Modelica: Fluid from source, valve, to sink

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This next model I'm working on has a source with high pressure driving the movement of gas to the low pressure sink. In between the source and sink is a linear valve which I plan to be a control valve in the future. So source--> valve--> sink. The model will not solve because there exists one more variable than the total number of equations. The unknown variable is valve.port_a.m_flow.

It is not clear where or how I would define this variable. I thought that the pressure difference between source and sink would be enough to drive the movement of fluid. I did, however, see under the valve advanced settings that m_flow = port_a.m_flow. Not sure if that helps but I'm still confused.