### 12.1.6. Using a General Set-up (CEL or User Fortran)

For any other real fluid calculation, you only need to set up a pure substance, set the Thermodynamic State to `Gas or Liquid`, set the properties option to `General Material`, then provide your own equation of state, by supplying an expression (CEL or User Fortran) for the fluid density and specific heat capacity. The flow solver automatically builds tables for both enthalpy and entropy

If you are unsure or not concerned about the real behavior of the specific heat capacity, then you can select the `Zero Pressure Polynomial` option, although note that this makes both specific heat and enthalpy a function of temperature only. For details, see Zero Pressure Polynomial.

In addition, when you set the density or specific heat capacity using expressions or User Fortran, you must be careful to ensure that the values are consistent. Because the flow solver uses exact differentials of enthalpy and entropy to calculate tables, the coefficients of the differential terms for both enthalpy and entropy must obey the exact differential property. For details, see Static Enthalpy in the CFX-Solver Theory Guide.

For transport properties of real gases, like dynamic viscosity and thermal conductivity, it is recommended that you use one of the built in temperature-dependent models based on the kinetic theory of gases. For example, `Sutherlands Formula` for both properties or, like the Real Gas equations, simply combine the `Rigid Non Interacting Sphere Model` or `Interacting Sphere Model` and `Modified Eucken Model`. For details, see Transport Properties. For real liquids, none of the presupplied transport property models are appropriate and another selection should be used.

#### 12.1.6.1. Saturation Curve Clipping

When using a general set-up, the flow solver makes no assumption about the form of the vapor pressure curve so it does not clip any properties at saturation. You will have to be careful to ensure that your equation of state is well-behaved if it approaches or crosses the vapor pressure curve to predict metastable states. Many equations of state are not well behaved inside the dome and robustness problems may result if the enthalpy and pressure (solved for by the flow solver) go past the spinodal curves.

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