backflow and outlet boundary condition
Hi,
I have a problem with my LES simulation. I have backflow at the outlet of my domain which results in instability and as a result Courant number increases very fast and the code blows up eventually. What is the best outlet b.c for velocity and pressure that are not reflective and allow the eddies to leave the domain without influencing way upstream? For the velocity, I thought of inletOutlet and advective b.c and for the pressure zeroGradient and waveTransmisive. It'd be appreciated if you could share your experience with me. Thanks |
I haven't used it myself but I think 'fixedmeanvalue' might be appropriate for the pressure boundary condition. Have a look through the below thread and see if you can find anyone with similar problems.
https://www.cfd-online.com/Forums/op...oam-4-0-a.html |
As the first guess I would fix the inflow (based on the fluxes). For that purpose you can use inletOutlet with an inletValue zero. The pressure could be fixed. However, I do not know your simulation settings and which kind of LES you are performing (incompressible / compressible / transonic / subsonic / hyper ...). So you have to decide which BC you want to use. I guess at the moment you have zeroGradient for the velocity. However, this is a common problem (reverse flow) and there are several possibilities.
Cheers... |
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Thanks Tobias. I am doing an incompressible LES with Smagorinsky model and my inflow data is taken from a precursor simulation. You are right, I currently have zeroGradient for both velocity and pressure and it blows up very fast. I will try to use InletOutet for the velocity outlet patch. |
Okay, but keep in mind: Setting p and U at one patch for incompressible cases is not a good thing in the numerical point of view. Good luck.
PS: If you are searching for explanations of the boundary conditions, look into the header files. There is a description ;) |
Eman,
You might take a look at the following thread... https://www.cfd-online.com/Forums/op...-question.html I had stability issue with mixed inflow/outflow at an outflow boundary, and going to a totalPressure boundary has essentially rectified the problem. |
I just want to mention something here. The totalPressureboundary condition should be used with a pressure based velocity boundary condition. However, the main problem in reverse flow is the following. If we have an inlet and an outlet and based on the flow structure, we have a back flow at the outlet, the main problem is that we do not know how the flow should flow inside the domain again. Of course based on the pressure field we can estimate something and hope that this is true but in fact we do not have any information. Simple example:
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Hi Eman,
Could you tell me how did you solve this backflow problem? I encounter the same issue. BC at outlet I set as below: U:zeroGradient, p:fixedValue; k:inletOutlet; nusgs:zeroGradient; |
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