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Cross-Sectional Area at Ports in Simscape

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In Simscape, several blocks require you to specify one or more 'Cross-sectional area at port(s) A' (and B, C and/or D). For example, in the 'Flow Resistance (G)' block there is a single value 'Cross-sectional area at ports A and B'. What is the effect of this area? The Help only gives "Flow area at the ports of the flow resistance. The ports are assumed to be identical in size.", which doesn't enlighten me much. In addition, the block Description (also in Help) states "The Flow Resistance (G) block models a general pressure drop in a gas network branch. The pressure drop is proportional to the square of the mass flow rate and to the density of the gas. The constant of proportionality is determined from a nominal operating condition specified in the block dialog box." Not a word about the cross-sectional area here.
Is there any other documentation that explains the role of this cross-sectional area parameter, for example whether it affects the pressure (ratio or difference) at which the flow chokes?

Accepted Answer

Chidvi Modala
Chidvi Modala on 10 Aug 2020
In general, "Cross-sectional area of port A/B" represents the area of the connection to that block. The cross-sectional area at ports A and B of the flow resistance block usually have small impact on the flow rate through the block. The parameter exists because we need to determine the direction of flow and provide a smoothing for the temperature at a connection node when the flow direction changes (i.e. flow reversal). The threshold for smoothing the nodal temperature based on a Mach number, so the cross-sectional area at the port is used to divide the mass flow rate to get the velocity and Mach number.
You can refer to Momentum Balance section under Description in this documentation.

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