Manufacturers usually provide curves that describe the static pressure as a function of flow rate for a vacuum pump. Also see Vacuum Pump for the node settings.

In this case (see Figure 3-4), the device’s inlet is the interior face situated between the blue (cube) and green (circle) domains while its outlet is an external boundary, here the circular boundary of the green domain. The lumped curve gives the flow rate as a function of the pressure difference between the interior face and the external boundary. This boundary condition implementation follows the Pressure Boundary Condition for outlets with the Suppress backflow option:

Here, V0 is the flow rate across the boundary and pvacuum pump(V0) is the static pressure function of flow rate for the vacuum pump. pref is the reference pressure that ensure that the absolute pressure is set to pvacuum pump. In vacuum pump models the reference pressure may be set to 0 Pa since the absolute pressure is close to 0 Pa by opposition to ambient conditions where pref is often set to 1 atm. Equation 3-38 corresponds to the compressible formulation. For incompressible flows, the term −(2/3)μ(∇ ⋅ u) vanishes. In 2D the thickness in the third direction, Dz, is used to define the flow rate. Vacuum pumps are modeled as rectangles in this case.