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mass in is not equal to mass out

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Old   March 29, 2016, 07:07
Default mass in is not equal to mass out
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I simulated flow in spillway using transient simulation, as well as velocity as entered is wrong (higher than actual), but the out put mass flow result is not equal to mass in as shown, my question is that is that velocity affect the mass inflow an out?, second the nonequal of mass in and out is related to total time?
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Old   March 29, 2016, 11:48
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I am not sure what your concern is based on the diagnostics you are showing.

You are running a transient simulation; therefore, there is a transient term in the mass conservation equation which is shown as "Negative Accumulation". Until this term is not zero, the inlet will never match the outlet since mass is being accumulated in the system.

Once the flow reaches steady conditions, the accumulation term should vanish and the inflow should match the outflow.

Hope the above helps,
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Old   March 29, 2016, 14:46
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thanks a lot, interested answer, so my boundary and physical set up correct, RMS courant number does not exceed 9.5, can you tell me what is the region of this imbalance (not this is after many trial to get this result with computational time 125hr)
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Old   March 29, 2016, 20:13
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CFX is an implicit solver, so Courant number is not a very useful measure of time step. You need to do a time step independance check, or use adaptive time stepping homing in on 3-5 coeff loops per iteration.

If you want to find where the fluid is being accumulated you will need to do some post-processing in CFD-Post.
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Old   March 30, 2016, 05:25
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You could also activate the "Conservation Target" in the solver control to enhance reaching a converged solution.

That is what it does:
http://www.cfd-online.com/Forums/cfx...on-target.html
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