using VOF. With rivers one typically has the inflow rate and downstream water surface elevation (WSEL) defined as known values.

Using interFoam with OpenFoam 2.1.x, I can generate a specified flow rate using funkySetBoundaryFields by creating a known area at the inlet by setting alpha =1 and using a fixed velocity so long as I make sure that this area is fully submerged under water. However, I am not sure the best way to "fix" the downstream depth given that it is free surface flow. Using a zeroGradient for the velocity at the outlet leads to the water surface changing due to changes in the calculated velocites from the initial values near the outlet. I have had some success duplicating my inlet approach described above at the outlet as well (fixed area, fixed velocity) so that my Flow In must equal my Flow Out. To clarify my approach, I have attached a screen shot showing my outlet condition for a simplified geometry of a contracted bridge crossing. If I set my initial water surface using setFields, then the total volume of water in the system is constant though the water surface may adjust locally within the system which is OK.

This approach seems to work (sometimes, but other times it seems to be unstable); however, I understand that one normally prescribes the velocity (or flow rate) at the inlet and pressure at the outlet. Because my case uses VOF, I'm not sure if or how one can specify a pressure at the outlet to control the water surface elevation.

Any thoughts on how to use pressure at the outlet to achieve my goal of having a fixed WSEL or any other suggested changes that I could make to the BCs would be very helpful. I have included my basic files if you wold like to see the specifics. Thanks in Advance.