Variable Selection and Unit Control Commands

• variable - Select variables to be processed
• vector - Define a vector variable
• units - Specify default dimensional units of output data

variable - Select variables to be processed

   [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.

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:

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:

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

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:

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

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:

Valid values for mass units are:

   kg, kilogram
   gram
   slug
   lb, lbm, pound

   m, meter
   cm, centimeter
   mm, millimeter
   ft, feet, foot
   in, inch

   K, Kelvin
   C, Centigrade, Celsius
   R, Rankine
   F, Fahrenheit

   N, Newtons
   dynes
   lbf (pounds force)

   Joule
   erg
   lbf-ft, ft-lbf
   BTU

Example

   ! English system, but override energy unit
   units english BTU

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