Hydraulic pipeline with resistive, fluid inertia, and fluid compressibility properties
The Segmented Pipeline block models hydraulic pipelines with circular cross sections. Hydraulic pipelines, which are inherently distributed parameter elements, are represented with sets of identical, connected in series, lumped parameter segments. It is assumed that the larger the number of segments, the closer the lumped parameter model becomes to its distributed parameter counterpart. The equivalent circuit of a pipeline adopted in the block is shown below, along with the segment configuration.
Pipeline Equivalent Circuit
The model contains as many Constant Volume Hydraulic Chamber blocks as there are segments. The chamber lumps fluid volume equal to
|Number of segments|
The Constant Volume Hydraulic
Chamber block is placed between two
branches, each consisting of a Hydraulic
Resistive Tube block and a Fluid
Inertia block. Every Hydraulic
Resistive Tube block lumps
portion of the pipe length, while Fluid Inertia
additional pipe length equal to aggregate
equivalent length of pipe local resistances, such
as fitting, elbows, bends, and so on).
The nodes to which Constant Volume
Hydraulic Chamber blocks are connected are
n is the number of
segments). Pressures at these nodes are assumed to
be equal to average pressure of the segment.
Intermediate nodes between Hydraulic
Resistive Tube and Fluid
Inertia blocks are assigned names
The Constant Volume Hydraulic
Chamber blocks are named
Hydraulic Resistive Tube blocks are
and Fluid Inertia blocks are named
The number of segments determines the number of computational nodes associated with the block. A higher number increases model fidelity but decreases simulation speed. Experiment with different numbers to obtain a suitable trade-off between accuracy and speed. Use the following equation as a starting point in estimating a suitable number of segments:
|Number of segments|
|Speed of sound in the fluid|
|ω||Maximum frequency to be observed in the pipe response|
The table contains an example of simulation of a pipeline where the first four true eigenfrequencies are 89.1 Hz, 267 Hz, 446 Hz, and 624 Hz.
|Number of Segments||1st Mode||2nd Mode||3rd Mode||4th Mode|
The error between simulated and actual eigenfrequencies is less than 5% if an eight-segment model is used.
The flow rate through the pipeline is positive if it is directed from port A to port B. The pressure differential is positive if pressure is higher at port A than at port B.
Basic Assumptions and Limitations
Flow is assumed to be fully developed along the pipe length.
- Pipe internal diameter
Internal diameter of the pipe. The default value is
- Pipe length
Pipe geometrical length. The default value is
- Number of segments
Number of lumped parameter segments in the pipeline model. The default value is
- Aggregate equivalent length of local resistances
This parameter represents total equivalent length of all local resistances associated with the pipe. You can account for the pressure loss caused by local resistances, such as bends, fittings, armature, inlet/outlet losses, and so on, by adding to the pipe geometrical length an aggregate equivalent length of all the local resistances. This length is added to the geometrical pipe length only for hydraulic resistance computation. Both the fluid volume and fluid inertia are determined based on pipe geometrical length only. The default value is
- Internal surface roughness height
Roughness height on the pipe internal surface. The parameter is typically provided in data sheets or manufacturer’s catalogs. The default value is
1.5e-5m, which corresponds to drawn tubing.
- Laminar flow upper margin
Specifies the Reynolds number at which the laminar flow regime is assumed to start converting into turbulent. Mathematically, this is the maximum Reynolds number at fully developed laminar flow. The default value is
- Turbulent flow lower margin
Specifies the Reynolds number at which the turbulent flow regime is assumed to be fully developed. Mathematically, this is the minimum Reynolds number at turbulent flow. The default value is
- Pipe wall type
The parameter can have one of two values:
Flexible Wall. If the parameter is set to
Rigid Wall, wall compliance is not taken into account, which can improve computational efficiency. The value
Flexible Wallis recommended for hoses and metal pipes where wall compliance can affect the system behavior. The default value is
- Static pressure-diameter coefficient
Coefficient that establishes relationship between the pressure and the internal diameter at steady-state conditions. This coefficient can be determined analytically for cylindrical metal pipes or experimentally for hoses. The parameter is used if the Pipe wall type parameter is set to
Flexible Wall, and the default value is
- Viscoelastic process time constant
Time constant in the transfer function that relates pipe internal diameter to pressure variations. By using this parameter, the simulated elastic or viscoelastic process is approximated with the first-order lag. The value is determined experimentally or provided by the manufacturer. The default value is
- Specific heat ratio
Gas-specific heat ratio for the Constant Volume Hydraulic Chamber block. The default value is
- Initial pressures at model nodes
Lets you specify the initial condition for pressure inside the pipe segments. The parameter can have one of two values:
The same initial pressure for all nodes— The initial pressure in all pipe segments is the same, and is specified by the Initial pressure parameter value. This is the default.
Custom— Lets you specify initial pressure individually for each pipe segment, by using the Initial pressure vector parameter. The vector size must be equal to the number of pipe segments, defined by the Number of segments parameter value.
- Initial pressure
Specifies the initial pressure in all pipe segments. The parameter is used if the Initial pressures at model nodes parameter is set to
The same initial pressure for all nodes, and the default value is
- Initial pressure vector
Lets you specify initial pressure individually for each pipe segment. The parameter is used if the Initial pressures at model nodes parameter is set to
Custom. The vector size must be equal to the number of pipe segments, defined by the Number of segments parameter value.
- Initial flow rate
Specifies the initial volumetric flow rate through the pipe. The default value is
Parameters determined by the type of working fluid:
Fluid kinematic viscosity
The block has the following ports:
Hydraulic conserving port associated with the pipe inlet.
Hydraulic conserving port associated with the pipe outlet.