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Laminar flow

Many hydraulic models have a laminar (leakage) flow:

 

flow = G * dp

 

with G the flow conductance and dp the pressure difference. The table below shows the flow rates for various conductance values and pressure differences of 10 [bar] and 400 [bar].

 

If the flow rate is very small (leakage flow), choose G below 1.0e-14 [m3/s.Pa]. If the flow rate is very large (open connection to a tank), choose G 1.0e-9 or higher if desired.

Leakage Flow

For the valve models in the library, the leakage flow is described with the laminar flow equation. For leakage flow the conductance is not often given in datasheets. Therefore the conductance is calculated out of the leakage flow at a nominal pressure drop:

 

Q_leak = G * p_nom

 

which gives:

 

G = Q_leak/P_nom

 

Turbulent flow

The turbulent flow in an orifice or valve can be described by:

 

flow = Cd * A * sqrt(  (2/rho) * dp)

 

with Cd the discharge coefficient, A the orifice area, rho the fluid density and dp the pressure difference. The discharge coefficient depends on the shape of the orifice and is generally not listed in data sheets. Therefore we will write the turbulent flow with nominal parameters. Given a nominal pressure drop p_nom we find a nominal flow:

 

Q_nom = Cd * A * sqrt( (2/ rho) * p_nom)

 

we can use this to rewrite the flow equation as:

 

flow = Q_nom * sqrt( dp / p_nom)

 

This is the flow equation, that is found in most datasheets. Therefore all the models in the hydraulics library where the turbulent flow equation is used (orifices, valves) use this equation.

Older 20-sim Models

If you have an old 20-sim model that uses the flow equation with a discharge coefficient and area, and you want to replace it with a new model using the nominal flows here is what you have to do: