what is 'Derivative of state in block at time t is not finite. There may be a singularity in the solution.' error? how to address it?

Hi

I am trying to simulate a thermal fluid system. As soon as I introduce gas-charged accumulator circled red in the circuit, it gives me following error

An error occurred while running the simulation and the simulation was terminated

Caused by:

Derivative of state 'pumpandpipe13082024b.Gas_Charged_Accumulator_TL.T_I' in block 'pumpandpipe13082024b/Gas-Charged Accumulator (TL)' at time 20.0 is not finite. The simulation will be stopped. There may be a singularity in the solution. If not, try reducing the step size (either by reducing the fixed step size or by tightening the error tolerances)

Also, when I introduce elevation gain from port A to port B in pipes TL circled red, i have the following error

An error occurred while running the simulation and the simulation was terminated

Caused by:

• Derivative of state 'pumpandpipe13082024b.Centrifugal_Pump_TL1.A.T' in block 'pumpandpipe13082024b/Centrifugal Pump (TL)1' at time 16.666666666666668 is not finite. The simulation will be stopped. There may be a singularity in the solution. If not, try reducing the step size (either by reducing the fixed step size or by tightening the error tolerances)

what do these errors mean?

[Muhammad Ali]

UPDATE!!!!

The problem has been resolved using Variable-step solver ode15s (Stiff/NDF) instead of fixed step solver ode14x.

However, another question arises ==> Does this mean ONLY that the problem was a stiff one? and no error in the model?

[Yifeng Tang]

I did find the gas-charged accumulator to be a bit more difficult to use, mainly because the definition for the parameters and initial conditions can be a bit hard to get right (read the documentation!). I prefer to use a Tank (TL) or Tank (G-TL) for similar behavior and easier parametrization. You will need some kind of tank/accumulator in a closed thermal liquid network. Otherwise it's quite likely to build up pressure or energy and error out, as the fluid has no extra volume to expand or shrink.

Using a stiff solver for fluids systems is almost a must. The nature of the equations typically requires stiff solver. If you leave it to variable-step, auto selection, you'll likely get "daessc", a solver designed to handle typical equations from a Simscape network. I find it to work fine most of the time.