Introduction to CFXdl.ptecgroup.ir/.../cfd/ANSYS-CFX/CFX12_03_Physics1.pdf · 2015-03-19 · Introduction to CFX . Boundary Conditions 3-2 ... a source term is added to the momentum

3-1ANSYS, Inc. Proprietary

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

Inventory #002598

Chapter 3

Domains andBoundary Conditions

Introduction to CFX

Boundary Conditions



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598


• 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



Inventory #002598


• 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

Boundary Conditions



Inventory #002598

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 r 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 + r gh

Gravity, g

Small pressure

changes drive the

flow field in the tank

Boundary Conditions



Inventory #002598

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



Inventory #002598

Training ManualDomain Types

• The additional domain tabs/settings

depend on the Domain Type selected

Boundary Conditions



Inventory #002598

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



Inventory #002598


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



Inventory #002598


Radiation Models

– For simulations when thermal radiation is significant

– See the Heat Transfer chapter for more details

Boundary Conditions



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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

reductionSuperficial Velocity = Volume Porosity * True Velocity This setting should be

consistent with the

velocity used when

the Loss Coefficients

(next slide) were

calculated

Boundary Conditions



Inventory #002598

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

ilossi

permi

UKUKdx

dp

2

r

ilossi

permi

UKUKdx

dp

2

r

iRiR

i

UCUCdx

dp21

Boundary Conditions



Inventory #002598

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



Inventory #002598

Training ManualMaterials

• A Material can be created/edited by right clicking “Materials”

in the Outline Tree

Boundary Conditions



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

Boundary Conditions

Boundary Conditions



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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

Inflow

allowed

Outflow

allowed

Boundary Conditions



Inventory #002598

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

Outflow

allowed

Boundary Conditions



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598

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



Inventory #002598


• 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



Inventory #002598


• 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

Introduction to CFXdl.ptecgroup.ir/.../cfd/ANSYS-CFX/CFX12_03_Physics1.pdf · 2015-03-19 · Introduction to CFX . Boundary Conditions 3-2 ... a source term is added to the momentum

Documents