MASS [FLOW] {RATE [ACTUAL | CORRECTED] | RATIO} value \ [PRESSURE | DIRECT [RELAXER rlxr]] [ORDER {ZERO|0|ONE|1|FIRST}] \ [FREQUENCY freq] zone_selector |
This keyword allows the user to specify mass flow at outflow boundaries in the flowfield.
The input keywords are as follows:
RATE | value represents the mass flow rate in lb_{m}/sec,
and may be actual (the default) or corrected, as set by the
ACTUAL or CORRECTED keyword.
The specified value must be positive.
The corrected air flow is defined as
where
and P_{x} and T_{x} are the total pressure and temperature at the duct exit, and P_{0} and T_{0} are equal to 14.7 psi and 520 °R, respectively. | ||
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RATIO | value represents the mass flow ratio.
Again, the specified value must be positive.
The actual mass flow is computed as
where A_{cap} is the capture area found in the .cgd file zonal parameters. This area can be set with the GMAN CAPTURE AREA command or from the boundary condition menu in the GUI. | ||
PRESSURE | A spatially-constant pressure is set at the boundary, and modified as the solution proceeds until the desired mass flow is achieved. This is the default. | ||
DIRECT | The momentum, and thus the mass flow, is modified directly, and the pressure adjusts as the solution proceeds. | ||
RELAXER | The specified mass-flow rate will be relaxed using the relaxation factor rlxr. This option only applies when DIRECT is specified. The default value for rlxr is 1.0 (i.e., no relaxation). | ||
ORDER | Either zeroth- or first-order extrapolation will be used, as specified. The default is zeroth-order. First-order may only be used with structured grids. | ||
FREQUENCY | freq controls how often the pressure is updated to drive the solution to the requested mass flow rate. This frequency is also used in updating the total pressure increment of ARBITRARY INFLOW HOLD_MASS boundaries The default is every 5 iterations. Note that the DIRECT option controls the momentum flux rather than the pressure and will be enforced at every iteration. |
Specification of the zone_selector is required for this keyword. If multiple zones are listed, the specified mass flow will be applied in each zone.
With the PRESSURE option, the pressure will be constant over the entire outflow boundary, resulting in poor solutions for flows that should have cross-flow pressure gradients in that region. With the DIRECT option, cross-flow pressure gradients may be present at the outflow boundary, and the mass flow will be equal to the user-specified value (for rlxr = 1) for all iterations.
For flows with negligible cross-flow pressure gradients, the results and convergence rates using the PRESSURE and DIRECT options are nearly the same. For a test case with a significant cross-flow pressure gradient near the outflow boundary, the computed pressures using the two options differed by as much as 10%. The PRESSURE option, although non-physical for this case, had a slightly better convergence rate.
Internally, to apply this boundary condition Wind-US does the following:
Extrapolation Notes
The default for all extrapolation is zeroth-order (i.e., conditions at the boundary are set to the values at the computational plane adjacent to the boundary). This results in a discontinuous slope in flow values near the outflow boundary, which may be important for flows with significant streamwise pressure gradients. First-order extrapolation yields smoother results.
For flows with little or no streamwise pressure gradient near the outflow boundary, the results using zeroth- and first-order extrapolation are essentially identical. Convergence rates and the final residual values are generally better with zeroth-order extrapolation, however, so the default zeroth-order extrapolation is recommended.
For flows with significant streamwise pressure gradients near outflow boundaries, zeroth-order extrapolation can give poor results at the outflow boundary, and in some cases these can affect values at the inflow boundary. First-order extrapolation is thus recommended for these flows.
Examples
MASS FLOW RATIO 0.95 ZONE 2 MASS FLOW RATE ACTUAL 180. ZONE 3 MASS FLOW RATE CORRECTED 220. ZONE 4
See Also: ARBITRARY INFLOW, COMPRESSOR FACE, DOWNSTREAM PRESSURE, DOWNSTREAM MACH, TEST 22, TEST 123, TEST 160
Last updated 30 Sep 2016