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The fluid properties model, contains the properties of hydraulic fluids with the following parameters:


kinematic viscosity {m2/s}
density {kg/m3}
bulk modulus {Pa}
vapour pressure {Pa}


You can drag and drop the FluidProperties model in the Editor and use all these parameters automatically in all other hydraulic components of your model.


Every fluid is stored as an implementation of the FluidProperties model. When you drag and drop the FluidProperties model in the Editor, you will be asked which implementation you want to use. Currently the following fluids are supported:





ISO VG 100

ISO VG 150






SAE 10W-30

SAE 10W-40

SAE 15W-40

SAE 75W-140

Sea Water

Skydrol 500B-4



Default Fluid

The default fluid is meant for use in hydraulic cylinders and hydraulic motors for (heavy) machines. The properties of the default fluid were used in 20-sim 4.8 and previous versions:


 kin_viscosity = 2.7e-5 {m2/s};// kinematic viscosity

 rho = 865.0 {kg/m3};                // density

 B = 1.6e9 {Pa};                        // effective bulk modulus

 p_vapour = -99900.0 {Pa};        // vapour presure


Custom Fluid

You can change the parameters of a fluid in the Parameters Editor.

Multiple Fluids

The fluid properties are valid at the level where the FluidProperties model is inserted. If you have multiple hydraulic circuits and you want to use multiple fluids, you have to put every circuit in a submodel and insert FluidProperties model into that submodel.


Kinematic Viscosity (kin_viscosity)

The kinematic viscosity is defined in SI units: m2/s. It is used in the line model to calculate the pressure drop over a hydraulic line because of friction.

Density (rho)

The density of a fluid is defined in SI units: kg/m3. It is used to derive the mass of a fluid in the hydraulic inertia model and line models and for the pressure drop over an orifice in the orifice and valve models.


The density of a fluid is defined in SI units: kg/m3.

Bulk modulus (B)

No liquid is fully incompressible. The compliance characteristics of the oil in a hydraulic system is a vital parameters affecting the response. The effect of compressibility is incorporated by entering the bulk modulus of the fluid:


dp = -B*dV/V


Here dp is the pressure , V the volume and B the bulk modulus. For small pressure variations the compressibility effect my be rewritten as:


p = B/V*int(flow)


The bulk modulus is a very uncertain parameter. It depends on the percentage of air dissolved in the fluid, the pressure and the temperature. In most textbooks a values of 1.5 to 1.75 [GPa] is used. 20-sim uses a default value of 1.6 [GPa]. If fluid should be simulated at elevated temperatures or with a high percentage or air dissolved, you should use a smaller value for the bulk modulus.

Vapour Pressure (p_vapour)

The 20-sim hydraulics library uses the common definition of pressure in Pa:


1 Pa = 1 N/m2 = 1e-5 bar


The air pressure at sea level is taken as the zero value:


0 Pa = air pressure


Consequently the pressure for a hydraulic fluid that starts to vaporize is negative. In 20-sim the default value of the vapour pressure is:


p_vapour = -0.999e5 Pa


The vapour pressure is the minimum pressure. Some models have a safeguard on the pressure to prevent it to become smaller. You can change the value of the vapour pressure but take care that always a value is chosen that is smaller than zero. Otherwise some of the models will not work correctly.