Introduction to CFX - dl.ptecgroup.irdl.ptecgroup.ir/.../cfd/ANSYS-CFX/CFX12_07_Physics2.pdf · Domain Interfaces 7-2 ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved

7-1ANSYS, Inc. Proprietary

© 2009 ANSYS, Inc. All rights reserved.April 28, 2009

Inventory #002598

Chapter 7

Interfaces, Sources andAdditional Variables

Introduction to CFX

Domain Interfaces



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Training Manual

• Domain Interfaces are used for:

– Connection of mismatched meshes (hex to tet for example)

• The meshes may be in the same or different domains

– “Domain” in Domain Interfaces is a little misleading

• A single mesh file may contain non-matching mesh regions and require

domain interfaces

– Changes in reference frames between domains

• Even if the mesh matches

– Connect different types of domains together (e.g. Fluid to Solid)

– Create periodic regions within a domain

Overview

Domain Interfaces



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Training ManualInserting Domain Interfaces

• To create a domain interface right-click

on the Flow Analysis or use the toolbar

icon

Domain Interfaces



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Training Manual

• After creating a domain interface 3 new

object are created in the outline tree

• The interface object is at the Flow

Analysis level

– This is the object you should edit to

make changes to the domain interface

• Within each domain a Side 1 or Side 2

boundary condition is automatically

created

– In general do not edit these objects

– They will be automatically updated when

changes are made to the interface object

Domain Interfaces and Boundary Objects

The Interface object

The Side 1 and Side 2

boundary conditions

Domain Interfaces



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Training Manual

• Domain Interfaces connect two sets

of surfaces together

– Side 1 and Side 2

• First select the domain combination

to be connected

• Then select the Side 1 and Side 2

surface sets

– The Domain (Filter) just limits the

scope of the Region List to make

selection easier

• The Interface Models and Mesh

Connection Method control how

data is transferred across the

interface

Wh

at?

Domain Interfaces Panel

Ho

w?

Domain Interfaces



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Training Manual

• The available Interface Models are:

• Translational Periodicity– Simulates geometries that have translational

periodicity

– Allows for either the mass flow rate or the pressure change across the interface to be specified

– The quantity not specified will be part of the solution

• Rotational Periodicity– Simulates rotationally periodic geometries

• General Connection– For all other types of connections

– A Frame Change/Mixing Model and a Pitch Change apply to rotating domain cases. These are discussed in the Moving Zones lecture

Interface Models

Domain Interfaces



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Training Manual

• 1:1

– Only use this option if you are sure that the nodes on Side 1 and

Side 2 of the interface match up exactly

– Not recommended for Fluid – Solid and Solid – Solid interfaces

• GGI

– Use this option when the nodes on the two sides are not aligned

– For best results both sides should have fairly similar mesh

length scales

– Fluxes are conserved across the interface

– If the size of the connection region for one side is different to

the other, the connection will be automatically made between

the mutually overlapping surfaces (for best results ensure both

sides fully overlap)

– Possible to perform a connection where there is a “slight” gap

or interference between the two sides of the GGI connection

• The gap should be small relative to the mesh length scale

– When solving, GGI connections use more memory and CPU than

1:1 connections

Mesh Connection Method

Domain Interfaces



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Training ManualMesh Connection Method

• Automatic

– This is generally the recommended option when available

• In some cases only the GGI option will be available

– It will try to make a 1:1 connection if possible, otherwise GGI

– The Mesh Match Tolerance under Edit > Options > Mesh

determines how close nodes need to be before a 1:1

connection can be made

• The default value of 0.005 (0.5%) is a fraction of the local mesh

length scale

– In some situations a GGI connection will be used even when

nodes match 1:1

• E.g. Fluid – Solid interfaces, since GGI connections are more

accurate in these situations

Domain Interfaces



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Training ManualPorous Interface Usage

• Domain interfaces involving porous domains are always treated as

GGI

• Total Pressure is unchanged across the interface

– Static pressure will show a discontinuity at the interface

• Total Enthalpy (Total Energy) is unchanged across interface

– May see a discontinuity in Enthalpy (Temperature) in high speed flows

Total Pressure

Velocity

Static Pressure

Domain Interfaces



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Training ManualAutomatic Domain Interfaces

• In some cases CFX-Pre will automatically

create domain interfaces within a single mesh

assembly if the mesh is from ANSYS Meshing:

– To connect multiple domains and non-matching

meshes within the assembly

– Right-click on Mesh > View by > Region Type

to see a list of assemblies in the mesh

• Always check the automatic interfaces to

make sure they are appropriate!

• You can disable automatic interface creation

from Case Options > General in the Outline

tree

• You always need to manually create interfaces between mesh assemblies

and when the mesh was not generated by ANSYS Meshing

• However, a mesh with multiple assemblies but 1:1 node connections will usually

be glued together to form 1 assembly, thus interfaces are not needed

Domain Interfaces



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Training ManualAutomatic Domain Interfaces

• The Connectivity entry in the Outline tree shows

mesh connections that have been detected by

CFX-Pre

– You can right-click to add/remove connections

– Automatic Domain Interface are created based on

detected connections

• Connections are created automatically &

manually in the Meshing application in

Workbench and passed to CFX-Pre

• However, you must force all Meshing

Connections to be 1:1 (i.e. 1 region connected to

1 region) for them to be successfully passed to

CFX-Pre

– In Meshing, select Connections from the Outlnie

tree and set Group By = None

Meshing

CFX-Pre

Sources


© 2009 ANSYS, Inc. All rights reserved.April 28 2009

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Training ManualSource Terms

• Sources add additional terms added to the solved 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 / removing work 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 rotating motion, a source term is used to add momentum to the flow

Source

Viscous workConvectionTransient

Conduction

Energy Transport Equation

Sources



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Training Manual

• Sources need to be applied at a 3D, 2D or 1D location

• A sub domain is a 3D region within a domain that can be used to specify values for volumetric sources

• Boundary sources permit the specification of sources as fluxes (source per unit area) on boundary condition surfaces

• Source points are sources that act on a single mesh element

3D, 2D & 1D Sources

Solid heater with

Energy source term

Dispersion of an

Additional Variable

from a Point Source

Sources



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Training Manual3D Sources – Subdomains

• To add a Subdomain right-click on a Domain > Insert > Subdomain

– A domain can contain many subdomains, if necessary

– Subdomains cannot span multiple domains

• Create separate subdomains for each domain

• In Basic Settings the Location is

specified

– 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

regions where source terms are

required

• In general create a separate 3D solid in

the geometry, then Form New Part in

DM – gives a continuous mesh with

distinct 3D regions

Sources



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Training Manual3D Sources – Subdomains

• On the Sources tab a source term for each equation can be set

– Momentum Sources have their own section on the Sources panel – see 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 the subdomain, e.g. [W]

• The optional Source Coefficient should be set (to improve convergence) if the source term is a function of the solved 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

Sources



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Training Manual3D Sources – Momentum Sources

ilossi

permi

uuKuKx

P

2

• 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 it is easier to define the momentum

source using a loss model

• This is based on Darcy’s Law, relating the pressure

drop to the velocity through a Permeability and a

Loss Coefficient or alternatively a Linear and

Quadratic Resistance Coefficient

Kperm = Permeability Coefficient Kloss = Loss Coefficient

/Kperm = Linear Resistance Coefficient

Kloss / 2 = Quadratic Resistance Coefficient

– Pressure Drop due to the Permeability or Linear Resistance Coeff. scales with velocity

• In Laminar flows pressure drop typically scales with velocity

– Pressure Drop due to the Loss or Quadratic Resistance Coeff. scales with velocity2

• In Turbulent flows pressure drop typically scales with velocity2

Sources



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Training Manual2D Sources – Boundary Sources

• 2D sources are associated with boundary

condition

• Each boundary condition has a Sources

tab

• Settings are the same as 3D sources

except either a Flux (source per unit area)

is specified or a Total Source (total amount

over the boundary)

• You cannot set Momentum sources on

boundaries

Sources



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Training Manual1D Sources – Source Points

• 1D Sources are created by right-clicking on the appropriate domain > Insert

> Source Point, or using the toolbar icon

• Settings are similar to 3D sources except

that you can only use the Total Source

option

• You cannot currently 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

Additional Variables

719ANSYS, Inc. Proprietary


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Training Manual

• Additional Variables (AV’s) are non-reacting scalar

components that may be transported through the flow

– They do not have any direct influence on the flow solution

– You can set boundary conditions and sources for transported


• Examples:

– A tracer such as a dye or smoke

• This is an example of a Transport Additional Variable. The AV is

transported with the flow, but does not influence the flow

– pH level

• This is an example of an Algebraic Additional Variable. The AV is

expressed as a function of other flow quantities through an algebraic

expression, rather than solving a transport equation

Additional Variables Overview




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Training ManualCreating Additional Variables

1. AV’s are created by right-clicking on Expressions, Functions and

Variables > Additional Variables, or using the toolbar icon

• Variable Type

– Specific: The AV is solved on a per-unit-mass basis

– Volumetric: The AV is solved on a per-unit-volume basis

– Unspecified: The AV is defined in terms of an algebraic expression

• Units: the units that describe the additional variable

• Tensor Type: Scalar or Vector as necessary

1




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Training ManualCreating Additional Variables

2. Once an AV has been created it must be included in the domain

– Enable the AV on the domain > Fluid Models panel

3. Then boundary and initial values must be set (except Algebraic AV’s)

23




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Training ManualDomain Options

• When including an AV in a domain the type

of equation to solve is specified:

• Transport Equation

– A full transport equation is used

– The transport of the AV occurs through both

convection and diffusion

• Setting the Kinematic Diffusivity controls

laminar diffusion

• Turbulent diffusion is always included

Transient Advection Diffusion Sources

SDUt

)()(

)(




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Training ManualDomain Options

• Diffusive Transport Equation

– The advection term is dropped from the full

transport equation – models a diffusion

process

• Poisson Equation

– The advection and transient terms are

dropped from the full transport equation

– Has uses in electromagnetics

Transient Diffusion Sources

SDt

)(

)(

Diffusion Sources

SD )(0

• Algebraic Equation / Vector Algebraic Equation

– An expression (scalar) or three expression (vector) define the AV

value throughout the domain




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Training ManualTips

• Additional Variables can be used to work-around some limitations:

– You must pass a variable to the integrated CEL functions (areaAve(),

voulmeInt(), etc). The following is not valid:

areaAve(Velocity * Density)@Inlet

because Velocity * Density is an expression, not a variable

– As a work-around you can create an Algebraic AV equal to the

expressions, then pass in the AV to the CEL function

• Creating an Additional Variable showing the age of the fluid in the

domain is often useful for post-processing

– This is done by creating a transport AV “Age” with units of [s]

• Inlet and initial values should be zero

• An AV source term with a value of 1 should be set throughout the domain

Case Options


© 2009 ANSYS, Inc. All rights reserved.February 23, 2009

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Training ManualGeneral Options

• Under Case Options in the Outline tree

Graphics Style and Labels and Markers

control the Viewer look

• General contains a number of useful

options:

– Automatic Default Domain and Automatic

Default Interfaces:

• Control the creation of these automatic objects

– Automatic Physics Update:

• By default CFX-Pre enforces all domains to use

the same physics. In some cases you may

want different physics in different domains. In

general this is only valid when the domains are

not directly connected, e.g. two fluid domains

containing different fluids separated by a solid

domain

Documents

Introduction to CFX - dl.ptecgroup.irdl.ptecgroup.ir/.../cfd/ANSYS-CFX/CFX12_07_Physics2.pdf · Domain Interfaces 7-2 ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved