31.5.3 Sample Custom Field Functions

When you are checking the results of your simulation, you may find it useful to define some of the following field functions:

• To define a function that determines the ratio of static pressure to inlet total pressure, use the relationship

  
  

  
  

  where $p$ is the static pressure calculated by the solver, $p_{\rm to}$ is the inlet total pressure, and $p_{\rm op}$ is the operating pressure for the problem. Use the solver-defined function Static Pressure for $p$, and the numerical value that you specified for Gauge Total Pressure in the Pressure Inlet dialog box for $p_{\rm to}$. Specify the value of the operating pressure to be the value that you set in the Operating Conditions dialog box. As discussed in Section  8.14, all pressures in ANSYS FLUENT are gauge pressures relative to the operating pressure. If the operating pressure is zero, as is generally the case for compressible flow calculations, the expression for the pressure ratio reduces to

  
  

  
  

• To define a function that determines the critical velocity ratio $v/a_*$, a parameter that is sometimes used in turbomachinery calculations, use the relationship

  
  

  
  

  In this relationship, $a_*$ is the critical velocity (i.e., the velocity that would occur for the same stagnation conditions if $M = 1$), $\gamma$ is the ratio of specific heats, and ${\rm PR}$ is the pressure ratio defined in Equation  31.5-2 for which you created your own function. For $\gamma$, ratio of specific heats, select Specific Heat Ratio (gamma) in the Properties... category. To include ${\rm PR}$, select Custom Field Functions... in the first drop-down list under Field Functions, and then select from the second list the function name that you assigned ${\rm PR}$.

• Suppose you have swirling flow in a pipe, aligned with the $z$ axis, and you want to calculate the flow rate of angular momentum through a cross-sectional plane:

  
  

  
  

  You can create a function for the product $r v_\theta$, where $r$ is the Radial Coordinate and $v_\theta$ is the Tangential Velocity. Then use the Surface Integrals dialog box to compute the flow rate of this quantity.

The custom field function containing model dependent functions (like temperature when the energy equation is enabled) will be computed only when those models are still active.