Matrix diffusion zones are defined via the Matrix Diffusion Zones... button on the Pipe dialog. When you press this button, the following dialog is displayed:
Clicking the Used checkbox to the left of each zone activates ("turns on") the zone. A lower zone can only be activated if the zone above it is active. Inactive (unused) zones are grayed out (and are not represented in the model).
For each zone, it is necessary to define a Medium, a Geometry, a Minimum Thickness, a Maximum Thickness, and a Fraction of Perimeter:
Medium: This is the porous medium for the matrix diffusion zone. It must be a previously defined Solid medium.
Geometry: This is the assumed diffusive geometry for the matrix zone. There are three options: Slab, Sphere, and Slot.
Minimum Thickness and Maximum Thickness: These two inputs define the thickness of the matrix diffusion zone. They have dimensions An output attribute for an element that defines the dimensionality (in terms of Length, Time and other fundamental dimensions) of the output. of length. The Maximum Thickness must be greater than or equal to the Minimum Thickness (otherwise it is ignored and only the Minimum Thickness is used). GoldSim assumes that the thickness of the zone varies uniformly over the pathway perimeter between the specified minimum and maximum values, such that every part of the pathway perimeter "sees" the full range of matrix thicknesses. Mathematically, this is represented in GoldSim by integrating the mobile to immobile zone flux for each of the matrix diffusion zones over the specified range of thickness.
Fraction of Perimeter: This is the fraction of the pathway's perimeter which is connected to this particular matrix diffusion zone. It is dimensionless, and must be greater than or equal to 0. It can, however, be greater than 1 (implying the perimeter with respect to matrix diffusion is larger than specified in the Pipe's main dialog).
For each zone, GoldSim internally computes the Effective Area and displays it to the right of each matrix diffusion zone. This represents the area in a cross-section perpendicular to the flow direction. By comparing this area to the specified pathway area, the user can better determine whether the diffusive geometry and thickness for the zone have been defined reasonably.
You can also define a diffusive skin for the pathway:
Skin medium: This specifies the Solid medium which is the "skin" material for the pathway, and is optional (i.e., the field can remain empty), . The skin medium must be a previously defined Solid medium. The skin is a diffusive layer in contact with the mobile zone. If a skin is specified, species The chemical (or non-chemical, such as bacterial or viral) constituents that are stored and transported through an environmental system in a contaminant transport model. In GoldSim, the Species element defines all of the contaminant species being simulated (and their properties). must diffuse through the skin before entering any of the matrix diffusion zones (i.e., the skin, if defined, is between the matrix diffusion zones and the mobile zone).
Skin Thickness: This defines the thickness of the skin for the pathway, and has dimensions of length. If no Skin Medium is specified, this field is ignored. The Skin Thickness must be greater than or equal to zero.