Simulate Fluid Flow Applications with the CFD Module
CFD Modeling Software for SinglePhase and Multiphase Flows
Define and solve models for studying systems containing fluid flow and fluid flow coupled to other physical phenomena with the CFD Module, an addon product to the COMSOL Multiphysics^{®} simulation platform.
The CFD Module provides tools for modeling the cornerstones of fluid flow analysis, including:
 Incompressible and compressible flows
 Laminar and turbulent flows
 Singlephase and multiphase flows
 Free and porous media flow and flow in open domains
 Thin film flow
These capabilities are implemented through structured fluid flow interfaces in order to define, solve, and analyze timedependent (transient) and steadystate flow problems in 2D, 2D axisymmetry, and 3D. In addition to the list above, the CFD Module includes tailored functionality for solving problems that include nonNewtonian fluids, rotating machinery, and high Mach number flow.
The ability to implement multiphysics in a model is important for fluid flow analyses. With the CFD Module, you can model conjugate heat transfer and reacting flows in the same software environment you use to analyze fluid flow problems — simultaneously. Additional multiphysics possibilities, such as fluidstructure interaction, are available when combined with other modules within the COMSOL^{®} product suite.
Did You Know? A physics interface is a user interface for a specific physics area that defines equations together with settings for mesh generation, solvers, visualization, and results.
What You Can Simulate with the CFD Module
Laminar and Creeping Flow
The Laminar Flow and Creeping Flow interfaces provide you with the functionality for modeling transient and steady flows at relatively low Reynolds numbers. A fluid viscosity may be dependent on the local composition and temperature or any other field that is modeled in combination with fluid flow. For nonNewtonian fluids, you can use the predefined rheology models for viscosity, such as Power Law and Carreau, for easy model setup.
In general, density, viscosity, and momentum sources can be arbitrary functions of temperature, composition, shear rate, and any other dependent variable, as well as derivatives of dependent variables. These settings make it possible to define arbitrary models for viscoelastic flow.
Turbulent Flow
A comprehensive set of Reynoldsaveraged NavierStokes (RANS) turbulence models are available in the corresponding fluid flow interfaces in the CFD Module. The following turbulent flow models are available for transient and steady flows:
TwoEquation Models
 kε model
The standard kε model with realizability constraints
 Realizable kε model
The kε model with modified coefficients satisfying realizability
 kω model
The revised Wilcox kω model (1998) with realizability constraints
 SST model
Combination of the kε model in the free stream and the kω model close to the walls
 LowRe kε model
AKN kε model, with the possibility to resolve the flow close to walls
Additional TransportEquation Models
 SpalartAllmaras model
Oneequation model with rotational correction, developed for aerodynamic applications
 v2f model
An extension of the kε model that accounts for turbulence anisotropy by solving for the wallnormal turbulence velocity fluctuations
Algebraic Turbulence Models
 Algebraic yPlus model
 The turbulent viscosity is evaluated by first solving for the wall distance in viscous units using the Reynolds number based on the local speed and dimensional wall distance
 Robust and computationally efficient, but not as accurate as other more sophisticated models
 LVEL model
 The turbulent viscosity is evaluated by first solving for the wallparallel velocity in viscous units using the Reynolds number based on the local speed and dimensional wall distance
 Robust and computationally efficient, but not as accurate as other more sophisticated models
Wall Treatment
You can combine the turbulent flow interfaces with different types of wall treatments, according to the following list:
 Wall functions
 Robust and applicable for coarse meshes
 Limited accuracy
 Smooth and rough walls
 Supported by kε, Realizable kε, and kω
 LowReynoldsnumber treatment
 Resolves the flow all the way down to the walls
 Accurate
 Requires a fine mesh
 Supported by all turbulence models except the standard kε and Realizable kε
 Automatic wall treatment
 Switches between lowRe treatment and wall functions
 Accurate according to local mesh resolution
 Inherits the robustness provided by wall functions
 Default for all turbulence models except standard kε and Realizable kε
UserDefined Turbulence Models
Change or extend the model equations directly in the graphical user interface (GUI) to create turbulence models that are not yet included.
Thin Film Flow
To describe flows in thin domains, such as the thin oil films between moving mechanical parts or fractured structures, the CFD Module provides the Thin Film Flow, Shell interface. This formulation is typically used for modeling lubrication, elastohydrodynamics, or the effects of fluid damping between moving parts due to the presence of gases or liquids (for example, in MEMS).
The Thin Film Flow, Shell interface formulates and solves the Reynolds equation for flow in narrow structures and formulates the mass and momentum balances using a function for the flow averaged across the thickness of the thin structure, which implies that the thickness does not have to be meshed. This functionality helps avoid meshing problems across the gap and thereby saves computation time.
Multiphase Flow
In separated multiphase flow systems, you can use surface tracking methods to model and simulate the behavior of bubbles and droplets in detail, as well as free surfaces. For such cases, the shape of the phase boundary can be described in detail, including surface tension effects, using surface tracking techniques for separated multiphase flow.
When bubbles, droplets, or particles are small compared to the computational domain and there is a large number of them, you can use dispersed multiphase flow models. These models keep track of the mass fraction of the different phases and the influence that the dispersed bubbles, droplets, or particles have on the transfer of momentum in the fluid in an averaged sense.
The following separated and dispersed multiphase flow models are available for transient and steady flows:
Separated Multiphase Flow Models
 Level Set method
 Used for laminar and turbulent flows
 Adaptive mesh refinement to resolve the phase boundary between phases
 Track free liquid surfaces in contact with gases in singlephase flows
 Phase Field method
 Used for laminar and turbulent flows
 Threephase flow model available for laminar flows
 Adaptive mesh refinement to resolve the phase boundary between phases
 Track free liquid surfaces in contact with gases in singlephase flows
Dispersed Multiphase Flow Models

Bubbly flow model
 Used for laminar and turbulent flows
 Used for a relatively small volume fraction (< 0.1) of dispersed gas bubbles in liquids
 Assumes that bubbles do not accelerate relative to the continuum liquid (equilibrium)
 Robust and computationally inexpensive

Mixture model
 Similar to the bubbly flow model, but more generic
 Accurately describes bubbles in liquids, liquidliquid emulsions, aerosols, and solid particles suspended in liquids, provided that the acceleration of the dispersed phase relative to the continuum phase can be neglected (equilibrium)
 More computationally expensive than the bubbly flow model, but still relatively inexpensive

EulerEuler model
 Used for laminar and turbulent flows
 Most general dispersed multiphase flow model
 Can be used to tackle bubbly flows, emulsions, liquid suspensions, aerosols, and solid particles suspended in gases
 Typical applications range from scrubbing gases with liquids to modeling fluidized beds
 Most computationally expensive
Porous Media Flow
The CFD Module makes it simple to simulate fluid flow in porous media using three different porous media flow models.
Porous Media Flow Models
 Darcy's law
 Robust and computationally inexpensive description of flows in porous structures
 Available for twophase flow
 Brinkman equations
 An extension of Darcy's law that accounts for the dissipation of kinetic energy by viscous shear
 Relevant for highly open structures with high porosity
 More general than the Darcy's Law interface, and therefore more computationally expensive
 Free and porous media flow
 Couple flow in porous domains with laminar or turbulent flows in open domains
 Formulates the Brinkman equations for the porous domain and the laminar or turbulent flow equations for the free flow
High Mach Number Flow
Model transonic and supersonic flows of compressible fluids in both laminar and turbulent regimes. The laminar flow model is typically used for lowpressure systems and automatically defines the equations for the momentum, mass, and energy balances for ideal gases. High Mach number flow is available for the kε and SpalartAllmaras turbulence models.
The COMSOL^{®} software automatically formulates the energy equation coupled to the momentum and mass balance equations for ideal gases. In both cases, when meshing these models, automatic mesh refinement resolves the shock pattern by refining around the regions with very high velocity and pressure gradients.
Fluid Flow in Rotating Machinery
Rotating machines, such as mixers and pumps, are common in processes and equipment where fluid flow occurs. The CFD Module provides rotating machinery interfaces that formulate the fluid flow equations in rotating frames and are available for singlephase laminar and turbulent flow. Either define and solve problems using the full timedependent description of the rotating system or use an averaged approach based on the frozen rotor approximation. This feature is computationally inexpensive and can be used to estimate averaged velocities, pressure changes, mixing levels, averaged temperature and concentration distributions, and more.
Generally speaking, the CFD Module can also solve fluid flow problems on any moving frame, not just rotating frames. You can use moving frames to solve a problem where a structure slides in relation to another structure with fluid flow in between, which is easy to set up and solve by employing a moving mesh.
Creating RealWorld Multiphysics Models
The CFD Module provides a dedicated physics interface for defining models of heat transfer in fluid and solid domains coupled to fluid flow in the fluid domain. These types of models are denoted conjugate heat transfer models, which implies that the fluid flow equations are defined and solved in the fluid domain, while the heat transfer equations are formulated and solved in both the solid and fluid domains.
For laminar flows and turbulence models using the lowReynoldsnumber wall treatment, the temperature is continuous across the solidfluid internal boundary, which is the default setting in the nonisothermal flow interfaces. To simulate turbulent conjugate heat transfer using turbulence models with wall functions, the Nonisothermal Flow interface automatically defines thermal wall functions.
The options of lowRe formulations and thermal wall functions make it very straightforward to define and solve conjugate heat transfer problems in combination with turbulent flow.
With the addition of the Structural Mechanics Module, fluidstructure interaction (FSI) problems can be defined and solved for both laminar and turbulent flow. Two FSI options are available in the CFD Module:
 Oneway FSI coupling, in which the flow creates a load on a structure, but the deformations are small enough to neglect their influence on the flow
 Twoway FSI coupling, in which the flow creates loads on a structure, but the deformations are large and influence the flow by changing the shape of the fluid domain
The twoway coupling defines a moving mesh problem in the fluid domain. The displacements at the solidfluid surfaces are determined by the balance of forces exerted by the fluid and counterforces exerted by the deforming solid structure. Steadystate and timedependent studies are available for both oneway and twoway FSI problems for laminar and turbulent flows.
You can use the CFD Module to model reacting systems for both turbulent and laminar flows. This allows for the study and design of reactors, mixers, and any other system where chemical reactions and flow occur. The reacting flow interfaces are able to describe multicomponent transport in diluted and concentrated mixtures. The mixtureaverage model for multicomponent transport is used for concentrated solutions.
The full MaxwellStefan multicomponent transport equations are available in combination with the Chemical Reaction Engineering Module. For turbulent reacting flows, the eddy dissipation model is used to describe turbulence fluctuations in the reaction terms for both diluted and concentrated solutions. To simulate multicomponent transport in concentrated mixtures, the Stefan term is also automatically taken into account; for example, at reacting boundaries.
The Mixer Module expands the capabilities of the CFD Module by adding multiphase flow and free surfaces for rotating machinery. Additionally, you can access a Part Library for impellers and vessels to streamline geometry creation. Both of these features are well suited for modeling processes in the pharmaceutical and food industries.
The multiphase flow interfaces for dispersed flow in the CFD Module treat the dispersed phase as a field where its volume fraction is a model variable. When combined with the Particle Tracing Module, you can use the CFD Module to model EulerLagrange multiphase flow models, where particles or droplets are modeled as rigid particles. With rigid particles modeled separately, the interaction between the fluid and the particles is bidirectional, where the particles affect the fluid flow as well. In addition, the EulerLagrange models are computationally inexpensive when studying a relatively small volume fraction of particles.
The Pipe Flow Module defines models for networks of pipes and channels where the fluid flow equations can be solved along lines and curves. By combining this product with the CFD Module, you can create highfidelity simulations that include pipes and channels connecting to 2D or 3D fluid domains with incompressible, weakly compressible, nonisothermal, and reacting flows.
General Functionality Adapted for Solving CFD Problems
When you build a simulation in COMSOL Multiphysics^{®}, you follow a consistent workflow across all addon modules. The CFD Module offers specialized functionality for fluid flow simulations to maximize the performance and accuracy you need for a CFD analysis. Here are a few of the CFDspecific features:
Geometry
Generate flow domains, such as a bounding box, around imported CAD geometries. You can automatically or manually remove details included in a CAD representation that are not relevant for fluid flow.
Materials
The CFD Module includes a Material Library with the most common gases and liquids. In combination with the Chemical Reaction Engineering Module, you can also access generic descriptions for physical properties of gases (such as viscosity, density, diffusivities, and thermal conductivity).
Meshing
The physicscontrolled mesh functionality in the CFD Module accounts for boundary conditions in fluid flow problems in order to compute accurate solutions. A boundary layer mesh is automatically generated in order to resolve the gradients in velocity that usually arise at the surfaces where wall conditions are applied.
Discretization
The fluid flow physics interfaces use a Galerkin leastsquares method to discretize the flow equations and generate the numerical model in space (2D, 2D axisymmetry, and 3D). The test function is designed to stabilize the hyperbolic terms and the pressure term in the transport equations. Shockcapturing techniques further reduce spurious oscillations. Additionally, discontinuous Galerkin formulations are used to conserve momentum, mass, and energy over internal and external boundaries.
Solvers
The flow equations are usually highly nonlinear. In order to solve the numerical model equations, the automatic solver settings select a suitable damped Newton method. For large problems, the linear iterations in the Newton method are accelerated by stateoftheart algebraic multigrid or geometric multigrid methods specifically designed for transport problems.
For transient problems, timestepping techniques with automatic time stepping and automatic polynomial orders are used to resolve the velocity and pressure fields with the highest possible accuracy, in combination with the aforementioned nonlinear solvers.
Postprocessing
The fluid flow interfaces generate a number of default plots to analyze the velocity and pressure fields. There is an extensive list of derived values and variables that can be easily accessed to extract analytical results.
Build Simulation Apps for Streamlined CFD Simulation
You can build user interfaces on top of any existing model using the Application Builder, included in COMSOL Multiphysics^{®}. This tool enables you to create apps for very specific purposes with welldefined inputs and outputs. Apps can be used for many different purposes:
 Automate difficult and repetitive tasks that can be linked to a single command by recording GUI operations, which can be complex parameterized sequences that may be difficult to reproduce without errors
 Create and update reports from a large number of parameterized simulations according to specific routines to grant the best possible reproducibility and quality
 Provide userfriendly interfaces for specific models to allow nonexperts in modeling and simulation to benefit from the accelerated understanding and optimization capabilities
 Increase access to models within an organization in order to maximize the return on investment from simulationdriven development and design
 Get a competitive edge by allowing your customers to get the best possible fit regarding the selection of your products, based on highfidelity models embedded in userfriendly apps that you provide
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