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Advantages of Using Isothermal Liquid Blocks

Isothermal liquid blocks model hydraulic systems where the working fluid temperature remains constant during simulation. You can use the isothermal liquid blocks in the Foundation > Isothermal Liquid or Fluids > Isothermal Liquid libraries to replace Hydraulic blocks in the Foundation library or Hydraulics (Isothermal) blocks in the Fluids library.

Upgrading your hydraulic blocks to isothermal blocks results in models with improved usability, increased accuracy, and enhanced simulation robustness. For more information about upgrading your models, see How to Upgrade Hydraulic Models.

Using isothermal liquid blocks provides improved usability, increased accuracy, and enhanced simulation robustness:

  • The isothermal liquid block equations provide smooth transitions, reduce zero-crossings, and improve handling of zero flow and flow reversal. These improvements increase simulation robustness.

  • The isothermal liquid blocks have the same functionality as the hydraulics blocks, and may have greater functionality.

  • Because isothermal liquid blocks model fluid density as a function of pressure, you can specify your working fluid by selecting the bulk modulus model and the desired option for modeling the amount of entrained air. The block equations reflect these settings, which increases the accuracy of the simulation.

  • The isothermal liquid blocks may perform the same functionality as multiple hydraulic blocks, and often have improved parameterization options. For example, you can choose multiple cross-sectional geometries when using the Laminar Leakage (IL) block. For isothermal liquid source blocks, you can configure each as either controlled or constant, and use the Flow Rate Source (IL) block to provide the options for generating either mass flow rate or volumetric flow rate. Similarly, you can use the Flow Rate Sensor (IL) block to measure the mass flow rate, volumetric flow rate, or both.

  • The isothermal liquid domain allows for cleaner model construction. By default, the isothermal liquid pipe and actuator blocks account for fluid compressibility, which reduces the likelihood of dry nodes and makes the simulation more robust. Additionally, you do not need to add extra Constant Volume Chamber blocks to your model to mitigate the effect of dry nodes.

  • In the isothermal liquid domain, the Across and Through variables are absolute pressure and mass flow rate. In the hydraulic domain, the variables are gauge pressure and volumetric flow rate. Using mass flow rate as the Through variable reduces the potential for small errors in mass conservation to accumulate over time due to the conversion between mass and volumetric quantities, which results in increased accuracy.

  • When modeling complex phenomena such as water hammers, the isothermal liquid blocks require a greater numerical cost per time step, but can take larger time steps, which may reduce overall simulation time.

Comparison of Hydraulic and Isothermal Block Performance

You can automatically convert large and complex models from the hydraulic to the isothermal liquid domain by using the hydraulicToIsothermalLiquid conversion tool. To see the effect of conversion on a model, see Boom Lift Model in Simscape. This is a model available on MATLAB File Exchange that uses Simscape™, Simscape Fluids™, and Simscape Multibody™. The Boom Lift Model contains these valves, hydraulic cylinders, and actuators:

Model Element Number in Model
Hydraulic cylinders12
Rotary actuators2

After using the hydraulicToIsothermalLiquid conversion tool, performing manual adjustments to address the HTML report issues, and removing superfluous Constant Volume Chamber blocks, the Isothermal Liquid model is more streamlined. On a Windows 10 desktop 3.6-GHz Intel® Xeon CPU with 64 Gb of memory, the Isothermal Liquid model runs 10% faster than the Hydraulic model in R2023a. For more information on upgrading your models and adjustments to improve numerical performance, see Conversion Messages After Converting Hydraulic to Isothermal Liquid Models and How to Upgrade Hydraulic Models.

The converted isothermal liquid model accurately reproduces the results of the hydraulic model. This figure compares the hydraulic (H) and isothermal liquid (IL) model results for the outrigger pressures.

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