Documentation

Variable Local Restriction (G)

Time-varying restriction in flow area

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  • Simscape / Foundation Library / Gas / Elements

Description

The Variable Local Restriction (G) block models the pressure drop due to a time-varying reduction in flow area, such as a valve, in a gas network. Choking occurs when the restriction reaches the sonic condition.

Ports A and B represent the restriction inlet and outlet. The input physical signal at port AR specifies the restriction area. The Minimum restriction area and Maximum restriction area parameters specify the lower and upper bounds for the restriction area.

The restriction is adiabatic. It does not exchange heat with the environment.

The restriction consists of a contraction followed by a sudden expansion in flow area. The gas accelerates during the contraction, causing the pressure to drop. The gas separates from the wall during the sudden expansion, causing the pressure to recover only partially due to the loss of momentum.

Local Restriction Schematic

Caution

Gas flow through this block can choke. If a Mass Flow Rate Source (G) block or a Controlled Mass Flow Rate Source (G) block connected to the Variable Local Restriction (G) specifies a greater mass flow rate than the possible choked mass flow rate, you get a simulation error. For more information, see Choked Flow.

See the Local Restriction (G) block reference page for the defining equations.

Variables

Use the Variables tab in the block dialog box (or the Variables section in the block Property Inspector) to set the priority and initial target values for the block variables prior to simulation. For more information, see Set Priority and Initial Target for Block Variables and Initial Conditions for Blocks with Finite Gas Volume.

Assumptions and Limitations

  • The restriction is adiabatic. It does not exchange heat with the environment.

  • This block does not model supersonic flow.

Ports

Input

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Input physical signal that controls the gas flow restriction area. The signal saturates when its value is outside the minimum and maximum restriction area limits, specified by the block parameters.

Conserving

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Gas conserving port associated with the inlet or outlet of the local restriction. This block has no intrinsic directionality.

Gas conserving port associated with the inlet or outlet of the local restriction. This block has no intrinsic directionality.

Parameters

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The lower bound for the restriction cross-sectional area. You can use this parameter to represent the leakage area. The input signal AR saturates at this value to prevent the restriction area from decreasing any further.

The upper bound for the restriction cross-sectional area. The input signal AR saturates at this value to prevent the restriction area from increasing any further.

Area normal to flow path at ports A and B. This area is assumed the same for the two ports.

Ratio of actual mass flow rate to the theoretical mass flow rate through the restriction. The discharge coefficient is an empirical parameter that accounts for nonideal effects.

Pressure ratio at which the gas flow transitions between laminar and turbulent regimes. The pressure loss is linear with respect to mass flow rate in the laminar regime and quadratic with respect to mass flow rate in the turbulent regime.

Introduced in R2016b

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