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OpenModelica / OpenModelica

OpenModelica is an open-source Modelica-based modeling and simulation environment intended for industrial and academic usage.
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Problem with 0 pressure head to the Mover_dp (Buildings_library) #12847

Open FrancescoPavarelli23 opened 2 months ago

FrancescoPavarelli23 commented 2 months ago

Description

The system is composed of: Boundary_pt, necessary to provide a pressure reference within a closed circuit; Mover_dp, which is the pump with head set by the ramp signal; Ramp, gives a signal of 0 Pa for the first 5 seconds, then reaches a head of 475 Pa in 10 seconds (475 Pa is the head required to achieve nominal flow in the system); Supply and return manifold, which only changes the direction of the flow and consists of a MixingVolume with pressure losses on the various branches. The medium used in the system is water.

The figure shows the diagrams of the system and the supply manifold.

System Impianto semplice

Manifold Collettore

Expected Behavior

The expected behavior would be to simulate the system with a head of 0 Pa, resulting in zero water flow. However, during the first 5 seconds of simulation, the program gives the following warning multiple times for each timestep, with only the last one shown below: 'Homotopy solver total pivot: Matrix (nearly) singular at time 5.001635. Continuing anyway. For more information please use -lv LOG_NLS_V.' The issue is that with more complex systems, this warning becomes a fatal error that causes the simulation to crash. Specifically, this error occurs: 'New front position is not smaller than previous first node. Process crashed.'

Version and OS

casella commented 2 months ago

A question related to the manifold implementation: the volume has an array connector, are the three pipes connected to the same element of it, or to three separate connectors?

FrancescoPavarelli23 commented 2 months ago

The three pipes are connected to three differents ports.

casella commented 1 month ago

@FrancescoPavarelli23 quadratic flow models are well-known to become singular at zero flow. The Modelica.Fluid pipe models have a range of countermeasures to avoid that, which should be set up with reasonable parameter values. Are you aware of that? You can check the documentation of the pipe model for more info.