In this section:

•Selecting the Right Interface

•Coupling to Other Physics Interfaces

Selecting the Right Interface

The two interfaces found under the Thin-Film Flow branch () of the Model Wizard describe momentum transport—the Lubrication Shell and Thin-Film Flow. Both can be added from the Model Wizard; either singularly or in combination with other interfaces such as mass and energy transfer. If you have the MEMS Module, the Film-Damping Shell and Thin-Film Gas Flow interfaces are also available.

The Lubrication Shell Interface () is used primarily to model slow-moving flow in very thin environments. The lubrication oil between two rotating cylinders is a classic example of this. The thin gaps in motors and generators is also an area of application.

The interface solves the a modified version of the Navier-Stokes equations, called the Reynolds equation, in 1D or 2D. This involves relations to simulate the thickness of the fluid-flow regime as well as a modification to the Reynolds equation that allows for the rarefaction of gases at the microscale.

While, for example, you would choose this interface in 3D, you are then defining all your physics in 2D, that is, at your 3D geometries boundaries.

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The Thin-Film Flow Interface () is basically the same as the Lubrication Shell interface except that it is only available in 2D and is applied to domains instead of boundaries.

TABLE 5-1: THE THIN-FILM FLOW INTERFACES FOR THE CFD MODULE

Coupling to Other Physics Interfaces

It is common to simulate applications that couple fluid flow to another type of phenomenon described in another physics interface. Although this is not often another type of flow, it can still involve physics interfaces supported in the CFD Module and COMSOL Multiphysics base package.

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T h e L u b r i c a t i o n S h e l l I n t e r f a c e

The Lubrication Shell interface (), found under the Thin-Film Flow branch () of the Model Wizard, is available in 2D, 2D axisymmetric, and 3D geometries. This physics interface is a boundary mode, which means that the boundary level is the highest level where this physics interface has equations and user interfaces; it does not have any user interfaces or equations at the domain level. Using boundary equations, this physics interface models the pressure in a narrow flow channel that is assumed to reside just outside of the boundary. The interface is used primarily to model slow-moving flow in very thin environments. The lubrication oil between two rotating cylinders is an example of this.

•Journal Bearing: Model Library path

CFD_Module>Thin-Film_Flow>journal_bearing

Model

The following default nodes are added when using this interface—Fluid-Film Properties,

Border, and Initial Values. Right-click the main interface node to add other features.

I N T E R F A C E I D E N T I F I E R

The interface identifier is a text string that can be used to reference the respective physics interface if appropriate. Such situations could occur when coupling this interface to another physics interface, or when trying to identify and use variables defined by this physics interface, which is used to reach the fields and variables in expressions, for example. It can be changed to any unique string in the Identifier field.

The default identifier for the first interface in the model is tffs.

B O U N D A R Y S E L E C T I O N

The default setting is to include All boundaries in the model to define the film-damping or lubrication shell. To choose specific boundaries, select Manual from the Selection list.

P H Y S I C A L M O D E L

The Equation type defaults to a Reynolds equation.

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