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-   -   Net mass flow inlet vs outlet (https://www.cfd-online.com/Forums/fluent/91493-net-mass-flow-inlet-vs-outlet.html)

 Nigui28 August 12, 2011 08:59

Net mass flow inlet vs outlet

Hi,

I simulated an open channel flow problem for a dam project using two inlets (upstream boundary conditions) and one outlet (downstream boundary condition). The total water mass flow from my 2 inlets is 330 m3/s, thus I expected to get approximately the same mass flow for my outlet which is totally logical (conservation of mass). My simulation converged very well but I found out a mass flow of 570 m3/s at my outlet which is 240 m3/s more than the mass flow that enter in the domain ??? How does the model can enter 230 m3/s more water in my domain ?? I double checked the mass flow rate of my outlet in FLUENT and in the POST and I found for both a same difference mass flow of 230 m3/s ?

I use the following parameter as model setup

- VOF with the open channel flow option checked
- Explicit solver
- mass flow rate as upstream boundary conditions (inlets)
- Specify water level as downstream boundary condition (outlet)
- Geo-reconstruct as Volume fraction solution method....default options for the other solution methods
- Outlet ventilation on the top of my domain

Is somebody as an idea why the conservation of mass is wrong for a simple simulation like this one....I really do not understand why ?

Thanks for your help !

Nicolas

 Nigui28 August 12, 2011 10:09

I think the best assumption to explain this mass flow difference is likely the fact that I patched with water in the initialization and this water volume (sort o wave from upstream to downstream) is now exiting the domain and give me a higher water mass flow than mass flow that enter inthe domain. I will continue to simulate and I suppose the mass flow will decrease at the outlet while all the initialization water volume will be out of the domain. Im still in the transient stage.....more time step to reach a sort of steady flow.

I answer myself but somebody can confirmed my assumptions :)

Thanks

Nicolas

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