Moisture Flow
Use the Moisture Flow multiphysics coupling () to simulate fluid flows in free and porous media, where the fluid properties (density, viscosity) depend on moisture content. Models can also include moisture transport in building materials. The physics interface supports low Mach numbers (typically less than 0.3).
The Moisture Flow interface solves for conservation of vapor concentration, mass, and momentum in air. It synchronizes the features from the Moisture Transport and Fluid Flow interfaces when a turbulent flow regime is defined. It also complements the Wall, Screen, and Interior Fan features from the flow interface to account for moisture effects.
When considering an hygroscopic porous medium filled with moist air and liquid water, the coupling between the Moisture Transport and Fluid Flow interfaces applies on the moist air phase only. This means that the synchronized fluid properties (density, viscosity) are the moist air properties, and that the porosity set in the Fluid Flow interface should account for the presence of the liquid water phase in the porous medium. This can be done by multiplying the dry material porosity by (1-mt.sl), where the mt.sl variable is the liquid water saturation defined in the Moisture Transport interface. In addition, the mass source or sink in the gas phase, due to evaporation or condensation, is handled by the Moisture Flow coupling node. This moisture source term, stored in the mt.G_evap variable, cancels out with the condensation term in the equilibrium formulation that couples the gas and liquid phases, but has to be accounted for in the fluid flow equations for the gas phase. Other moisture sources, that may be defined in a Moisture Source node and stored in the mt.Gtot variable, are not accounted for, and should be manually added in the fluid flow interface if needed. See Moisture Flow in Hygroscopic Porous Medium domains for details.
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 mf1.
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 moisture flow, or select All domains as needed.
Coupled Interfaces
This section defines the physics involved in the multiphysics coupling. The Fluid flow and Moisture Transport lists include all applicable physics interfaces.
The default values depend on how this 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 chosen 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 node from a physics interface. If the physics interface is removed from the Model Builder — for example, Moisture Transport in Air is deleted — then the Moisture Transport list defaults to None as there is nothing to couple to.
Moisture Transport at Walls
This section is available for free and porous media flows, when the Mixture type for moist air is set to Concentrated species in the moisture transport interface.
By selecting the Account for Stefan velocity at walls check box, you add a velocity contribution to the fluid flow due to the vapor flux at walls.
When prescribing a vapor flux gw at boundaries in the moisture transport interface, using the Moisture Content, Moisture Flux, Wet Surface, or Moist Surface features, the Moisture Flow coupling node computes the Stefan velocity uStefan and applies it in the Wall features of the fluid flow interface, at the corresponding boundaries.
The Stefan velocity is defined as
where ρg is the moist air density.
At low temperature conditions, the vapor content in moist air stays relatively small, even for high relative humidity, and the Stefan flow may be neglected. By default, the check box is not selected.
Moisture Transport Turbulence Model
This section is available for free media flow, when the fluid flow interface uses a turbulence model. In this case, the diffusive moisture flux is defined as
to account for the turbulent mixing caused by the eddy diffusivity in moisture convection, with the turbulent diffusivity defined as
where νT is defined by the flow interface, and the turbulent Schmidt number ScT depends on the Moisture transport turbulence model.
Select an option from the Moisture transport turbulence model list: Kays–Crawford (the default) or User-defined turbulent Schmidt number.
For User-defined turbulent Schmidt number, enter a Turbulent Schmidt number ScT (dimensionless). The turbulent kinematic viscosityνT is taken directly from the fluid flow interface.
For Kays–Crawford, see Kays–Crawford Model for Turbulent Diffusivity for details about the definition of ScT.
The Turbulence model type used by the fluid flow interface can be displayed by selecting the Show or Hide Physics Property Settings button at the right of the Fluid flow list.
When an interface is selected from the Moisture Transport list, some of its model inputs are forced with values from the Moisture Flow node. In addition, it defines how the turbulence has to be accounted for, depending on the Fluid flow interface’s turbulence settings. Therefore, each moisture transport or fluid flow interface should be used in at most one Moisture Flow node. In cases where multiple fluid flow interfaces are used, an equal number of moisture transport interfaces and Moisture Flow nodes are needed to define proper multiphysics couplings.
If a physics interface is deleted and then added to the model again, then in order to reestablish the coupling, you need to choose the physics interface again from the Fluid flow or Moisture Transport lists. This behavior is applicable to all multiphysics coupling nodes that would normally default to the once present interface. See Multiphysics Modeling Workflow in the COMSOL Multiphysics Reference Manual.
Evaporative Cooling of Water: Application Library path Heat_Transfer_Module/Phase_Change/evaporative_cooling
Drying of a Potato Sample: Application Library path Heat_Transfer_Module/Phase_Change/potato_drying
Location in User Interface
Context Menus
 Multiphysics>Moisture Flow
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when any version of a Single-Phase Flow (or Brinkman Equations, or Free and Porous Media Flow, Brinkman) interface with Fluid Properties feature is active together with a Moisture Transport interface with Moist Air feature.
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when any version of a Single-Phase Flow (or Brinkman Equations, or Free and Porous Media Flow, Brinkman, or Darcy’s Law) interface with Fluid and Matrix Properties feature is active together with a Moisture Transport interface with Hygroscopic Porous Medium feature.