[Wenhao Dong]

Hello CFD community
I am doing some simulation for very classical open channel water flow cases in hydraulic engineering field.
For the past several days, I was struggling to apply suitable boundary conditions to get a good simulation results using interFoam. I would like to impose typical flow rate (discharge) BC for inlet and a fixed water depth BC for outlet. By the way, I think this is the very common BCs in open channel experiments. However, I could not find a direct BC for fixed water depth outlet.
For the inlet, I just adopted variableHeightFlowRateInletVelocity with specific flow rate which is already known through experimental conditions.
As for the outlet, I was first trying to use the outletPhaseMeanVelocity BC and setting the Umean as Q/h (inlet flow rate/outlet water depth). The simulation went well and I got quite good results in terms of the velocity field, however, the water surface elevation seemed to be a problem because this outlet BC adjusted the alpha.water to achieve Umean, resulting in a variable water depth (not same as the experimental outlet water depth).
Then I tried to follow another approach by dividing the whole outlet patch into two parts: air and water. For the water patch, it is exactly same height as the experimental water depth, and I applied the totalPressure BC with specific p0 = rho*g*h, so that I can get a fixed water depth. This approach seems to be fine as it achieved what I want, a fixed water depth outlet BC. But the results show that still the water surface elevation for the whole flow field is quite different with experimental results. (velocity field no big differneces, still suits the expermental results)
So I seek your kind help about how to deal with BCs, and for your information I list all my BCs as following:

alpha.water
inlet-variableHeightFlowRate
walls-zeroGradient
outletWater-zeroGradient
outletAir-zeroGradient
atmosphere-inletOutlet
p_rgh
inlet-fixedFluxPressure
walls-fixedFluxPressure
outletWater-totalPressure with p0=rho*g*h
outletAir-totalPressire with p0=0
atmosphere-totalPressure with p0=0
U
inlet-variableHeightFlowRateInletVelocity with flow rate Q
walls-noSlip
outletWater-fluxCorrectedVelocity with same flow rate Q as inlet
outletAir-pressureInletOutletVelocity
atmosphere-pressureInletOutletVelocity

And I am using the k omega SST turbulence model.

Best regards,