What Is the Reference Pressure?

The reference pressure is simply a device used to reduce the numerical roundoff error in the numerical calculations involving pressure. This is necessary since the differences in pressure are important, and these differences can be small relative to the absolute value of the pressure, especially in fully incompressible or very low-Mach number flows. By subtracting off a suitable constant reference pressure, one obtains a working pressure that is less prone to roundoff errors.

For simulations that use the Constant Density model or the Polynomial Density model, the actual value of the reference pressure has no relevance to the calculations. However, when using the Ideal Gas model, the reference pressure is used as in Eqns. (35) and (36). In all cases, the reference pressure is used in the definition of the Absolute Pressure field function according to Eqn. 36.

Reference Pressure Properties

Value

The value of the reference pressure. The customizer triggers the define value action.

Does STAR-CCM+ Require a Reference Pressure Location to Be Specified?

The concept of reference pressure is completely independent of any location. However, there are two related situations where locations might be relevant.

If there is no pressure boundary, say in the case of a cavity or a case with a mass-flow split outlet, then the equation for pressure will have Neumann conditions on all boundaries. If one does not "pin down" the pressure somewhere in the domain, the pressure can float arbitrarily high, causing numerical problems. It does not matter where the pressure is specified, or what it is specified to be. However, it must be specified at the same point for all steady iterations and regardless of whether serial or the number of parallel partitions. STAR-CCM+ uses its own algorithm to find a reference location at the cell adjacent to the boundary face that has the smallest X, Y, Z coordinates. This frees the user from having to specify the location.

In buoyant flows, the definition of an operating altitude is required. This is a position vector defining the location at which the static and piezometric pressures are equal. The sections on modeling gravity for variable density and constant density flows explain this further.