Orifice with Variable Area Slot

Hydraulic variable orifice shaped as rectangular slot




The block models a variable orifice created by a cylindrical sharp-edged spool and a rectangular slot in a sleeve. The flow rate through the orifice is proportional to the orifice opening and to the pressure differential across the orifice. The flow rate is determined according to the following equations:





A(h)={b·h+Aleakfor h>0Aleakfor h<=0 



qFlow rate
pPressure differential
pA, pBGauge pressures at the block terminals
CDFlow discharge coefficient
A(h)Instantaneous orifice passage area
bWidth of the orifice slot
x0Initial opening
xSpool displacement from initial position
hOrifice opening
orOrifice orientation indicator. The variable assumes +1 value if a spool displacement in the globally assigned positive direction opens the orifice, and –1 if positive motion decreases the opening.
ρFluid density
νFluid kinematic viscosity
pcrMinimum pressure for turbulent flow
RecrCritical Reynolds number
DHInstantaneous orifice hydraulic diameter
AleakClosed orifice leakage area

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. Positive signal at the physical signal port S opens or closes the orifice depending on the value of the parameter Orifice orientation.

Basic Assumptions and Limitations

  • Fluid inertia is not taken into account.

Dialog Box and Parameters

Orifice width

The width of the rectangular slot. The default value is 1e-2 m.

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.

Initial opening

Orifice initial opening. The parameter can be positive (underlapped orifice), negative (overlapped orifice), or equal to zero for zero lap configuration. The value of initial opening does not depend on the orifice orientation. The default value is 0.

Orifice orientation

The parameter is introduced to specify the effect of the orifice control member motion on the valve opening. The parameter can be set to one of two options: Opens in positive direction or Opens in negative direction. The value Opens in positive direction specifies an orifice whose control member opens the valve when it is shifted in the globally assigned positive direction. The parameter is extremely useful for building a multi-orifice valve with all the orifices being controlled by the same spool. The default value is Opens in positive direction.

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.

Leakage area

The total area of possible leaks in the completely closed valve. The main purpose of the parameter is to maintain numerical integrity of the circuit by preventing a portion of the system from getting isolated after the valve is completely closed. The parameter value must be greater than 0. The default value is 1e-12 m^2.

 Restricted Parameters

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 to control spool displacement.

The flow rate is positive if fluid flows from port A to port B. Positive signal at the physical signal port S opens or closes the orifice depending on the value of the parameter Orifice orientation.

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