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The mass flow rate changes TOO much!

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Old   March 30, 2015, 07:54
Default The mass flow rate changes TOO much!
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Hello,
I'm trying to simulate a rotor/stator flow. I used the "stage" frame change option and the "High Resolution" Advection Scheme option.
The simulation ran fine with 1E-8 RMS residuals and 1E-6 MAX Residuals, but there's something that looks really strange to me: moving from the rotor to the stator the flow rate changes a lot (from 3000 kg/s to 3093 kg/s). Obviously the stator is set to "stationary", while the rotor is "rotating" with a given angular velocity.
Do you have any explanation about this behaviour?
Thanks so much!
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Old   March 30, 2015, 08:42
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Has the imbalance for the P-Mass equation (continuity equation) converged as well ? What is the mass imbalance reported in the output file, and in the Solver Manager ? Is it consistent with the values you see in CFD-Post ?
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Old   March 30, 2015, 09:15
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Quote:
Originally Posted by Opaque View Post
Has the imbalance for the P-Mass equation (continuity equation) converged as well ? What is the mass imbalance reported in the output file, and in the Solver Manager ? Is it consistent with the values you see in CFD-Post ?
Thanks for your answer!
The P mass converged at 1E-8 RMS and 2.8E-6 MAX

In the output file I read:
Domain Interface TR_TM Side 2 = -3.0114E+3 (inlet of the Rotor) and
Domain Interface TM_RA Side 2 = -3.0338E+3 (inlet of the stator)

What do you mean with "mass imbalance reported in the Solver Manager"? The output file collects all the infos reported in the Solver Manager during the calc process, doesn't it? (or maybe I've misunderstood your question)

Thanks again
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Old   March 30, 2015, 09:51
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We are talking about .7% mass imbalance here.

In the SolverManager, create a new Monitor Plot, select IMBALANCE, and plot the P-Mass equation. Is the imbalance still converging ? , i.e. it can be reduced further if given more iterations or it has long converged and remained fixed for many iterations.

If you increase the time step once converged this far, does the imbalance change.
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Old   March 30, 2015, 10:12
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Quote:
Originally Posted by Opaque View Post
We are talking about .7% mass imbalance here.

In the SolverManager, create a new Monitor Plot, select IMBALANCE, and plot the P-Mass equation. Is the imbalance still converging ? , i.e. it can be reduced further if given more iterations or it has long converged and remained fixed for many iterations.

If you increase the time step once converged this far, does the imbalance change.
Thanks, Opaque, for you advices.
In Solver Manager I've created a New Monitor, I've choosen "Plot Monitor" and then "Imbalance". Then I've selected P Mass Imbalance (%) Equation" for the Rotor Domain.
The graph has converged to 0 (8.8E-8 to be sharp).

I've repeated the same procedure with the P-mass Imbalance Equation for the stator domain and obtained the same thing...

Yes, it's true, it's a matter of .7%. The strange thing is that with other simulations (in which convergence was just 1E-5 RMS) i didn't see these imbalances.
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