The Transitional Flow (tran) interface (), found under the Rarefied Flow branch () when adding a physics interface, can be used to model isothermal flows across the full range of Knudsen numbers from the laminar flow limit to the molecular flow limit. The method is less computationally efficient than other physics interfaces targeting specific Knudsen number regimes. In the transitional flow regime, at Knudsen numbers between 0.1 and 10, this physics interface is the available solution in COMSOL Multiphysics. The physics interface solves the Boltzmann BGK equation, which is a simplified form of the Boltzmann equation, using the lattice Boltzmann method. When larger lattices are used in the lattice Boltzmann method it can be viewed as a discrete velocity method for solving the Boltzmann BGK equation.

When this physics interface is added, these default nodes are also added to the Model Builder: Flow Properties, Wall, and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and volume forces. You can also right-click Transitional 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 tran.

Select the Neglect scattering term (Molecular flow) check box to neglect scattering of the gas molecules within the domain. This is appropriate for models in the molecular flow regime.

Select an option from the Quadrature list. Several predefined quadratures are available. The notation for the quadratures is of the form DXQY where X and Y are integers. X represents the number of dimensions whilst Y represents the number of velocities in the quadrature. A transitional flow simulation can be considered accurate if the results do not change with increased Y and with reduced mesh size. This dialog defines the dependent variables: tran.f1...tran.fn (SI unit: 1/m3). These are defined on the flow properties node). There are n dependent variables defined, where n is the number of velocities in the quadrature.

Select a Form of equilibrium function — First order, Second order (the default), or Third order. The equilibrium function is the discretized form of the Maxwell–Boltzmann distribution for the gas at its local velocity and temperature.

To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. Select the Performance index: 1, 2, 4 (the default), 8, 16, or 32. This setting determines the number of dependent variables in each segregated group generated by the default solver. Select the Always use dependent variable grouping check box to use the segregated solver even for smaller quadratures in 2 dimensions.