This Variable Orifice component adheres to ISO standard 6358. The conductance and critical pressure ratio are controlled by input signals. The component calculates the mass flow rate through the orifice based on the upstream and downstream pressures, respectively. When the pressure ratio is almost unity, the compressibility effects are almost not valid and the component is assumed to have a linear relationship with the pressure drop.

	Formulation Approaches
	Two approaches were taken for the formulation of the flow equation inside the component. When the boolean parameter $\mathrm{Use} \mathrm{Restriction} \mathrm{Area}$ is true, the flow area at the local restriction can be provided which will be converted to sonic conductance using Gidlund's approximation.

If Use Restriction Area is not selected the inp_C is given as conductance, else inp_Ar can be given as input.

$$\begin{gathered} \mathrm{inp\_C}\= C \\ \mathrm{inp\_b} \= b \end{gathered}$$$p\=p_A-p_B$$$$$$$$\mathrm{hflow\_\_x} \= \mathrm{inStream}\left(\mathrm{portx}\cdot \mathrm{hflow}\right)$$$

Orifice Gas Equation

$$\begin{gathered} \mathrm{mflow} \= \{\begin{array}{cc}k\cdot \mathrm{p1}\cdot \left(1-\frac{\mathrm{p\_\_2}}{\mathrm{p\_\_1}}\right)\sqrt{\left(\frac{\mathrm{T\_\_env}}{\mathrm{T\_\_ref}}\right)}& \frac{\mathrm{p2}}{\mathrm{p1}}\ge \mathrm{b\_\_lamreg}\\ \mathrm{p\_\_1}\cdot C\cdot \mathrm{\ρ\_\_0}\cdot \sqrt{\left(\frac{\mathrm{T\_\_env}}{\mathrm{T\_\_ref}}\right)\cdot \(1-{\left(\frac{\frac{\mathrm{p\_\_2}}{\mathrm{p\_\_1}}-b}{1-b}\right)}^2}& \mathrm{b\_\_lamreg}\> \frac{\mathrm{p\_\_2}}{\mathrm{p\_\_1}}\>b\\ \mathrm{p\_\_1}\cdot C\cdot \mathrm{\ρ\_\_0}\cdot \sqrt{\frac{\mathrm{T\_\_0}}{\mathrm{T\_\_1}}}& \frac{\mathrm{p\_\_2}}{\mathrm{P\_\_1}}\le b\end{array} \\ \mathrm{Where}\, \mathrm{p\_\_1} \to \mathrm{Upstream} \mathrm{pressure}\, \mathrm{p\_\_2} \to \mathrm{Downstream} \mathrm{pressure} \end{gathered}$$

Linear Gain k is given by

$k \= \frac{C}{1-\mathrm{b\_\_lamreg}}\cdot \mathrm{\ρ\_\_0}\sqrt{1- {\left(\frac{\left(\mathrm{b\_\_lamreg}-b\right)}{\left(1-b\right)}\right)}^2}$

When $\mathrm{Use} \mathrm{Restriction} \mathrm{Area}$ is checked, Gidlund's approximation is used to calculate the sonic conductance using the restriction area

$$\begin{gathered} \mathrm{inp\_Ar} \= \mathrm{Ar} \\ C\=\frac{0.125\cdot 4\cdot \mathrm{Ar}}{\mathrm{\pi }} \end{gathered}$$