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Chapter 3
Domains andBoundary Conditions
Introduction to CFX
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Training ManualDomains
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 are included
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.
Rotor Stator
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Training ManualHow to Create a Domain (as shown earlier)
Define Domain Properties
Right-click on the domain and pick Edit
Or right-click on Flow Analys is 1to insert a new domain
When edit ing an item a new tab panel opens
contain ing the proper ties. You can switch
between open tabs.
Sub-tabs contain
various different
proper t ies
Comp lete the required
fields on each sub-tab
to def ine the domain
Optional f ields are
activated by enabling
a check box
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Training ManualDomain Creation
General Options panel: Basic Settings Location:Only assemblies and 3D
primitives
Domain Type:Fluid, Solid, or Porous
Coordinate Frame:select coordinate
frame from which all domain inputs will be
referenced to
Not to be confused with the reference
frame, which can be stationary or rotating
The default Coord 0frame is usually used
Fluids and Particles Definitions: select
the participating materials
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Training Manual
Ex. 2: Preference= 100,000 Pa
Domain CreationReference Pressure
General Options panel:Domain Models
Reference Pressure
Represents the absolute pressure datum from
which all relative pressures are measured
Pabs= Preference+ Prelative
Pressures specified at boundary and initial
conditions are relative to the Reference Pressure
Used to avoid problems with round-off errors which
occur when the local pressure differences in a fluid
are small compared to the absolute pressure level
PressurePressure
Ex. 1: Preference
= 0 Pa
Pref
Prel,max=100,001 Pa
Prel,min=99,999 Pa
Prel,max=1 Pa
Prel,min=-1 Pa
Pref
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Training ManualDomain Creation - Buoyancy
General Options panel: Buoyancy
When gravity acts on fluid regions with differentdensities a buoyancy force arises
When buoyancy is included, a source term is addedto the momentum equations based on the differencebetween the fluid density and a reference density
SM,buoy=(ref)g
refis the reference density. This is just the datumfrom which all densities are evaluated. Fluid withdensity other than refwill have either a positive ornegative buoyancy force applied.
See below for more on the reference density
The (ref) term is evaluated differently dependingon your chosen fluid:
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Training ManualDomain Creation - Buoyancy
Full Buoyancy Model
Evaluates the density differences directly
Used when modeling ideal gases, real fluids, ormulticomponent fluids
A Reference Density is required
Use an approximate value of the expected domaindensity
Boussinesq Model
Used when modeling constant density fluids
Buoyancy is driven by temperature differences
(ref) = - ref(TTref)
A Reference Temperature is required
Use an approximate value of the averageexpected domain temperature
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Training ManualDomain Creation - Buoyancy
Buoyancy Ref. Density
The Buoyancy Reference Densityis used to avoid
round-off errors by solving at an offset level
The Reference Pressure is used to offset theoperating pressure of the domain, while theBuoyancy Reference Densityshould be used tooffset the hydrostatic pressure in the domain
The pressure solution is relative to rref g h, where hisrelative to the Reference Location
If rref= the fluid density (r), then the solution becomesrelative to the hydrostatic pressure, so when visualizingPressureyou only see the pressure that is driving theflow
Ab solute Pressurealways includes both thehydrostatic and reference pressures
Pabs= Preference+ Prelative+ rref g h
For a non-buoyant flow a hydrostatic pressure does
not exist
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Training ManualPressure and Buoyancy Example
Consider the case of flow through a tank
The inlet is at 30 [psi] absolute
Buoyancy is included, therefore a
hydrostatic pressure gradient exists
The outlet pressure will be approximately
30 [psi] plus the hydrostatic pressure
given by g h
The flow field is driven by small dynamicpressure changes
NOT by the large hydrostatic pressure or the
large operating pressure
To accurately resolve the small dynamic
pressure changes, we use the Reference
Pressureto offset the operating
pressure and the Buoyanc y 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
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Training ManualDomain 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,
the CFX-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
implicit part of the solution
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Training ManualDomain Types
The additional domain tabs/settings
depend on the Domain Type selected
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Training ManualDomain Type: Fluid Models
Heat Transfer
Specify whether a heat transfer model isused to predict the temperature throughout
the flow
Discussed in Heat Trans fer Lecture
Turbulence Specify whether a turbulence model is
used to predict the effects of turbulence in
fluid flow
Discussed in Turbulence Lecture
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Training ManualDomain Type: Fluid Models
Reaction or Combustion Models
CFX includes combustion models to allow thesimulation of flows in which combustion
reactions occur
Available only if Option = Material Definitionon
the Basic Settings tab
Not covered in detail in this course
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Training ManualDomain Type: Fluid Models
Radiation Models
For simulations when thermal radiation issignificant
See the Heat Transfer chapter for moredetails
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Training ManualDomain 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 Trans fer Lecture
Radiation
Only the Monte Carlo radiation model is
available in solids
Theres no radiation in solid domains if it is
opaque!
Solid Motion
Used onlywhen you need to account foradvection of heat in the solid domain
Solid motion must be tangential to its
surface everywhere (for example, an object
being extruded or rotated)
Tubular heat exchanger
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Training Manual
Images Courtesy of Babcock and Wilcox, USA
Domain Type: Porous Domains
Used to model flows where thegeometry is too complex to
resolve with a grid
Instead of including the geometric
details, their effects are
accounted for numerically
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Training ManualDomain Type: Porous Domains
Area Porosity
The area porosity (the fraction of physicalarea that is available for the flow to go
through) is assumed isotropic
Volume Porosity
The local ratio of the volume of fluid to the
total physical volume (can vary spatially)
By default, the velocity solved by the code
is the superficial fluid velocity. In a porous
region, the true fluid velocity of the fluid
will be larger because of the flow volume
reduction
Superficial Velocity = Volume Porosity * True VelocityThis setting should be
consistent with the
velocity used when
the Loss Coefficients
(next slide) were
calculated
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Training ManualDomain Type: Porous Domains
Loss Model
Isotropic: Losses equal in all directions
Directional Loss: For many applications,
different losses 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
ilossi
permi
UKUKdx
dp
2
r
ilossi
permi
UKUKdx
dp
2
r
iRiR
i
UCUCdxdp
21
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Training ManualMaterials
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
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Training ManualMaterials
A Material can be created/edited by right clicking Materials
in the Outline Tree
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Training ManualMulticomponent/Multiphase Flow
ANSYS CFX has the capability to model fluid mixtures
(multicomponent) and multiple 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
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Training ManualMulticomponent Flow
Each component fluid may have a distinct set of physical
properties
The ANSYS CFX-Solver will calculate appropriate average values
of the properties for each control volume in the flow domain, for
use in calculating the fluid flow
These average values will depend both on component property
values and on the proportion of each component present in the
control volume
In multicomponent flow, the various components of a fluid share
the same mean velocity, pressure and temperature fields, and
mass transfer takes place by convection and diffusion
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Training ManualCompressible Flow Modelling
Activated by selecting an Ideal Gas, Real Fluid, or a General Fluidwhose 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,especially when shocks are present
Click to load a
real gas library
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Boundary Conditions
B d C di t i
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Training ManualDefining Boundary Conditions
You must specify information on the dependent (flow) variables at the
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 boundary
condition 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 your
solution
B d C di t i
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Training ManualAvailable Boundary Condition Types
Inlet Velocity Components -Static Temperature (Heat Transfer)
Normal Speed -Total Temperature (Heat Transfer)
Mass Flow Rate -Total Enthalpy (Heat Transfer) Total Pressure (stable) -Relative Static Pressure (Supersonic)
Static Pressure -Inlet Turbulent conditions
Outlet Average Static Pressure -Normal Speed
Velocity Components -Mass Flow Rate
Static Pressure
Opening Opening Pressure and Dirn -Opening Temperature (Heat Transfer)
Entrainment -Opening Static Temperature (Heat Transfer)
Static Pressure and Direction -Inflow Turbulent conditions
Velocity Components
Wall No Slip / Free Slip -Adiabatic (Heat Transfer)
Roughness Parameters -Fixed Temperature (Heat Transfer)
Heat Flux (Heat Transfer) -Heat Transfer Coefficient (Heat Transfer)
Wall Velocity (for tangential motion only)
Symmetry No details (only specify region which corresponds to the symmetry plane
Inlet
Opening
Outlet
Wall
Symmetry
B d C di t i
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Training Manual
Right-click on the domain to insert BCs
How to Create a Boundary Condition
After completing
the boundary
condition, it
appears in the
Outline tree
below its domain
B d C di t i
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Training ManualInlets and Outlets
Inlets are used predominantly for regions where inflow is expected;however, inlets also support outflow as a result of velocity specified
boundary conditions
Velocity specified inlets are intended for incompressible flows
Using velocity inlets in compressible flows can lead to non-physical results
Pressure and mass flow inlets are suitable for compressible andincompressible flows
The same concept applies to outlets
Velocity Specified Condition Pressure or Mass Flow Condition
Inlet Inlet
Inflow
allowed
Inflow
allowed
Outflow
allowed
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Training ManualOpenings
Artificial walls are not erected with the opening type boundary, as
both inflow and outflow are allowedYou are required to specify information that is used if the flow
becomes locally inflow
Do not use opening as an excuse for a poorly placed boundary
See the following slides for examples
Pressure Specified Opening
Inlet
Inflow
allowedOutflow
allowed
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Training ManualSymmetry
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
symmetry
planes
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Training Manual
Fuel
Air
Manifold box1Nozzle
1
23
Specifying Well Posed Boundary Conditions
1 Upstream of manifold
Can use uniform profilessince natural profiles will
develop in the supply pipes Requires more elements
2 Nozzle inlet plane
Requires accurate velocityprofile data for the air andfuel
3 Nozzle outlet plane Requires accurate velocity
profile data and accurateprofile data for the mixturefractions of air and fuel
Consider the following case in which contain separate air and fuelsupply pipes
Three possible approachesin locating inlet boundaries:
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Training ManualSpecifying Well Posed Boundary Conditions
If possible, select boundary
location and shape such that
flow either goes in or out
Not necessary, but will typically
observe better convergence
Should not observe large
gradients in direction normal to
boundary
Indicates incorrect boundarycondition location
Upper pressure boundary modified to
ensure that flow always enters domain.
This outlet is poorly located. It should
be moved further downstream
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Training Manual
Boundaries placed over recirculation zones
Poor Location: Apply an opening to allow inflow
Better Location: Apply an outlet with an accurate velocity/pressure profile
(difficult)
Ideal Location: Apply an outlet downstream of the recirculation zone to allow
the flow to develop. This will make it easier to specify accurate flow
conditions
Specifying Well Posed Boundary Conditions
Opening
Outlet
Outlet
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Training Manual
Turbulence at the Inlet
Nominal turbulence intensities range from 1% to 5% but will dependon your specific application.
The default turbulence intensity value of 0.037 (that is, 3.7%) is
sufficient 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, consider
extending the domain upstream to allow the walls to generate
turbulence and the flow to become developed
Specifying Well Posed Boundary Conditions
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Training Manual
External Flow
In general, if the building has height H and width W, you would want your
domain 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
normal to any of the boundaries of the computational domain. If there are,
then it would be wise to enlarge the size of your domain.
Specifying Well Posed Boundary Conditions
w
h
5h
10HAt least 2H
10w
Concentrate mesh in
regions of high
gradients
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Training Manual
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 duct will tend to flow along one side above a certain
Reynolds number
Specifying Well Posed Boundary Conditions
No Symmetry Plane Symmetry Plane
Coanda effect
not allowed
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Training Manual
When there is 1 Inlet and 1 Outlet
Most Robust: Velocity/Mass Flow at an Inlet; Static Pressure at an Outlet.
The Inlet total pressure is an implicit result of the prediction.
Robust:Total Pressure at an Inlet; Velocity/Mass Flow at an Outlet. The
static pressure 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.
This combination is not recommended, as the inlet total pressure level andthe mass flow are both an implicit result of the prediction (the boundary
condition combination is a very weak constraint on the system).
Specifying Well Posed Boundary Conditions
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Training ManualSpecifying 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
over constrains 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 = 0
boundary condition
With a domain Reference Pressure of 1 [atm]
Inlets that draw flow in from the atmosphere often use a Total
Pressure = 0 boundary condition (e.g. an open window)
With a domain Reference Pressure of 1 [atm]
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Training ManualSpecifying Well Posed Boundary Conditions
Mass flow inlets result in a uniform velocity profile over the inlet
Fully developed flow is not achieved You cannot specify a mass flow profile
Mass flow outlets allow a natural velocity profile to develop based on
the upstream conditions
Pressure specified boundary conditions allow a natural velocity
profile to develop