In the transitional flow regime, the continuum approximation for the gas breaks down completely, and the Knudsen layer occupies a significant fraction — if not all — of the flow domain. Historically flows in this regime have been modeled by the direct simulation Monte Carlo (DSMC) method, which computes the trajectories of large numbers of randomized particles through the system. The discrete velocity method, which represents the Boltzmann equation, has also been used. The discrete velocity method has dependent variables that exist in six dimensions (three in real space and three in velocity space) by a discrete number of three-dimensional equations, each corresponding to a discrete velocity (the set of discrete velocities is sometimes referred to as the quadrature). This approach produces an increased number of degrees of freedom in the problem (corresponding to the number of discrete velocities used) but restricts the problem to only three dimensions. The velocity degrees of freedom are coupled together through scattering terms in the equation system. More recently the lattice Boltzmann technique has been shown to be an optimized form of the discrete velocity method (close to the continuum limit) to solve a simplified form of the Boltzmann equation known as the Boltzmann BGK equation. COMSOL Multiphysics uses a modified form of the lattice Boltzmann method to solve transitional flows. Unlike the DSMC method, the lattice Boltzmann method is not subject to statistical noise — this is an advantage for low velocity gas flows. The implementation available in COMSOL Multiphysics is limited to isothermal flows because the BGK equation has a greatly simplified scattering model with a single parameter: the relaxation time. The relaxation time can be chosen to produce the correct viscosity (the approach used in the Transitional Flow interface) or the correct thermal conductivity but not both simultaneously. More complicated scattering models (with more than one parameter) are required for nonisothermal modeling. Diffuse reflection of gas molecules is also assumed at all surfaces.

The Transitional Flow Interface () should be used to compute fluid flows with Knudsen numbers greater than 0.1. Lower Knudsen number flows can be solved, but ultimately the problem becomes very highly coupled in the Navier–Stokes regime, and a fully coupled solver must be used. For three-dimensional flows and two-dimensional flows with quadratures larger than the D2Q12 quadrature, it becomes impractical to use the fully coupled solver with the computing power that is typically available.