13.5. Function Calculator

The Function Calculator is used to provide quantitative information about the results. To use the Function Calculator:

  1. Select the function to evaluate from this list.

  2. Choose the location for the calculation.

    Only locations valid for the selected function will be available.

  3. If multiple cases are open, choose which cases the Function Calculator should act upon.

  4. If applicable to the function, choose a variable from the list.

    For most functions, you can click in the Variable box and enter an expression. The expression can include variables and any valid CEL (CFX Expression Language) function. For example, abs(Velocity u) could be entered so that the calculation is performed using the absolute values of the variable Velocity u. For details, see CEL Operators, Constants, and Expressions and CFX Expression Language (CEL).

    User variables are also available. For details, see Variables Workspace.

  5. Select the direction if applicable to the function.

    For some functions None is available.

    For details, see Quantitative Function List.

  6. If applicable to the function, select the appropriate fluid.

    For multiphase results, you can select which fluids to use in your calculation for selected functions. The All Fluids option can be selected to perform the calculation using all of the fluids in the results.

    Note:  When calculating mass flow rate for a Fluent file, the option Mixture gives the same results as All Fluids. These two options appear because have different origins (Fluent and CFD-Post respectively); you may choose either for your calculations.

Click Calculate to calculate the result. Choose whether to base the calculation on hybrid or conservative values. Most quantitative calculations are best performed using conservative variable values. For details, see Hybrid and Conservative Variable Values.

Note:  If the function result is a temperature, and if C or K are selected as temperature units, the result’s units will be K. If F or R are selected, the temperature will be returned in R.

This has an implication for calculations of temperature differences measured in C or F. Expressions are always evaluated in absolute temperature units (K or R) and then, if necessary, the result is converted to the user-selected units. For example, if you evaluate 1[C] - 1[C], internally it is evaluated as 274[K] - 274[K], which is 0[K] and is reported as such (with the units forced to be in an absolute scale). In plots (where CFD-Post cannot force the units to be K), the software cannot tell whether the result is a temperature difference or just the temperature, so the result is converted to user-selected units (in this case, -273[C]) and a value of -273 is reported in the plot legend. Thus when analyzing temperature differences, set the preferred temperature units to be in an absolute scale (K or R) in the Edit > Options > Units dialog box.

Important:  There are some important limitations concerning calculations performed on CFX-4 results files. For details, see CFX-4 Dump Files.