Divergence due to Oscillating flow in a Wake

Mar 3, 2012


Issue:

A steady-state analysis runs, but never appears to completely converge.

Solution:

Solution

Determine if there is a significant wake region in your flow. Typically this wake region will be caused by flow over a bluff body that is shedding vortices. Known as a von Karmen vortex street, the formation and shedding of these vortices is a transient phenomenon.

The good news is that you don’t necessarily have to run your analysis as transient. On a macroscopic level, the phenomenon is well contained to the wake region. But it does mean that your convergence plot may not completely flatten, particularly in the cross flow directions.

To assess convergence in this case, you will need to inspect the mass flow rate and velocity to assess how much they change.  

  • Create a Summary Plane at the inlet and outlet, and watch the Critical Values in the Decision Center.
  • Examine the velocity contours from one iteration to the next to see if the global solution is relatively unchanging.
  • Look at the magnitude of the change in the cross flow velocities. If the magnitude of change in the cross flow quantities is relatively small and it is only the normalizing of the convergence graph that is making them appear to be wildly varying, you probably have a converged solution.

If, however, the wake is a significant part of the flow field, you will need to make sure that it is adequately represented. Ensure there is enough mesh to capture the recirculation. It may be necessary to add an additional flow volume downstream of the bluff body that encompasses the wake region. This makes focusing the mesh in this area much simpler, and reduces the chances of over-meshing other (non-critical) regions of the model.

In addition, we recommend extended the flow volume past the end of the wake. This region does not typically require a very fine mesh, but it will allow the wake to be completely defined before the flow encounters the constant pressure outlet boundary condition.

 

Products:

Simulation CFD

Versions:

2010;2011;2012;2013;2014

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