[aja1345]

Hi,

I am simulating an . I am simulating an axial turbine by using Fluent. The convergence of solution by checking the residuals of the numerically solved governing equations. Moreover, in order enerated e default convergence criterion of each residual is reduced in order to allow the monitored quantities to stagnate at consistent values. U. Solver is ansys Fluent. Thisis na steady. The computation was carried out with changing the rotational speed er a constant axial velocity.


Residuals are as follows:

http://upsara.com/images/ak6c_residuals3.jpg
http://upsara.com/images/nlfk_residuals4.jpg
http://upsara.com/images/qif_residuals5.jpg

And also, mass flow rate, torque (in direction of Y) and inlet and outlet total pressure are as follows(in order to judge on convergence):


Torque (in direction of Y):


http://upsara.com/images/8q40_y_momment3.jpg
http://upsara.com/images/zf53_y_momment4.jpg


Inlet and outlet total pressure:

http://upsara.com/images/89os_inlet_..._pressure3.jpg
http://upsara.com/images/uep0_inlet_..._pressure4.jpg


Mass flow rate in inlet and outlet:


http://upsara.com/images/5ep1_mass_flow_rate3.jpg


Now, I have some question about Residuals:


As you see in first figure, residuals decrease until about iteration of 1500 and then after iteration of 1500, residuals increase. I want to know that this event is logical or not. I already thought that residuals should decrease decidedly because of convergence. But in this simulation, residuals increases and decreases!! What is reason for this event?
According above figures, Is solution converged?
I am grateful that guide me about these cases.


Thanks.

[aja1345]

Other figures:

[MrCaplin]

Hi,

I've got a "similar" problem:

I'm doing a simulation of an internal (gas)flow with the following set-up:

3D
unstructured mesh
steady
incompressible
turbulence: k-e, realizable
second order upwind scheme

If you need more information please let me know!

The problem:
Residuals were dropping for about 3000 iterations but then they started to increase.
Continuity residual which was the highest went from about 5*e-4 to 8*e-3 and is steadily increasing.

Is there a problem with the setup or shall let the calculation continue?

Thanks in advance!

[CeesH]

It is unlikely your simulation converged, but never ever judge so by the residuals alone.

Check integral parameters (average velocity, temperature, gas fraction, whatever is suitable for your problem). In a steady state solution, these should become constant over the iterations (where constant means oscillating within a strict margin, for example, oscillating with 0.01% with respect to the mean or so - up to you to select what is acceptable)

[MrCaplin]

Originally Posted by CeesH

It is unlikely your simulation converged, but never ever judge so by the residuals alone.

Check integral parameters (average velocity, temperature, gas fraction, whatever is suitable for your problem). In a steady state solution, these should become constant over the iterations (where constant means oscillating within a strict margin, for example, oscillating with 0.01% with respect to the mean or so - up to you to select what is acceptable)

I hope you were talking about my post.
Thanks for the tipp. I started monitoring the average velocity in addition to the residuals. I also always check mass fluxes.

Anyway, I started a new thread since I have a couple of more questions.
http://www.cfd-online.com/Forums/flu...questions.html