Variable Area Hydraulic Orifice

Hydraulic variable orifice created by cylindrical spool and sleeve


Hydraulic Elements


The Variable Area Hydraulic Orifice block models a variable orifice created by a cylindrical sharp-edged spool and a variable-area slot in a sleeve. The area of the orifice is expected to be computed outside the block and imported via the AR physical signal connection. The Minimum area parameter specifies the minimum orifice area value. If the input signal falls below this level (for example, turns negative), the area is saturated to this value. The flow rate through the orifice is proportional to the orifice area and the pressure differential across the orifice.

The flow rate is determined according to the following equations:






qFlow rate
pPressure differential
pA, pBGauge pressures at the block terminals
CDFlow discharge coefficient
AOrifice passage area
DHOrifice hydraulic diameter
ρFluid density
νFluid kinematic viscosity
pcrMinimum pressure for turbulent flow
RecrCritical Reynolds number

The block positive direction is from port A to port B. This means that the flow rate is positive if it flows from A to B and the pressure differential is determined as p=pApB.

Basic Assumptions and Limitations

  • Fluid inertia is not taken into account.

Dialog Box and Parameters

Parameters Tab

Flow discharge coefficient

Semi-empirical parameter for orifice capacity characterization. Its value depends on the geometrical properties of the orifice, and usually is provided in textbooks or manufacturer data sheets. The default value is 0.7.

Critical Reynolds number

The maximum Reynolds number for laminar flow. The transition from laminar to turbulent regime is assumed to take place when the Reynolds number reaches this value. The value of the parameter depends on the orifice geometrical profile. You can find recommendations on the parameter value in hydraulics textbooks. The default value is 12, which corresponds to a round orifice in thin material with sharp edges.

Minimum area

Leakage area of the completely closed orifice. If the input signal falls below this level (for example, turns negative), the area is saturated to this value. The parameter value must be greater than zero. The default value is 1e-12 m^2.

Variables Tab

Use the Variables tab in the block dialog box (or the Variables section in the block Property Inspector) to set the priority and initial target values for the block variables prior to simulation. For more information, see Set Priority and Initial Target for Block Variables.

Global Parameters

Parameters determined by the type of working fluid:

  • Fluid density

  • Fluid kinematic viscosity

Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.


The block has the following ports:


Hydraulic conserving port associated with the orifice inlet.


Hydraulic conserving port associated with the orifice outlet.


Physical signal port that provides the value of the orifice area.

Introduced in R2009b

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