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7.8.3 Calculation Procedure at Pressure Outlet Boundaries

At pressure outlets, FLUENT uses the boundary condition pressure you input as the static pressure of the fluid at the outlet plane, $p_s$, and extrapolates all other conditions from the interior of the domain.



Density-Based Solver Implementation


In the density-based solver, the pressure at the faces of the pressure outlet boundary condition is computed using a pressure splitting procedure based on the $AUSM^+$ scheme of Liou [ 214].

For subsonic compressible flow leaving the exit pressure boundary, the pressure is computed using a weighted average of the left and right state of the face boundary. This weighting is a blend of fifth-order polynomials based on the exit face normal Mach number [ 214]. Therefore, the face pressure $P_f$ is function of ( $P_c$, $P_e$, $M_n$), where $P_c$ is the interior cell pressure neighboring the exit face f, $P_e$ is the specified exit pressure, and $M_n$ is the face normal Mach number.

Figure 7.8.2: Pressures at the Face of a Pressure Outlet Boundary
figure

If the flow becomes locally supersonic, then the face pressure value $P_f$ is extrapolated from the interior cell pressure.

For incompressible flows, the face pressure is computed as an average between the specified pressure and interior pressure.


 P_f = 0.5(P_c + P_e) (7.8-2)

With this boundary implementation, the exit pressure is not constant along the pressure outlet boundary. However, upon flow convergence, the average boundary pressure will be close to the specified static exit pressure.

figure   

This implementation is not available when you use the general NRBC option, or when you enable the turbo-specific NRBC model.


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