fluent slides

© Fluent Inc. 2/20/011

Fluent Software TrainingTRN-00-002

Fluent Inc.



Orientationu Agenda for Week:

u Facilitiesl Training rooms, Lunch rooms, and Bathrooms

u Trainingl Problem Setup

n Creating the model: geometry and mesh generationn Using the Solver: setup and execution

l Establish contact with support staffl Begin your own CFD project!

n We recommend doing all Gambit and relevant Fluent tutorials first.

Monday

Solver-1

Tuesday

Solver-2

Wednesday

Gambit-1

Thursday

Gambit-2

Friday

1:1 Consult



Detailed Agenda (Day 1)

8:00-8:30 Introduction to Fluent Inc.8:30-9:00 Introduction to CFD Analysis9:00-9:30 Demonstration: Overview of the CFD Process9:30-11:15 Tutorial Session I11:15-12:00 Solver Basics12:00-1:00 Lunch1:00-2:00 Boundary Conditions2:00-4:00 Tutorial Session II4:00-5:00 Turbulence Modeling



Detailed Agenda (Day 2)

8:00-9:00 Solver Settings9:00-10:30 Tutorial Session III10:30-11:15 Heat Transfer and Thermal Boundary Conditions11:15-12:00 User Defined Functions (Optional)12:00-1:00 Lunch1:00-2:00 Tutorial Session IV: Post-processing2:00-3:30 Multiphase Modeling (Optional)3:30-5:00 Combustion Modeling (Optional)



About Fluent: History

Creare Inc.Creare Inc.CFD Group

Creare Inc.

Fluent Inc.

Aavid ThermalTechnologies


Fluent Inc.


Fluent Inc.IncorporatingFDI andPOLYFLOW

1961

1983

1988

19951964

1996

u Fluent now has more than 10 offices located in Europe, Asia, andthe U.S.A.



Organization

Aero

Sales Support Consulting

Auto Material Chemical Power HVAC

• Engineers and staff are assigned to industry teams.

• Each team has their own sales, support, and consulting groups.



Fluent Support Onlineu User Services Center

l Access through fluent.comn must be registered user

l Servicesn Release Informationn Download Updatesn Documentationn Supported Platformsn Defects/Workaroundsn Presentationsn Training

u ftp’ing files to supportl ftp to ftp.fluent.coml log on as ftp and use email address for passwordl cd to incoming/xxx and put appropriate support files (binary).

n xxx = support engineer identifier



Advanced Training on CD-ROM

u Advanced training available onCD-ROM

u Can be purchased onlinel Turbulence Modelingl User Defined Functionsl Combustionl Multiphase



u Attend the annual UGM and:l meet with the users and staff of

Fluentl attend short-coursesl learn of other Fluent applications

presented by usersl provide input to future development

of software

u Worldwide User Group Meetings:l USA (Manchester NH, MI, CA)

n typ. Mid-June

l European Meetingsn typ. Mid-September through early October

l Asia-Pacific Meetingsn typ. Mid-October through early November

User Group Meetings

© Fluent Inc. 2/20/0110


FLUENT 4.5 and FLUENT 5

u General purpose FVM solversl Fluent 5 Applications:

n Internal and externalautomotive flows

n High speed aerodynamicsn Rocket flowsn Turbomachineryn Reactor Vessels

l Fluent 4.5 Applications:n Cyclonesn Bubble Columnsn Mixing tanksn Fluidized Beds

Surface pressuredistribution in an

automotive enginecooling jacket.

Instantaneous solids concentration in ariser indicating uniform distribution ofcatalyst at the riser head.

© Fluent Inc. 2/20/0111


FIDAPu General purpose FEM solver

l FIDAP Applications:n Polymer processing: non-

Newtonian flow in extrusiondies

n Thin film coating flowsn Biomedical: oxygenators,

blood pumps, deformingarteries

n Semiconductor crystal growthn Other metal, glass, and

chemical processing problems

Example: Pulsatile flow in an arterywith a compliant vein graft.

u=u(t),sinusoidalinlet velocity

rigidwall

rigidwall

compliantwall

initial mesh

rigid arterydeforming artery

Time history plot of wall shear rate-Deformations cannot be neglected!

Velocity contour plot

© Fluent Inc. 2/20/0112


POLYFLOW

u FEM solver for laminar, viscousflows for complex rheologies andfree surfacel POLYFLOW Applications:

n Extrusion, coextrusion, diedesign

n Blow molding, thermoformingn Film casting, glass sheet

forming/stretching, fiberdrawing

n Chemical reactions, foamingn Viscoelastic flows (“memory

effects”)

Inverse Die Design:Determines die geometrybased upon desiredextruded shape.

Requested part shape and calculated dielip shape for a rubber car door seal.

Blow molding simulation of agas tank using the membraneelement.

© Fluent Inc. 2/20/0113


IcePak

u IcePak is focused on electronicscooling design:l Cooling airflow, heat

conduction, convection andradiation heat transfer

u The user interface and automaticmeshing are tailored forapplications such as:l Cabinet designl Fan placementl Board-level designl Heat sink evaluation

Flow pathlines and temperaturedistribution in a fan-cooled computercabinet.

© Fluent Inc. 2/20/0114


u Simplifies the design and analysis ofventilation systems

u Accurate, quick, and easy-to-use design toolthat empowers designers and professionals,without extensive backgrounds in computerapplications, to utilize the powers ofadvanced CFD tools

u Optimize your designs or pinpoint problemsbased on accurate predictions of airflowpatterns, thermal conditions, comfortconditions, and/or contamination controleffectiveness

Airpak

© Fluent Inc. 2/20/0115


u MixSim is a specialized user interface that allows quick and easy set-upof mixing tank simulations.

u The tank size, bottom shape, baffle configuration, number and type ofimpellers, etc. are specified directly.

u The mesh and complete problem definition are then automaticallycreated.

MixSim

u Other features include:l Impeller libraries from leading equipment

manufacturersl Transient sliding mesh, steady-state multiple

reference frame modelsl Non-Newtonian rheology

© Fluent Inc. 2/20/0116


u A single, integratedpre-processor forCFD analysis.l Geometry

creationl Mesh generationl Mesh quality

examinationl Boundary zone

assignment

Pre-processor: Gambit

© Fluent Inc. 2/20/0117


u A pre-processor fortet/hybrid meshgeneration.

u Useful whenstarting withtriangular surfacemesh.

Pre-processor: TGrid

© Fluent Inc. 2/20/0118


u Separate CD for each product (e.g.,FLUENT 5, TGrid, etc.) containingall the manuals for that product.

u Three formats available:l HTML

n for general viewing, searching,limited printing

l Adobe Acrobat PDFn for high quality printing of one

or many pages

l Adobe PostScriptn for high quality printing of one or many pages

u A script is included which can (optionally) install the documentation.

Documentation CD’s

Fluent 5 CD Documentation html homepage

© Fluent Inc. 2/20/0119


Evaluation Forms

u Evaluation formsare provided inyour folder.

u Your feedbackhelps us improveour trainingmaterial andmethods.

x

© Fluent Inc. 2/20/01A1


Introduction to CFD Analysis



Background

u FLUENT solvers are based on thefinite volume method.l Domain is discretized into a

finite set of control volumesor cells.

l General conservation (transport) equationfor mass, momentum, energy, etc.,

are discretized into algebraic equations.

l All equations are solved to render flow field.

∫∫∫∫ +⋅∇Γ=⋅+∂∂

VAAV

dVSdddVt φφρφρφ AAV

unsteady convection diffusion generation

Eqn.continuity 1

x-mom. uy-mom. venergy h

φ

Fluid region ofpipe flowdiscretized intofinite set ofcontrol volumes(mesh).

controlvolume



CFD Analysis: Basic Steps

u Problem Identification and Pre-Processing1. Define your modeling goals.2. Identify the domain you will model.3. Design and create the grid.

u Solver Execution4. Set up the numerical model.5. Compute and monitor the solution.

u Post-Processing6. Examine the results.7. Consider revisions to the model.



Define Your Modeling Goals

u What results are you looking for, and how will they be used?u What physical models will need to be included in your analysis?

l Multiphase?

u What degree of accuracy is required?u How quickly do you need the results?u Do you require a unique modeling capability?

l User-defined subroutines (written in FORTRAN) in FLUENT 4.5

l User-defined functions (written in C) in FLUENT 5



Identify the Domain You Will Model

u How will you isolate a piece of the complete physical system?u Where will the computational domain begin and end?u What boundary conditions are needed?u Can the problem be simplified to 2D?



Design and Create the Grid

u Should you use a quad/hex grid, a tri/tet grid, a hybrid grid, or a non-conformal grid?

u What degree of grid resolution is required in each region of thedomain?

u Can you take advantage of MixSim, IcePak, or Airpak?u Will you use adaption to add resolution?u How many cells are required for the problem?u Do you have sufficient computer memory?

triangle

quadrilateraltetrahedron pyramid

prism or wedgehexahedron



Tri/Tet vs. Quad/Hex Meshes

u For simple geometries, quad/hexmeshes can provide high-qualitysolutions with fewer cells than acomparable tri/tet mesh.

u For complex geometries, quad/hexmeshes show no numericaladvantage, and you can save meshingeffort by using a tri/tet mesh.



Hybrid Mesh Example

u Valve port gridl Specific regions can be

meshed with differentcell types.

l Both efficiency andaccuracy are enhancedrelative to a hexahedralor tetrahedral meshalone.

l Tools for hybrid meshgeneration are availablein Gambit and TGrid.

Hybrid mesh for anIC engine valve port

tet mesh

hex mesh

wedge mesh



Non-Conformal Mesh Exampleu Nonconformal mesh: mesh in which grid nodes do not match up

along an interface.l Useful for ‘parts-swapping’ for design study, etc.l Helpful for meshing complex geometries.

u Example:l 3D Film Cooling Problem

n Coolant is injected into a ductfrom a plenum

s Plenum is meshed withtetrahedral cells.

s Duct is meshed withhexahedral cells.

Plenum part can be replaced with newgeometry with reduced meshing effort.



Set Up the Numerical Model

u For a given problem, you will need to:l Select appropriate physical models.

n Turbulence, combustion, multiphase, etc.

l Define material properties.n Fluidn Solidn Mixture

l Prescribe operating conditions.l Prescribe boundary conditions at all boundary zones.l Provide an initial solution.l Set up solver controls.l Set up convergence monitors.



Compute the Solution

u The discretized conservation equations are solved iteratively.l A number of iterations are usually required to reach a converged

solution.

u Convergence is reached when:l Changes in solution variables from one iteration to the next are

negligible.n Residuals provide a mechanism to help monitor this trend.

l Overall property conservation is achieved.

u The accuracy of a converged solution is dependent upon:l Appropriateness and accuracy of the physical models.l Grid resolution and independencel Problem setup

u A converged and grid-independent solution on a well-posedproblem will provide useful engineering results!



Examine the Results

u Examine the results to review solution and to extract usefulengineering data.

u Visualization can be used to answer such questions as:l What is the overall flow pattern?l Is there separation?l Where do shocks, shear layers, etc. form?l Are key flow features being resolved?l Are physical models and boundary conditions appropriate?l Are there local convergence problems?

u Numerical reporting tools can be used to calculate quantitative results:l Lift and dragl Average heat transfer coefficientsl Surface-averaged quantities



Tools to Examine the Results

u Graphical toolsl Grid, contour, and vector plotsl Pathline and particle trajectory plotsl XY plotsl Animations

u Numerical reporting toolsl Flux balancesl Surface and volume integrals and averagesl Forces and moments



Consider Revisions to the Modelu Are physical models appropriate?

l Is flow turbulent?l Is flow unsteady?l Are there compressibility effects?l Are there 3D effects?

u Are boundary conditions correct?l Is the computational domain large enough?l Are boundary conditions appropriate?l Are boundary values reasonable?

u Is grid adequate?l Can grid be adapted to improve results?l Does solution change significantly with adaption, or is the solution grid

independent?l Does boundary resolution need to be improved?



Review for Demo

u Problem Identification and Pre-Processing1. Define your modeling goals.2. Identify the domain you will model.3. Design and create the grid.

u Solver Execution4. Set up the numerical model.5. Compute and monitor the solution.

u Post-Processing6. Examine the results.7. Consider revisions to the model.



FLUENT DEMOu Startup Gambit

l load databasel define boundary zonesl export mesh

u Startup Fluentl GUIl Problem Setupl Solvel Post-Processing



Unix Operating System Basicsu FLUENT/GAMBIT user interface is same on Unix and NTu Basic Unix commands issued in xterm window:

l pwd - prints the name current working directoryl ls - lists the files in the current directoryl cd - change working directories (cd .. to go up one directory).

u Directoriesl Home directory is /home/fluent.l Tutorial mesh files are in /home/fluent/tut.l Before running tutorial, copy appropriate mesh file into home directory.

n e.g., from home directory: cp tut/elbow/elbow.msh .

u To start Fluent 5: % fluent 2d &u To start Fluent 4.5: % fluent -r4.5 &u !Note: It is recommended that you restart FLUENT for each tutorial to

avoid mixing solver settings from different tutorials.

© Fluent Inc. 2/20/01B1


Solver Basics



CFD Analysis: Solver Execution

u Solver Execution:l Import and scale mesh file*.l Select physical models.l Define material properties*.l Prescribe operating conditions.l Prescribe boundary conditions.l Provide an initial solution.l Set solver controls.l Set up convergence monitors.l Compute and monitor solution.

u Post-Processing*

l Feedback into Solverl Engineering Analysis

* Covered in this lecture; remaining steps covered in later lectures.



On Startup: License Manager

u License Manager is ‘librarian’ to software process check out.u license.dat file defines how many processes can be checked out.

l limit to each software process (e.g., FEATURE fluent)l limits total processes checked out (FEATURE fluentall)

u Typical floating/network license file:

SERVER host_name host_id tcp_ip_port#DAEMON Fluentd Fluentd.exeFEATURE fluent Fluentd 5.300 16-jan-2001 2 CC064F90570633323E03 ""FEATURE tgrid Fluentd 1.000 16-jan-2001 2 9CF65FB082E75D09614C ""FEATURE fluent-nox Fluentd 1.000 16-jan-2001 2 DCC6CF4074A895463A29 ""FEATURE uns Fluentd 1.000 16-jan-2001 2 7CB6DFA053802EB023CF ""FEATURE rng-premix Fluentd 1.000 16-jan-2001 2 EC56BF90A7888F6D55FC ""FEATURE gambit Fluentd 1.200 16-jan-2001 2 9C763F707B4DB6553904 ""FEATURE fluent-post Fluentd 1.000 16-jan-2001 1 1CB61F6094D4AEEFA599 ""FEATURE fluentall Fluentd 1.000 16-jan-2001 2 1CD63FB04BB6EBBC62D6 ""

expiration dateavailable processes

version #; ignored byfluent, but do not change!



User Inputs

u GUI commands have a corresponding TUI command.l Advanced commands are only available through TUI.l ‘Enter’ displays command set at current level.l ‘q’ moves up one level.

u Journal/Transcript write capability.



Mouse Functionalityu Mouse button functionality depends on solver and can be configured

in the solver.Display Õ Mouse Buttons...

u Default Settings:l 2D Solver

n Left button translates (dolly)n Middle button zoomsn Right button selects

l 3D Solvern Left button rotates about 2-axesn Middle button zooms

s Middle click on point in screen centers point in window(an alternative to no translate option)

n Right button selects



Reading Mesh: Mesh Components

u Cell = control volume into whichdomain is broken upl computational domain is defined by

mesh that represents the fluid andsolid regions of interest.

u Face = boundary of a cellu Edge = boundary of a faceu Node = grid pointu Zone = grouping of nodes, faces,

and/or cellsl Boundary data assigned to face zones.l Material data and source terms

assigned to cell zones.face cell

node

edge

Simple 2D mesh

Simple 3D mesh

node

face

cell

cellcenter



Reading Mesh: Zones

u Example: Face and cell zonesassociated with Pipe Flowthrough orifice plate.

inlet

outlet

wall

orifice(interior)

Orifice_plate andorifice_plate-shadow

Fluid (cell zone)

Default-interior iszone of internal cellfaces (not used).



Scaling Mesh and Units

u All physical dimensions initially assumed to be in meters.l Scale grid accordingly.

u Other quantities can also be scaledindependent of other units used.l Fluent defaults to SI units.



u Fluid Flow and Heat Transferl Momentum, Continuity, and Energy Equationsl Radiation Models

u Turbulencel RANS based models

including k-e and RSM.l LES

u Chemical Species Transportand Reacting Flowsl Species transport equationl Finite Rate Chemistryl PDF Modeling

n laminar flamelet

l Premixed Turbulent Combustionl Surface Reaction and CVD

Models in Fluent 5 (1)

Pressure contours in near ground flight

Temperature contours for kiln burner retrofitting.



u Multiple Phase Flowsl Liquid/Solid Phase Change Modell Discrete Phase Modell VOF modeling of immiscible fluidsl Cavitationl Algebraic Slip Mixture Modell Eulerian-Eulerian and Eulerian-Granular (Fluent 4.5)

u Flows involving Moving Partsl Moving boundaries (normal translation not allowed,

exception: Fluent 4.5)l Moving zones

n Rotating/Multiple Reference Framen Mixing Planen Sliding Mesh Model

u User-Defined Scalar Transport

Models in Fluent 5 (2)Gas outlet

Oil outlet

Inlet

Water outletContours of oil volume fractionin three phase separator.

Pressure contours for squirrel cage blower.



Material Types and Property Definitionu Material properties defined in Materials Panel.

l Single-Phase, Single Species Flowsn Define fluid/solid propertiesn Real gas model (NIST’s REFPROP)

l Multiple Species (Single Phase) Flowsn Mixture Material concept employed

s Mixture properties (composition dependent)defined separately from constituent’s properties.

s Constituent properties must be defined.

n PDF Mixture Material concepts PDF lookup table used for mixture properties.

– Transport properties for mixture defined separately.s Constituent properties extracted from database.

l Multiple Phase Flows, Single Speciesn Define fluid/solid properties.

l Multiple Phase, Multiple Species flows in Fluent 4.5 only.



Fluid Densityl For constant density, incompressible flow:

n ρ = const.

l For incompressible flow:n ρ = poperating/RT

s Select incompressible-ideal-gas in Define Õ Materials...

s Set poperating close to mean pressure in problem.

l For compressible flow:n ρ = pabsolute/RT

s For low Mach number flows, set poperating close to meanpressure in problem to avoid round-off errors.

s Use Floating Operating Pressure for unsteady flows withlarge, gradual changes in absolute pressure (seg. only).

l Density can also be defined as a function of Temperaturen polynomial or piece-wise polynomialn Boussinesq model discussed in heat transfer lecture.

l Density can also be defined using UDF’s.

Energyequationautomaticallyenabled.



Material Assignmentu Materials are assigned to cell zone where

assignment method depends upon modelsselected:l Single-Phase, Single Species Flows

n Assign material to fluid zone(s) inFluid Panel.

l Multiple Species (Single Phase) Flowsn Assign mixture material to fluid zones in

Species Model Panel or in Pre-PDF.n All fluid zones consist of ‘mixture’.

l Multiple Phase Flows, Single Speciesn Primary and secondary phases selected

in Multiphase Model Panel.n All fluid zones consist of ‘mixture’.



Post-Processingu Post-Processing functions typically

operate on surfaces.l Surfaces are automatically created

from zones.l Additional surfaces can be created.

l Facets are generated as the result ofthe intersection of the new surfaceand the original grid.

u Example: Planar slice through axisof Pipe/Orifice mesh.



Post-Processing: Node Valuesu Fluent stores most field variable

data at cell centers.u Node values of the grid are either:

l calculated as the average ofneighboring cell data, or,

l defined explicitly (when available)with boundary condition data.

u Node values on surfaces areinterpolated from grid node data.

u data files store:l data at cell centersl node value data for primitive

variables at boundary nodes.

u Enable Node Values to interpolatefield data to nodes.



Flux Reports and Surface Integralsu Flux Reports

l use boundary conditiondata and therefore providemore accurate values.

u Surface Integralsl interpolates from cell

center data- slightly lessaccurate.

l Many options available,e.g.,



Grid Adaptionu Grid adaption adds more cells where needed to

resolve the flow field.u Fluent adapts on cells listed in register.

l Registers can be defined based on:n Gradients of flow or user-defined variablesn Isovalues of flow or user-defined variablesn All cells on a boundaryn All cells in a regionn Cell volumes or volume changesn y+ in cells adjacent to walls

l To assist adaption process, you can:n Combine adaption registersn Draw contours of adaption functionn Display cells marked for adaptionn Limit adaption based on cell size

and number of cells:



Adaption Example: 2D Planar Shell

2D planar shell - initial grid

u Adapt grid in regions of high pressure gradient to better resolve pressurejump across the shock.

2D planar shell - contours of pressureinitial grid



Adaption Example: Final Grid and Solution

2D planar shell - contours of pressurefinal grid

2D planar shell - final grid



Parallel Solveru With 2 or more processes,

Fluent can be run onmultiple processors.

u Can run on a dedicated,multiprocessor machine,or a network of machines.

u Mesh can be partitionedmanually orautomatically.

u Some models not yetported to parallel solver.l See release notes.

Partitioned grid for multi-element airfoil.

© Fluent Inc. 2/20/01C1


Boundary Conditions



Outlineu Overviewu Inlet and Outlet Boundaries

l Velocityn Profilesn Turbulence Parameters

l Pressure Boundaries and others...

u Wall, Symmetry, Periodic and Axis Boundariesu Internal Cell Zones

l Fluidn Porous Median Moving Cell Zones

l Solid

u Internal Face Boundaries



Overview

u Boundary Conditions:l Boundaries direct motion of flow.l Boundary Conditions are a required

component of mathematical model.

u Specify fluxes into computational domain.l e.g., mass, momentum, and energy

u Fluid/Solid regions represented by cellzones.l Material and Source terms are assigned to

cell zones.

u Boundaries and internal surfaces arerepresented by face zones.l Boundary data are assigned to face zones.

Example: Face and Cell zonesassociated with Pipe Flowthrough orifice plate

inlet

outlet

wall

orifice(interior)

orifice_plate andorifice_plate-shadow

fluid



Setting Boundary Conditionsu Zones and zone types are initially defined in

pre-processor.u To change zone type for a particular zone:

Define Õ Boundary Conditions...

l Choose the zone in Zone list.n Can also select boundary zone using right

mouse button in Display Grid window.

l Select new zone type in Type list.

u To set boundary conditions for particular zone:l Choose the zone in Zone list.l Click Set... button

u Boundary condition data can be copied from one zone to another.u Boundary condition data can be stored and retrieved from file.

l file ® write-bcl file ® read-bc



Flow Inlets and Outletsu Wide range of boundary conditions types permit flow to enter and exit

solution domain:l General

n Pressure inletn Pressure outlet

l Incompressiblen Velocity inletn Outflow

u Boundary data required depends on physical models selected.u General guidelines:

l Select boundary location and shape such that flow either goes in or out.n Not necessary, but will typically observe better convergence.

l Should not observe large gradients in direction normal to boundary.n Indicates incorrect set-up.

l Minimize grid skewness near boundary.

l Compressible flowsn Mass flow inletn Pressure far-field

l Specialn Inlet vent, outlet vent,

intake fan, exhaust fan



Velocity Inlets

u Defines velocity vector and scalarproperties of flow at inlet boundaries.

u Useful when velocity profile isknown at inlet.l uniform profile is default

u Intended for incompressible flows.l Total (stagnation) properties of flow

are not fixed.n Stagnation properties vary to accommodate prescribed velocity distribution.

l Using in compressible flows can lead to non-physical results.

u Avoid placing velocity inlet too close to a solid obstruction.l Can force the solution to be non-physical, e.g., imposes velocity field, etc., at

boundary that may not be intended.



Using Profiles

u Alternative to UDF’s for definingboundary profiles.l Profiles can define spatial and time

varying boundary conditions.

u Profiles can be generated by:l Writing a profile from another CFD

simulationl Creating an appropriately formatted text

file with location information andboundary condition data.

u Profiles can be manipulated through:l Define à Profiles

u Profiles data applied to boundarythrough ‘hooks’.



Determining Turbulence Parametersu When turbulent flow enters domain at inlet, outlet, or at a far-field

boundary, FLUENT 5 requires boundary values for:l Turbulent kinetic energy k l Turbulence dissipation rate ε

u Four methods available for specifying turbulence parameters:l Set k and ε explicitlyl Set turbulence intensity and turbulence length scalel Set turbulence intensity and turbulent viscosity ratiol Set turbulence intensity and hydraulic diameter

u Intensity and length scale depend on conditions upstream, e.g.:l Exhaust of a turbine

Intensity = 20 % Length scale = 1 - 10 % of blade spanl Downstream of perforated plate or screen

Intensity = 10 % Length scale = screen/hole sizel Fully-developed flow in a duct or pipe

Intensity = 5 % Length scale = hydraulic diameter



Pressure Boundary Conditions

u Pressure boundary conditions requiregauge pressure inputs:

u Operating pressure input is set under:l Define → Operating Conditions

u Useful when:l flow rate and/or velocity is not

known (e.g., buoyancy-driven flows).

l “free” boundary in an external orunconfined flow needs to be defined.

operatinggaugeabsolute ppp += gaugepressure

operatingpressure

pressurelevel

operatingpressure

absolutepressure

vacuum



Pressure Inlet Boundary (1)u Defines total pressure, temperature,

and other scalar quantities at flowinlets.

u Supersonic/Initial Gauge Pressure:l Defines static pressure at boundary

for locally supersonic flows.l Used, if necessary, to initialize flow field for incompressible flows.

u Total temperature:l must be defined for compressible flows.l is used, if necessary, to set static temperature for incompressible flows.

)1/(2)2

11( −−

+= kkstatictotal M

kpp

2

21

vpp statictotal ρ+= incompressible flows

compressible flows



Pressure Inlet Boundary (2)

u Flow Direction must be defined.l Can get non-physical results if you don’t specify a reasonable direction.

u Suitable for compressible and incompressible flows.l Pressure inlet boundary is treated as loss-free transition from stagnation

to inlet conditions.l Mass flux through boundary varies depending on interior solution and

specified flow direction.

u Outflow can occur at pressure inlet boundaries.l Flow direction taken from interior solution.l Exhaust static pressure is defined by value specified for gauge total

pressure wherever outflow occurs.



Pressure Outlet Boundary (1)u Defines static (gauge) pressure at

the outlet boundary.l Interpreted as static pressure of

environment into which flowexhausts.

u Radial equilibrium pressuredistribution option available.

u Backflow can occur at pressureoutlet boundaries:l during solution process or as part of solution.l Backflow is assumed to be normal to the boundary.l Convergence difficulties minimized by realistic values for backflow quantities.l Value specified for static pressure used as total pressure wherever backflow

occurs.



Pressure Outlet Boundary (2)

u For incompressible flows:l The static pressure input defines the boundary pressurel All other flow quantities are extrapolated from the interior.

u For compressible flows:l The static pressure input is ignored if locally supersonic.l All flow quantities are extrapolated from interior.

u Pressure Outlet must be used when problem is set up with PressureInlet.



Outflow Boundary

u Flow exiting domain at Outflow boundary has zero normalgradients for all flow variables except pressure.

u FLUENT extrapolates required information from interior.u Useful when:

l Details of flow velocity and pressure not known prior to solution offlow problem.

l Appropriate where exit flow is close to fully developed condition.

u Note: Use of Pressure Outlet (instead of Outflow) often results inbetter rate of convergence when backflow occurs duringiteration.



Restrictions on Outflow Boundariesu Outflow Boundaries cannot be used:

l with compressible flows.l with the Pressure Inlet boundary condition (use Velocity Inlet instead):

n Combination does not uniquely set a pressure gradient over the whole domain.

l in unsteady flows with variable density.

u Do not use outflowboundaries where:l Flow enters domainl Gradients in flow

direction are significantl Conditions downstream

of exit plane impactflow in domain

outflowconditionill-posed

outflow conditionnot obeyed

outflowconditionobeyed

outflowconditioncloselyobeyed



Modeling Multiple Exitsu Using Outflow boundary condition:

l Mass flow divided equally among alloutflow boundaries by default.

l Flow Rate Weighting (FRW) set to 1 bydefault.

l For uneven flow distribution:n specify Flow Rate Weighting for each

outflow boundary: mi=FRWi/ΣFRWi.n static pressure varies among exits to

accommodate flow distribution.

u Can also use Pressure Outlet boundariesto define exits.

pressure-inlet (p0,T0) pressure-outlet(ps)2

velocity-inlet (v,T0)pressure-outlet(ps)1

or

FRW2

velocityinlet

FRW1



Other Inlet/Outlet Boundary Conditionsu Mass Flow Inlet

l Used in compressible flows to prescribe mass flow rate at inlet.l Not required for incompressible flows.

u Pressure Far Fieldl Available when density is calculated from the ideal gas law.l Used to model free-stream compressible flow at infinity, with free-stream

Mach number and static conditions specified.

u Exhaust Fan/Outlet Ventl Model external exhaust fan/outlet vent with specified pressure jump/loss

coefficient and ambient (discharge) pressure and temperature.

u Inlet Vent/Intake Fanl Model inlet vent/external intake fan with specified loss coefficient/

pressure jump, flow direction, and ambient (inlet) pressure andtemperature.



Wall Boundaries

u Used to bound fluid and solid regions.u In viscous flows, no-slip condition

enforced at walls:l Tangential fluid velocity equal

to wall velocity.l Normal velocity component = 0

u Thermal boundary conditions:l several types available.l Wall material and thickness can be defined for 1-D or in-plane thin plate heat

transfer calculations.

u Wall roughness can be defined for turbulent flows.l Wall shear stress and heat transfer based on local flow field.

u Translational or rotational velocity can be assigned to wall.l Shear stress can also be specified.



Symmetry Boundaries

u Used to reduce computational effort in problem.u Flow field and geometry must be symmetric:

n Zero normal velocity at symmetry planen Zero normal gradients of all variables at symmetry plane

u No inputs required.l Must take care to correctly define symmetry boundary locations.

u Also used to model slip walls in viscous flow

symmetryplanes



Periodic Boundaries

u Used when physical geometry of interest and expected pattern offlow/thermal solution have periodically repeating nature.l Reduces computational effort in problem.

u Two types available in FLUENT 5.l ∆p = 0 across periodic planes.

n Rotationally or translationally periodic.s Rotationally periodic boundaries require axis of rotation be defined in

fluid zone.

l ∆p is finite across periodic planes.n Translationally periodic only.n Models fully developed conditions.n Specify either mean ∆p per period or net mass flow rate.

l By default, periodic boundaries defined in Gambit are assumed to betranslational in FLUENT 5.



Periodic Boundaries: Examples

computationaldomain

Streamlines ina 2D tube heatexchanger

flowdirection

Translationally periodic boundaries

4 tangentialinlets

Rotationally periodic boundaries

l ∆p = 0: l ∆p > 0:



Axis Boundaries

u Used:l At centerline (y=0) of an

axisymmetric gridl Where multiple grid lines meet

at a point in a 3D O-type grid

u Specify:l No inputs required

AXISboundary



Cell Zones: Fluidu Fluid zone = group of cells for

which all active equations aresolved.

u Fluid material input required.l Single species, phase.

u Optional inputs allow settingof source terms:l mass, momentum, energy, etc.

u Define fluid zone as laminar flowregion if modeling transitional flow.

u Can define zone as porous media.u Define axis of rotation for rotationally periodic flows.u Can define motion for fluid zone.



Porous Media Conditions

u Porous zone modeled as special type of fluid zone.l Enable Porous Zone option in Fluid panel.l Pressure loss in flow determined via user inputs

of resistance coefficients to lumped parametermodel.

u Used to model flow through porous mediaand other “distributed” resistances, e.g.,l Packed bedsl Filter papersl Perforated platesl Flow distributorsl Tube banks



Moving Zonesu Single Zone Problems:

l Rotating Reference Frame Modeln define zone as Moving Reference Framen limited applicability

u Multiple Zone Problems:l Each zone defined as moving reference frame:

n Multiple Reference Frame Models least accurate, least demanding on CPU

n Mixing Plane Models field data are averaged at the outlet of one zone

and used as inlet boundary data to adjacent zone.

l Each zone defined as Moving Mesh:n Sliding Mesh Model

s must also define interface.s Mesh positions are calculated; time-accurate simulationss relative motion must be tangential (no normal translation)



Cell Zones: Solidu “Solid” zone = group of cells for which only

heat conduction problem solved.l No flow equations solved

u Material being treated as solid may actually befluid, but it is assumed that no convectiontakes place.

u Only required input is material typel So appropriate material properties used.

u Optional inputs allow you to set volumetricheat generation rate (heat source).

u Need to specify rotation axis if rotationallyperiodic boundaries adjacent to solid zone.

u Can define motion for solid zone



Internal Face Boundaries

u Defined on cell facesl Do not have finite thicknessl Provide means of introducing step change in flow properties.

u Used to implement physical models representing:l Fansl Radiatorsl Porous jump

n Preferable over porous media- exhibits better convergence behavior.

l Interior wall



Summary

u Zones are used to assign boundary conditions.u Wide range of boundary conditions permit flow to enter and exit

solution domain.u Wall boundary conditions used to bound fluid and solid regions.u Repeating boundaries used to reduce computational effort.u Internal cell zones used to specify fluid, solid, and porous regions.u Internal face boundaries provide way to introduce step change in flow

properties.

fluent slides

Documents

2d planar shell

reduce computational effort

prescribe operating conditions

high quality printing

prescribe boundary conditions

boundary condition data

flow rate weighting

fluid zones consist