19.6 Monitoring the Solution

At low speeds, combustion couples to the fluid flow through density. The Lagrangian PDF transport algorithm has random fluctuations in the density field, which in turn causes fluctuations in the flow field. For steady-state flows, statistical fluctuations are decreased by averaging over a number of previous iterations in the Run Calculation task page (Figure  19.6.1).

Figure 19.6.1: The Run Calculation Task Page for Composition PDF Transport

Averaging reduces statistical fluctuations and stabilizes the solution. However, ANSYS FLUENT often indicates convergence of the flow field before the composition fields (temperatures and species) are converged. You should lower the default convergence criteria in the Residual Monitors dialog box, and always check that the Total Heat Transfer Rate in the Flux Reports dialog box is balanced. It is also recommended that you monitor temperature/species on outlet boundaries and ensure that these are steady.

The Lagrangian PDF method has the additional solution controls: Iterations in Average and Iteration Increment. By increasing the Iterations in Average, fluctuations are smoothed out and residuals level off at smaller values. However, the composition PDF method requires a larger number of iterations to reach steady-state. It is recommended that you use the default of 50 Iterations in Average until the steady-state solution is obtained. Then, to gradually decrease the residuals, increase the Iterations in Average by setting a Iteration Increment to a value from 0 to 1 (the value 0.2 is recommended). Subsequent iterations will increase the Iterations in Average by the Iteration Increment.