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Modeling Isothermal Liquid Systems

Intended Applications

The Isothermal Liquid library contains basic elements, such as orifices, chambers, and hydromechanical converters, as well as sensors and sources. Use these blocks to model hydraulic systems where the liquid temperature is assumed constant, for applications such as:

  • Hydraulic actuation of mechanical systems

  • Isothermal liquid transport through pipe networks

  • Hydraulic turbines for power generation

In the isothermal liquid domain, the working fluid is assumed to be at a constant temperature during simulation. To model thermodynamic effects, use the blocks from the Thermal Liquid and Two-Phase Fluid libraries. For more information on fluid domains available for Simscape™ multidomain system modeling, see Simscape Fluid Domains.

You specify the properties of the working fluid in a connected loop by using the Isothermal Liquid Properties (IL) block. This block lets you choose between several modeling options (see Isothermal Liquid Modeling Options).

Network Variables

In the isothermal liquid domain, the Across variable is pressure and the Through variable is mass flow rate. Note that these choices result in a pseudo-bond graph, because the product of pressure and mass flow rate is not power.

Blocks with Fluid Volume

Components in the isothermal liquid domain are modeled using control volumes. The control volume encompasses the fluid inside the component and separates it from the surrounding environment and other components. Mass flow rates across the control surface are represented by ports. The fluid volume inside the component is represented using an internal node, which provides the pressure inside the component. This internal node is not visible, but you can access its parameters and variables using Simscape data logging. For more information, see About Simulation Data Logging.

The following blocks in the Isothermal Liquid library are modeled as components with a fluid volume. In the case of Reservoir (IL), the volume is assumed to be infinitely large.

BlockGas Volume
Constant Volume Chamber (IL)Finite
Pipe (IL)Finite
Rotational Mechanical Converter (IL)Finite
Translational Mechanical Converter (IL)Finite
Reservoir (IL)Infinite

Other components have relatively small volumes, so that fluid entering the component spends negligible time inside the component before exiting. These components are considered quasi-steady-state and they do not have an internal node.

Reference Node and Grounding Rules

Unlike mechanical and electrical domains, where each topologically distinct circuit within a domain must contain at least one reference block, isothermal liquid networks have different grounding rules.

Blocks with a fluid volume contain an internal node, which provides the pressure inside the component and therefore serves as a reference node for the isothermal liquid network. Each connected isothermal liquid network must have at least one reference node. This means that each connected isothermal liquid network must have at least one of the blocks listed in Blocks with Fluid Volume. In other words, an isothermal liquid network that contains no fluid volume is an invalid network.

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