cfx12 03 physics1

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April 28, 2009Inventory #002598

Chapter 3

Domains andBoundary Conditions

Introduction to CFX

Boundary Conditions



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 solid domain.

e.g. To account for rotational motion, the rotor is placed in a rotating domain.

Rotor Stator

Boundary Conditions



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 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

Boundary Conditions



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 0 frame is usually used

– Fluids and Particles Definitions: select the participating materials

Boundary Conditions



Training Manual

Ex. 2: Preference= 100,000 Pa

Domain Creation – Reference 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

Boundary Conditions



Training ManualDomain Creation - Buoyancy

• 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:

Boundary Conditions



Training ManualDomain Creation - Buoyancy• 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

Boundary Conditions



Training ManualDomain Creation - Buoyancy• 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 ref g h, where h is relative to the Reference Location

• If ref = the fluid density (), 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 + ref g h

– For a non-buoyant flow a hydrostatic pressure does not exist

Boundary Conditions



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 dynamic pressure changes

• 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

30 psi

h

~30 psi + gh

Gravity, g

Small pressure changes drive the

flow field in the tank

Boundary Conditions



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

Boundary Conditions



Training ManualDomain Types

• The additional domain tabs/settings depend on the Domain Type selected

Boundary Conditions



Training ManualDomain Type: Fluid Models

• 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

Boundary Conditions




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

Boundary Conditions




Radiation Models– For simulations when thermal radiation is

significant– See the Heat Transfer chapter for more

details

Boundary Conditions



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 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

Boundary Conditions



Training Manual

Images Courtesy of Babcock and Wilcox, USA

Domain Type: Porous Domains

• 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

Boundary Conditions



Training ManualDomain Type: Porous Domains

• 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 This setting should be

consistent with the velocity used when

the Loss Coefficients (next slide) were

calculated

Boundary Conditions



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 Darcy’s Law• Permeability and Loss Coefficients

• Linear and Quadratic Resistance Coefficients

ilossipermi

UKUKdx

dp

2

ilossipermi

UKUKdx

dp

2

iRiRi

UCUCdx

dp21

Boundary Conditions



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

Boundary Conditions



Training ManualMaterials

• A Material can be created/edited by right clicking “Materials” in the Outline Tree

Boundary Conditions



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

Multiphase• 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

Boundary Conditions



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

Boundary Conditions



Training ManualCompressible Flow Modelling

• 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



Boundary Conditions

Boundary Conditions



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 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

Boundary Conditions



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

Boundary Conditions



Training Manual

• Right-click on the domain to insert BC’s

How to Create a Boundary Condition

After completing the boundary condition, it appears in the Outline tree below its domain

Boundary Conditions



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 and incompressible flows

• The same concept applies to outlets

Velocity Specified Condition Pressure or Mass Flow Condition

Inlet Inlet

Inflow allowed

Inflowallowed

Outflowallowed

Boundary Conditions



Training ManualOpenings

• 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

Outflowallowed

Boundary Conditions



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

Boundary Conditions



Training Manual

Fuel

Air

Manifold box1Nozzle

1

23

Specifying Well Posed Boundary Conditions

1 Upstream of manifold• Can use uniform profiles

since natural profiles will develop in the supply pipes

• Requires more elements

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

• Consider the following case in which contain separate air and fuel supply pipes

• Three possible approachesin locating inlet boundaries:

Boundary Conditions



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 boundary condition location

Upper pressure boundary modified to ensure that flow always enters domain.

This outlet is poorly located. It should be moved further downstream

Boundary Conditions



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


Opening

Outlet

Outlet

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


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

h

5h

10HAt least 2H

10w

Concentrate mesh in regions of high

gradients

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


No Symmetry Plane Symmetry Plane

Coanda effect not allowed

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).


Boundary Conditions




• 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]

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

cfx12 03 physics1

Documents

fluid domain

solid domain

new domain

rotating domain

domain inputs

domain properties right

fluid density

domain optional fields