# 4.13.1. Boundary Fluxes

All boundary condition blocks support adding integrated flux postprocessors for all equations, for either total flux, advective flux, or diffusive flux. The post-processed quantity will be a global variable you can output in a heartbeat or results output block, or use in a subsequent global function or in another boundary condition string function (e.g. to tie the flux out of one BC to the incoming flux of another BC).

    Begin open boundary condition on Surface surface_4
      # ...

      # Postprocess [TOTAL | ADVECTIVE | DIFFUSIVE] flux of [EQ_NAME] as [GVAR_NAME]
      Postprocess total flux of continuity as mdot4
    End

Total flux is calculated by constructing the equation residual on the boundary nodes, minus boundary condition contributions. This approach is the most accurate, and will always return the exact flux actually applied through the boundary. If you have a specified flux, this post-processed value may differ from the specified value by an amount on the order of your linear solver tolerance, and will be an accurate indicator of how well converged your equation is. One exception to this is `continuity`, which will automatically use the advective approach for total flux.

Advective flux is calculated by using the `mdot` calculated on the boundary from the continuity equation, multiplied by the advected quantity on the boundary. The advective flux is not relevant on boundary conditions with no advective flux (e.g. wall, symmetry), so requesting it there will result in an error.

Diffusive flux is calculated as the difference between total flux (using the residual approach) and the advective flux.

The complete list of flux post-processing commands can be found in each boundary condition block in the command summary.

## 4.13.1.1. Enthalpy

When post-processing the flux of enthalpy, there is an additional optional command to calculate average temperature on the boundary as well. To activate this option, add a second variable name (`Tavg` here).

    Begin open boundary condition on Surface surface_4
      # ...

      # Postprocess [TOTAL | ADVECTIVE | DIFFUSIVE] flux of enthalpy as [H_NAME] [T_NAME]
      Postprocess total flux of enthalpy as hFlux Tavg
    End

## 4.13.1.2. Species

When post-processing the flux of species, the post-processor runs for all species and you must supply aliases for all the resolved species.

Note

The `fracbal` species (the last one) does not have an equation and should not be included in the post-processed list. If you need its flux, you can calculate total flux and subtract the resolved species.

    Begin open boundary condition on Surface surface_4
      # ...

      # Example when your species are CO2, O2, N2
      Postprocess total flux of species as mdot_CO2_s4 mdot_O2_s4
      Postprocess total flux of continuity as mdot_s4
    End

    Begin postprocess Global
      Output name = mdot_N2_s4
      Function = "mdot_s4 - mdot_CO2_s4 - mdot_O2_s4"
    End