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shanon September 21, 2010 09:06

k-w SST transitional model problem
I am performing low Reynolds flow over eppler 361 airfoil. the sst transitional k-w turbulence model has been used.
the first lenght of grid near the wall is about 10^-5 C of airfoil. the flow Reynolds number is set to be 100000. In such Reynolds Numbers, the laminar separation bubbles form near the leading edge of the airfoil, causing more complicated flow.
the problem is when I intend to model the flow, I see some strange behavior in residuals;i.e. the continuity residual is in order of 0.001 and w equation residual shows some peaks of fluctuation (sth like electrocardiogram:)). these fluctuation peaks are of order such that they rise the residual by 4~5 orders.
As I have tried my best to resolve the mesh near the airfoil as much as possible, and the y+ value shows that the grid is fine enough; I do not know how to fix the problem.
So would anybody help me on the subject, I would be grateful.

Trev September 22, 2010 11:31

Use grid adaption to see where your mesh might need improving. Use the gradient of velocity (assuming incompressible flow otherwise check the gradients of total pressure) and set the upper threshold to 10% of the maximum gradient. You can then mark these cells and display them to find where you need to refine the mesh. Fluent will probably tell you to refine the BL mesh but you can ignore this if your y+=1 approximately as you know the resolution is adequate. Use the minimum cell volume options to blank out the BL to give you a clear view of what cells need refining.

I would also refine the mesh in Gambit, Gridgen or what ever you use to avoid Fluent putting too many cells in during the refinement or reduce the refinement level to 1.

If problems still persist reduce the URFs for pressure and momentum heavily and also reduce the turbulence related URFs quite alot if steady state. Otherwise just reduce the time step accordingly. Also make sure your turbulent parameters for the boundary conditions are set correctly to avoid getting the turbulent viscosity ratio limited warning in cells.

Last thing would be to monitor your mass flow rate, lift and drag coefficients and wait for them to stabilise before deeming you have a converged solution.

Hopefully this should be enough to sort out your problems.


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