### 15.5.18. Details on Mass Flow Related Functions

Note that the `massFlowAve` and `massFlowAveAbs` functions provide the same result, and that the denominator evaluates to the net mass flow through the 2D locator, only when all of the flow passes through the 2D locator in the same general direction (that is, when there is no backflow). If there is any backflow through the 2D locator, the denominator in the function for `massFlowAveAbs` evaluates to a value of greater magnitude than the net mass flow through the 2D locator (although this is not necessarily harmful to the resulting average value).

Note: The values of variables other than mass flow are stored at the mesh nodes and are applied to the locator nodes by linear interpolation. For the mass flow variable, CFD-Post uses either:

- Integration point mass flow data, or

- Approximate mass flow values based on mesh nodal values of velocity (and density, if available)

according to Table 15.5: Source of mass flow data.

Table 15.5: Source of mass flow data

Flow condition | massFlowAve | massFlowAveAbs
--- | --- | ---
No recirculation (backflow) present | Integration point data if available (CFX results only), otherwise approximate mass flow values | Integration point data if available (CFX results only), otherwise approximate mass flow values
recirculation (backflow) present | Integration point data if available (CFX results only), otherwise approximate mass flow values | Approximate mass flow values

Note: The `massFlow`, `massFlowAve`, and `massFlowAveAbs` functions in CFD-Post can be inaccurate when evaluated on a physical locator (cut plane, isosurface, etc.) that cuts through a GGI interface, due to inaccuracies in evaluating the conserved mass flow (from integration point data) along the intersection of the physical locator and the GGI interface. In this situation, calculating the approximate mass flow would be more accurate. The approximate mass flow can be calculated with an expression of the form: `sum(Approximated Mass Flow)@[locator]`. For example, `sum(Approximated Mass Flow)@Plane 1`.

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