Simcenter STAR-CCM+ How to get the Mass Flux through a Derived Part

Either when analyzing results or troubleshooting the simulation, it can be quite useful to access the mass flux that goes through a small passage or an area of our domain. If however, an internal interface was not created since the beginning for that section of interest, a derived part - section needs to be created in order to visualize the field values at such a location. 

For a boundary or interface the mass flux can easily be visualized in a Scalar Scene selecting the native Mass Flux Field Function. Sadly, the same can not be done for a derived part section:

Additionally, if we try to create a surface average or integral report of the mass flux at the derived part, the value is not available:



In order to obtain the mass flux at a derived part, a new a user defined field function must be creted. The following expression will provide you the mass flux at the desired Derived Part Section:

${Density} * dot($${Velocity}, $${Normal})

Perhaps even more useful is to get the species mass flux in a Multi-Component gas simulation. Such field function allow for the determination of the specie mass flow at a given section derived part when used together with a surface integral report. Multiplying the previous expression with the Specie Mass Fraction gives us the desired expression:

${MassFraction} * ${Density} * dot($${Velocity}, $${Normal})