### 11.6.4. Convergence and Solution Monitoring of Transient Blade Row Flow Problems

Due to the periodic nature of the flow in blade row configurations, ensure that the solution is monitored sufficiently in order to determine when a transient periodic state has been reached. The following monitoring options can be used:

- Use flow field monitors to check pressure, temperature and velocity variation. The solution is usually deemed converged when the monitor repeats the same pattern over a common period. Sometimes local flow field monitors do not show perfect repeatability within a common period due to local flow instabilities. The Frequency Filtering option was added to the Fourier Transformation model to avoid instabilities. The setting for enabling frequency filtering is described in Frequency Filtering in the CFX-Pre User's Guide.

- Use monitors of integrated quantities such as pressure surface loads, stage pressure ratio, or stage efficiency. Integrated quantity monitors tend to be less influenced by local instabilities and can give a better sense of the overall solution convergence than local flow field monitors. Monitoring of instantaneous integrated quantities is not recommended for Time Transformation. Instead, you should monitor integrated quantities using monitor averaging.

- Use monitors on inlet and outlet flow rates.

- Use monitor averaging to assess the convergence of the periodic monitors. By selecting an appropriate averaging range, you can monitor the average of the variation to determine if the solution has reached a transient periodic state.

Ensure that a transient solution is converging properly at each time step:

- Check that the residuals of all equations are converging sufficiently at every time step.

- For time accurate solutions, convergence must be achieved without reaching the maximum number of coefficient loops. If convergence is not achieved, a mesh issue could be the problem preventing the solution from converging, requiring a re-examination of the mesh quality.

Also see Using Interrupt Control in Cases with Transient Convergence Behavior in the CFX-Solver Modeling Guide.

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