Local Resistance

(To be removed) Hydraulic resistance specified by loss coefficient

The Hydraulics (Isothermal) library will be removed in a future release. Use the Isothermal Liquid library instead. (since R2020a)

For more information on updating your models, see Upgrading Hydraulic Models to Use Isothermal Liquid Blocks.

Library

Local Hydraulic Resistances

Description

The Local Resistance block represents a generic local hydraulic resistance, such as a bend, elbow, fitting, filter, local change in the flow cross section, and so on. The pressure loss caused by resistance is computed based on the pressure loss coefficient, which is usually provided in catalogs, data sheets, or hydraulic textbooks. The pressure loss coefficient can be specified either as a constant, or by a table, in which it is tabulated versus Reynolds number.

The pressure drop between port A and port B is:

$Δ p = p_{A} - p_{B},$

where:

p is the pressure drop.
p_A is the gauge pressure at port A.
p_B is the gauge pressure at port B.

If the Model parameterization parameter is set to By semi-empirical formulas, the pressure drop is related to the volumetric flow rate by the expression:

$q = A \sqrt{\frac{2}{K \cdot ρ}} \cdot \frac{p}{{(p^{2} + p_{c r}^{2})}^{1 / 4}}$

where:

q is the volumetric flow rate.
A is the flow area.
K is the pressure loss coefficient of the flow resistance.
ρ is the fluid density.
p_Cr is the minimum pressure for turbulent flow.

If the Model parameterization parameter is set to By loss coefficient vs. Re table, the pressure drop is related to the volumetric flow rate by the expression:

$p = K (Re) ρ \frac{q | q |}{2 A^{2}}$

where K is now a function of the Reynolds number (Re). Its value is specified in the block dialog box in tabulated form against the Reynolds number:

$Re = \frac{q \cdot D_{H}}{A \cdot ν}$

where:

D_H is the hydraulic diameter of the flow resistance:

$D_{H} = \sqrt{\frac{4 A}{π}}$
ν is the kinematic viscosity.

For a constant pressure loss coefficient, the minimum pressure for turbulent flow, p_cr, 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:

p_cr = (p_avg + p_atm)(1 – B_lam)

p_avg = (p_A + p_B)/2

where

p_avg	Average pressure between the block terminals
p_atm	Atmospheric pressure, 101325 Pa
B_lam	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:
$p_{c r} = K \frac{ρ}{2} {(\frac{{Re}_{c r} \cdot ν}{D_{H}})}^{2}$
where
Re_cr Critical Reynolds number (Critical Reynolds number parameter value)

The block provides two parameterizations:

By semi-empirical formulas — The pressure loss coefficient is assumed to be constant for a specific flow direction. The flow regime can be either laminar or turbulent, depending on the Reynolds number.
By loss coefficient vs. Re table — The pressure loss coefficient is specified as a function of the Reynolds number. The flow regime is assumed to be turbulent at all times. You must ensure that the loss coefficient data corresponds to this flow regime.

The resistance can be symmetrical or asymmetrical. In symmetrical resistances, the pressure loss practically does not depend on flow direction and one value of the coefficient is used for both the direct and reverse flow. For asymmetrical resistances, the separate coefficients are provided for each flow direction. If the loss coefficient is specified by a table, the table must cover both the positive and the negative flow regions.

Connections A and B are conserving hydraulic ports associated with the block inlet and outlet, respectively.

The block positive direction is from port A to port B. This means that the flow rate is positive if fluid flows from A to B, and the pressure loss is determined as $Δ p = p_{A} - p_{B},$ .

Variables

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

Nominal values provide a way to specify the expected magnitude of a variable in a model. Using system scaling based on nominal values increases the simulation robustness. Nominal values can come from different sources, one of which is the Nominal Values section in the block dialog box or Property Inspector. For more information, see Modify Nominal Values for a Block Variable.

Basic Assumptions and Limitations

Fluid inertia is not taken into account.
If you select parameterization by the table-specified relationship K=f(Re), the flow is assumed to be completely turbulent.

Parameters

Parameters Tab

Resistance area

The smallest passage area. The default value is 1e-4 m^2.

Model parameterization

Select one of the following methods for specifying the pressure loss coefficient:

By semi-empirical formulas — Provide a scalar value for the pressure loss coefficient. For asymmetrical resistances, you have to provide separate coefficients for direct and reverse flow. This is the default method.
By loss coefficient vs. Re table — Provide tabulated data of loss coefficients and corresponding Reynolds numbers. The loss coefficient is determined by one-dimensional table lookup. You have a choice of two interpolation methods and two extrapolation methods. For asymmetrical resistances, the table must cover both the positive and the negative flow regions.

Pressure loss coefficient for direct flow

Loss coefficient for the direct flow (flowing from A to B). For simple ideal configurations, the value of the coefficient can be determined analytically, but in most cases its value is determined empirically and provided in textbooks and data sheets (for example, see [1]). The default value is 2. This parameter is used if Model parameterization is set to By semi-empirical formulas.

Pressure loss coefficient for reverse flow

Loss coefficient for the reverse flow (flowing from B to A). The parameter is similar to the loss coefficient for the direct flow and must be set to the same value if the resistance is symmetrical. The default value is 2. This parameter is used if Model parameterization is set to By semi-empirical formulas.

Laminar transition specification

If Model parameterization is set to By semi-empirical formulas, 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.

Laminar flow pressure ratio

Pressure ratio at which the flow transitions between laminar and turbulent regimes. The default value is 0.999. This parameter is visible only if the Laminar transition specification parameter is set to Pressure ratio.

Critical Reynolds number

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 150. This parameter is visible only if the Laminar transition specification parameter is set to Reynolds number.

Reynolds number vector

Specify the vector of input values for Reynolds numbers as a one-dimensional array. The input values vector must be strictly increasing. The values can be nonuniformly spaced. The minimum number of values depends on the interpolation method: you must provide at least two values for linear interpolation, at least three values for smooth interpolation. The default values are [-4000, -3000, -2000, -1000, -500, -200, -100, -50, -40, -30, -20, -15, -10, 10, 20, 30, 40, 50, 100, 200, 500, 1000, 2000, 4000, 5000, 10000]. This parameter is used if Model parameterization is set to By loss coefficient vs. Re table.

Loss coefficient vector

Specify the vector of the loss coefficient values as a one-dimensional array. The loss coefficient vector must be of the same size as the Reynolds numbers vector. The default values are [0.25, 0.3, 0.65, 0.9, 0.65, 0.75, 0.90, 1.15, 1.35, 1.65, 2.3, 2.8, 3.10, 5, 2.7, 1.8, 1.46, 1.3, 0.9, 0.65, 0.42, 0.3, 0.20, 0.40, 0.42, 0.25]. This parameter is used if Model parameterization is set to By loss coefficient vs. Re table.

Interpolation method

Select one of the following interpolation methods for approximating the output value when the input value is between two consecutive grid points:

Linear — Select this option to get the best performance.
Smooth — Select this option to produce a continuous curve with continuous first-order derivatives.

For more information on interpolation algorithms, see the PS Lookup Table (1D) block reference page. This parameter is used if Model parameterization is set to By loss coefficient vs. Re table.

Extrapolation method

Select one of the following extrapolation methods for determining the output value when the input value is outside the range specified in the argument list:

Linear — Select this option to produce a curve with continuous first-order derivatives in the extrapolation region and at the boundary with the interpolation region.
Nearest — Select this option to produce an extrapolation that does not go above the highest point in the data or below the lowest point in the data.

For more information on extrapolation algorithms, see the PS Lookup Table (1D) block reference page. This parameter is used if Model parameterization is set to By loss coefficient vs. Re table.

Restricted Parameters

When your model is in Restricted editing mode, you cannot modify the following parameters:

Model parameterization
Interpolation method
Extrapolation method
Laminar transition specification

All other block parameters are available for modification. The actual set of modifiable block parameters depends on the value of the Model parameterization parameter at the time the model entered Restricted mode.

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.

Ports

The block has the following ports:

A: Hydraulic conserving port associated with the resistance inlet.
B: Hydraulic conserving port associated with the resistance outlet.

References

[1] Idelchik, I.E., Handbook of Hydraulic Resistance, CRC Begell House, 1994

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2006a

collapse all

R2023a: To be removed

The Hydraulics (Isothermal) library will be removed in a future release. Use the Isothermal Liquid library instead.

For more information on updating your models, see Upgrading Hydraulic Models to Use Isothermal Liquid Blocks.

Local Resistance

Library

Description

Variables

Basic Assumptions and Limitations

Parameters

Parameters Tab

Restricted Parameters

Global Parameters

Ports

References

Extended Capabilities

C/C++ Code Generation Generate C and C++ code using Simulink® Coder™.

Version History

R2023a: To be removed

See Also

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.