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How to Check Convergence of a CFD Simulation?

1 – Check the Convergence Plot
In an iterative solution, residuals are the solution imbalances. For numerical accuracy, one should expect residuals to be small as possible. Usually residuals below 1e-3 is a good starting point to move to the next check.
Residuals in convergence plot

Convergence plot also gives an idea regarding the stability of the solution. Observing sudden spikes shows that the solution encounterd problems. Results of such simulations are not reliable.

convergence - quantities

2 – Check the Quantities of Interest
In an iterative solution, calculation process starts with initial estimations on quantities (pressure, velocity, temperature, etc.). In every iteration, quantities are updated with respect to the previous iteration results. In a converged steady-state solution, one should expect to see that quantities come to a level and don’t change anymore.

The following graph belongs to a pressure drop simulation. Quantity of interest for this simulation is the pressure difference. First, simulation was performed for 300 iterations and area average pressure values at the inlet and outlet are calculated:

While the pressure on one surface is stable at zero (outlet), the other one is close to 150Pa and yet is not converged. The result cannot be trusted, further iterations are needed until the quantity is converged.


Using “Continue to run” button on simulation Run, simulation is re-run until 1000 iterations.


Results show that both inlet pressure value is converged around 500. iteration. Since both convergence plot as well as the the plot for quantity of interest are converged, one should monitor the solution fields to check if the results look realistic.

 

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