MATLAB: Do I observe negative hydraulic pressure and similar non-physical quantities when simulating a model with SimHydraulics 1.6 (R2009b)

Simscape Fluids

I have built a simple hydrostatic system using SimHydraulics 1.6 (R2009b). I am able to simulate my model without any errors, however, I observe physically unattainable quantities, such as negative hydraulic pressure (of large magnitudes).

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Models built using SimHydraulics may display physically unattainable values during simulation because of an irregularity with the model itself. SimHydraulics does not stop simulation if one or more variables assume non-physical value(s) such as gauge pressures below -1 bar, negative value of fluid volume in the reservoir, etc. This is so that, at the end of simulation, the user knows the extent of the irregularity rather than just the fact that the reservoir is short of fluid or pressure drop below vaporization level.

In order to resolve this situation, you must analyze your model and iteratively modify your design to smooth out the irregularities.

Related Solutions

MATLAB: Do I observe zero output for the model in SimHydraulics 1.6 (R2009b)

To resolve this issue, tighten the 'Consistency tolerance' parameter in the Solver Configuration block by setting it to the default value of 1e-9.

Sensor output is zero because the 'Consistency tolerance' is large enough to not allow transient initialization. This parameter affects the nonlinear solver used for computing initial conditions and for transient initialization. It determines how accurately the algebraic constraints are to be satisfied at the beginning of simulation and after every discrete event (for example, a discontinuity resulting from a valve opening, a hard stop hitting the stop, and so on).

Decreasing the parameter value (that is, tighter tolerance) results in a more reliable time simulation.

MATLAB: How to model flow rate caused by a decreasing volume as in a piston with SimHydraulics 1.1 (R2006b)

There are three flow rate components associated with the moving piston:

    - flow rate caused by the piston motion,             q = A * v,
    - flow rate caused by the fluid compressibility,   q = V/E * dp/dt
    - flow rate due to leakage, if any                         q = K_l * p.

The flow rate associated with the piston velocity is accounted in the hydro-mechanical converter while the compressibility-caused flow is accounted in the variable volume chamber block. It gives you the possibility to build cylinder models with various level of idealization of a very limited set of building blocks.

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Related Solutions

MATLAB: Do I observe zero output for the model in SimHydraulics 1.6 (R2009b)

MATLAB: How to model flow rate caused by a decreasing volume as in a piston with SimHydraulics 1.1 (R2006b)

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