Specific and ideal pressure and flow

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Damas Limoge
Damas Limoge on 20 May 2013
Answered: Yifeng Tang on 18 Aug 2022
I am trying to create a flow and pressure based on two constants. I thought I knew how to do this, but the results don't match. In the attached image, I have an ideal pressure source connected in parallel to an ideal flow source. My assumption was that there was no flow through the pressure source, similar to the ideal pressure sensor, but that's clearly not the case. The pressure source is adding power, and that causes the measured flow (shown on display "D1") to be significantly different from the induced flow (0.008 instead of 0.001). I've also connected them in series, which yields the correct flow, but an incorrect pressure.
How can I make a circuit where I specifically control the flow and pressure?
I've also tried creating a custom block, but to no avail.

Answers (1)

Yifeng Tang
Yifeng Tang on 18 Aug 2022
In general, one cannot prescribe both the pressure at and the flow through a node at the same time. If a pressure is prescribed, using an ideal pressure source, the flow through the source is determined by the downstream flow resistance. If a flow is prescribed, using an ideal flow source, the pressure at the source outlet is determined by the pressure needed to generate such flow through the downstream resistances.
In this model, the pressure at the outlets of the two sources, and the inlet of the check valve, is prescribed by the pressure source. The check valve will let through a certain amount of flow calculated from its parametrization (e.g. opening pressure & orifice area). This flow may be greater or less than the prescribed flow at the flow source. The deficit or surplus will flow through the pressure source.
If you have both the reference flow and pressure, the proper way to use such information is to parametrize the check valve so that both can be satisfied when using either a pressure source or a flow source.

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