| Simscape™ | |
Hydraulic Elements
The Constant Volume Chamber block models a fixed-volume chamber with rigid or flexible walls, to be used in hydraulic valves, pumps, manifolds, pipes, hoses, and so on. Use this block in models where you have to account for some form of fluid compressibility. You can select the appropriate representation of fluid compressibility using the block parameters.
Fluid compressibility in its simplest form is simulated according to the following equations:
where
| q | Flow rate into the chamber |
| Vf | Volume of fluid in the chamber |
| Vc | Geometrical chamber volume |
| E | Fluid bulk modulus |
| p | Gauge pressure of fluid in the chamber |
If pressure in the chamber is likely to fall to negative values and approach cavitation limit, the above equations must be enhanced. In this block, it is done by representing the fluid in the chamber as a mixture of liquid and a small amount of entrained, nondissolved gas. The mixture bulk modulus is determined as:
where
| El | Pure liquid bulk modulus |
| pα | Atmospheric pressure |
| α | Relative gas content at atmospheric pressure, α = VG/VL |
| VG | Gas volume at atmospheric pressure |
| VL | Volume of liquid |
| n | Gas-specific heat ratio |
The main objective of representing fluid as a mixture of liquid and
gas is to introduce an approximate model of cavitation, which takes place
in a chamber if pressure drops below fluid vapor saturation level. As it is
seen in the graph below, the bulk modulus of a mixture decreases at
, thus considerably
slowing down further pressure change. At high pressure,
, a small
amount of nondissolved gas has practically no effect on the system behavior.
Cavitation is an inherently thermodynamic process, requiring consideration of multiple-phase fluids, heat transfers, etc., and as such cannot be accurately simulated with Simscape™ software. But the simplified version implemented in the block is good enough to signal if pressure falls below dangerous level, and to prevent computation failure that normally occurs at negative pressures.
If it is known that cavitation is unlikely in the system under design, you can set the relative gas content in the fluid properties to zero, thus increasing the speed of computations. Use the Hydraulic Fluid or the Custom Hydraulic Fluid block to set the fluid properties.
If chamber walls have noticeable compliance, the above equations must be further enhanced by representing geometrical chamber volume as a function of pressure:
where
| d | Internal diameter of the cylindrical chamber |
| L | Length of the cylindrical chamber |
| Kp | Proportionality coefficient (m/Pa) |
 | Time constant |
| s | Laplace operator |
Coefficient Kp establishes relationship between pressure and the internal diameter at steady-state conditions. For metal tubes, the coefficient can be computed as (see [1]):
where
| D | Pipe external diameter |
| EM | Modulus of elasticity (Young's modulus) for the pipe material |
| ν | Poisson's ratio for the pipe material |
For hoses, the coefficient can be provided by the manufacturer.
The process of expansion and contraction in pipes and especially in hoses is a complex combination of nonlinear elastic and viscoelastic deformations. This process is approximated in the block with the first-order lag, whose time constant is determined empirically (for example, see [2]).
As a result, by selecting appropriate values, you can implement four different models of fluid compressibility with this block:
Chamber with rigid walls, no entrained gas in the fluid
Cylindrical chamber with compliant walls, no entrained gas in the fluid
Chamber with rigid walls, fluid with entrained gas
Cylindrical chamber with compliant walls, fluid with entrained gas
The block allows two methods of specifying the chamber size:
By volume — Use this option for cylindrical or non-cylindrical chambers with rigid walls. You only need to know the volume of the chamber. This chamber type does not account for wall compliance.
By length and diameter — Use this option for cylindrical chambers with rigid or compliant walls, such as circular pipes or hoses.
The block has one hydraulic conserving port associated with the chamber inlet. The block positive direction is from its port to the reference point. This means that the flow rate is positive if it flows into the chamber.
The model is based on the following assumptions:
No inertia associated with pipe walls is taken into account.
Chamber with compliant walls is assumed to have a cylindrical shape. Chamber with rigid wall can have any shape.
The parameter can have one of two values: By volume or By length and diameter. The value By length and diameter is recommended if a chamber is formed by a circular pipe. If the parameter is set to By volume, wall compliance is not taken into account. The default value of the parameter is By volume.
The parameter can have one of two values: Rigid or Compliant. If the parameter is set to Rigid, wall compliance is not taken into account, which can improve computational efficiency. The value Compliant is recommended for hoses and metal pipes, where compliance can affect the system behavior. The default value of the parameter is Rigid. The parameter is used if the Chamber specification parameter is set to By length and diameter.
Volume of fluid in the chamber. The default value is 1e-4 m^3. The parameter is used if the Chamber specification parameter is set to By volume.
Internal diameter of the cylindrical chamber. The default value is 0.01 m. The parameter is used if the Chamber specification parameter is set to By length and diameter.
Length of the cylindrical chamber. The default value is 1 m. The parameter is used if the Chamber specification parameter is set to By length and diameter.
Coefficient Kp that establishes relationship between pressure and the internal diameter at steady-state conditions. The parameter can be determined analytically or experimentally. The default value is 1.2e-12 m/Pa. The parameter is used if Chamber wall type is set to Compliant.
Time constant in the transfer function relating pipe internal diameter to pressure variations. With this parameter, the simulated elastic or viscoelastic process is approximated with the first-order lag. The parameter is determined experimentally or provided by the manufacturer. The default value is 0.01 s. The parameter is used if Chamber wall type is set to Compliant.
Gas-specific heat ratio. The default value is 1.4.
Initial pressure in the chamber. This parameter specifies the initial condition for use in computing the block's initial state at the beginning of a simulation run. For more information, see Computing Initial Conditions. The default value is 0.
The parameter is determined by the type of working fluid selected for the system under design. Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.
Nondissolved gas relative content determined as a ratio of gas volume to the liquid volume. The parameter is determined by the type of working fluid selected for the system under design. Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.
The block has one hydraulic conserving port associated with the chamber inlet.
[1] Meritt, H.E., Hydraulic Control Systems, John Wiley & Sons, New York, 1967
[2] Holcke, Jan, Frequency Response of Hydraulic Hoses, RIT, FTH, Stockholm, 2002
The Constant Volume Chamber Test Rig demo (sh_constant_chamber_test_rig) is specifically designed to demonstrate the Constant Volume Chamber block behavior at different regimes. The chamber is placed between two linear hydraulic resistances and subjected to an abrupt pressure change from 0 to 5 MPa at the beginning of simulation.
If air content is set to zero and chamber walls are rigid, the pressure change can easily be determined analytically. The Simulink Transfer Fcn block simulates this regime for a metal cylindrical pipe with 0.03 m internal diameter, 0.036 m external diameter, and 16.5 m length. The output of this block serves as a reference during all other regimes. You can investigate chamber characteristics, for example, by changing air content in the fluid, by switching from rigid to compliant walls, by changing the viscoelastic time constant or the pressure-diameter coefficient.
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