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(General Overview) (General Purpose Commands) (File Specification Commands) (Domain Selection Commands) (Variable Selection and Unit Control Commands) (File and Report Generation Commands) (Plotting Commands) (GENPLOT File Format) (rake file Format) (Equations Used by CFPOST)

Variable Selection and Unit Control Commands

variable - Select variables to be processed

variable name [unit-override] [; name [unit-override] ...]

    name   The name of a variable to be selected. The specified name must either be present in the input files or must be one that can be derived from other information in the files. See the list below for the variables that can be derived.

unit-override   Specifies one or more unit-specifiers that may be specified on the unit command, a normalization quantity or a user specified scale and offset of the form:
   [origin origin] [scale scale]

The variable command is used to define the variables that are to be processed and optionally in what units of measure the data is to be presented. The unit-override specifications only affect the variable being selected; they do not alter the default units of measure.

variable commands accumulate, that is, each variable command adds to the list of variables that will be made available for subsequent processing. The list is passed on in the order that it was created. Up to 128 variables may be selected at any one time. The list of selected variables and their units of measure can be displayed with the show command and cleared with the clear command.

The following list represents variables that, if requested and not present in the input files, will be derived from other information in the files if sufficient information is available. Real gas effects are taken into account if the solution file has the necessary information.

Note that some variables require the specification of a surface, or normal direction into the flow field, and others require knowledge of "up" and "down" axes orientation. Variables denoted with (s) following their description indicate that the normal direction must be specified properly using the subset or surface command. Variables denoted with (o) are affected by the axes orientation set by the orientation command.

Variables Available

name  Description  Direction

xx coordinate
yy coordinate
zz coordinate
dxΔx, measured from first point in subset
dyΔy, measured from first point in subset
dzΔz, measured from first point in subset
dsArc length, measured from first point in subset
x/cNormalized x (only valid for GENPLOT files)
y/bNormalized y (only valid for GENPLOT files)
rho0Stagnation density
p0Stagnation pressure
CpPressure coefficient
CptTotal pressure coefficient
deltapDelta pressure
ppPitot pressure
qDynamic pressure
T0Stagnation temperature
MMach number
MaEquivalent isentropic Mach number
MtTurbulent Mach number
aSpeed of sound
ux velocity
vy velocity
wz velocity
VVelocity magnitude
Vxyuv crossflow velocity magnitude
Vyzvw crossflow velocity magnitude
Vxzuw crossflow velocity magnitude
e0Absolute stagnation energy per unit volume
ekKinetic energy per unit volume
eiSensible internal energy per unit volume
hSensible enthalpy
h0Sensible stagnation enthalpy
SStrain rate magnitude
Sxx, Sxy, SxzStrain rate tensor components
Syx, Syy, SyzStrain rate tensor components
Szx, Szy, SzzStrain rate tensor components
WRotation rate magnitude or vorticity magnitude
Wxx, Wxy, WxzRotation rate tensor components
Wyx, Wyy, WyzRotation rate tensor components
Wzx, Wzy, WzzRotation rate tensor components
wxx component of vorticity
wyy component of vorticity
wzz component of vorticity
localphaLocal α (angle of attack)o
locbetaLocal β (sideslip angle)o
kappaThermal conductivity coefficient
QHeat transfer rates
muTotal viscosity coefficient
mulLaminar viscosity coefficient
mutTurbulent viscosity coefficient
kk from k-ε or SST turbulence model
epsilonε from k-ε turbulence model
omegaω from SST turbulence model
alphatTurbulent thermal diffusivity
var_ei, eps_eiVariance and dissipation rate of internal energy
var_h0, eps_h0Variance and dissipation rate of total enthalpy
var_h, eps_hVariance and dissipation rate of static enthalpy
deltaBoundary layer thicknesss
delta1 or delta*Boundary layer displacement thicknesss
delta2 or THETABoundary layer momentum thicknesss
delta3Boundary layer energy thicknesss
deltauBoundary layer velocity thicknesss
THETAincBoundary layer incompressible momentum thicknesss
CfSkin friction coefficients
Cfxx component of skin friction coefficients
Cfyy component of skin friction coefficients
Cfzz component of skin friction coefficients
tauTotal (laminar+turbulent) shear stresss
tauxx component of total shear stresss
tauyy component of total shear stresss
tauzz component of total shear stresss
uu, uv, uwTurbulent stress componentss
vu, vv, vwTurbulent stress componentss
wu, wv, wwTurbulent stress componentss
uu+, uv+, uw+Non-dimensional turbulent stress componentss
vu+, vv+, vw+Non-dimensional turbulent stress componentss
wu+, wv+, ww+Non-dimensional turbulent stress componentss
y+Non-dimensional boundary layer wall coordinates
u+Non-dimensional boundary layer velocitys
k+Non-dimensional turbulent kinetic energys
epsilon+Non-dimensional turbulent dissipation rates
RtTurbulent Reynolds number
RyTurbulent Reynolds number based on y
cmuRGCμ, Rumsey-Gatski algebraic stress coefficients
fmuJLfμ, Jones-Launder damping functions
fmuCHfμ, Chien damping functions
fmuSPfμ, Speziale damping functions
fmuLBfμ, Lam-Bremhorst damping functions
C-speciesMass fraction of species
X-speciesMole fraction of species
VeffEffective collision frequency
MWMolecular weight
RGas constant
cpSpecific heat at constant pressure
cvSpecific heat at constant volume
gammaRatio of specific heats
hfHeat of formation
betaEffective gamma
ZCompressibility factor

In addition to the above, for flows in a rotating reference frame (computed, for example, using Wind-US with the ROTATE keyword), the following variables may be requested for properties in the rotating reference frame: p0r, T0r, Mr, ur, vr, wr, Vr, and e0r. Note that the names are the same as the corresponding variable from the above list for the inertial (i.e., non-rotating) reference frame.

Also, due to the special significance of conservation variables, any name of the form rho*xxxx will be derived if sufficient information is present to derive rho and xxxx.

In addition to any of the unit-specifiers that may be specified on the units command, any one of the following normalization factors may be supplied:

Variable Normalization Factors

unit-override  Description

rhoinfFreestream density
rho0infFreestream stagnation density
pinfFreestream pressure
p0infFreestream stagnation pressure
ppinfFreestream pitot pressure
qinfFreestream dynamic pressure
TinfFreestream temperature
T0infFreestream stagnation temperature
hinfFreestream enthalpy
h0infFreestream stagnation enthalpy
ainfFreestream speed of sound
MinfFreestream Mach number
uinfFreestream velocity
muinfFreestream viscosity
kinfFreestream k from the k-ε turbulence model
einfFreestream ε from the k-ε turbulence model

The use of the origin and scale unit override allows the user to create their own units or nonrmalizations. The values specified for origin and scale are used as follows:

x′ = ( xorigin ) × scale

The user must be aware that if x is a dimensional quantity, origin and scale must take into account the fact that x is in SI units. A typical use would be the display of x normalized by the chord length.

Variable names may be preceeded by a minus sign to swap their sign. The minus sign is applied in SI units before any optional origin, scale, unit conversion, or normalization. The minus sign should not be used on variables written to a common file or interpolation file, because CFPOST will not be able to read those variables later. See the interpolate command for more details.

Example 1

   ! select pressure normalized by the freestream pressure
   ! also select Mach number
   variable p pinf; M
   ! select u in feet/second (assume second is default time unit)
   variable u feet
   ! select turbulent shear stress, swapping the sign
   variable −rho*uv
   ! four variables have been selected!

Example 2

   ! select u velocity normalized by freestream velocity
   variable u uinf
   ! enable English units
   units english
   ! select v velocity in feet/second
   variable v ft sec
   ! select w velocity in centimeters/second
   variable w cm

Example 3

   ! select pressure in psi. Note that if one simply selected
   ! English units then the result would be in psf!
   variable p lbf inches
   ! select heat transfer rate in BTU/(foot^2-hour)
   variable Q feet hour BTU

Example 4

   ! Select Cp and x. x is normalized by the chord.
   ! The chord is 100 inches = 2.54 meters, therefore
   !    scale = 1/2.54 = .3937
   ! The leading edge was at 250 inches = 6.35 meters, therefore
   !    origin = 6.35
   variable x origin 6.35 scale .3937

See Also: The units command for how to define units of measure; the show command for how to display the variable list; the clear command for how to clear the variable list; the subset and surface commands for how to specify the surface and normal direction into the flow field for variables marked with (s); the orientation command for how to specify the axes orientation for variables marked with (o); "Equations Used by CFPOST" for the formulas used for deriving variables.

vector - Define a vector variable

vector variable [unit-override]

    variable   The name of a vector variable.

unit-override   A normalization quantity or one or more unit-specifiers that may be specified on the units command.

The vector command is used to specify the variable to be included in the plot file for generating a vector plot. The following vector variables are available:

Vector Variables

variable  Description

CfSkin friction

See Also: The units command for how to define units of measure; the genplot surface command for how to generate a file for vector plotting; the plot vectors command for how to perform vector plotting.

units - Specify default dimensional units of output data

units unit-specifier unit-specifier ...

    unit-specifier   The name of a unit or system of units.

The units command defines the default units of measure to be used for the subsequent presentation of data. CFPOST maintains units of measure for four basic units (mass, length, time and temperature), and two derived units (force and energy). All other units are derived from these basic units. For instance, units of pressure are derived from the force unit and the length unit. Similarly, velocity is derived from length and time units. CFPOST uses SI units internally as its default system of units. The default output system of units is FSS.

If a "system of units" is specified as a unit-specifier, all six of the units of measure are set to the values defined for the selected "system of units". If a "unit name" is specified as a unit specifier, only the corresponding unit of measure is modified.

Some commands produce geometry information with the geometry variables being explicitly requested (e.g.: artis, plot3d). On some other commands the user is providing locations, lengths, or areas (e.g.: cut, integrate, rake). In these situations, CFPOST needs to know the length unit in which the information is being given. The program does this by maintaining a default length unit. The default length unit is the length unit that is in effect just before the first variable is added to the variable list or, if no variables have been selected, at the time that a command that requires the default length unit is executed. Note that the first variable is either the first one added in an execution of CFPOST or the first one added after the variable list has been cleared.

Valid values for "system of units" are:

Systems of Units

unit-specifier  Description

SISystem International (meter, kilogram, second, degrees Kelvin, Newton, Joule)
MKSA synonym for SI
metricA synonym for SI
CGSCentimeter, gram, second, degrees Kelvin, dyne and erg
FSSFoot, slug, second, degrees Rankine, pound force, foot-pound force
britishA synonym for FSS
englishA synonym for FSS
FPSFoot, pound mass, second, degrees Rankine, pound force, foot-pound force

Valid values for mass units are:

   kg, kilogram
   lb, lbm, pound
Valid values for length units are:
   m, meter
   cm, centimeter
   mm, millimeter
   ft, feet, foot
   in, inch
Valid values for temperature units are:
   K, Kelvin
   C, Centigrade, Celsius
   R, Rankine
   F, Fahrenheit
Valid values for force units are:
   N, Newtons
   lbf (pounds force)
Valid values for energy units are:
   lbf-ft, ft-lbf


   ! English system, but override energy unit
   units english BTU

See Also: The variable command for how to override the current units of measure.

Last updated 4 Sep 2019