Open Channel Flow Boundary Conditions

TWaung · September 30, 2011, 15:50

Hi,

I am trying to model a simple open channel flow with an energy extraction device in the channel.

I am having difficulty specifying the inlet and outlet boundary conditions. Due to the presence of the energy extraction device (like an actuator disk modeled using a momentum source or porous region) , there is a head drop across the device.

I would like to set the water elevation at one boundary and then let the water elevation sort itself out at the other boundary.

I started with a velocity inlet and pressure outlet. I set the volume fraction ratios at the outlet using a field function. Then I set a constant inlet velocity ( I wasn't sure what to do with the VOF settings for the inlet). The resulting water elevations outlet were way off of the target...

Does anyone have suggestions on how to go about setting the inlet and outlet boundary conditions?

Here are some of the settings I am using

Physics Models:
- Implicit Unsteady
- K-e turbulence
- Eulerian Multiphase
- VOF
Phases
- Air, Water

Thanks for any suggestions!

abdul099 · October 5, 2011, 19:56

Velocity inlet is fine. But what the hell do you mean, you are unsure about the VOF settings there? Don't you know your inlet conditions? Just specify a velocity profile and volume fractions and go ahead.
It should also be possible to try a pressure outlet as inlet condition when you're unsure about the inlet velocity. In this case, apply an appropriate pressure field (with hydrostatic pressure) and the volume fractions.

I'm not sure about a pressure outlet as outlet condition, as you have to specify conditions. The pressure boundary also allows an inflow when necessary, so it could occur and you have to specify values which often should be the result, no boundary condition. The water level at the outlet will be influenced from your outlet condition, especially from the pressure field you applied on the outlet. Therefore the result might be different due to your boundary condition, but that's no issue of the solver.
I think, I would try a get around this by choosing a flow split outlet (but I haven't tried, so better check it).

TWaung · October 7, 2011, 00:37

Thanks for suggesting using the flow split outlet! That seems to be working well.

September 30, 2011, 15:50	Open Channel Flow Boundary Conditions	#1
TWaung New Member Join Date: Mar 2009 Posts: 14 Rep Power: 17	Hi, I am trying to model a simple open channel flow with an energy extraction device in the channel. I am having difficulty specifying the inlet and outlet boundary conditions. Due to the presence of the energy extraction device (like an actuator disk modeled using a momentum source or porous region) , there is a head drop across the device. I would like to set the water elevation at one boundary and then let the water elevation sort itself out at the other boundary. I started with a velocity inlet and pressure outlet. I set the volume fraction ratios at the outlet using a field function. Then I set a constant inlet velocity ( I wasn't sure what to do with the VOF settings for the inlet). The resulting water elevations outlet were way off of the target... Does anyone have suggestions on how to go about setting the inlet and outlet boundary conditions? Here are some of the settings I am using Physics Models: - Implicit Unsteady - K-e turbulence - Eulerian Multiphase - VOF Phases - Air, Water Thanks for any suggestions!

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October 5, 2011, 19:56		#2
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 21	Velocity inlet is fine. But what the hell do you mean, you are unsure about the VOF settings there? Don't you know your inlet conditions? Just specify a velocity profile and volume fractions and go ahead. It should also be possible to try a pressure outlet as inlet condition when you're unsure about the inlet velocity. In this case, apply an appropriate pressure field (with hydrostatic pressure) and the volume fractions. I'm not sure about a pressure outlet as outlet condition, as you have to specify conditions. The pressure boundary also allows an inflow when necessary, so it could occur and you have to specify values which often should be the result, no boundary condition. The water level at the outlet will be influenced from your outlet condition, especially from the pressure field you applied on the outlet. Therefore the result might be different due to your boundary condition, but that's no issue of the solver. I think, I would try a get around this by choosing a flow split outlet (but I haven't tried, so better check it).

October 7, 2011, 00:37		#3
TWaung New Member Join Date: Mar 2009 Posts: 14 Rep Power: 17	Thanks for suggesting using the flow split outlet! That seems to be working well.