Modeling semi-permeable surfaces

You can model semi-permeable membranes such as GORE-TEX® textiles with the following methods:

Humidity based permeability

Diffuses humidity to ambient air given a diffusion resistance value. There is no explicit flow or head loss associated with this permeability type. You specify a constant or temperature-varying diffusion resistance value and the ambient humidity using the External Conditions modeling object.

Mixture based permeability

Allows for the exchange of humid air across the semi-permeable surface by equalizing pressures across the interface. The humid air can cross the interface in either direction depending on the pressure difference across the interface. The equation for this model uses the specified constant head loss value and the global ambient pressure.

The boundary flow surface is semi-permeable in the sense that the velocity is zero at the boundary, thus there is no convection between the enclosure and the external ambient region. The humid air is exchanged only through diffusion.

Estimating the head loss for mixture based permeability

You can estimate the head loss factor from manufacturer data using the following equation:

h = \frac{2 \Delta p A^2_v}{\rho Q^2}				where:∆p is the typical pressure difference across the interface given in manufacturer specificationsAv is the vent area that can be computed from the design and dimensions given in manufacturer specificationsρ is the density of air that can be approximated to 1.0 kg/m3Q is the typical volume flow rate given in manufacturer specifications

Where do I find it?

Application	Pre/Post
Prerequisite	You must specify the appropriate solver, use the parallel flow solver, and model humidity.
Command Finder	Flow Surface
Simulation Navigator	Right-click Simulation Object node→New Simulation Object→Flow Surface
Location in dialog box	Type→Boundary Flow Surface→Semi-Permeable Interface

How do I

Create a flow surface

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Modeling semi-permeable surfaces, Simcenter 3D 2021.1 Series

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