Relaxation Factors in RANS Flow Solvers

Solver relaxation factors control how far to advance the solution of the primary fields (e.g., U, p/rho, k, epsilon) per iteration for steady-state RANS solvers. If they are set too high the flow simulation will diverge, i.e., not converge. Conversely, if they are set too low then the flow simulation will require an excessive number of iterations to converge.

Relaxation factors are available when the RANS Flow or Professional Caedium add-ons are activated and you have configured a steady-state CFD simulation such as the "Incompressible Flow Through a Pipe into a Box" tutorial. To access the relaxation factors follow the procedure below.

Procedure

  1. Right-click an edge or face of the flow volume in the View Window, then double-click the flow volume in the Select dialog, and select Properties from the menu.
  2. In the Properties Panel select the Volume tab and expand the Substance->Solver:RANS Flow->Relaxation Factors properties.


Relaxation Factors

Notice the relaxation factors for the primary flow variables U, epsilon, k, and p/rho - they should all be greater than zero and less than one.

Troubleshooting

Before modifying the relaxation factors check your mesh for problems. The majority of convergence problems are caused by a poor mesh.

If you are confident that your mesh is good and your residual monitor still shows erratic behavior without any sign of convergence or if the flow solver reports a singularity, try halving the p/rho and U relaxation factors and restarting the simulation. Perform this routine recursively until the simulation shows some sign of converging. If after a number of such cycles there is no improvement in convergence, then try halving the remaining turbulent variable relaxation factors and restarting the simulation, and repeat until you see signs of convergence. If this procedure does not help, then review your initial conditions, reference values, and boundary conditions.

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