The Thin-Film Flow (tff) interface (), found under the Thin-Film Flow branch () when adding a physics interface, is used to solve the Reynolds equation or the modified Reynolds equation in a narrow channel that is represented by a surface within the geometry. It is used for lubrication, elastohydrodynamics, or gas damping simulations when the fluid channel is thin enough to apply the Reynolds equation or the modified Reynolds equation. The physics interface is available for 3D geometries.

This physics interface is defined on boundaries, which means that the boundary level is the highest level for which this physics interface has equations and features; it does not have any features or equations on the domain level. The boundary represents a reference surface on which the thin-film flow equation is solved. On one side of the boundary level there is a wall and on the other a base surface, neither of which is represented in the geometry. The wall and base surfaces are orientated with respect to the reference surface normal as shown in Figure 9-1. Fluid flows in the gap between the wall and the base.

Using equations on the reference surface, the physics interface computes the pressure in a narrow gap between the wall and the base. When modeling the flow, it is assumed that the total gap height, h = hw+hb, is much smaller than the typical lateral dimension L of the reference surface. The physics interface is used to model laminar flow in thin gaps or channels. A lubricating oil between two rotating cylinders is an example of this.

When this physics interface is added, the following default nodes are also added in the Model Builder — Fluid-Film Properties, Border, and Initial Values. Then, from the Physics toolbar, you can add other nodes that implement, for example, boundary conditions. You can also right-click Thin-Film Flow to select physics features from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different 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 physics interface in the model) is tff.

Select the Fluid type — Liquid (Reynolds equation), Liquid with cavitation, or Gas (modified Reynolds equation).

For Liquid with cavitation enter the Cavitation transition width (SI unit: Pa). The default is 1 MPa.

Under normal operating conditions, gases dissolved in lubricants cause cavitation when the pressure in the lubricant drops below the saturation pressure. See the theory section for Cavitation for more information.

Some options are only available with certain COMSOL products (see https://www.comsol.com/products/specifications/)

Enter a Reference pressure level pref. The default value is 1[atm]. This pressure represents the ambient pressure, which is not accounted for when computing fluid loads.

Select the Pressure discretization — Linear, Quadratic, Cubic, or Quartic to change the order of the shape functions for the pressure.

The dependent variable (field variable) is the Pressure pfilm. The name can be changed but the names of fields and dependent variables must be unique within a component.