Hello, I went through the Fluent Theory Guide and other Ansys learning resources, however I couldn't find the answers to the following issues.
Let's discuss the problem on the example of a general steady RANS k-omega case.
No slip Wall
U is set to zero, k is set to zero, omega is given by a specific equation.
1. What about pressure? Does Fluent use the zeroGradient condition for pressure on a wall? It's done by default in OpenFOAM.
2. NS momentum equations are 2nd order in space, so the boundary conditions should involve the normal derivative (e.g. dUn/dn=0), if I am not mistaken ?
3. Consequently, the turbulence transport equations are 2nd order in space as well, so should their respective derivatives be specified on a wall?
Velocity Inlet
U, k and omega are specified.
4. What about pressure? zeroGradient?
5. Refering to question 2,3 - should a Neumann BC be specified for U, k, omega?
Pressure outlet
Fluent Theory guide says that all variables (apart from pressure) are extrapolated from the interior. However, that does not sound right. Imagine a highly diffusive system, wherein turbulence (or any other scalar) is diffused upstream, faster than the downstream convection. In such a case, turbulence properties at the outlet would affect the solution in the domain.
6. As a consequence, what BC are imposed for U, k, omega? Mentzer suggests "intervals" which freestream turbulence properties should be taken from. https://turbmodels.larc.nasa.gov/sst.html
Symmetry
Fluent specifies zero convective/diffusive flux across the symmetry plane.
7. We should specify a value of k and omega at the symmetry anyway... right? It has to be done in OpenFOAM...
Thank you very much for your time and effort, feel free to make a comment on that.
The basic of boundary conditions implementation may found in CFD books please have a look at the books for finite difference you will find all mathematics their.
The answer you are seeking entirely depends upon the case you are simulating. Overall, your problem should be a closed problem(without unkowns).
For some cases, only field initial pressure may be needed, for some, inlet outler pressures are needed.
While building a case, you will automatically fulfill all conditions for starting simulation. Also, you can have a look at all governing PDEs, all variables are dependent and you can find one field from another.
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