cfx-intro 14.5 l04 domains bcs sources

8/13/2019 CFX-Intro 14.5 L04 Domains BCs Sources

1/48

2012 ANSYS, Inc. December 17, 2012 1 Release 14.5

14. 5 Release

Introduction to ANSYS

CFX

Lecture 04

Domains, Boundary Conditions andSources


2/48


Domains

Domains Boundary Conditions Sources

Domains are regions of space in which the equations of fluid flow

or heat transfer are solved

Only the mesh components which are included in a domain areincluded in the simulation

e.g. A simulation of a copper heating coil in water

will require a fluid domain and a soliddomain.

e.g. To account for rotational motion, the rotor is

placed in a rotating domain.


3/48


How to Create a Domain

Define Domain Properties

Right-click on the domain and pick Edit Or right-click on Flow Analysis 1 to insert a new domain

When edit ing an item a new tab panel

opens containing the prop ert ies. You

can switch between open tabs.

Sub-tabs con tain

various different

propert ies

Comp lete the required

fields on each sub-tab

to define the dom ain

Optional f ields are

activated by enabling a

check box



4/48


5/48


Domain CreationReference Pressure

Basic Settings: Reference Pressure

Represents the absolute pressure datum from whichall relative pressures are measured

Pabs= Preference+ Prelative

Pressures specified at boundary and initial conditionsare relative to the Reference Pressure

Used to avoid problems with round-off errors whichoccur when the local pressure differences in a fluid

are small compared with the absolute pressure level


Ex. 2: Preference= 100,000 Pa

PressurePressure

Ex. 1: Preference= 0 Pa

Pref

Prel,min=99,999 Pa

Prel,max=1 Pa

Prel,min=-1 Pa

Pref


6/48


Domain CreationBuoyancy

Basic Settings: Buoyancy

When gravity acts on fluid regions with differences indensity, a buoyancy force arises

When buoyancy is included, a source term is added tothe momentum equations. It is based on the

difference between the fluid density and a reference

density

SM,buoy=(ref)

refis the reference density. This is the datum fromwhich all densities are evaluated. Fluid with densityother than refexperience either a positive or negative

buoyancy force.

The (ref) term is evaluated differently dependingon the type of fluid



7/48 2012 ANSYS, Inc. December 17, 2012 7 Release 14.5


Full Buoyancy Model

Evaluates the density differences directly

Used when modeling ideal gases, real fluids,multicomponent fluids or multiphase systems

A Buoyancy Reference Density is required

In single-phase models use an average value

of the expected domain density

Boussinesq Model Used when modeling constant density fluids

Buoyancy is driven by temperature differences

(ref) = - ref(TTref)

A Buoyancy Reference Temperature is required

Use an approximate value of the averageexpected domain temperature





Buoyancy Reference Density

The Buoyancy Reference Density is used to avoidround-off errors

Careful selection of the value will remove thehydrostatic head from the Pressuresolution

If rref= 0 [kg m^-3] the full hydrostatic variation

appears in the Pressurefield If rref= the fluid density (r), then no hydrostatic

pressure appears in the Pressurefield and the onlygradients are those driving the flow

Absolute Pressure always includes both the hydrostatic

and reference pressuresPabs= Preference+ Prelative+rref g h

For a non-buoyant flow a hydrostatic pressure does notexist




Pressure and Buoyancy Example

Consider the case of flow through a tank

The inlet is at 30 [psi] absolute Buoyancy is included and so a hydrostatic pressuregradient exists

The outlet pressure will be approximately30 [psi] plus the hydrostatic pressure given byr g h

The flow field is driven by small dynamic pressurechanges

NOT by the large hydrostatic pressure or thelarge operating pressure

To resolve the small dynamic pressure changes

accurately, we use the Reference Pressuretooffset the operating pressure and the Buoyancy

Reference Density to offset the hydrostatic

pressure


30 psi

h

~30 psi + gh

Gravity, g

Small pressure

changes drive the

flow field in the tank



Domain Creation General Options panel: Domain Motion

You can specify a domain that is rotating about an axis

When a domain with a rotating frame is specified, theCFX-Solver computes the appropriate Coriolis and

centrifugal momentum terms and solves a rotating

frame total energy equation

Mesh Deformation Used for problems involving moving boundaries or

moving subdomains

Mesh motion could be imposed or arise as an implicitpart of the solution



11/48


Domain Types The additional domain tabs/settings

depend on the Domain Type selected



12/48


Domain Type: Fluid Models Heat Transfer

Specify whether a heat transfer model forconvection and conduction is used to

predict the temperature throughout the

flow

Discussed in Heat Transfer Lecture

Turbulence Specify whether a turbulence model is used

to predict the effects of turbulence in fluid

flow

For many flows Shear Stress Transport (SST)is recommended

Discussed in Turbulence Lecture



13/48


Domain Type: Fluid Models

Reaction or Combustion Models CFX includes combustion models to allow

the simulation of flows in which combustion

reactions occur

Available only if Option = MaterialDefinition on the Basic Settings tab

Not covered in detail in this course



14/48


Domain Type: Fluid Models

Radiation Models For simulations when thermal radiation is

significant

See the Heat Transfer Lecture for moredetails



15/48


Domain Type: Solid Models Solid Domains are used to model regions that

contain no fluid or porous flow (for example,

the walls of a heat exchanger)

Heat Transfer (Conjugate Heat Transfer) Discussed in Heat Transfer Lecture

Radiation

Only the Monte Carlo radiation model is availablein solids

Theres no radiation in a solid domain if it isopaque!

Solid Motion

Used onlywhen you need to account foradvection of heat in the solid domain

Solid motion must be tangential to its surfaceeverywhere (for example, an object being

extruded or rotated)



16/48


Domain Type: Porous Domains

Used to model flows where thegeometry is too complex to resolvewith a grid

Instead of including the geometricdetails, their effects are accounted for

numerically



17/48


Domain Type: Porous Domains Area Porosity

The area porosity (the fraction of physical area thatis available for flow) is assumed isotropic

Volume Porosity The local ratio of the volume of fluid to the total

physical volume (can vary spatially)

Loss Model

In a porous region, the True Velocity of the fluid islarger than the Superficialvelocity because of the

reduction in flow volumeSuperficial Velocity = Volume Porosity * True Velocity

Select the Loss Velocity Type that is consistent withthe velocity to which the chosen loss coefficients

apply



18/48


Loss Model (continued) Isotropic: Losses equal in all directions

Directional Loss: For many applications, differentlosses are induced in the streamwise and transverse

directions. (Examples: honeycombs and porous plates)

Losses are applied using Darcys Law

Permeability and Loss Coefficients

Linear and Quadratic Resistance Coefficients

Laminar flow: pressure drop typically scales withvelocity

Turbulent flow: pressure drop typically scales withvelocity2

iRiR

i

UCUCdx

dp

21

ilossi

permi

UKUKdx

dp

2

r

Domain Type: Porous Domains



19/48


Materials

Create a name for the fluid to be used

Select the material to be used in the domain Currently loaded materials are available in the drop-down list

Additional Materials are available by clicking



20/48


Materials

A Material can be created/edited by right clicking Materials inthe Outline tree



21/48


Multicomponent/Multiphase Flow

ANSYS CFX has the capability to model fluid mixtures (multicomponent) andmultiple phases


Multicomponent(more details on next slide)One flow field for the mixture

Variations in the mixture accounted for by variable mass

fractions

Applicable when components are mixed at the molecular

level

MultiphaseEach fluid may possess its own flow field

(not available in CFD-Flo product) or all

fluids may share a common flow field

Applicable when fluids are mixed on a

macroscopic scale with a discernibleinterface between the fluids.

Creating multiple fluids will

allow you to specify fluid

specific and fluid pair models


22/48


Multicomponent Flow

ANSYS CFX has the capability to model fluid mixtures (multicomponent) andmultiple phases

For a multicomponent fluid mixture the ANSYS CFX-Solver will calculateappropriate average values of the properties for each control volume in theflow domain and use these in calculating the fluid flow

These average values will depend both on component property values and onthe proportion of each component present in the control volume

In multicomponent flow the various components of a fluid share the samemean velocity, pressure and temperature fields, and mass transfer takes place

by convection and diffusion



23/48


Compressible Flow Modeling Activated by selecting an Ideal Gas, Real Fluid, or a General Fluid whose density

is a function of pressure

Can solve for subsonic, supersonic and transonic flows

Supersonic/transonic flow problems Set the heat transfer option to Total Energy

Generally more difficult to solve than subsonic/incompressible flow problems, especiallywhen shocks are present. For Mach numbers > 2 increasing the maximum number or

continuity loops per time step can help. Expert parameter, max continuity loops.


Click to load a

real gas library

Max continuity loops = 2


24/48


14. 5 Release


CFX

Boundary Conditions


25/48


Defining Boundary Conditions

You must specify information for the dependent (flow) variables atthe domain boundaries

Specify fluxes of mass, momentum, energy, etc. into the domain.

Defining boundary conditions involves: Identifying the location of the boundaries (e.g. inlets, walls, symmetry)

Supplying information at the boundaries

The data required at a boundary depends upon the boundarycondition type and the physical models employed

You must be aware of types of the boundary condition available andlocate the boundaries where the flow variables have known values

or can be reasonably approximated

Poorly defined boundary conditions can have a significant impact on yoursolution



26/48


Available Boundary Condition Types


Inlet

Opening

Outlet

Wall

Symmetry

Inlet

Velocity Components

Normal Speed

Mass Flow RateTotal Pressure (stable)

Relative Static Pressure (Supersonic)

Static Pressure

Static Temperature

Total Temperature (Heat Transfer)

Total Enthalpy (Heat Transfer)

Inlet Turbulent conditions

Outlet

Average Static PressureVelocity ComponentsStatic Pressure

Normal Speed

Mass Flow Rate

Opening

Opening Pressure and DirnEntrainmentStatic Pressure and DirectionVelocity Components

Opening Temperature (Heat Transfer)Opening Static Temperature (Heat Transfer)Inflow Turbulent conditions

Wall

No Slip / Free SlipRoughness ParametersWall Velocity (tangential motion only)

Adiabatic (Heat Transfer)Fixed Temperature (Heat Transfer)Heat Flux (Heat Transfer)Heat Transfer Coefficient (Heat Transfer)

Symmetry

No details (only specify region which

corresponds to the symmetry plane


27/48


How to Create a Boundary Condition

Right-click on the domain to insert BCs


After completingthe boundary

condition, it

appears in the

Outline tree

below its domain


28/48


Inlets and Outlets

Inlets are used predominantly for regions where inflow is expected; however,inlets also support outflow when velocity is specified

Velocity specified inlets are intended for incompressible flows

Using velocity inlets with compressible flows can lead to non-physical results

Pressure and mass flow inlets are suitable for compressible and incompressibleflows

The same concept applies to outletsDomains Boundary Conditions Sources

Velocity Specified Condition Pressure or Mass Flow Condition

Inlet Inlet

Inflow

allowed

Inflow

allowed

Outflow

allowedArtificial wall

prevents outflow


29/48


Openings

The opening type boundary allows both inflow and outflow

You have to provide information on conditions, e.g. temperature, turbulence,composition, that apply to fluid flowing into the domain

Do not use opening as an excuse for a poorly placed boundary See the following slides for examples


Pressure Specified Opening

Inlet

Inflow

allowed

Outflow

allowed


30/48


Symmetry

Used to reduce computational effort in problem.

No inputs are required. Flow field and geometry must be symmetric:

Zero normal velocity at symmetry plane

Zero normal gradients of all variables at symmetry plane

Must take care to correctly define symmetry boundary locations

Can be used to model slip walls in viscous flow


symmetryplanes


31/48


Specifying Well Posed Boundary Conditions

Consider the following case which contains separate air and fuel supply pipes

Three possible approaches in locating inlet boundaries:


1 Upstream of manifold

Can use uniform profiles sincenatural profiles will develop inthe supply pipes

Requires more elements2 Nozzle inlet plane

Requires accurate velocityprofile data for the air and fuel

3 Nozzle outlet plane

Requires accurate velocityprofile data and accurateprofile data for the mixturefractions of air and fuel


32/48



If possible, select boundary locationand shape such that flow either goes in

or out

Not necessary, but will typically observebetter convergence

Should not observe large gradients indirection normal to boundary

Indicates incorrect boundary conditionlocation


Upper pressure boundary modified to

ensure that flow always enters domain.

This outlet is poorly located. It should

be moved further downstream


33/48



Boundaries placed over recirculation zones Poor Location: Apply an opening to allow inflow

Better Location: Apply an outlet with an accurate velocity/pressureprofile (difficult)

Ideal Location: Apply an outlet downstream of the recirculation zone toallow the flow to develop. This will make it easier to specify accurate

flow conditions


Opening

Outlet

Outlet


34/48



Turbulence at the Inlet Nominal turbulence intensities range from 1% to 5% but will depend on

your specific application.

The default turbulence intensity value of 0.037 (that is, 3.7%) issufficient for nominal turbulence through a circular inlet, and is a good

estimate in the absence of experimental data.

For situations where turbulence is generated by wall friction, considerextending the domain upstream to allow the walls to generate

turbulence and the flow to become developed



35/48


Specifying Well Posed Boundary Conditions External Flow

In general, if the building has height H and width W, you would want yourdomain to be at least 5H high, 10W wide, with at least 2H upstream of the

building and 10 H downstream of the building.

You would want to verify that there are no significant pressure gradients normalto any of the boundaries of the computational domain. If there are, then it

would be wise to enlarge the size of your domain.


w

h

5h

10HAt least 2H

10w

Concentrate mesh inregions of high

gradients


36/48


Specifying Well Posed Boundary Conditions Symmetry Plane and the Coanda Effect

Symmetric geometry does not necessarily mean symmetric flow

Example: The coanda effect. A jet entering at the center of a symmetrical ductwill tend to attach to one side above a certain Reynolds number


No Symmetry Plane Symmetry Plane

Coanda effect

not allowed


37/48


Specifying Well Posed Boundary Conditions When there is 1 Inlet and 1 Outlet

Most Robust: Velocity/Mass Flow at an Inlet; Static Pressure at an Outlet. TheInlet total pressure is an implicit result of the prediction.

Robust: Total Pressure at an Inlet; Velocity/Mass Flow at an Outlet. The staticpressure at the Outlet and the velocity at the Inlet are part of the solution.

Sensitive to Initial Guess: Total Pressure at an Inlet; Static Pressure at an Outlet.The system mass flow is part of the solution

Very Unreliable: Static Pressure at an Inlet; Static Pressure at an Outlet. Thiscombination is not recommended as the inlet total pressure level and the mass

flow are both an implicit result of the prediction (the boundary conditioncombination is a very weak constraint on the system)



38/48


Specifying Well Posed Boundary Conditions At least one boundary should specify Pressure (either Total or Static)

Unless its a closed system

Using a combination of Velocity and Mass Flow conditions at all boundaries overconstrains the system

Total Pressure cannot be set at an Outlet It is unconditionally unstable

Outlets that vent to the atmosphere typically use a Static Pressure = 0boundary condition

With a domain Reference Pressure of 1 [atm]

Inlets that draw flow in from the atmosphere often use a Total Pressure = 0boundary condition (e.g. an open window)

With a domain Reference Pressure of 1 [atm]



39/48


Specifying Well Posed Boundary Conditions Mass flow inlets produce a uniform velocity profile over the inlet

Fully-developed flow is not achieved

You cannot specify a mass flow profile

For a mass flow outlet, the mass flow distribution, by default, is based onthe upstream profile and the pressure distribution is an implicit part of the

solution. Options to modify are: Constant fluxuniform mass flow (used when flow highly tangential to outlet)

Shift pressureoption to constrain pressure profile

Pressure specified boundary conditions allow a natural velocity profile todevelop



40/48


14. 5 Release


CFX

Source Terms


41/48


Source Terms Sources add additional terms to the transport equations

They provide a source (or sink) of the solved variable, e.g. A source term added to the energy transport equation represents a source of heat

A source / sink term added to the momentum equations represent adding / removingwork to / from the system e.g. a pump / turbine

Source terms are often used as black-boxes The details of the process producing the source are not simulated

E.g. instead of modelling a fan by resolving the blades and simulating the rotatingmotion, a source term is used to add momentum to the flow


Source

Viscous workConvectionTransient

Conduction

Energy Transport Equation


42/48


3D, 2D and 1D Sources Sources can be applied at a 3D, 2D or

1D location

A Subdomain is a 3D region within adomain that can be used to specifyvalues for volumetric sources

Boundary sources permit thespecification of sources as fluxes(source per unit area) on boundarycondition surfaces

Source points are sources that act on asingle mesh element


Solid heater with

energy source term

Dispersion of an

Additional Variable

from a point source


43/48


3D Sources - Subdomains To add a Subdomain right-click on a

Domain > Insert > Subdomain

A domain can contain many subdomains, ifnecessary

Subdomains cannot span multiple domains

Create separate subdomains for eachdomain

In Basic Settings the Locationisspecified

This can be any 3D mesh region in the

domain, including the whole domain

When creating your geometry and mesh

you should account for any regionswhere source terms are required

In general create a separate 3D solid in the

geometry, then Form New Part in

DesignModelergives a continuous mesh

with distinct 3D regions



44/48


3D Sources - Subdomains On the Sources tab a source term for each

equation can be set

Momentum Sources have their own section on theSources panelsee next slide

Sources may be constants or expressions Sinks are just negative sources

The source Option can be: Source: An amount per unit volume, e.g. [W/m^3]

Total Source: The total amount applied to thesubdomain, e.g. [W]

The optional Source Coefficient should be set

(to improve convergence) if the source term is a function of thesolved variable

E.g. an energy source which is a function of temperature

Set to the derivative of the source with respect to the solved variable



45/48


3D SourcesMomentum Sources

Momentum Sources can be set using a: General Momentum Source: similar to how sources are

set for other equations

Loss Model: when modeling porous materials, screens,etc.

This is based on Darcys Law and so the parameters are

similar to those for a porous domain

Unlike a porous domain, there is no account made for areduction in flow volume and so there is no velocity option for

calculation of pressure gradient


ilossi

permi

uuKuKx

P

2

r


46/48


2D SourcesBoundary Sources

2D sources are associated with a boundary

condition

Each boundary condition has a Sourcestab

Settings are the same as 3D sources excepteither a Flux(source per unit area) isspecified or a Total Source(total amount

over the boundary)

You cannot set momentum sources onboundaries



47/48


1D SourcesSource Points 1D Sources are created by right-clicking on

the appropriate domain > Insert > Source

Pointor using the toolbar icon

Settings are similar to 3D sources exceptthat you can only use the Total Source

option

You cannot set a momentum Source at a

point Source points are actually implemented as

3D sources on a single mesh element

Mesh refinement will refine the source point



48/48

Workshop

Workshop 02 Mixing Tee with Particle TransportModelling Addition of Lagrangian particle tracking to the

previous workshop

OR

Workshop03 Mixing Tube Mixing of hot and cold fluids

Inlet velocity profile using Profile Boundary Condition Temperature-dependent viscosity

Use of Instance Transforms in CFD-Post

cfx-intro 14.5 l04 domains bcs sources

Documents