Hi,
It sounds like you're aiming to get a deeper context into the physics that drive the Simscape local restrictor component, specifically in terms of two-phase (2P) flow and momentum balance.
The momentum balance equation is derived from first principles internally. Conceptually, the momentum balance equation in Local Restriction (2P) is derived from a control volume (CV) analysis, where we model the flow from the inlet to the restriction as a flow contraction and the flow from the restriction to the outlet as a flow expansion.
For the flow contraction from inlet to restriction R, the momentum balance is:
where 
is approximated as 
= 
is approximated as = For the flow expansion from restriction R to outlet, the momentum balance is:
Integrating these equations with the defined velocities produces the equations utilized in the model. This approach is purely based on momentum balance analysis.
