Fluxes

There is a need to point out that, although a material volume boundary moves such that the flux of a property across its surface is zero, it is not practical in all cases to track a material volume and the velocity at which it moves.

In this case, a different reference velocity may be assigned to the material volume, such that there is a non-zero flux of the property of interest across the control volume boundary.

This article should address what that flux looks like, and the form it takes (e.g. an extra term in Reynolds Transport Theorem).<ref name=Taylor_1993"> Taylor, R.; Krishna, R. (1993). Multicomponent Mass Transfer. Wiley and Sons.  </ref>