[Dreoasteh]

Hello everyone,

I'm running a low Re LES case, you can see the geometry in the attached image.

After 4s I started to have backflow from the outlet patch into my domain which made my case unstable. I was using zeroGradient for U and fixedValue 0 for p at the outlet.

I have been searching for a solution and I have found the following:

1. Change boundary conditions at outlet. Here I found two options:
   • inletOutlet for U and fixedValue 0 for p.
   • pressureInletOutletVelocity for U and totalPressure 0 for p.
   
   If there is reverse flow both options change behaviour from zeroGradient for U to not allow flow to enter the domain from the outlet. I have generally read that the second option is preferred although I don't understand the reason why.
2. Modify the solver (pimpleFoam) to have a constant K multiplying the viscosity in the whole domain. Then use setFields to define K=1 in the whole domain except for a layer of cells (~5-10) at the outlet where K=100. This way no reverse flow will be carried upstream.

So my question is, which is the correct approach? Do any of you have experience with either one to solve the issue of reverse flow from the outlet into the domain?

Thank you!