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Interaction between open channel flow and porous flow

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Old   June 8, 2020, 11:42
Default Interaction between open channel flow and porous flow
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Narendra
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##FLUENT. I have two infinitely long and wide flow zones- (1) free flow zone and (2) porous flow zone and both zones make an interface so that water can flow from one zone to other zone. Ideally itís 2D problem but I donít get mass conservation (inflow ≠ outflow) in 2D simulation and solution is unstable so I am solving it as 3D problem and assigning both side walls (span wise direction) as symmetric BC. (Since flow is 2D on symmetric surface and I know as we move away from symmetric surface, there will be small velocity component in span wise direction as I am using 3D solver. I am interested for velocity component in streamwise direction only as other two components are negligible). I also assign top as symmetric BC. (Let me know if you need more explanation on this).
I have created geometry horizontal and to consider slope of the bed, I calculate gravity components in streamwise and vertical direction. I am interested for flow in both zones together. For STEADY UNIFORM flow,
1) Can I assign two periodic boundary conditions as shown in figure? Is interface an issue? I have same pressure drop between inlets and outlets for both zones.

2) Assume it's case for flow in river with sand bed and slope is only in length wise direction. For Periodic boundary conditions inputs, I calculate pressure drop using slope in river (rho*g*tan(theta)). Now should I still activate gravity in both vertical and flow direction? Or should I make pressure drop zero and activate gravity?

3) Interface is very smooth, is there any way we can make it rough so it will represent rough sand (diameter around 3mm)?

4) FLUENT always uses bottom wall of porous bed to calculate Y+ or Y* but as porosity decreases, the interface starts behaving like a wall but Fluent wouldnít consider that. I want to generate log profile in free flow region when porosity is less then around 0.25. In natural porous river bed, log profile is observed (with slip velocity at interface) even if porosity is as high as around 0.35. Is it possible to generate the log profile using software?
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