This module primarily models the transport and conversion of mass in chemical reactors and other reacting systems. Many of these applications include transport through convection, and the module includes a number of physics interfaces for the simulation of fluid flow, particularly within porous media. As with all other physical descriptions simulated by COMSOL Multiphysics, any description of fluid flow can be directly coupled to any other physical process. This is particularly relevant for systems based on turbulent or multiphase flow, which are supported by the CFD Module. For detailed descriptions of the fluid-flow interfaces listed below, see the CFD Module User’s Guide.

The Fluid Flow branch () has a number of subbranches to describe momentum transport. The Laminar Flow Interface (), found under the Single-Phase Flow branch () when adding a physics interface, is used to model laminar flow described by the Navier–Stokes equations. It is also possible to extend this physics interface to study nonisothermal or non-Newtonian flow.

The Creeping Flow Interface (), also under the Single-Phase Flow branch (), is used to model flow fluid flows at very low Reynolds numbers, also referred to as Stokes flow. This typically occurs in fluid systems with high viscosity or small geometrical length scales (for example in microfluidics and MEMS devices).

The rest of the available physics interfaces for fluid flow are found under the Porous Media and Subsurface Flow branch ().

The Darcy’s Law Interface () is used to model fluid movement through interstices in a porous medium where a homogenization of the porous and fluid media into a single medium is done. This physics interface combines the continuity equation and an equation of state for the pore fluid (or gas), can be used to model low velocity flows, for which the pressure gradient is the major driving force.

Darcy’s law can be used in porous media where the fluid is mostly influenced by the frictional resistance within the pores. When the size of the interstices are larger and the fluid is influenced by itself, the kinetic potential from fluid velocity, pressure, and gravity must be also be considered, and The Brinkman Equations Interface () is the appropriate physics interface to use.

The Brinkman equations extend Darcy’s law to describe the dissipation of the kinetic energy by viscous shear, similar to the Navier–Stokes equation. Consequently, they are well suited to transitions between slow flow in porous media, governed by Darcy’s law, and fast flow in channels described by the Navier–Stokes equations. The equations and boundary conditions that describe these types of phenomena are also found in The Free and Porous Media Flow, Brinkman Interface ().

The Free and Porous Media Flow, Darcy Interface () models porous media flow connected to free flow domains. This multiphysics interface couples the Laminar Flow interface with the Darcy’s Law interface over their common boundaries.