[Taruk]

Hello everyone

I am doing a Cfd analyse of a nozzle and i got a convergent of the solution of order -3 and it doesn't change the residual value even if i leave it for a high number of iteration....
Do you think the solution is wrong??

Thank you

[DA6righthand]

That depends on several things. A lot of things.

I recently ran a handful of cfd simulations with Fluent of con-di nozzles. I ran my simulations as steady, coupled pressure based solver, with all discretizations set to second order upwind. My residuals would decrease to 1e-5 or lower after roughly 1000 iterations but at some point, the residuals stopped decreasing and essentially remained constant. My simulations would not reach convergence to 1e-6. Depending on the nozzle pressure ratio, my simulations would converge to some value between 1e-5 and 1e-6 and stay constant.

My recommendations would be to better initialize the solution and monitor mass flow rate between inlet and outlet to see if they're close to the same (my simulations showed a difference of 0.001 kg/s between inlet and outlet flow rates).

[Taruk]

Thank you for the reply

I also monitor the mass flow inlet and outlet and it basically remain constant( very very small fluctuation around the 0 value) but the thing that I'm worried about is that the continuity residual which is oscillating with an order of -3 while other parameters like k is Order -4 and omerga -3

[DA6righthand]

Below is a convergence history for one of my simulations



All of my nozzles were rectangular single expansion ramp nozzles which are inherently unsteady (the external jet oscillates). That is what causes the erratic oscillations of residuals. Notice my continuity residual was also the highest. I ran my simulation out to 3000 iterations. I considered my simulations converged after seeing that residuals stopped decreasing and mass flow at inlet and outlet were essentially the same (1.3949 and 1.3959 kg/s, respectively). That being said, I initialized first with hybrid initialization and then full multi-grid initialization. This allowed me to obtain a solution faster. Hope this helps!

[DA6righthand]

I edited the picture. I don't think it was showing up correctly before

[FMDenaro]

What about the order of magnitude of the initial residuals?

[Taruk]

I got convergent.....I just changed one BC....I initialize my case with an hybrid initialization and the order of residuals are of the order of -7

So if I got a residual plot like the above one...it is not wrong right???