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nate.simula

Time Steps, Time Increment, Convergence - Steady State

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Hi-


I am curious as to some guidelines on the max number of time steps and the initial time increment for a steady state problem.  Initial time increment is 1e10 s but can this be adjusted to improve convergence?  My problem is terminated at the max number of time steps rather than converging and terminating.  I am new to AcuSolve so I am unsure on how to determine if the problem is converging.  I have read some things indicating the Residual and Solution ratios need to be less than the specified convergence tolerance.  In one tutorial I did, the convergence tolerance = 0.001 but when I looked at the Residual ratios, they were not less than this value and the solution still converged  and terminated before reaching the max number of time steps.


 


Also, the results seemed reasonable for my problem that terminated at the max number of time steps - Should these results be regarded as inaccurate?


 


Any help appreciated!


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The concept of convergence is subjective - different users will give different responses.  Some general comments:


1. Residual Ratio - an overall sense of how well the solution matches the equations being solved


2. Solution Ratio - an overall sense of how much the solution changes from time step to time step (for steady-state)


 


In general we would want both of these to be quite low.  'Low' is subject to interpretation, and the default convergence tolerance of 0.001 is usually good for AcuSolve results.  By default, the Residual Ratio would need to be below the specified tolerance (0.001 default) for pressure, velocity, temperature, species.  Other quantities could be a factor of 10 higher (0.01 default).  By default the Solution Ratio could be a factor of 10 higher than the residual ratio values.  Again - that is the default behavior.  You can review the settings in the CONVERGENCE_CHECK_PARAMETERS command which will be included in the <problem>.ss.inc file generated by the AUTO_SOLUTION_STRATEGY command.  This information will also be in the <problem>.<run>.echo file.


 


You would also want to track the results of interest and see how they are coming to a 'constant' value for steady-state.  If the default convergence has been met, but the solution of interest is still changing by more than is acceptable to you, then reduce the convergence tolerance and allow it to converge more.  (This is something of a convergence sensitivity study.)  By the same token, if the results of interest have come to what you consider a steady solution, but the default convergence has not been reached, you may still consider the solution to be acceptable.


 


Convergence itself does not necessarily mean you have an accurate solution.  You would still want to perform a mesh sensitivity study to see how the results of interest change as the mesh is refined (volume size, surface size, boundary layer parameters, etc.).  You could also perform other sensivity studies (effect of changes in boundary conditions, initial conditions, etc.)


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