Transcript CFX12 03 Physics1

Chapter 3
Domains and
Boundary Conditions
Introduction to CFX
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Boundary Conditions
Domains
Training Manual
• Domains are regions of space in which the equations of fluid flow or heat
transfer are solved
Rotor
e.g. A simulation of a copper heating coil in water
will require a fluid domain and a solid domain.
Stator
e.g. To account for rotational motion, the rotor is
placed in a rotating domain.
• Only the mesh components which are included in a domain are included
in the simulation
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Boundary Conditions
How to Create a Domain (as shown earlier)
Training Manual
Define Domain Properties
–
–
Right-click on the domain and pick Edit
Or right-click on Flow Analysis 1 to insert a new domain
When editing an item a new tab panel opens
containing the properties. You can switch
between open tabs.
Sub-tabs contain
various different
properties
Complete the required
fields on each sub-tab
to define the domain
Optional fields are
activated by enabling
a check box
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Boundary Conditions
Domain Creation
Training Manual
• 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 0 frame is usually used
– Fluids and Particles Definitions: select
the participating materials
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Boundary Conditions
Domain Creation – Reference Pressure
Training Manual
• 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
Pref
Pressure
Pressure
Prel,max=100,001 Pa
Prel,max=1 Pa
Prel,min=99,999 Pa
Prel,min=-1 Pa
Pref
Ex. 1: Preference= 0 Pa
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Ex. 2: Preference= 100,000 Pa
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Boundary Conditions
Domain Creation - Buoyancy
Training Manual
• General Options panel: Buoyancy
– When gravity acts on fluid regions with different
densities a buoyancy force arises
– When buoyancy is included, a source term is added
to the momentum equations based on the difference
between the fluid density and a reference density
SM,buoy=(ρ – ρref)g
– ρref is the reference density. This is just the datum
from which all densities are evaluated. Fluid with
density other than ρref will have either a positive or
negative buoyancy force applied.
• See below for more on the reference density
– The (ρ – ρref) term is evaluated differently depending
on your chosen fluid:
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Boundary Conditions
Domain Creation - Buoyancy
Training Manual
• Full Buoyancy Model
– Evaluates the density differences directly
– Used when modeling ideal gases, real fluids, or
multicomponent fluids
– A Reference Density is required
• Use an approximate value of the expected domain
density
• Boussinesq Model
– Used when modeling constant density fluids
– Buoyancy is driven by temperature differences
(ρ – ρref) = - ρref β(T – Tref)
– A Reference Temperature is required
• Use an approximate value of the average
expected domain temperature
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Boundary Conditions
Domain Creation - Buoyancy
Training Manual
• Buoyancy Ref. Density
– The Buoyancy Reference Density is used to avoid
round-off errors by solving at an offset level
– The Reference Pressure is used to offset the
operating pressure of the domain, while the
Buoyancy Reference Density should be used to
offset the hydrostatic pressure in the domain
• The pressure solution is relative to rref g h, where h is
relative to the Reference Location
• If rref = the fluid density (r), then the solution becomes
relative to the hydrostatic pressure, so when visualizing
Pressure you only see the pressure that is driving the
flow
– Absolute Pressure always includes both the
hydrostatic and reference pressures
Pabs = Preference + Prelative + rref g h
– For a non-buoyant flow a hydrostatic pressure does
not exist
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Boundary Conditions
Pressure and Buoyancy Example
Training Manual
• 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 r g h
– The flow field is driven by small dynamic
pressure changes
30 psi
Small pressure
changes drive the
flow field in the tank
h
• NOT by the large hydrostatic pressure or the
large operating pressure
• To accurately resolve the small dynamic
pressure changes, we use the Reference
Pressure to offset the operating
pressure and the Buoyancy Reference
Density to offset the hydrostatic
pressure
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~30 psi + r gh
Gravity, g
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Boundary Conditions
Domain Creation
Training Manual
• 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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Boundary Conditions
Domain Types
Training Manual
• The additional domain tabs/settings
depend on the Domain Type selected
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Boundary Conditions
Domain Type: Fluid Models
Training Manual
• Heat Transfer
– Specify whether a heat transfer model 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
– Discussed in Turbulence Lecture
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Boundary Conditions
Domain Type: Fluid Models
Training Manual
Reaction or Combustion Models
– CFX includes combustion models to allow the
simulation of flows in which combustion
reactions occur
– Available only if Option = Material Definition on
the Basic Settings tab
– Not covered in detail in this course
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Boundary Conditions
Domain Type: Fluid Models
Training Manual
Radiation Models
– For simulations when thermal radiation is
significant
– See the Heat Transfer chapter for more
details
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Boundary Conditions
Domain Type: Solid Models
Training Manual
• 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
available in solids
– There’s no radiation in solid domains if it is
opaque!
• Solid Motion
– Used only when you need to account for
advection 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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Boundary Conditions
Domain Type: Porous Domains
Training Manual
• Used to model flows where the
geometry is too complex to
resolve with a grid
• Instead of including the geometric
details, their effects are
accounted for numerically
Images Courtesy of Babcock and Wilcox, USA
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Boundary Conditions
Domain Type: Porous Domains
Training Manual
• Area Porosity
– The area porosity (the fraction of physical
area 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 Velocity
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This setting should be
consistent with the
velocity used when
the Loss Coefficients
(next slide) were
calculated
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Boundary Conditions
Domain Type: Porous Domains
Training Manual
• 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 Darcy’s Law
 dp
dx i


K perm
U i  K loss
r
2
Ui
• Permeability and Loss Coefficients
 dp

dx i

K perm
U i  K loss
r
2
Ui
• Linear and Quadratic Resistance Coefficients
 dp
dx i
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 C R 1U i  C R 2  U i
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Boundary Conditions
Materials
•
•
•
•
Training Manual
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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Boundary Conditions
Materials
Training Manual
• A Material can be created/edited by right clicking “Materials”
in the Outline Tree
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Boundary Conditions
Multicomponent/Multiphase Flow
Training Manual
• ANSYS CFX has the capability to model fluid mixtures
(multicomponent) and multiple phases
Multicomponent (more details on next slide)
Multiphase
• 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
• Each 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 discernible
interface between the fluids.
Creating multiple fluids will
allow you to specify fluid
specific and fluid pair models
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Boundary Conditions
Multicomponent Flow
Training Manual
• 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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Boundary Conditions
Compressible Flow Modelling
Training Manual
• 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,
especially when shocks are present
Click to load a
real gas library
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Boundary Conditions
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Boundary Conditions
Defining Boundary Conditions
Training Manual
• 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 and
locate 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
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Boundary Conditions
Available Boundary Condition Types
Training Manual
• Inlet
–
–
–
–
–
Velocity Components
Normal Speed
Mass Flow Rate
Total Pressure (stable)
Static Pressure
-Static Temperature (Heat Transfer)
-Total Temperature (Heat Transfer)
-Total Enthalpy (Heat Transfer)
-Relative Static Pressure (Supersonic)
-Inlet Turbulent conditions
• Outlet
–
–
–
Average Static Pressure
Velocity Components
Static Pressure
Outlet
Wall
Inlet
-Normal Speed
-Mass Flow Rate
• Opening
–
–
–
–
Opening Pressure and Dirn
Entrainment
Static Pressure and Direction
Velocity Components
-Opening Temperature (Heat Transfer)
-Opening Static Temperature (Heat Transfer)
-Inflow Turbulent conditions
Symmetry
Opening
• 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
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Boundary Conditions
How to Create a Boundary Condition
•
Training Manual
Right-click on the domain to insert BC’s
After completing
the boundary
condition, it
appears in the
Outline tree
below its domain
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Boundary Conditions
Inlets and Outlets
Training Manual
• 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 Condition
Pressure or Mass Flow Condition
Inlet
Inlet
Inflow
allowed
Inflow
allowed
Outflow
allowed
• 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 and
incompressible flows
• The same concept applies to outlets
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Boundary Conditions
Openings
Training Manual
• Artificial walls are not erected with the opening type boundary, as
both inflow and outflow are allowed
• You 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
allowed
Outflow
allowed
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Boundary Conditions
Symmetry
Training Manual
• 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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Boundary Conditions
Specifying Well Posed Boundary Conditions
Training Manual
• Consider the following case in which contain separate air and fuel
supply pipes
Air
• Three possible approaches
in locating inlet boundaries:
1
1 Upstream of manifold
• Can use uniform profiles
since natural profiles will
develop in the supply pipes
• Requires more elements
2
3
2 Nozzle inlet plane
• Requires accurate velocity
profile data for the air and
fuel
3 Nozzle outlet plane
• Requires accurate velocity
profile data and accurate
profile data for the mixture
fractions of air and fuel
Nozzle
Manifold box
1
Fuel
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Boundary Conditions
Specifying Well Posed Boundary Conditions
• If possible, select boundary
location and shape such that
flow either goes in or out
Training Manual
Upper pressure boundary modified to
ensure that flow always enters domain.
– Not necessary, but will typically
observe better convergence
• Should not observe large
gradients in direction normal to
boundary
– Indicates incorrect boundary
condition location
This outlet is poorly located. It should
be moved further downstream
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Boundary Conditions
Specifying Well Posed Boundary Conditions
•
Training Manual
Boundaries placed over recirculation zones
–
Poor Location: Apply an opening to allow inflow
Opening
–
Better Location: Apply an outlet with an accurate velocity/pressure profile
(difficult)
Outlet
–
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
Outlet
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Boundary Conditions
Specifying Well Posed Boundary Conditions
Training Manual
• 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%) 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
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Boundary Conditions
Specifying Well Posed Boundary Conditions
•
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.
w
Concentrate mesh in
regions of high
gradients
h
5h
10w
At least 2H
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Boundary Conditions
Specifying Well Posed Boundary Conditions
•
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
Coanda effect
not allowed
Symmetry Plane
No Symmetry Plane
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Boundary Conditions
Specifying Well Posed Boundary Conditions
•
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 and
the mass flow are both an implicit result of the prediction (the boundary
condition combination is a very weak constraint on the system).
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Boundary Conditions
Specifying Well Posed Boundary Conditions
Training Manual
• At least one boundary should specify Pressure (either Total or Static)
– Unless it’s 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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Boundary Conditions
Specifying Well Posed Boundary Conditions
Training Manual
• 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
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