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Hydraulic Fluid

Working fluid properties, set by selecting from list of predefined fluids


Hydraulic Utilities


The Hydraulic Fluid block lets you specify the type of hydraulic fluid used in a loop of hydraulic blocks. It provides the hydraulic fluid properties, such as kinematic viscosity, density, and bulk modulus, for all the hydraulic blocks in the loop. These fluid properties are assumed to be constant during simulation time. The density is determined by the type of fluid, while kinematic viscosity additionally requires that the temperature is specified.

The bulk modulus value shown in the block dialog box is the bulk modulus of pure liquid, and is determined by the type of fluid and by the temperature. When the fluid properties are used in hydraulic blocks, such as Constant Volume Hydraulic Chamber (Simscape) or Variable Hydraulic Chamber (Simscape), the fluid is represented as a mixture of liquid and a small amount of entrained, nondissolved gas, which is specified in the Hydraulic Fluid block as Relative amount of trapped air. The mixture bulk modulus in these blocks is determined as:



ElPure liquid bulk modulus
pαAtmospheric pressure
αRelative gas content at atmospheric pressure, α = VG/VL
VGGas volume at atmospheric pressure
VLVolume of liquid
nGas-specific heat ratio

The main objective of representing fluid as a mixture of liquid and gas is to introduce an approximate model of cavitation, which takes place in a chamber if pressure drops below fluid vapor saturation level. As it is seen in the graph below, the bulk modulus of a mixture decreases at ppa, thus considerably slowing down further pressure change. At high pressure, p>>pa, a small amount of nondissolved gas has practically no effect on the system behavior.

Cavitation is an inherently thermodynamic process, requiring consideration of multiple-phase fluids, heat transfers, and so on, and as such cannot be accurately simulated with Simscape™ Fluids™ software. But the simplified version implemented in the block is good enough to signal if pressure falls below dangerous level, and to prevent computation failure that normally occurs at negative pressures.

The Hydraulic Fluid block offers a selection of predefined fluids. See Examples for how you can get information on the fluid properties used in the block. Once you select a fluid name, you can also specify the temperature of the fluid and the relative amount of entrained, nondissolved gas.

The Hydraulic Fluid block has one port. You can connect it to a hydraulic diagram by branching a connection line off the main line and connecting it to the port. When you connect the Hydraulic Fluid block to a hydraulic line, the software automatically identifies the hydraulic blocks connected to the particular loop and propagates the hydraulic fluid properties to all the hydraulic blocks in the loop.

Each topologically distinct hydraulic loop in a diagram requires the properties of its working fluid to be specified. You can specify these properties either by selecting a predefined fluid using a Hydraulic Fluid block, or by specifying parameters of a custom fluid using a Custom Hydraulic Fluid block, which is available in the Simscape Foundation library. If no Hydraulic Fluid block or Custom Hydraulic Fluid block is attached to a loop, the hydraulic blocks in this loop use the default fluid, which is equivalent to fluid defined by a Custom Hydraulic Fluid block with the default parameter values.


Hydraulic fluid

Hydraulic fluid type. Select one of the predefined fluids:

  • Skydrol LD-4 (default)

  • Skydrol 500B-4

  • Skydrol-5

  • HyJet-4A

  • Fluid MIL-F-83282

  • Fluid MIL-F-5606

  • Fluid MIL-F-87257

  • Oil-10W

  • Oil-30W

  • Oil-50W

  • Oil SAE-30

  • Oil SAE-50

  • Transmission fluid ATF (Dexron III)

  • ISO VG 22 (ESSO UNIVIS N 22)

  • ISO VG 32 (ESSO UNIVIS N 32)

  • ISO VG 46 (ESSO UNIVIS N 46)

  • Brake fluid DOT3

  • Brake fluid DOT4

  • Brake fluid DOT5

  • Gasoline

  • Diesel fuel

  • Jet fuel

  • Water-Glycol 60/40

  • Water

Relative amount of trapped air

Amount of entrained, nondissolved gas in the fluid. The amount is specified as the ratio of gas volume at normal conditions to the fluid volume in the chamber. In practice, the relative amount of trapped air is always greater than 0. If set to 0, ideal fluid is assumed, but you will get a warning upon simulation. The default value is 0.005.

System temperature

Fluid temperature (C). The default value is 60.

Viscosity derating factor

Proportionality coefficient that you can use to adjust fluid viscosity, if needed. Specify a value between 0.5 and 1.5. The default value is 1.

Pressure below absolute zero

Determines how the block handles the out-of-range assertion during simulation:

  • Error — If the pressure falls below absolute zero, the simulation stops and you get an error message. This is the default.

  • Warning — If the pressure falls below absolute zero, you get a warning but the simulation continues. Use this option when modeling systems where cavitation can occur in extreme cases.

 Restricted Parameters


The block has one hydraulic conserving port.


You can get information on the fluids and their properties through the MATLAB® command line. In the following example, the first command brings you the list of available fluids, and the second command plots the properties of a selected fluid from the list, in this case, Skydrol LD-4.

  1. In the MATLAB Command Window, type:

    props = sh_stockfluidproperties

    The system responds with a list of available fluids:

    props = 
         skydrol_ld_4: [1x1 struct]
        skydrol_500_4: [1x1 struct]
            skydrol_5: [1x1 struct]
               hy_jet: [1x1 struct]
              f_83282: [1x1 struct]
               f_5606: [1x1 struct]
              f_87257: [1x1 struct]
              oil_10w: [1x1 struct]
              oil_30w: [1x1 struct]
              oil_50w: [1x1 struct]
           oil_sae_30: [1x1 struct]
           oil_sae_50: [1x1 struct]
           atf_dexron: [1x1 struct]
            iso_vg_32: [1x1 struct]
             gasoline: [1x1 struct]
          diesel_fuel: [1x1 struct]
             jet_fuel: [1x1 struct]
         water_glycol: [1x1 struct]
  2. To plot the properties of the first fluid in the list, Skydrol LD-4, type:


    The plot window opens:

Fluid properties for the Skydrol family of hydraulic fluids were obtained from literature provided by the manufacturer, Solutia, Inc. More information is available on their website at:

Introduced in R2006a

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