Introduction to Cfd Module

INTRODUCTION TO

CFD Module

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Part number: CM021302

I n t r o d u c t i o n t o t h e C F D M o d u l e 19982014 COMSOL

Protected by U.S. Patents listed on www.comsol.com/patents, and U.S. Patents 7,519,518; 7,596,474; 7,623,991; and 8,457,932. Patents pending.

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Version: October 2014 COMSOL 5.0

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Aspects of CFD Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

The CFD Module Physics Interfaces . . . . . . . . . . . . . . . . . . . . . . 11

Physi

The Mo

Tutoria

Mode

Dom

Resu

Tutoria

Mode

Dom

Note

Resu

Refer | 3

cs Interface Guide by Space Dimension and Study Type . . . 18

del Libraries Window . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

l ExampleBackstep . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

l Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

ain Equations and Boundary Conditions . . . . . . . . . . . . . . . . . 25

lts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

l ExampleWater Purification Reactor . . . . . . . . . . . . . 34

l Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

ain Equations and Boundary Conditions . . . . . . . . . . . . . . . . . 35

s About the Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . 36

lts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

ence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Introduction

The CFD Module is used by engineers and scientists to understand, predict, and design for fluid flow in closed and open systems. At a given cost, these types of simulations typically lead to new and better products and improved operations of devices and processes compared to purely empirical studies involving fluid flow. As a part of an investigation, simulations give accurate estimates of flow patterns, pressure losses, forces on submerged objects, temperature distributions, and variations in fl

Figure 1: Flow ribbyields the flow andand validation of t

The CFD Motime-dependenFormulations physics interfaproblems. Thequantities, sucThere are diffefor example, lareacting flows.Introduction | 5

uid composition within a system.

ons and velocity field magnitudes from a simulation of an Ahmed body. The simulation pressure fields and calculates the drag coefficient as a benchmark for the verification urbulence models.

dules general capabilities include modeling stationary and t fluid flow problems in two- and three-dimensional spaces.

for different types of flow are predefined in a number of Fluid Flow ces, which allow you to set up and solve a variety of fluid-flow se physics interfaces define a fluid-flow problem using physical h as velocity and pressure, and physical properties, such as viscosity. rent Fluid Flow physics interfaces that cover a wide range of flows, minar and turbulent single-phase, multiphase, non-isothermal, and

6 | Introduction

The physics interfaces build on conservation laws for momentum, mass, and energy. These laws are expressed in terms of partial differential equations, which are solved by the module together with the specified initial and boundary conditions. The equations are solved using stabilized finite element formulations for fluid flow, in combination with damped Newton methods and, for time-dependent problems, different time-dependent solver algorithms. The results are presented in the graphics window through predefined plots relevant for CFD, expressions of physical quantities that you can freely define, and derived tabulated quantities (for example, average pressure on a surface or drag coefficients) oThe workflowfollowing stepof flow, defineselect a solver,COMSOL Deautomatically physics interfaThe CFD Motheir different types of flow. models for praFluid Flow phon how to accThis introductbuilding. It coFluid Flow phtutorial exampPurification R

Aspects of CThe physical nnumbers suchnumber. A greanalyzing thesThe Reynoldsviscous (internnumber, the inreversed bounimmediate. Asmore confinedbtained from a simulation. in the CFD Module is quite straightforward and is described by the s: define the geometry, select the fluid to be modeled, select the type boundary and initial conditions, define the finite element mesh, and visualize the results. All these steps are accessed from the sktop. The mesh and solver steps are usually carried out using default settings that are tuned for each specific Fluid Flow ce.dules model library describes the Fluid Flow physics interfaces and features through tutorial and benchmark examples for the different Here you find models of industrial equipment and devices, tutorial ctice, and benchmark models for verification and validation of the

ysics interfaces. Go to The Model Libraries Window for information ess these resources.ion is intended to give you an accelerated start in CFD model ntains examples of the typical use of the module, a list of all the ysics interfaces including a short description of each, and two les, Tutorial ExampleBackstep and Tutorial ExampleWater eactor, to introduce the workflow.

FD Simulationsature of a flow field may be characterized by a set of dimensionless as the Reynolds number, the Mach number, and the Grashof at deal of information about the flow field can be gained by e numbers. number, for example, expresses the ratio of the inertia forces to the al friction) forces. For vanishingly small values of the Reynolds ertia forces are negligible and the flow is reversible, in the sense that dary conditions lead to reversed flow. The energy dissipation is the Reynolds number increases, viscous effects become more and to boundaries, internal shear layers, and wakes. The size and other

characteristics of such regions are determined by the Reynolds number. Eventually, for very large values of the Reynolds number, the flow becomes fully turbulent. In contrast to laminar flow at high Reynolds numbers, viscous dissipation is active everywhere in a turbulent flow field but acts on very small flow structures. The energy is transfered from the large-scale flow structures to the small-scale flow structures through a cascade of eddies. Due to the requirement of resolving all these flow scales, direct numerical simulation of industrially relevant turbulent flows is currently not a feasible approach. Instead, turbulence models are applied when analyzing these flows. For very small values of the Reynolds number the CFD ModLaminar FlowThe Mach numsound of the flof compressiblrarefaction waHigh Mach NTemperature vby friction forcthermal convemomentum eqsquare of the Rthe Grashof nuone). For nonare available. You can use thmodel movingphysics interfamany particlesinterface is ablwell as transienratios of the preacting flow aPorous MediaContrary to exwhere measuregives the big flow and pressdesign.Figure 2 showof the panel, cforces that woIntroduction | 7

ule offers the Creeping Flow interface; for intermediate values, the interface; and for large values, the Turbulent Flow interfaces.

ber expresses the ratio of the speed of the fluid to the speed of owing medium. This dimensionless number measures the relevance e effects in the flow field, predicting occurrences of shock waves and ves. For Mach numbers greater than 0.3, the laminar and turbulent umber Flow interfaces are available. ariations caused by heat transfer, compression work, or work done es result in an inhomogeneous density field which may trigger

ction. The significance of thermally induced buoyancy forces in the uation is characterized by the ratio of the Grashof number to the eynolds number (for Reynolds numbers greater than one), or of mber to the Reynolds number (for Reynolds numbers smaller than

-vanishing values of this ratio, the Non-Isothermal Flow interfaces

e Two-Phase Flow interfaces in the Multiphase Flow branch to , deformable interfaces separating two different fluids. The other ces in this branch are mainly intended for modeling suspensions of , droplets or bubbles. Among the latter, the Euler-Euler Model e to handle high concentration levels with frequent collisions, as ts in the relative velocity between the phases (that is, non-vanishing

article relaxation time to the macroscopic flow time-scale). For nd flow in porous media, the Chemical Species Transport, and

and Subsurface Flow branches are available.perimental analyses, which are most often performed in a laboratory ments are limited to a small number of points, a CFD simulation picture view of the flow field. A qualitative interpretation of the ure fields is usually the first step toward creating or improving a

s the flow field around a solar panel. The presence of a wake in front aused by another panel in the solar power plant, may induce lift uld not be present if the panel were analyzed alone.

8 | Introduction

Three-dimensional graphics such as surface, streamline, ribbon, arrow, and particle-tracing plots, as well as animations that include any combination of the aforementioned features, are examples of tools you can use for qualitative studies.

Figure 2: Turbulen

In addition toCFD Module such as the aveof bodies subjIn Figure 3 anmedical devicedamage bloodcontrolling tht fluid flow around a solar panel solved using the CFD Module.

the qualitative big picture view, simulations performed with the give accurate quantitative estimates of properties of the flow field, rage flow at a given pressure difference, the drag and lift coefficients ected to a flow, or the air quality in a ventilated room.d Figure 4, the pressure losses are estimated for a nozzle used in s. The shear stresses and fluid forces in the nozzle system may cells in medical equipment, and must be accounted for when e flow.

Figure 3: Pressure

Figure 4: Pressure Introduction | 9

field and flow field in a model of a nozzle relevant for designs in medical applications.

difference between inlet and outlet at various average flow rates through the nozzle.

10 | Introductio

The CFD Module has a vast range of tools for evaluating quantitative results. For example, it comes with built-in functionality for evaluating surface and volume averages, maximum and minimum values, and derived values (functions and expressions of the solution), as well as for generating tables and x-y plots. Derived values such as drag and lift coefficients and other values relevant for CFD are predefined in the module.Qualitative studies typically form the basis for understanding, which in turn can spark new ideas. These ideas can then lead to significant improvements to products and processes, often in quantum leaps. Quantitative studies, on the other hand, form the basisproducts and pn

for optimization and control, which can also greatly improve rocesses but usually do so through a series of many smaller steps.

The CFD Module Physics Interfaces

The Fluid Flow physics interfaces in this module are based on the laws for conservation of momentum, mass, and energy in fluids. The different flow models contain different combinations and formulations of the conservation laws that apply to the physics of the flow field. These laws of physics are translated into partial differential equations and are solved together with the specified initial and boundary conditions.A physics interthe fluid propeconstraints. Eathe conservatiogeometric enticomponents), Figure 5 showSettings windoProperties 1 nselected geomto the Materiadynamic viscodefined by thequantities, sucadds the bounThe CFD Module Physics Interfaces | 11

face defines a number of features. These features are used to specify rties, boundary conditions, initial conditions, and possible ch feature represents an operation describing a term or condition in n equations. Such a term or condition can be defined on a

ty of the component, such as a domain, boundary, edge (for 3D or point.s the Model Builder, including a Laminar Flow interface, and the w for the selected Fluid Properties 1 feature node. The Fluid

ode adds the marked terms to the component equations in a etric domain. Furthermore, the Fluid Properties 1 feature may link ls feature node to obtain physical properties such as density and sity, in this case the fluid properties of water. The fluid properties, Water, liquid material, can be functions of the modeled physical h as pressure and temperature. In the same way, the Wall 1 node dary conditions at the walls of the fluid domain.

12 | The CFD M

Figure 5: The ModProperties for the component equatwith a dotted line.fluid properties.

The CFD Modifferent typesinterfaces for rfor heat transfebranch.odule Physics Interfaces

el Builder including a Laminar Flow interface (left), and the Settings window for Fluid selected feature node (right). The Equation section in the Settings window shows the ions and the terms added by the Fluid Properties 1. The added terms are underlined The arrows also explain the link between the Materials node and the values for the

dule includes a large number of Fluid Flow physics interfaces for of flow. It also includes Chemical Species Transport physics eacting flows in multicomponent solutions, and physics interfaces r in solids, fluids, and porous media found under the Heat Transfer

Figure 6 shows the Fluid Flow interfaces as they are displayed when you add a physics interface (see also Physics Interface Guide by Space Dimension and Study Type for further information). A short description of the physics interfaces follows.

Figure 6: The phys

SINGLE-PHASEThe Laminar Fintermediate RThe CFD Module Physics Interfaces | 13

ics interfaces for the CFD Module as shown in the Model Wizard.

FLOWlow interface ( ) is primarily applied to flows at low to eynolds numbers. This physics interface solves the Navier-Stokes

14 | The CFD M

equations, for incompressible and weakly compressible flows (up to Mach 0.3). The Laminar Flow interface also allows for simulation of non-Newtonian flow.The physics interfaces under the Turbulent Flow branch ( ) model flows at high Reynolds numbers. These physics interfaces solve the Reynolds-averaged Navier-Stokes (RANS) equations for the averaged velocity and pressure fields. The turbulent flow physics interfaces provide different options for modeling the turbulent viscosity. There are several turbulence models availabletwo algebraic turbulence models, the Algebraic yPlus and L-VEL models, and five transport-equation models, including a standard k- model, a k- model, an SST (Shear Stress TSpalart-Allmar(Mach < 0.3) The Algebraicviscosity modenearest wall. Finternal flows,models are comaccurate than turbulence mois a good comCPU time). Tgives more accwalls. HoweveSST model commodel, makingReynolds numespecially closeaerodynamic aused in other aresolution is nnumber k- mprovided by thThe Creepingvery low Reynapplicable whemicrofluidics dThe Rotating where one or propellers. Th(Mach < 0.3),using the stanodule Physics Interfaces

ransport) model, a low Reynolds number k- model and the as model. Similarly to the Laminar Flow interface, compressibility is selected by default. yPlus and L-VEL turbulence models are so-called enhanced ls. A turbulent viscosity is computed from the local distance to the or this reason, the algebraic turbulence models are best suited for such as in electronic cooling applications. Algebraic turbulence

putationally economical, and more robust but, in general, less transport-equation models. Among the transport-equation dels, the standard k- model is the most widely used since it often promise between accuracy and computational cost (memory and he k- model is an alternative to the standard k- model and often urate results, especially in recirculation regions and close to solid r, the k- model is also less robust than the standard k- model. The

bines the robustness of the k- model with the accuracy of the k- it applicable to a wide variety of turbulent flows. The Low ber k- model is more accurate than the standard k- model, to walls. The Spalart-Allmaras model is specifically designed for pplications, such as flow around wing profiles, but is also widely pplications due to its high robustness and decent accuracy. Higher

eeded in the near-wall region for the SST model, the Low Reynolds odel, and the Spalart-Allmaras model. Thus, the better accuracy ese models comes at a higher computational cost.

Flow interface ( ) approximates the Navier-Stokes equations for olds numbers. This is often referred to as Stokes flow and is n viscous effects are dominant, such as in very small channels or evices.

Machinery interfaces ( ) are applicable to fluid-flow problems more of the boundaries rotate, for example in mixers and around e physics interfaces support incompressible and compressible laminar Newtonian and non-Newtonian flow, and turbulent flow dard k- model.

MULTIPHASE FLOWThe physics interfaces under the Bubbly Flow branch ( ) model two-phase flow where the fluids form a gas-liquid mixture, and the content of the gas is less than 10%. There is support for both laminar flow and turbulent flow using an extended version of the k- turbulence model that accounts for bubble-induced turbulence. For laminar flow, the physics interface supports non-Newtonian liquids. The Bubbly Flow interfaces also allow for mass transfer between the two phases.The physics interfaces under the Mixture Model branch ( ) are similar to the Bubbly Flow interfaces but assume that the dispersed phase consists of solid particles or liqsupport for boThe Mixture MThe Euler-Eulsame cases as tlow concentrainterface can hcase of solid pexample, fluidThe Two-PhasField interfacefluid-fluid intemethod and thphysics interfa(Mach < 0.3) both fluids canflow, using the

NONISOTHERThe Non-Isotmodel flow at temperature anconvection, winterface for ware set up autoThe Non-IsotReynolds-Averfluids and in soAlgebraic yPluan SST modelmodel.The CFD Module Physics Interfaces | 15

uid droplets. The continuous phase has to be a liquid. There is th laminar flow and turbulent flow using the k- turbulence model. odel interfaces also allow for mass transfer between the two phases.

er Model interface ( ) for two-phase flow is able to handle the he Bubbly Flow and Mixture Model interfaces, but is not limited to tions of the dispersed phase. In addition, the Euler-Euler Model andle large differences in density between the phases, such as the articles in air. This makes the model suitable for simulations of, for ized beds.e Flow, Level Set interface ( ) and the Two-Phase Flow, Phase ( ) are both primarily applied to model two fluids separated by a rface. The moving interface is tracked in detail using the level-set e phase-field method, respectively. Similarly to other Fluid Flow

ces, these physics interfaces support both compressible and incompressible flows. They support laminar flow where one or be non-Newtonian. The physics interfaces also support turbulent standard k- turbulence model, and Stokes flow.

MAL FLOWhermal Flow, Laminar Flow interface ( ) is primarily applied to low to intermediate Reynolds numbers in environments where the d flow fields have to be coupled. A typical example is natural

here thermal buoyancy forces drive the flow. This is a multiphysics hich the component couplings between fluid flow and heat transfer matically.

hermal Flow, Turbulent Flow interfaces ( ) solve the aged Navier-Stokes (RANS) equations coupled to heat transfer in lids. There is support for all the fluid-flow turbulence models the s model, the L-VEL model, the standard k- model, a k- model, , a low Reynolds number k- model, and the Spalart-Allmaras

16 | The CFD M

The Conjugate Heat Transfer interfaces ( ) are also included with the CFD Module and are almost identical to the Non-Isothermal Flow interfaces. They only differ in the default domain feature selected -Heat transfer in Solids instead of Fluid.

HIGH MACH NUMBER FLOWThe High Mach Number Flow, Laminar Flow interface ( ) solves the continuity, momentum, and energy equations for fully compressible laminar flow. This physics interface is typically used to model low-pressure systems, for which the Mach numThe High Macontinuity, mofully compressturbulence quturbulence mo

POROUS MEDThe Brinkmanwhere the influboth the StokForchheimer dflow can be eitless than 0.3.The Darcys Lmedia for casesmall.The Two-Phasfor each fluid pa capillary expporous media.The Free and Popen channelscatalytic conve

REACTING FLThe Laminar Ffunctionality ointerfaces. Maa single physicodule Physics Interfaces

ber can be large but the flow remains laminar.ch Number Flow, Turbulent Flow interfaces ( ) solve the mentum and energy equations for the averaged flow variables in ible turbulent flow, coupled to transport equations for the antities. There are two versions: one that couples to the k- del and one that couples to the Spalart-Allmaras turbulence model.

IA AND SUBSURFACE FLOW Equations interface ( ) models flow through porous media ence of shear stresses is significant. This physics interface supports

es-Brinkman formulation, suitable for very low flow velocities, and rag, which is used to account for effects at higher velocities. The her incompressible or compressible, provided the Mach number is

aw interface ( ) models relatively slow flows through porous s where the effects of shear stresses perpendicular to the flow are

e Darcys Law interface ( ) sets up two Darcy-law equations, one hase in the porous medium. It couples the two, for example using

ression. It is tailored to model effects such as moisture transport in

orous Media Flow interface ( ) models porous media containing connected to the porous media, such as in fixed-bed reactors and rters.

OW

low interface ( ) under the Reacting Flow branch combines the f the Single-Phase Flow and Transport of Concentrated Species

ss and momentum transport in a reacting fluid can be modeled from s interface, with the couplings between the velocity field and

mixture density set up automatically. This physics interface is applicable to flow in the laminar regime.The Turbulent Flow physics interfaces ( ) under the Reacting Flow branch apply the Reynolds-Averaged Navier-Stokes (RANS) equations together with the functionality in the Transport of Concentrated Species interface. They model mass and momentum transport in turbulent reacting fluid flow. The supported RANS models comprise the standard k- model, a k- model, and a low Reynolds number k- model.

REACTING FLThe Reacting models dilutedThis is a multipfield is set up amatrix can be The Reacting interface ( )mixtures transcouplings betwaddition, effecthe porosity.

THIN-FILM FLThe Thin Filmin a thin layer interfaces complanar structurwhich means tdomain level. Tcylinders is an The CFD Module Physics Interfaces | 17

OW IN POROUS MEDIAFlow in Porous Media, Transport of Diluted Species interface ( ) reacting mixtures transported by free and/or porous media flow. hysics interface for which the component coupling for the velocity

utomatically. In addition, effective diffusion coefficients in a porous calculated from the porosity.Flow in Porous Media, Transport of Concentrated Species is a multiphysics interface that models concentrated reacting ported by free and/or porous media flow. The component een velocity field and mixture density are set up automatically. In

tive diffusion coefficients in a porous matrix can be calculated from

OW

Flow interfaces ( ) model the flow of liquids or gases confined on a surface. Applying equations defined on a surface, these physics pute the average velocity and pressure across the layer in narrow es. The physics interfaces are thus boundary physics interfaces, hat the boundary level is the highest level; they do not have a he simulation of the flow of a lubrication oil between two rotating

example of a possible application for this physics interface.

18 | The CFD M

Physics Interface Guide by Space Dimension and Study Type

PHYSICS INTERFACE ICON TAG SPACE DIMENSION

AVAILABLE PRESET STUDY TYPE

Chemical Species Transport

Transport of Diluted Species(1)

tds all dimensions stationary; time dependent

Transport of Concentrated S

Reactin

Laminar Flow

Turbulen

Turbulent Flo

Turbulent Flo

Turbulent Flo

Turbulent FloRe k-

Reactin

Transport of Diluted Spec

Transport of ConcentratedSpecies

Fluid Flow

Single-

Laminar Flowodule Physics Interfaces

peciestcs all dimensions stationary; time dependent

g Flow

rspf 3D, 2D, 2D axisymmetric

stationary; time dependent

t Flow

w, k- rspf 3D, 2D, 2D axisymmetric


w, k- rspf 3D, 2D, 2D axisymmetric


w, SST rspf 3D, 2D, 2D axisymmetric

stationary with initialization; transient with initialization

w, Low rspf 3D, 2D, 2D axisymmetric


g Flow in Porous Media

iesrfds 3D, 2D, 2D

axisymmetricstationary; time dependent

rfcs 3D, 2D, 2D

axisymmetricstationary; time dependent

Phase Flow

(1) spf 3D, 2D, 2D axisymmetric


Turbulent Flow

Turbulent Flow, Algebraic yPlus

spf 3D, 2D, 2D axisymmetric


Turbulent Flow, L-VEL



Turbulent Flo

Turbulent Flo

Turbulent Flo

Turbulent FloRe k-

Turbulent FloSpalart-Allma

Creeping Flow

Rotating

Rotating MacLaminar Flow

Rotating MacTurbulent Flo

Thin-Fi

Thin-Film Flo

Thin-Film FloDomain

Thin-Film FloEdge


AVAILABLE PRESET STUDY TYPEThe CFD Module Physics Interfaces | 19

w, k- spf 3D, 2D, 2D axisymmetric


w, k- spf 3D, 2D, 2D axisymmetric


w, SST spf 3D, 2D, 2D axisymmetric


w, Low spf 3D, 2D, 2D axisymmetric


w, ras





Machinery, Fluid Flow

hinery, rmspf 3D, 2D frozen rotor ; time dependent

hinery, w, k-

rmspf 3D, 2D frozen rotor ; time dependent

lm Flow

w, Shell tffs 3D stationary; time dependent; frequency domain; eigenfrequency

w, tff 2D stationary; time dependent; frequency domain; eigenfrequency

w, tffs 2D and 2D axisymmetric

stationary; time dependent; frequency domain; eigenfrequency

20 | The CFD M

Multiphase Flow

Bubbly Flow

Laminar Bubbly Flow

bf 3D, 2D, 2D axisymmetric


Turbulent BuFlow

Mixture

Mixture ModLaminar Flow

Mixture ModTurbulent Flo

Euler-Eu

Euler-Euler MLaminar Flow

Two-Pha

Laminar TwoFlow, Level S

Turbulent Two-Phase FLevel Set

Two-Pha

Laminar TwoFlow, Phase F

Turbulent Two-Phase FPhase Field


AVAILABLE PRESET STUDY TYPEodule Physics Interfaces

bbly bf 3D, 2D, 2D axisymmetric


Model

el, mm 3D, 2D, 2D axisymmetric


el, w

mm 3D, 2D, 2D axisymmetric


ler Model

odel, ee 3D, 2D, 2D axisymmetric


se Flow, Level Set

-Phase et

tpf 3D, 2D, 2D axisymmetric

transient with phase initialization

low, tpf 3D, 2D, 2D

axisymmetrictransient with phase initialization

se Flow, Phase Field

-Phase ield

tpf 3D, 2D, 2D axisymmetric

transient with phase initialization

low, tpf 3D, 2D, 2D

axisymmetrictransient with phase initialization

Porous Media and Subsurface Flow

Brinkman Equations br 3D, 2D, 2D axisymmetric


Darcys Law dl all dimensions stationary; time dependent

Two-Phase DLaw

Free and PorMedia Flow

Non-Iso

Laminar Flow

Turbulen

Turbulent FloAlgebraic yPl

Turbulent FloL-VEL

Turbulent Flok-(2)

Turbulent Flok-(2)

Turbulent FloSST(2)

Turbulent FloRe k-(2)


High M

Laminar Flow



arcys tpdl 3D, 2D, 2D axisymmetric


ous fp 3D, 2D, 2D axisymmetric


thermal Flow

2 3D, 2D, 2D axisymmetric


t Flow

w, us

3D, 2D, 2D axisymmetric


w, 3D, 2D, 2D axisymmetric








w, Low 3D, 2D, 2D axisymmetric


w, ras(2)



ach Number Flow

hmnf 3D, 2D, 2D axisymmetric


22 | The CFD M

Turbulent Flow, k- hmnf 3D, 2D, 2D axisymmetric


Turbulent Flow, Spalart-Allmaras

hmnf 3D, 2D, 2D axisymmetric


Heat Transfer

Heat Transfer inFluids(1)

Heat Transfer in Media

Conjuga

Laminar Flow

Turbulen

Turbulent FloAlgebraic yPl

Turbulent FloL-VEL

Turbulent Flok-(2)

Turbulent Flok-(2)

Turbulent FloRe k-(2)

Turbulent FloSST(2)



AVAILABLE PRESET STUDY TYPEodule Physics Interfaces

ht all dimensions stationary; time dependent

Porous ht all dimensions stationary; time dependent

te Heat Transfer

(2) 3D, 2D, 2D axisymmetric


t Flow

w, us









w, Low 3D, 2D, 2D axisymmetric




w, ras(2)



Mathematics

Moving Interface

Level Set ls all dimensions transient with phase initialization

Phase Field

(1) This physics ifunctionality for

(2) This physics iphysics interface



pf all dimensions time dependent; transient with phase initialization

nterface is included with the core COMSOL package but has added this module.

nterface is a predefined multiphysics coupling that automatically adds all the s and coupling features required.

24 | The Mode

The Model Libraries Window

To open a CFD Module model library model, click Blank Model in the New screen. Then on the Model or Main toolbar click Model Libraries . In the Model Libraries window that opens, expand the CFD Module folder and browse or search the contents. Click Open Model to open the model in COMSOL Multiphysics or click Open PDF Document to read background about the model including the step-by-step inlibrary can hav Full MPH-

window theexceeds 25Myou positio

Compact Mand solutiosolutions foand to meshversionswupdate youwindow witthe Model LMPH-file isincludes a D

To check all aLibrary ( ) f(Mac and LinuThe rest of thiExampleBacand the Tutorproblem usingl Libraries Window

structions to build it. The MPH-files in the COMSOL model e two formatsFull MPH-files or Compact MPH-files.files, including all meshes and solutions. In the Model Libraries se models appear with the icon. If the MPH-files size B, a tip with the text Large file and the file size appears when

n the cursor at the models node in the Model Libraries tree.PH-files with all settings for the model but without built meshes

n data to save space on the DVD (a few MPH-files have no r other reasons). You can open these models to study the settings and re-solve the models. It is also possible to download the full ith meshes and solutionsof most of these models when you

r model library. These models appear in the Model Libraries h the icon. If you position the cursor at a compact model in ibraries window, a No solutions stored message appears. If a full

available for download, the corresponding nodes context menu ownload Full Model item ( ).

vailable Model Libraries updates, select Update COMSOL Model rom the File>Help menu (Windows users) or from the Help menu x users). s guide uses two models from the model library. The Tutorial kstep, which starts on the next page, solves a laminar flow problem

ial ExampleWater Purification Reactor examines a turbulent flow the Turbulent Flow, k- interface.

Tutorial ExampleBackstep

This tutorial model solves the incompressible Navier-Stokes equations in a backstep geometry. A characteristic feature of fluid flow in geometries of this kind is the recirculation region that forms where the flow exits the narrow inlet region. The model demonstrates the modeling procedure for laminar flows in the CFD Module.

Model GeomThe model coDue to symme

Figure 7: The mod

Domain EquThe flow in thinterface.The inlet flowinlet boundaryfully developed

Wall

S

InletTutorial ExampleBackstep | 25

etrynsists of a pipe connected to a block-shaped duct (see Figure 7). try, it is sufficient to model one eighth of the full geometry.

el geometry showing the symmetry.

ations and Boundary Conditionsis system is laminar and you should therefore use the Laminar Flow

is fully developed laminar flow, described by the corresponding condition. This boundary condition computes the flow profile for laminar flow in a channel of arbitrary cross section. The boundary

Wall

ymmetry

Symmetry

Outlet

26 | Tutorial Ex

condition at the outlet sets a constant relative pressure. Furthermore, the vertical and inclined boundaries along the length of the geometry are symmetry boundaries. All other boundaries are solid walls described by a no-slip boundary condition.

ResultsFigure 8 shows a combined surface and arrow plot of the flow velocity. This plot does not reveal the recirculation region in the duct immediately beyond the inlet pipes end. For this purpose, a streamline plot is more useful, as shown in Figure 9.

Figure 8: The veloampleBackstep

city field in the backstep geometry.

Figure 9: The recir

The followingplots.

Model Wi

The first step tinterface and sLaminar FlowNote: These inwith minor dif

1 To open thethe softwareCOMSOL mthe Model WThe Model COMSOL iNew fro

The next wiTutorial ExampleBackstep | 27

culation region visualized using a velocity streamline plot.

instructions show how to formulate, solve, and reproduce these

zard

o build a model is to open COMSOL, then select the physics pecify the type of analysis you want to doin this case, a stationary, analysis.structions are for the user interface on Windows but also apply, ferences, to Linux and Mac.

software, double-click the COMSOL icon on the desktop. When opens, you can choose to use the Model Wizard to create a new odel or Blank Model to create one manually. For this tutorial, click izard button.

Wizard guides you through the first steps of setting up a model. If s already open, you can start the Model Wizard by selecting m the File menu and then clicking Model Wizard .

ndow lets you select the dimension of the modeling space.

28 | Tutorial Ex

2 In the Space Dimension window click the 3D button .3 In the Select Physics tree under Fluid Flow>Single-Phase Flow click

Laminar Flow (spf) .4 Click Add and then click the Study button .5 In the tree under Preset Studies, click Stationary . 6 Click the Done button .

Global De

The first task iparameter to r1 On the ModNote: On Linunear the top o

2 In the Settintable enter t- In the Na- In the Ex- In the De

Geometry

You can build a file containinconvenience.Note: The locainstallation. Fomight be simil

1 On the GeoampleBackstep

f init ions - Parameters

s to define a parameter for the inlet velocity. Then you will use this un a parametric study.el toolbar click Parameters . x and Mac, the Model toolbar refers to the specific set of controls f the Desktop.

gs window for Parameters locate the Parameters section. In the he following settings:me text field, enter v0pression text field, enter 1[cm/s]scription text field, enter Inlet velocity

1

the backstep geometry from geometric primitives. Here, instead use g the sequence of geometry features that has been provided for

tion of the file used in this exercise varies based on your r example, if the installation is on your hard drive, the file path ar to C:\Program Files\COMSOL50\models\.

metry toolbar choose Insert Sequence .

2 Browse to the model library folder and double-click the file backstep_geom_sequence.mph.

3 Go to the Model toolbar and click Build All .

The geometry sequence is now inserted into your component and should look like the geometry in Figure 7.

Materials

1 On the Mod2 Go to the A

3 In the Add 4 On the Mod

The physical pthe domain se

Laminar F

Inlet 11 On the Phy2 Select BounTutorial ExampleBackstep | 29

el toolbar click Add Material .dd Material window. In the tree under Built-In click Water, liquid.

Material window, click Add to Component.el toolbar click Add Material again to close the window.

roperties are now available for the CFD simulation. This also defines ttings. The next step is to specify the boundary conditions.

low

sics toolbar click Boundaries and choose Inlet .dary 1, which represents the inlet.

30 | Tutorial Ex

3 Under Boundary Condition from the Boundary condition list, select Laminar inflow.

4 Under Laminar Inflow in the Uav text field, type v0 (which you defined as a Global Parameter).

Symmetry 11 On the Phy

Boundaries 2 Select Boun

Outlet 11 On the Phy

Boundaries The default

2 Go to the S3 On the Sett

Select the N

The sequence the figure. Th

All other bounampleBackstep

sics toolbar click and choose Symmetry .

daries 2 and 3 only.

sics toolbar click and choose Outlet .

outlet condition specifies a zero relative pressure.ettings window for Outlet. Select Boundary 7 only.ings window for Outlet, locate the Pressure Conditions section. ormal flow check box.

of nodes in the Model Builder under Laminar Flow should match e D in the upper left corner of a node means it is a default node.

daries now have the default wall condition.

Mesh 1

1 In the Model Builder click Mesh 1 .2 In the Settings window for Mesh locate the

Mesh Settings section. From the Element size list, choose Coarse.The physics-induced mesh automatically introduces a mesh that is a bit finer on the walls than th

3 Click the Bu

The figure belmesh using thmatches the fiTutorial ExampleBackstep | 31

e free stream mesh.ild All button .

ow shows the boundary layer mesh at the walls. Zoom in to the e zoom function on the Graphics toolbar to confirm that it gure.

32 | Tutorial Ex

Study 1

1 On the Model toolbar click Compute .When Compute is selected, COMSOL Multiphysics automatically chooses a suitable solver for the problem.

Results

Two plots are pressure conto

Velocity (spf)1 In the Mod2 Right-click 3 Go to the V4 On the Velo5 Go to the S

- Under Co- From the

6 Click the Zo

The plot in FiTo see the recampleBackstep

automatically created, one slice plot for the velocity and one ur plot on the wall.

el Builder under Results , expand the Velocity (spf) node.Slice 1 and choose Delete. Click Yes.elocity (spf) toolbar and click Surface .city (spf) toolbar, click Arrow Surface .ettings window for Arrow Surface.loring and Style from the Arrow length list, select Logarithmic.

Color list, select Yellow.

om Extents button on the Graphics window toolbar.

gure 8 on page 26 displays in the Graphics window.irculation effects, create a streamline plot of the velocity field.

3D Plot Group 31 On the Model toolbar click Add Plot Group and choose

3D Plot Group .2 Go to the 3D Plot Group 3 toolbar and click Streamline .3 In the 3D Plot Group Settings window, scroll to the Selection section, and click

the Active button next to the Selection list.4 Select Boundary 1 only.5 In the Settings window for Streamline locate the Coloring and Style section.

From the Li6 Right-click

The plot in FigTutorial ExampleBackstep | 33

ne type list, choose Tube.Streamline 1 and choose Color Expression .

ure 9 on page 27 displays in the Graphics window.

34 | Tutorial Ex

Tutorial ExampleWater Purification Reactor

Water purification for turning natural water into drinking water is a process that consists of several steps. At least one step must be a disinfectant step. One way to achieve efficient disinfection in an environmentally friendly way is to use ozone. A typical ozone purification reactor is about 40 m long and resembles a mace with partial walls or baffles that divide the space into room-sized compartments (Ref. 1). When water flows through the reactor, turbulent flow is created along its winding path water with ozoinactivate micrpurification stIn analyzing athe turbulent be used for fureactions. Thismodel solves fTurbulent FloampleWater Purification Reactor

around the baffles towards the exit pipe. The turbulence mixes the ne gas, which enters through diffusers just long enough to o-pollutants. When the water leaves the reactor, the remaining

eps filter off or otherwise remove the reacted pollutants.n ozone purification reactor, the first step is to get an overview of flow field. The results from the turbulent-flow simulation can then rther analyses of residence time and chemical species transport and requires adding more physics features to the model. The current or turbulent flow in a water treatment reactor by applying the w, k- interface.

Model GeometryThe model geometry along with some boundary conditions is shown in Figure 10. The full reactor has a symmetry plane, which is utilized to reduce the size of the component.

Figure 10: Model

Domain EquBased on the idiameter of th

Here is the kthe flow is turbuse the k- moboth relativelyadvanced turbthat it employresolving the vexcept the inle

Inlet

OutletSymmetryTutorial ExampleWater Purification Reactor | 35

geometry. All boundaries except the inlet, outlet and symmetry plane are walls.

ations and Boundary Conditionsnflow velocity, which is 0.1 m/s, and a length scale L equal to the e inlet, the Reynolds number is

inematic viscosity. The high Reynolds number clearly indicates that ulent and a turbulence model must be applied. In this case, you will del. This is commonly used in industrial applications, because it is robust and computationally inexpensive compared to more ulence models. One major reason the k- model is inexpensive is s wall functions to describe the flow close to walls instead of ery steep gradients there. All boundaries are walls in Figure 10 t, the outlet, and the symmetry plane.

Re U L

------------- 0.1 0.41 10 6---------------------- 4 105= = =

36 | Tutorial Ex

The inlet velocity is prescribed as a plug flow profile. The turbulent intensity is set to 5 % and the turbulent length scale is specified according to Table 3-3 in Theory for the Turbulent Flow Interfaces in the CFD Module Users Guide. A constant pressure is prescribed on the outlet.

Notes About the ImplementationA three-dimensional turbulent flow can take a rather long time to solve, even using a turbulence model with wall functions. To make this tutorial feasible, the mesh is deliberately selected to be relatively coarse and the results are hence not mesh-indepeninvestigated in

ResultsThe velocity fiinlet hits the trecirculation zinto the reactomore fluid is e

Figure 11: VelocityampleWater Purification Reactor

dent. In any component, the effect of refining the mesh should be order to ensure that the model is well-resolved.

eld in the symmetry plane is shown in Figure 11. The jet from the op of the first baffle, which splits the jet. One half creates a strong one in the first chamber. The other half continues downstream r and gradually spreads out. The velocity magnitude decreases as ntrained into the jet.

field in the symmetry plane.

Figure 12 gives a more complete picture of the mixing process in the reactor. The streamlines are colored by the velocity magnitude, and their widths are proportional to the turbulent viscosity. Wide lines hence indicate a high degree of mixing. The turbulence in this model is mainly produced in the shear layers between the central jet and the recirculation zones. The mixing can be seen to be relatively weak in the beginning of the reactor and to increase further downstream.

Figure 12: Streamviscosity.

Reference1. http://www.

Model Wi

The first step tinterface and sTurbulent Flo1 Open COM

Then click tTutorial ExampleWater Purification Reactor | 37

lines colored by velocity. The width of the streamlines is proportional to the turbulent

comsol.com/stories/hofman_water_purification/full/

zard

o build a model is to open COMSOL, then select the physics pecify the type of analysis you want to doin this case, a stationary, w, k- analysis.SOL Multiphysics. On the New page click Model Wizard . he 3D button .

38 | Tutorial Ex

2 In the Select Physics tree under Fluid Flow>Single-Phase Flow>Turbulent Flow, click Turbulent Flow, k- (spf) .

3 Click Add and then click the Study button 4 In the tree under Preset Studies, click Stationary . 5 Click the Done button .

Global Definit ions - Parameters

The first task ito run parame1 On the Mod

The Model Desktop.

2 Go to the Ssettings:- In the Na- In the Ex- In the De

Geometry

You can builda file containinconvenience.Note: The locainstallation. Fomight be simil

1 On the Geo2 Browse to twater_purampleWater Purification Reactor

s to define a parameter for the inlet velocity. Parameters can be used tric studies.el toolbar click Parameters .

toolbar refers to the specific set of controls near the top of the

ettings window for Parameters. In the table, enter the following

me text field, enter u_inpression text field, enter 0.1[m/s]scription text field, enter Inlet velocity

1

the reactor geometry from geometric primitives. Here, instead use g the sequence of geometry features that has been provided for

tion of the file used in this exercise varies based on your r example, if the installation is on your hard drive, the file path ar to C:\Program Files\COMSOL50\models\.

metry toolbar click Insert Sequence .he model library folder and double-click the file ification_reactor_geom_sequence.mph.

3 On the Model toolbar click Build All .

The geometry sequence is now inserted into your component and should look like the figure below.

Materials

1 On the Mod2 Go to the A

Water, liquid3 In the Add 4 On the Mod

The physical pthe domain seTutorial ExampleWater Purification Reactor | 39

el toolbar click Add Material .dd Material window. In the tree under Built-In click .

Material window, click Add to Component.el toolbar, click Add Material again to close the window.

roperties are now available for the CFD simulation. This also defines ttings. The next step is to specify the boundary conditions.

40 | Tutorial Ex

Turbulent Flow, k-

Inlet 11 On the Physics toolbar click Boundaries and choose Inlet .2 Select Boundary 1, which represents the inlet.3 In the Settings window for Inlet locate the

Velocity section. In the U0 text field, type u_in.

4 Locate the TIn the LT te

Symmetry 11 On the Phys

and choose 2 Select Boun

Outlet 11 On the Phys

and choose 2 Select Boun

The sequence Turbulent Flothe upper left All boundariesOutlet 1 now

Mesh 1

The physics-inthe walls thanadds a boundaThe finer meshturbulence is pzones. The bocomputationa1 In the ModampleWater Purification Reactor

urbulence Conditions section. xt field, type 0.07*0.2[m].

ics toolbar click Boundaries Symmetry .dary 3 only.

ics toolbar click Boundaries Outlet .dary 28 only.

of nodes in the Model Builder under w, k- should match the figure. The D in corner of a node means it is a default node. not selected in Inlet 1, Symmetry 1, or have the default wall condition.

duced mesh automatically introduces a mesh that is a bit finer on the free stream mesh. It also refines the mesh at sharp corners and ry layer mesh. on the walls is not critical in this model since most of the roduced in the shear layers between the jet and the recirculation undary layer mesh can also be coarsened in order to save l time.el Builder under Component 1 click Mesh 1 .

2 In the Settings window for Mesh, from the Element size list, choose Coarser.

Size 11 Go to the Mesh toolbar and click Edit .

A mesh sequence as shown below appears. It contains suggestions made by the physics interface. The asterisk on each of the mesh features indicates that the features are not yet built.

2 Right-click

Boundary Lay1 In the Mod

Layers 1 2 In the Settin

Layer PropeLayer Prope- In the Nu

field, type- In the Th

field, type3 Click the Bu4 In the Mode

node.

The mesh is nothe figure belodepending on you use. The mWindows combut not identicMac or Linux Tutorial ExampleWater Purification Reactor | 41

Size 1 and choose Disable .

er Properties 1el Builder, expand the Component 1>Mesh 1> Boundary node, then click Boundary Layer Properties 1 .gs window for Boundary rties locate the Boundary rties section.mber of boundary layers text 2.ickness adjustment factor text 6.ild All button.l Builder, collapse the Mesh 1

w complete and should match w. The mesh can differ slightly which computer architecture esh in the figure is built on a

puter, and will look similar, al, if built on, for example, a computer.

42 | Tutorial Ex

Study 1

Next, solve fordesktop comp1 On the ModWhen Computhe problem.

Results

Three plots arcontour plot ofor the wall funwell resolved tInterfaces in functions.The followingFirst, create a ampleWater Purification Reactor

the flow field. This takes approximately 15 minutes on a quad-core uter.el toolbar click Compute .

te is selected, COMSOL automatically chooses a suitable solver for

e automatically created, one slice plot for the velocity, one pressure n the walls and one boundary plot of the wall lift-off in viscous units ctions. The last one is important since it gives an indication of how he flow is at the walls. See Theory for the Turbulent Flow the CFD Module Users Guide for further details on the wall

steps reproduce Figure 11.data set that corresponds to the non-wall boundaries.

Data Sets1 On the Results toolbar click More Data Sets and choose Surface .2 Select Boundaries 1, 3, and 28 only which correspond to the non-wall

boundaries.

Velocity (spf)1 In the Model Builder expand the Results>Velocity (spf) node.2 Right-click Slice 1 and choose Disable .3 In the Mod4 In the Settin

Data set list5 On the Velo6 In the Settin

list, choose 7 Locate the C

- From the- From the

8 On the Velovelocity mag

9 On the Velo10In the Settin

locate the C- From the

Logarithm- Select the

associated- In the Nu300.

- From the

11In the Mod(spf) .

12In the Settinclick to expa- From the- In the TitTutorial ExampleWater Purification Reactor | 43

el Builder click Velocity (spf) .gs window for 3D Plot Group locate the Data section. From the

, choose Surface 3.city (spf) toolbar click Surface .gs window for Surface locate the Data section. From the Data set

Surface 2, the reactor walls. oloring and Style section.

Coloring list, choose Uniform. Color list, choose Gray.city (spf) toolbar click Surface to generate a surface plot of the nitude.city (spf) toolbar click Arrow Surface .gs window for Arrow Surface oloring and Style section. Arrow length list, choose ic.

Scale factor check box. In the text field, type 1.4.mber of arrows text field, type

Color list, choose White.

el Builder click Velocity

gs window for 3D Plot Group nd the Title section.

Title type list, choose Manual.le text area, type Velocity field.

44 | Tutorial Ex

13Click the Plot button .

Also reproduc1 In the Mod

Copy .

3D Plot Grou1 On the Mod

Group .2 In the Mod

Surface .3 On the 3D 4 In the Settin

Active butto5 Select BounampleWater Purification Reactor

e Figure 12 as follows.el Builder under Velocity (spf), right-click Surface 1 and choose

p 4el toolbar click Add Plot Group and choose 3D Plot

el Builder right-click 3D Plot Group 4 and choose Paste

Plot Group 4 toolbar click Streamline .gs window for Streamline go to the Selection section and click the n next to the Selection list. dary 1 which is the inlet. The streamlines now start at this boundary.

6 In the Settings window for Streamline: - Locate the Streamline Positioning

section. In the Number text field, type 45.

- Locate the Coloring and Style section. From the Line type list, choose Ribbon.

- In the Width expression text field, type spf.nuT*

- Select theIn the ass

7 Right-click Streamline 1Color Expre

8 In the SettinExpression csection.- Select the- In the Mi- In the Ma

9 In the Mod10In the Settin

- From the- In the TitproportiTutorial ExampleWater Purification Reactor | 45

1[s/m]. Width scale factor check box. ociated text field, type 100.

Results>3D Plot Group 4> and choose ssion .gs window for Color lick to expand the Range

Manual color range check box.nimum text field, type 0. ximum text field, type 0.1.

el Builder click 3D Plot Group 4 .gs window for 3D Plot Group click to expand the Title section.

Title type list, choose Manual.le text area, type Streamlines colored by velocity. Width onal to turbulent viscosity.

46 | Tutorial Ex

11On the 3D Plot Group 4 toolbar click Plot .

This concludeampleWater Purification Reactor

s the introduction to the COMSOL CFD Module.

IntroductionAspects of CFD Simulations

The CFD Module Physics InterfacesPhysics Interface Guide by Space Dimension and Study Type

The Model Libraries WindowTutorial ExampleBackstepModel GeometryDomain Equations and Boundary ConditionsResultsModel WizardGlobal Definitions - ParametersGeometry 1MaterialsLaminar FlowMesh 1Study 1Results

Tutorial ExampleWater Purification ReactorModel GeometryDomain Equations and Boundary ConditionsNotes About the ImplementationResultsReferenceModel WizardGlobal Definitions - ParametersGeometry 1MaterialsTurbulent Flow, k-eMesh 1Study 1Results

Documents

Introduction to Cfd Module