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I am simulating a axial turbine and number of cell are about 3000000.

Under relaxation factors are as follows:

Pressure: 0.1

momentum:0.1

Turbulent kinetic energy: 0.5

Turbulent dissipation rate: 0.5

Velocity formulation is relative.

Turbulence model is K-E relizable. The convergence of solution is monitored by checking the residuals of the numerically solved governing equations. Moreover, in order to judge the convergence, the behaviour of other quantities, such as the total pressure at the inlet and outlet boundaries, and torque coefficient generated by the rotor, are also monitored. Here, the default convergence criterion of each residual is reduced in order to allow the monitored quantities to stagnate at consistent values. It seems that convergence is ok but there is reverse flow!! why?

residuals :

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torque coefficient:

enter image description here

total pressure at the inlet and outlet boundaries:

enter image description here

Mass flow rate:

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I continued this run and is as follows:

enter image description here

enter image description here

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Residuals are almost constant. What is the reason?

Is convergence OK?

I am grateful that guide me about this case.

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Reverse flow is usually not a big deal, there's even an option in the text user interface to suppress the warning. If you're using higher order discretization in your solver, you may want to instead try a first order approach first. Get that to converge, then run a few iterations with second order and see if you still get reversed flow. The solution obtained from the first-order method will provide good initial conditions for the second-order method.

The y-axis on your mass flow rate is hard to see, but it looks like the convergence is hovering around zero. All the values on the y-axis look to be zero, which means that graph is zoomed in quite far. Based on your residuals plot, I think your solution is pretty well converged at this point.

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  • $\begingroup$ Thank for your answer. I will perform your statement. i think that you are right. According to your statement, Is there any problem in this solution? In other words, in some simulation, it can be exist reverse flow and solution converges. Am i right? I thought that a converged solution have not any revers flow. but according your statement, it can be exist reverse flow and solution converges. Carlton, I didn't understood the following sentences, can you explain more? "there's even an option in the text user interface to suppress the warning." $\endgroup$ – user19061 Nov 10 '15 at 18:22
  • $\begingroup$ Other thing is that torque and inlet and outlet pressure are not stagnated. $\endgroup$ – user19061 Nov 10 '15 at 18:30
  • $\begingroup$ I cannot cite any professional sources that say how much reverse flow is acceptable, however I can say from personal experience that many of my Fluent simulations, both steady and transient, contain warnings about reverse flow yet still produce correct results. I would not worry about reverse flow too much; the residuals of mass/momentum/energy/etc. are much more important for gauging solution convergence. To suppress reverse flow warnnigs, type into the text user interface (TUI): solve set flow-warnings? no $\endgroup$ – Carlton Nov 10 '15 at 18:33
  • $\begingroup$ Other thing is that torque and inlet and outlet pressure are not stagnated. upsara.com/images/mgxf_66666.jpg upsara.com/images/i4pm_555.jpg $\endgroup$ – user19061 Nov 10 '15 at 18:37
  • $\begingroup$ Thanks, According " solve set flow-warnings? no", there is not any warnings about reverse flow. But Do you sure that this work affect on final solution? upsara.com/images/yi33_delet_revers_flow.jpg $\endgroup$ – user19061 Nov 10 '15 at 18:48

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