This pressure drop component defines a simple, generic orifice, where the loss factor ζ is provided for one flow direction (e.g., from loss table of a book):

Δp = 0.5*ζ*ρ*v*|v| = 8*ζ/(π^2*D^4*ρ) * m_flow*|m_flow|

where

- Δp is the pressure drop: Δp = port_a.p - port_b.p
- D is the diameter of the orifice at the position where ζ is defined (either at port_a or port_b). If the orifice has not a circular cross section, D = 4*A/P, where A is the cross section area and P is the wetted perimeter.
- ζ is the loss factor with respect to D that depends on the
geometry of the orifice. In the turbulent flow regime, it is
assumed that ζ is constant.

For small mass flow rates, the flow is laminar and is approximated by a polynomial that has a finite derivative for m_flow=0. - v is the mean velocity.
- ρ is the upstream density.

Since the pressure loss factor zeta is provided only for a mass flow from port_a to port_b, the pressure loss is not correct when the flow is reversing. If reversing flow only occurs in a short time interval, this is most likely uncritical. If significant reversing flow can appear, this component should not be used.

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