### 1.2.8. Setting a Reference Pressure

In CFX, you must specify a Reference Pressure for your simulation. The Reference Pressure is specified on the Basic Settings tab of the Domains form, but is a property of the entire simulation so all domains must use the same value. Each time you create a new domain or apply a change to an existing domain, the Reference Pressure in that domain is applied to all domains.

All relative pressure specifications set in CFX are measured relative to this Reference Pressure value. The Reference Pressure will affect the value of every other pressure set in the simulation.

The reference pressure is used to avoid problems with round-off errors. These can occur when the dynamic pressure change in a fluid, which is what drives the flow, are small compared to the absolute pressure level.

For example, low speed atmospheric air flow may have dynamic pressure changes of only a few Pascals or less, but the changes are relative to the atmospheric pressure of around 100,000 Pa. If you are dealing only in absolute pressure terms, these small pressure changes can get lost in round-off errors when performing calculations (for example, a change of 1 Pa is a change to the sixth significant digit). To rectify the situation, you should set a sensible Reference Pressure level. In this case, the local atmospheric pressure of 100,000 Pa is suitable. This value will be used as the new datum (instead of 0 Pa) about which all pressures are calculated. A change of 1 Pa will now be a change to the first significant digit.

As a counterexample, a reference pressure of 0 Pa can be used without any problems when the dynamic pressure changes are significant compared to the absolute pressure level. When modeling a liquid flow where nothing depends on the pressure level, there is no need to specify an atmospheric reference pressure.

When boundary and initial conditions are specified, they are set relative to the reference pressure. If you require a boundary to have an absolute pressure level of 100,000 Pa, you could:

- Set a relative pressure value of 0 Pa for the boundary if the reference pressure is 100,000 Pa or,

- Set a relative pressure value of 100,000 Pa for the boundary if the reference pressure is 0 Pa.

Note: The Reference Pressure may be evaluated as a CEL expression. However, Reference Pressure values that vary over time are not supported.

If the system variable called `Pressure` (or `p`) is used in an expression defining material properties (for example, when setting a custom equation of state for density), this variable is interpolated as absolute pressure rather than relative pressure.

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