LOADS [PRESSURE [OFFSET] {FREESTREAM | val}] PRINT [PLANES] [TOTALS] [ZONES] [LIFT | DRAG] [FREQUENCY freq] \ [MAKE_FRC file] [REFERENCE AREA aref] [REFERENCE LENGTH lref] [REFERENCE MOMENT [CENTER] xc yc zc] ZONE nzn SURFACE {I val | J val | K val} word1 [word2 [word3...]] SUBSET I range J range K range word1 [word2 [word3...]] GROUP group_name word1 [word2 [word3...]] ENDLOADS |
LOADS [PRESSURE [OFFSET] {FREESTREAM | val}] PRINT [PLANES] [TOTALS] [ZONES] [LIFT | DRAG] [FREQUENCY freq] \ [MAKE_FRC file] [REFERENCE AREA aref] [REFERENCE LENGTH lref] [REFERENCE MOMENT [CENTER] xc yc zc] ZONE nzn SURFACE {I|U} val word1 [word2 [word3...]] GROUP group_name word1 [word2 [word3...]] ENDLOADS |
Flowfield properties may be integrated during the course of a Wind-US run to check convergence and solution quality. The values printed to the list file represent force coefficients F/A_{r}q_{r} and moment coefficients M/L_{r}A_{r}q_{r} in the x, y, and z coordinate directions, where F and M are the force and moment, and L_{r}, A_{r}, and q_{r} are the reference values of length, area, and dynamic pressure. Integrated mass and momentum fluxes may also be computed. The values are written into the list output (.lis) file.
The various elements of the LOADS keyword block are defined
below.
Unless noted otherwise, the keywords apply to both structured and
unstructured grids.
PRESSURE [OFFSET] {FREESTREAM | val} |
This keyword controls the pressure integration. By default, the code uses P − P_{∞} in all pressure integrations, where P_{∞} is the static pressure at freestream conditions. This keyword permits the user to specify the pressure offset value, such that:
FREESTREAM | Use P − P_{∞} | |
---|---|---|
val | Use P − val |
PRINT [PLANES] [TOTALS] [ZONES] [LIFT | DRAG] [FREQUENCY freq] \ [MAKE_FRC file] |
This keyword controls the output data from the integration. Options are turned "on" by including the appropriate keyword.
PLANES | Output the result of the integration for each surface and/or subset specified. | ||
---|---|---|---|
TOTALS | Output integration grand totals over all the zones at the end of each cycle. | ||
ZONES | Output integration totals for each zone. | ||
LIFT | DRAG | Output lift, drag, and side forces instead of (x, y, z) force components. Directions for the lift, drag, and side force components are computed from the angles of attack and sideslip specified using the FREESTREAM keyword and the directions specified by the aerodynamic axes. | ||
FREQUENCY | Output integration results (except for grand totals) every freq iterations. The default value is 5. | ||
MAKE_FRC | Write grand totals to the summary file file at the end of each cycle, in addition to the .lis file. This requires that PRINT TOTALS also be specified. |
REFERENCE AREA aref REFERENCE LENGTH lref REFERENCE MOMENT [CENTER] xc yc zc |
These keywords specify the reference area aref in square inches;
the reference length lref in inches; and the coordinates
(xc, yc, zc) of the reference moment center
in inches.
The default values are 1.0 for aref, 1.0 for lref, and
(0.0, 0.0, 0.0) for (xc, yc, zc).
ZONE nzn |
This keyword specifies the zone number nzn for the surfaces
and/or subsets which follow.
SURFACE {I val | J val | K val} word1 [word2 [word3...]] |
This form of the SURFACE keyword applies to structured grids, and specifies the coordinate surface in zone nzn over which the integration is to be performed. The parameter val is a coordinate index specifying the surface.
The word parameters are keywords specifying what flowfield properties are to be computed. The following keywords are currently available.
FORCE | Compute pressure forces. | ||
---|---|---|---|
MOMENT | Compute moments. | ||
VISCOUS | Also compute viscous forces (when FORCE is specified) and moments (when MOMENT is specified). | ||
MASS | Compute mass flow. | ||
MOMENTUM | Compute momentum. |
Two additional word parameters may be used to control values computed on structured grids.
NORMAL inorm | inorm = ±1, specifying the desired normal vector direction; +1 for the increasing index direction and −1 for the decreasing index direction. inorm defaults toward the interior of the grid if the integration surface is on a boundary of the zone. | ||
---|---|---|---|
NOSLIP | Integrate only over points where the total velocity is 0. |
At least one flowfield property must be specified. For integration surfaces that are not part of a zonal boundary, the normal direction must also be specified.
Note that when VISCOUS is specified, the viscous forces and
moments are printed to the .lis file as separate values; they are
not summed with the pressure forces and moments.
Note also that you can't compute just the viscous values.
E.g., to get the viscous forces you must also get the pressure forces.
Specifying just SURFACE J 1 VISCOUS will result in no forces
being printed to the .lis file for the j = 1 surface.
SURFACE {I|U} val word1 [word2 [word3...]] |
This form of the SURFACE keyword applies to unstructured grids, and specifies the unstructured surface in zone nzn over which the integration is to be performed. The parameter val is the surface number.
The word parameters are keywords specifying what flowfield properties are to be computed. The following keywords are currently available.
FORCE | Compute pressure forces. | ||
---|---|---|---|
MOMENT | Compute moments. | ||
VISCOUS | Also compute viscous forces (when FORCE is specified) and moments (when MOMENT is specified). | ||
MASS | Compute mass flow. | ||
MOMENTUM | Compute momentum. | ||
HEAT | Compute heat flux. This keyword is only available for isothermal walls. |
At least one flowfield property must be specified.
SUBSET I range J range K range word1 [word2 [word3...]] |
This keyword may be used with structured grids to define a subset of a computational surface in zone nzn over which the integration is to be performed. The range parameters take one of the following forms:
index1 index2 | Starting and ending indices in the specified direction. LAST may be used for the last index. | ||
---|---|---|---|
ALL | Equivalent to 1 LAST. |
For three-dimensional cases, the starting and ending indices for one (and only one) of the I, J, or K parameters must be the same. For two-dimensional cases, the K parameter must be specified as either K 1 1 or K ALL; one or both of the I and J parameters may have different starting and ending indices.
The word parameters are the same as those described above for the
SURFACE keyword for structured grids.
GROUP group_name word1 [word2 [word3...]] |
This keyword may be used to reference a named surface group, which is stored in the common grid (.cgd) file. Since surface groups may contain surfaces from multiple zones, the ZONE keyword is not needed.
The word parameters used to specify flowfield properties are the same as those described above for the SURFACE keyword for structured grids.
Example
LOADS PRESSURE OFFSET 0.0 PRINT PLANES ZONES TOTALS LIFT FREQUENCY 2 REFERENCE AREA 100. REFERENCE LENGTH 12. REFERENCE MOMENT CENTER 35. 36. 78. GROUP wing FORCE MOMENT VISCOUS ZONE 4 SUBSET I ALL J 1 1 K 15 LAST FORCE MOMENT VISCOUS NOSLIP ZONE 10 SURFACE I 1 MOMENTUM MASS ENDLOADS
Extracting Loads from the List Output File
As discussed above, integrated values are output to the *.lis file for four types of domain regions: Planes, Groups, Zones, and Totals. For each domain type, several properties may be requested: Force, Moment, Mass, Momentum, and Heat Flux. Each line of loads data that is written to the *.lis file contains: an identifier, one or more integer values, one or more real values, and units (if any). The list of valid identifiers are shown below.
Planes | Groups | Zones | Totals |
---|---|---|---|
*IPFORCE* | *IGFORCE* | *IFORCE* | *IZFORCE* |
*IPLIFT* | *IGLIFT* | *ILIFT* | *IZLIFT* |
*IPMNT* | *IGMNT* | *IMNT* | *IZMNT* |
*IPMASS* | *IGMASS* | *IMASS* | *IZMASS* |
*IPMNTUM* | *IGMNTUM* | *IMNTUM* | *IZMNTUM* |
*IPHEAT* | *IHEAT* | *IZHEAT* |
Each domain type uses a particular identifier prefix. The integer values written to the load record include the iteration number, zone number, and load plane number (from the list of load planes specified in the *.dat file) as appropriate. This information can be summarized as follows:
Domains | Identifier Prefix | Integer Values |
---|---|---|
Planes | IP | iteration, zone, plane |
Groups | IG | iteration |
Zones | I | iteration, zone |
Totals | IZ | iteration |
The real values written to the load record represent the integrated values of the physical property being reported. For vector quantities, values are written for each component. Units are included for dimensional values, but not for coefficient data. Note again that the default force and moment coefficients are based solely on the pressure terms. Only when VISCOUS is specified are the additional viscous coefficients output. The table below summarizes the property values that are output.
Property | Print Option | Identifier | Variables | Units |
---|---|---|---|---|
FORCE | — | FORCE | c_{Fp(x,y,z)}; c_{Fv(x,y,z)} | — |
FORCE | {LIFT|DRAG} | LIFT | c_{Fp(drag,lift,side)}; c_{Fv(drag,lift,side)} | — |
MOMENT | — | MNT | c_{Mp(x,y,z)}; c_{Mv(x,y,z)} | — |
MASS | — | MASS | mdot_{(x,y,z,magnitude)} | slugs/s |
MOMENTUM | — | MNTUM | Total_Thrust_{(x,y,z)}; Gross_Thrust_{(x,y,z)} | pounds |
HEAT | — | HEAT | Heatflux | BTU/s |
The resplt utility can be used to filter and extract the loads data to a GENPLOT file. This method gives the user control over which properties are extracted and for which domain regions. Using techniques similar to those described in the Monitor Convergence section of the tutorial, one can plot the GENPLOT file with CFPOST. Since GENPLOT files are text-based, they can also be easily manipulated for use in other plotting applications. The Genplot File Format is described in the CFPOST User's Guide.
See Also: FIXED_CL, POSTPROC, WALL FUNCTION,
Last updated 1 Apr 2016