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Non-physical unlimited negative pressure results in Fixed Displacement Pump/Motor

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Itay Gefen
Itay Gefen on 18 Feb 2020
Answered: Yifeng Tang on 18 Aug 2020
In am using a fixed Displacement motor in a simulation where in some conditions may act as a pump.
I noticed a non-physical behavior in case the motor inlet is closed while still turning (shaft is turned by an external torque), The torque load rise unlimited.
After analyzing and checking the motor in the attached simple simulation, I notice that the motor creates a non-physical *UNLIMITED* negative pressure in the inlet.
This is non-physical because in an ideal vacuum there shall be a zero pressure in the inlet and thus the pressure difference cannot exceed 1-bar.
This behavior is obviously misleading and may lead to mistakes in simulation results.
I also checked it with a pump and got same results.
I attached the simple sim file and results.
Did anyone notice that? Am I missing something or this is a bug that needs a fix?
Thanks!
Itay
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Answers (1)

Yifeng Tang
Yifeng Tang on 18 Aug 2020
Hi Itay,
I believe this has to do with the fact that the A port of the pump is connected to a "cap", with no volume. If you connect a constant volume chamber instead, which should be conceptually the same as a sealed inlet, the model will err out when the pressure drops below -1atm. But the error is not a feature of the pump itself, but a warning/error mechanism of the hydraulic domain. You may toggle that in the Hydraulic Fluid block to have issue an warning instead of an error and again allow the pressure to go below -1atm.
Hope this helps,
Yifeng

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