# Howto calculate mass flux with variable density flow

From OpenFOAMWiki

In traditional CFD, mass flow at a boundary is calculated by writing explicit loops. In OpenFOAM, these loops can be invoked implicitly. Also, most OpenFOAM solvers include a variable phi that represents the mass flux. So calculating the mass flux only requires the following steps:

- ) Identifying the boundary where we want to do the summation
- ) Implicitly doing the sum over the specified boundary.

The following code fragment shows how to do this for a boundary named "velocity_inlet."

// find the identification number (e.g. label) for our boundary of interest. label inletPatch = mesh.boundaryMesh().findPatchID("velocity_inlet"); // if we don't have such a patch, warn the user if (inletPatch==-1) { Info << "Failure to find patch named velocity_inlet for mass flow calc." <<endl; } else // calculate the result and do output { // the sum operator implicity loops over the boundary faces and stores the result in inFlux // note inflow is negative according to dot product. Lets multiply by -1 to // make it positive. scalar inFlux = -1.0* sum(phi.boundaryField()[inletPatch]); Info << " Inflow= " << inFlux <<" [kg/s] " ; }