Free and Porous Media Flow Coupling (Multiphysics Coupling)
The Free and Porous Media Flow Coupling multiphysics coupling node () bidirectionally links the Laminar Flow and Darcy’s Law interfaces.
Settings
The Label is the default multiphysics coupling feature name.
The Name is used primarily as a scope prefix for variables defined by the coupling node. Refer to such variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different coupling nodes or physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first multiphysics coupling feature in the model) is nsd1.
Domain Selection
When nodes are added from the context menu, you can select Manual (the default) from the Selection list to choose specific domains to define the multiphase flow coupling, or select All domains as needed.
When the Free and Porous Media Flow Coupling is added as an effect of adding a Free and Porous Media Flow, Darcy multiphysics interface, the selection is the intersection of the boundaries of the participating physics interfaces.
Only boundaries that are active in the participating physics interfaces (which are chosen from the Coupled Interfaces selection list) can be selected.
Coupled Interfaces
This section defines the physics involved in the multiphysics coupling. The Free flow and Porous media flow lists include all applicable physics interfaces.
The default values depend on how the coupling node is created.
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If it is added from the Physics ribbon (Windows users), Physics contextual toolbar (Mac and Linux users), or context menu (all users), then the first physics interface of each type in the component is selected as the default.
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If it is added automatically when a multiphysics interface is selected in the Model Wizard or Add Physics window, then the two participating physics interfaces are selected.
You can also select None from either list to uncouple the Free and Porous Media Flow Coupling node from a physics interface. If the physics interface is removed from the Model Builder, for example Darcy’s Law is deleted, then the Porous media flow list defaults to None as there is nothing to couple to.
Coupling Settings
This section defines the settings for the Free flow boundary condition.
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When No slip is selected, the tangential component of the fluid velocity at the free-porous interface on the free-flow side (computed in the Free flow interface) is set to zero.
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When Continuous tangential velocity is selected, the tangential velocity component of the flow on both sides of the free-porous interface is continuous. In addition, continuity of normal stresses and conservation of mass across the interface are ensured.
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When Beavers–Joseph is selected, the difference of the tangential velocity components of the flow on both sides of the free-porous interface is proportional to the shear stress at the free-porous interface on the free-flow side. For this option, enter a value or expression for the Beavers and Joseph parameter aBJ (dimensionless) that determines the proportionality. In addition, select the checkbox Use projected permeability to use only the component of the permeability tensor in the direction of the difference of the tangential velocities on both sides of the interface, instead of the trace of the permeability tensor, in determining the proportionality.
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When Beavers–Joseph–Saffman is selected, the tangential velocity component of the flow at the free-porous interface on the free-flow side is proportional to the shear stress at the interface on the free-flow side. For this option also, enter a value or expression for the Beavers and Joseph parameter aBJ (dimensionless) that determines the proportionality. In addition, select the checkbox Use projected permeability to use only the component of the permeability tensor in the direction of the difference of the tangential velocities on both sides of the interface, instead of the trace of the permeability tensor, in determining the proportionality.
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When Viscous slip is selected, the difference of the tangential velocity components of the flow on the free-flow side and a user-supplied interfacial velocity is proportional to the shear stress at the free-porous interface on the free-flow side. Enter values for the Slip length Ls (SI unit: m, the default is 1e-7 m), and an Interfacial velocity ub (SI unit: m/s, the default is 0 m/s).
In all the above cases, additionally continuity of normal stresses and conservation of mass across the interface are ensured.
Select the Include pressure jump across free-porous boundary checkbox to specify an additional pressure jump pj (SI unit: Pa, the default is 0 Pa), in order to model, for example, a resistive layer or other pressure losses at the free-porous interface.
Please find related information in the CFD Module User’s Guide:
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The Darcy’s Law Interface
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Theory for the Darcy’s Law Interface
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The Laminar Flow Interface
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Theory for the Single-Phase Flow Interfaces
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Theory for the Free and Porous Media Flow, Darcy Interface