At low speeds, combustion couples to the fluid flow through density. The Monte Carlo 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 Iterate panel (Figure 18.3.4).
Averaging reduces statistical fluctuations and stabilizes the solution. However, 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 panel, and always check that the Total Heat Transfer Rate in the Flux Reports panel is balanced. It is also recommended that you monitor temperature/species on outlet boundaries and ensure that these are steady.
By default FLUENT performs one finite-volume iteration and then one PDF transport particle step. This should be optimal for most cases; however, control is provided to perform multiple finite-volume iterations ( Number of FV Sub-Iterations) and multiple PDF transport particle steps ( Number of PDF Sub-Iterations).
By increasing the Iterations in Time 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 Time Average until the steady-state solution is obtained. Then, to gradually decrease the residuals, increase the Iterations in Time Average by setting a Time Average Increment to a value from 0 to 1 (the value 0.2 is recommended). Subsequent iterations will increase the Iterations in Time Average by the Time Average Increment.