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-   -   Is my mass flow considered as balance? (http://www.cfd-online.com/Forums/main/10149-my-mass-flow-considered-balance.html)

MT October 25, 2005 03:15

Is my mass flow considered as balance?
 
Hi,

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?

Thanks.

ganesh October 25, 2005 10:25

Re: Is my mass flow considered as balance?
 
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.

Regards,

Ganesh

Mani October 25, 2005 14:44

Re: Is my mass flow considered as balance?
 
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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