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The CFD Module is a numerical simulation platform for computational fluid dynamics (CFD) that accurately describes your fluid flow processes and engineering designs. Using the CFD Module, you can model most aspects of fluid flow, including compressible, nonisothermal, non-Newtonian, multiphase, and porous media flows — all in the laminar and turbulent flow regimes. For full control over your CFD models, you also have the option to input your own equations into the software. The CFD Module can be used together with the other modules from the COMSOL^{®} product suite to run multiphysics simulations where fluid flow is important.

The CFD Module GUI grants you full access to all steps in the modeling process. This includes the following steps:

- Selecting the appropriate description of the flow, for example single-phase or two-phase, laminar or turbulent flows, etc.
- Creating or importing the model geometry
- Defining the fluid properties
- Adding source and sink terms, or editing the underlying equations of the fluid model, if required
- Selecting mesh elements and controlling the density of the mesh at different positions
- Selecting solvers and tuning them, if required

Tools for defining the different descriptions for fluid flow are packaged and available in easy-to-use physics interfaces. Under the hood, these interfaces define the conservation of momentum, mass, and energy equations that describe fluid flow, accounting for the contribution from multiphysics couplings to other physics. Furthermore, they formulate a stabilized form of these equations, which can be used by COMSOL to create finite element discretization for space and finite differences for time derivatives for stationary or time dependent problems. The stabilized formulations are adapted to the selected description and the functions for the fluid properties by the physics interfaces, which also suggest solver configurations and solver settings for the type of flow described. Tailor-made physics interfaces are available for the following types of fluid flow:

**Single-Phase Flow:**The CFD Module solves multiple variations of the Navier-Stokes equations to model flows in all velocity regimes. This includes the modeling of low-velocity fluids, or creeping flow (Stokes flow), laminar and weakly-compressible flow, and turbulent flow. Turbulent flow is modeled using the Reynolds-Averaged Navier-Stokes (RANS) equations and includes the k-ε, low-Reynolds k-ε, k-ω, SST (Shear Stress Transport), and Spalart-Allmaras turbulence models.

You have the option to manipulate all of the variables in the Navier-Stokes equations and the terms in the turbulence models. You can, for example, include equations based on model variables from other coupled physics interfaces. Many additional tools exist for assisting in the solution process of turbulence models. Among these are tools for the specification of wall functions, including wall roughness, automatic boundary layer meshing, hybrid meshes, and other tools for adapting mesh density and placement.**Nonisothermal Flow:**Thermally-induced buoyancy forces are considered by default in both laminar and turbulent flows when coupled to heat transfer. The CFD Module includes ready-made multiphysics interfaces for nonisothermal and conjugate heat transfer. The module can combine arbitrary multiphysics couplings to define weakly compressible flows, i.e. flows with Mach numbers below 0.3.**Compressible Flow:**The CFD Module is also able to model compressible fluids for Mach numbers greater than 0.3, where temperature variations caused by heat transfer, compression work, or work done by friction forces, result in significant compressibility effects, like shocks, for instance. The built-in adaptive meshing capabilities within COMSOL Multiphysics help greatly with resolving shock waves and areas of great change in the fluid flow profile.**Two-Phase Flow:**Physics interfaces and supporting equations are at your disposal for modeling two-phase flow. When this includes tracking the moving interfaces separating two or more immiscible fluids, the CFD Module utilizes the Phase Field and Level Set methods.

The CFD Module also includes physics interfaces for dispersed two-phase fluid flow models for describing flows that contain suspensions of many particles, droplets, or bubbles through the Bubbly Flow, Mixture Model, and Euler-Euler Model methods. The latter method handles high concentrations of bubbles that collide frequently and contain significant variations in relative velocity between the phases. The Heat Transfer Module also includes interfaces for the modeling of condensation and humid air, where phase changes are described using built-in step functions in COMSOL Multiphysics.**Porous Media Flow:**With the CFD Module, you can also model the transport of single-phase and two-phase fluids in porous media, by utilizing Darcy's Law and Brinkman's extension to Darcy's Law. Darcy's Law is appropriate for porous media where the pores are small enough to negate viscous effects, so that flow is driven by a pressure difference, while the Brinkman equations include terms accounting for viscous effects. An internal condition also exists that allows for modeling the interface between free channel fluid flow and the porous media.**Rotating Machinery:**The Rotating Machinery interfaces include modeling tools to describe rotating parts that dynamically change the geometry, such as the vanes in a mixer or fins in a propeller rotating in a fluid domain. There is also a Frozen Rotor interface that approximates the rotation by including additional terms in the fluid flow equations instead of changing the geometry during the simulation. Using far less computational resources than solving for the actual rotation, this physics interface adds centrifugal or coriolis forces to the formulation of the stationary Navier-Stokes equations, and provides good approximations for modeling applications like turbines, centrifugal separators, and mixers. An interface for Swirl Flow is also available for modeling rotating flows. In this physics interface, an out-of-plane swirl velocity component is included for axisymmetric models yielding a three-dimensional velocity vector defined in a 2D geometry, which also reduces computational requirements compared to full 3D modeling.**Thin-Film Flow:**A specialized physics interface is included in the CFD Module to model the flow of liquids or gases confined in thin layers between two surfaces, or on the one surface, for example to model lubrication.**Non-Newtonian Flow:**The CFD Module includes the Carreau and Power-Law models, but also allows you to define your own equations or bring in external data to describe the viscosity and shear rate of polymer and other non-Newtonian fluids. You may, for example, define viscoelastic models in this way. Step functions are built into COMSOL Multiphysics, and can be utilized for modeling large or sudden changes in the fluid properties, for example to describe Bingham fluids.**Flow Through Thin Screens:**Modeling processes that include perforated plates, grilles, and wire-gauzes are made easier using the built-in Thin Screens feature. This includes correlations for refraction and resistance coefficients that consider the effects of flow through a screen, and before and after a screen for laminar or turbulent flow.**Fluid Flow and Heat Transfer:**The CFD Module includes a Conjugate Heat Transfer interface for describing fully coupled heat transfer in solids and fluids, including laminar and turbulent fluid flow. By default, this solves for nonisothermal flow and can be coupled to any other physics interface that includes temperature, such as interfaces for surface-to-surface radiation in the Heat Transfer Module, Joule heating, and heat of reactions in Chemical Species Transport interfaces. In addition, physics interfaces for heat transfer in porous media combine the conduction in the solid matrix to the conduction and convection in the fluid phase while accounting for the tortuous path taken by the fluid, and the heat dispersion this entails.**Reacting Flow:**A specialized interface that couples both laminar and turbulent flow to the transport of chemical species in dilute and concentrated solutions is included, and you can couple it with interfaces that describe chemical reaction from the Chemical Reaction Engineering Module

In addition to the built-in formulations, you can also define expressions that are arbitrary functions of the modeled variables, and type them into the edit fields for introducing source or sink terms, for describing material properties, or for defining boundary conditions. For example, you may want to define physical properties using your own functions to describe the influence of composition, temperature, shear rate, or any other model variable. You may also want to define boundary conditions and source or sink terms that are functions of the modeled variables or that couple to other physics. You can do this directly in the GUI, without the need for introducing cumbersome user subroutines. The ability to define arbitrary equations of the modeled variables, such as averaging over domain cross sections or in domain control volumes, also allows for the characterization of fluid flow through using different representations of the Reynolds, Mach, and Grashof numbers.

In a further step to adding source or sink terms and defining physical properties as functions, you can also manipulate the underlying equations in the previously described physics interfaces to modify the description of the flow and to create even more non-standard couplings to other physics interfaces.

Many of the physics interfaces also support sophisticated descriptions of boundary conditions. Apart from specifying Slip and No Slip boundary conditions, you can also set up walls to simulate Sliding and Moving Wall conditions, as well as walls that are Leaking and, even, Open, where the fluid is assumed to make its own free boundary. Wall functions and their relevant tuning parameters can be defined for turbulent models. For inlets and outlets, a velocity or a velocity profile can be configured along with Pressure, Stresses, or Mass Flow conditions, as well as a Periodic Flow boundary condition that links the outlet flow from one boundary to the inlet of another. Periodic boundary conditions useful when modeling a unit cell in a geometry consisting of an assembly of repetitive unit cells.

Flow is an integral part of many different processes and applications, and must be understood and optimized often with respect to how it affects other processes. The effective cooling of a computer’s hard drive, the dispersion of energy within the damping film of an accelerometer, and the transport of species through the different parts of a chemical reactor are examples where fluid flow is a contributor to a process described by other physics. Yet, in reality, the heat emanating from the electronic devices affects the fluid's density. The accelerometer's elasticity imposes an oscillation on the flow, and the reactions change chemical composition and potentially the fluid flows' driving pressure. This means that you must also include their effects for a completely accurate description of the overall process.

COMSOL Multiphysics and the CFD Module help in describing such processes through the seamless coupling of all involved physics, and through allowing unhindered access to the model equations directly in the GUI. Also at your disposal are the two-way coupled fluid structure interaction (FSI) formulations. These allow you to model scenarios where the fluid deforms a structure, and where this structure's reaction to its deformation in turn influences the fluid flow. All the physics interfaces in the CFD Module can be coupled with any of the other modules in the COMSOL Product Suite to provide the standard platform for applications where computational fluid dynamics need to be considered.

COMSOL also provides modules that model flow in alternate ways to the CFD Module, but which can still be easily coupled to utilize the benefits from both. One example of this is the Pipe Flow Module, which models fully developed flow in 2D and 3D piping networks using edge elements, with one tangential average velocity component along the edges, for describing the pipe sections. This allows you to model flow in a pipe network connected to tanks in a process, but avoid meshing the cross section of the pipes in the network, which would result in large 3D meshes. COMSOL contains a feature that seamlessly allows the mapping of data from edges, to surfaces, and volumes, and vice versa, to connect pipe networks to fully meshed 2D or 3D geometries. In this way, you can consider the computational fluid dynamics properties of a certain unit within a whole network of piping, and adjust the operating conditions of both in connection to each other.

Since all physics are modeled using the same, standard graphical user interface and workflow, CFD engineers can easily communicate with other engineers analyzing different characteristics of the same component or process, such as structural, electrical, or chemical properties. All you have to do is send over the file, switch off the physics not being investigated, add another physics interface or two, and continue modeling. And, of course, couple these new physics interfaces to the one describing fluid flow for a full multiphysics simulation of the component or process.

Simulating computational fluid dynamics in equipment or processes is often a workflow where you approach a final, accurate solution in steps. The CFD Module contains many different tools, features, settings, and interfaces to assist you through all the steps of your workflow.

The CAD Import Module or one of the LiveLink™ products assists you in bringing in the geometry of your part, component, or process to be simulated from a third-party CAD software. These products allow you to subsequently manipulate your geometry to help reduce small features and artifacts that may not be important for the flow, but that complicate the meshing of CFD simulations.

Once you have your 3D CAD design within the CFD Module, you may not want to immediately launch yourself into performing 3D simulations. COMSOL Multiphysics supports the ability to create a 2D modeling workspace from 3D geometries. By working on a 2D geometry, such as a representative cross section, you will be able to familiarize yourself with a number of the parameters in your simulation. Without using the large computational resources that a 3D model would require, you can:

- Investigate the effect fluid properties have on the overall simulation
- Decide what the appropriate turbulence model to use is
- Determine the placement of appropriate meshing and boundary layer meshing
- Select the solvers and settings to use
- Study the effects of multiphysics couplings on the fluid flow
- Estimate the accuracy you may expect from a 3D model

With a greater understanding of your system, you can then perform the full 3D simulation using the knowledge and optimized settings gained from the 2D model. This feature is also especially useful for treating 3D CAD designs that are symmetric or axisymmetric, avoiding 3D modeling altogether, and reducing the computational requirements substantially.

Meshing is often a critical step in modeling computational fluid dynamics in devices or processes. The mesh must be good enough to provide accuracy, but not too fine so as to drain computational resources. COMSOL Multiphysics provides many different tools to ensure a good mesh for fluid flow simulations. This includes creating unstructured, structured and swept meshes, which allow for flexibility in considering the modeling domain's geometric dimensions and their ratio, and the effects on the flow's directions. The CFD Module also utilizes boundary layer meshing to insert structured layers of mesh along boundaries such as walls, and integrate them into surrounding structured or unstructured meshes to become an overall hybrid mesh.

The CFD Module makes use of most of the linear, nonlinear, time-dependent, and parametric solvers found within COMSOL Multiphysics. This includes direct solvers for solving 2D and small 3D models, which have good abilities to easily converge, and iterative solvers for larger or more complex models. Preconditioning and multigrid solvers are available to work in collaboration with other solvers to ensure solutions. Advanced solver functionality, such as the inclusion of crosswind and streamline diffusion, and smoothing methods, are available, and their values can be fine-tuned along with most of the other solver settings. The CFD Module also utilizes elements of different orders in one and the same simulation, and may apply lower order elements to solve one variable, such as pressure, and higher order elements to solve the other variables.

The solver scheme also allows for better approximations of initial values for a solving process. This includes setting up solver schemes that will solve for an easier description of flow, such as the laminar flow field within a certain modeling domain, and apply this solution as the initial guess to a turbulent flow description. A solution using the Frozen Rotor interface can be used as the initial guess to a full simulation of the rotating modeling domain, saving you a lot of computational resources.

The CFD Module calculates properties intrinsic to fluid flow, such as: flow patterns; pressure losses; forces on objects subjected to flow, drag, and lift; temperature distribution; and variations in fluid composition in a system. Moreover, it provides qualitative postprocessing involving surface, streamline, ribbon, arrow, and qualitative particle tracing plots as well as animations. Data from all parameters and variables in the underlying equations, and extra terms are accessible to be extracted and plotted against any other parameter or variable. This includes postprocessing derived values, like drag and lift coefficients. By including and coupling physics from the Particle Tracing Module in the solving of your CFD applications, you can consider the effects of particles on both the flow itself (Lagrange-Euler), and on each other, through collisions and their own momentum.