[pchoopanya]

Dear all,

Please take a look at my residuals plot of a 3D flow past a row of compressor blade cascade, using Spalart-Allmaras turbulence model.

As you can see in the plot the residuals dived satisfactorily, then flattened out, then they rose up again, and again...

Luckily, I also activated point monitors of pressure and velocity, and lift and drag coefficients, and they all had stabilised, unlike the residuals.

There was no modification to the settings at all during the iteration process. Why did the residuals rise again after being seemingly flattened out/converged?

The residuals plot does not look great in terms of their value, but other point monitors and force coefficients had converged, can I conclude that my simulation has already converged? Or should I continue the iteration with new under-relaxation factors to get as low as possible residual values?

Judging the point monitors, I am happy with my solution but I am just being curious at what made the residuals rise as I kept iterating without making any change to the settings?

[LuckyTran]

You'll never know why the residuals jumped around unless you follow what the solution was doing at that time; and this depends also on what the initial guess was.

Flattened residuals is a hint that the solution is no longer changing, which it sounds like you've verified using monitors. If your solution has converged then it has converged.

URF's are for stabilizing the solution. If it ain't diverging leave it alone. If your monitors converged then your solution is converged. Remember, the values of residuals measure imbalances & does not tell you anything about convergence. If you want different values of converged residuals, then start tinkering with your settings & mesh to try to improve the quality of the solution.

[Aeroma]

What if the residuals had a spike and then started a diverging trend? I am running a supersonic ramjet engine simulation. Residuals were stable for almost 2500 iterations and at that point the temperature plot is still increasing and the mass flow rate lines had still not met. Then it has a drastic increase and a drop followed by a diverging trend while the temperature still increased while thrust were flattened out and mass flow rates at different sections (inlet and outlet) were still coming closer and had not met each other. I thought the mass flow rates at the inlet and before nozzle must be good indicators for convergence since it is a ramjet engine.