DOWNSTREAM PRESSURE { \ value [EXTRAPOLATE [SUPERSONIC]] | \ FREESTREAM [EXTRAPOLATE [SUPERSONIC]] | \ FROZEN | \ EXTRAPOLATE [ALWAYS] | \ value VARIABLE i j k [RELAXER rlxr] | \ value UNSTEADY {SINUSOIDAL | USERSPEC} Deltap freq phase \ } [ORDER {ZERO|0|ONE|1|FIRST}] zone_selector |
DOWNSTREAM PRESSURE { \ value [EXTRAPOLATE [SUPERSONIC]] | \ FREESTREAM [EXTRAPOLATE [SUPERSONIC]] | \ FROZEN | \ EXTRAPOLATE [ALWAYS] \ } zone_selector |
This keyword controls the specification of a pressure boundary condition at outflow boundaries.
Specification of the zone_selector is required for this keyword.
There are several modes for structured grids, defined as follows:
DOWNSTREAM PRESSURE value [EXTRAPOLATE [SUPERSONIC]] \ [ORDER {ZERO|0|ONE|1|FIRST}] zone_selector |
This mode specifies the pressure value (in psi) to be applied at the outflow boundary in the specified zone(s). The EXTRAPOLATE [SUPERSONIC] option tells Wind-US to extrapolate all flow conditions to the boundary for points where the flow is supersonic. Either zeroth- or first-order extrapolation will be used, as specified by ORDER.
For boundary points where the flow is subsonic,
DOWNSTREAM PRESSURE FREESTREAM [EXTRAPOLATE [SUPERSONIC]] \ [ORDER {ZERO|0|ONE|1|FIRST}] zone_selector |
This mode is the same as the previous one, except that the pressure
value is taken from the FREESTREAM
keyword.
DOWNSTREAM PRESSURE FROZEN \ [ORDER {ZERO|0|ONE|1|FIRST}] zone_selector |
This mode maintains the downstream pressure at the current value from the solution.
DOWNSTREAM PRESSURE EXTRAPOLATE [ALWAYS] \ [ORDER {ZERO|0|ONE|1|FIRST}] zone_selector |
This mode extrapolates all flow conditions for all points on
the boundary, regardless of whether or not the flow there is supersonic.
DOWNSTREAM PRESSURE value VARIABLE i j k [RELAXER rlxr] \ [ORDER {ZERO|0|ONE|1|FIRST}] zone_selector |
This mode imposes the pressure value (in psi) only at the specified (i,j,k) boundary point in the specified zone(s). The indices (i,j,k) should correspond to a point on the outflow boundary, although at present no check is made to verify that they do. The pressure at the remaining boundary points will spatially vary according to the variation of pressure at the solution points adjacent to the boundary, and may be over- or under-relaxed using the input relaxation factor rlxr. The default value for rlxr is 1.0 (i.e., no relaxation).
The procedure is as follows:
DOWNSTREAM PRESSURE value UNSTEADY \ {SINUSOIDAL | USERSPEC} Deltap freq phase \ [ORDER {ZERO|0|ONE|1|FIRST}] zone_selector |
This mode allows an unsteady pressure to be applied at the
outflow boundary in the specified zone(s).
A sinusoidal or user-defined oscillation may be specified.
The parameters are defined as:
value | Baseline pressure, p_{s} (psi) | ||
---|---|---|---|
Deltap | Amplitude of oscillation, Δp (psi) | ||
freq | Frequency of oscillation, ω (Hz) | ||
phase | Phase angle of oscillation, φ (deg) |
If SINUSOIDAL is used, specifying a sinusoidal pressure oscillation, the pressure is computed from
If USERSPEC is used, specifying a user-defined pressure oscillation, a table must be input immediately following the DOWNSTREAM PRESSURE keyword, with the following format:
PROFILE npts per(1) amp(1) per(2) amp(2) ... per(npts) amp(npts)where npts is the number of points in the profile (max 65), per is the normalized period (i.e., from 0.0 to 1.0), and amp is the normalized amplitude (i.e., from −1.0 to 1.0). As an example, the following normalized pressure oscillation:
PROFILE 6 0.0 0.0 0.2 1.0 0.3 1.0 0.7 -1.0 0.8 -1.0 1.0 0.0
Once the downstream pressure is determined for the current time level, the density and momentum at each boundary point, plus the effective gamma, compressibility factor, and speed of sound, are extrapolated from the interior to the boundary using either zeroth- or first-order extrapolation, as specified. The energy at each boundary point is then computed, consistent with the pressure and the extrapolated values of density, etc.
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.
See Also: COMPRESSOR FACE, DOWNSTREAM MACH, MASS FLOW, FREESTREAM, TEST 123
There are a couple of possible modes for unstructured grids, defined as follows:
DOWNSTREAM PRESSURE value [EXTRAPOLATE [SUPERSONIC]] zone_selector |
This mode specifies the pressure value to be applied at the
outflow boundary in the specified zone(s).
The EXTRAPOLATE [SUPERSONIC] option tells Wind-US to extrapolate
all flow conditions to the boundary for points where the flow is
supersonic.
For unstructured grids, only zeroth-order extrapolation is available.
DOWNSTREAM PRESSURE FREESTREAM [EXTRAPOLATE [SUPERSONIC]] zone_selector |
This mode is the same as the previous one, except that the pressure
value is taken from the FREESTREAM
keyword.
DOWNSTREAM PRESSURE FROZEN zone_selector |
This mode maintains the downstream pressure at the current value from the solution.
DOWNSTREAM PRESSURE EXTRAPOLATE [ALWAYS] zone_selector |
This mode extrapolates all flow conditions for all points on the boundary, regardless of whether or not the flow there is supersonic.
See Also: MASS FLOW, FREESTREAM, TEST 123
Last updated 1 Apr 2016