# how does CFX deal with mass conservation?

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March 21, 2012, 20:59
how does CFX deal with mass conservation?
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zhiwei liu
Join Date: Mar 2009
Posts: 22
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Hi,

As we know, CFX uses control volume finite volume method (CVFEM) to discretize the equations. These equations are integrated over each control volume,and Gauss' Divergence theorem is applied to convert volume integrals to surface integrals. The solution field must be approximated at integration points. CFX used finite-element shape functions to perform these approximations.
So, at transport equation, the convection term is discretized as:

where, rho, density;U, velocity; dnj, surface area of j direction; np, number of integral points.
But, using the exported CFX conservative velocity, i found mass is not conserved in some control volume when i calculate the mass flux through all the control volume faces.

So, i wondered, is the velocity at the iintegration point performed using shape function ? Or how does it use the mass flux at integration point when calculating the transport equation such as heat transfer?

Thanks!
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