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Is my mass flow considered as balance?

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Old   October 25, 2005, 02:15
Default Is my mass flow considered as balance?
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I've calculated the mass flow at inlet and outlet.

inlet=0.375433 kg/s outlet=-0.375963 kg/s

Difference is about 0.2%

Is that considered as satisfactory? Can I say the mass flow is conserved? Or I need to run a few more steps again?

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Old   October 25, 2005, 09:25
Default Re: Is my mass flow considered as balance?
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Dear MT,

The definition of conservation tells us that mass_in = mass_out. The present case has difference in the 4th decimal, and looks okay, but if it is not a converged solution, you have to run your code till convergence. You could also check the mass flow difference on a coarser/finer grid compared to the present one, for a converged solution case and see if you get a larger/lesser difference comapred to the present. And more importantly, the conservation would reflect in the solution, as in the shock location if you deal with compressible flows.


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Old   October 25, 2005, 13:44
Default Re: Is my mass flow considered as balance?
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You will always observe a small error simply because of unwanted effects of artificial/numerical dissipation. Those effects are not accounted for in your mass balance. An error of 0.2% in a converged solution seems acceptable. If you're not happy with it, you can try to further reduce it by reducing artificial dissipation (or choosing a higher-order scheme). Just keep in mind, as ganesh pointed out, that the accuracy of your mass balance alone does not guarantee an accurate flow solution.
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