Moisture Flow

Use the 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 and interfaces when a turbulent flow regime is defined. It also complements the , and 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.

Settings

The is the default multiphysics coupling feature name.

The 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 field. The first character must be a letter.

The default (for the first multiphysics coupling feature in the model) is mf1.

Domain Selection

When nodes are added from the context menu, you can select (the default) from the list to choose specific domains to define the moisture flow, or select as needed.

Coupled Interfaces

This section defines the physics involved in the multiphysics coupling. The and lists include all applicable physics interfaces.

The default values depend on how this coupling node is created.

•

If it is added from the ribbon (Windows users), 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.

•

If it is added automatically when a multiphysics interface is chosen in the or window, then the two participating physics interfaces are selected.

You can also select from either list to uncouple the node from a physics interface. If the physics interface is removed from the — for example,is deleted — then the list defaults to as there is nothing to couple to.

Moisture Transport at Walls

This section is available for free and porous media flows, when the is set to in the moisture transport interface.

By selecting the 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 coupling node computes the Stefan velocity uStefan and applies it in the 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 .

Select an option from the list: (the default) or .

For , enter a ScT (dimensionless). The turbulent kinematic viscosityνT is taken directly from the fluid flow interface.

For , see Kays-Crawford Model for Turbulent Diffusivity for details about the definition of ScT.

The used by the fluid flow interface can be displayed by selecting the button at the right of the 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

Location in User Interface

Context Menus

•

when any version of a (or , ) interface with feature is active together with a interface with feature.

•

when any version of a (or , ) interface with feature is active together with a interface with feature.