Hydraulic variable orifice created by cylindrical spool and sleeve
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:
|pA, pB||Gauge pressures at the block terminals|
|CD||Flow discharge coefficient|
|A||Orifice passage area|
|pcr||Minimum pressure for turbulent flow, when the block transitions from laminar to turbulent regime|
The minimum pressure for turbulent flow, pcr, is calculated according to the laminar transition specification method:
By pressure ratio — The transition from laminar to turbulent regime is defined by the following equations:
pcr = (pavg + patm)(1 – Blam)
pavg = (pA + pB)/2
|pavg||Average pressure between the block terminals|
|patm||Atmospheric pressure, 101325 Pa|
|Blam||Pressure ratio at the transition between laminar and turbulent regimes (Laminar flow pressure ratio parameter value)|
By Reynolds number — The transition from laminar to turbulent regime is defined by the following equations:
|DH||Orifice hydraulic diameter|
|ν||Fluid kinematic viscosity|
|Recr||Critical Reynolds number (Critical Reynolds number parameter value)|
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 .
To set the priority and initial target values for the block variables prior to simulation, use the Variables tab in the block dialog box (or the Variables section in the block Property Inspector). For more information, see Set Priority and Initial Target for Block Variables.
Fluid inertia is not taken into account.
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
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
Select how the block transitions between the laminar and turbulent regimes:
Pressure ratio —
The transition from laminar to turbulent regime is smooth and depends
on the value of the Laminar flow pressure ratio parameter.
This method provides better simulation robustness.
Reynolds number —
The transition from laminar to turbulent regime is assumed to take
place when the Reynolds number reaches the value specified by the Critical
Reynolds number parameter.
Pressure ratio at which the flow transitions between laminar
and turbulent regimes. The default value is
This parameter is visible only if the Laminar transition
specification parameter is set to
The maximum Reynolds number for laminar flow. 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. This parameter is visible
only if the Laminar transition specification parameter
is set to
Parameters determined by the type of working fluid:
Fluid kinematic viscosity
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.