CFX-Intro_14.5_L08_MovingZones

© 2012 ANSYS, Inc. December 17, 2012 1 Release 14.5

14. 5 Release

Introduction to ANSYS CFX

Lecture 08 Domain Interfaces & Moving Zones


• Lecture Themes:

– Domain Interfaces are important features that connect dissimilar domains or dissimilar meshes

– Many CFD applications across industries involve systems or devices with moving parts. CFX offers many different models for rotating machinery, for arbitrary prescribed motion and for objects whose motion is determined by the flow.

• Learning Aims: –How to define interfaces and periodic boundary conditions –The models available for rotating machinery such as the multiple reference frame

models

• Learning Objectives:

– You will be able to use domain interfaces and will become familiar with the CFX models for systems with moving parts and when a particular model is applicable.

Introduction

Introduction Domain Interfaces Rotating Zones Dynamic Mesh Summary


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Chapter 08-1 Domain Interfaces


Overview

• Where two 3-d mesh regions meet there are two possible treatments

• Two elements share one face – no interpolation necessary – called a conformal or one-to-one mesh interface

• Every element has its own face – interpolation of fluxes is necessary – called non-conformal or unmatched and a Generalised Grid Interface (GGI) is required to connect the regions

conformal non-conformal



Overview • Domain Interfaces are used for:

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

• a single mesh file may contain non-matching mesh regions and require non-conformal interfaces

– Changes in frames of reference between domains

• even if the mesh matches

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

– Create periodic regions within a domain



Inserting Domain Interfaces • To create a domain interface right-

click on ‘Flow Analysis’ or use the toolbar icon



Domain Interfaces and Boundary Objects • After creating a domain interface 3 new objects

appear 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

– They will be automatically updated when changes are made to the interface object

– In general do not edit these objects but you can:

• specify sources

• set wall boundary conditions for non-overlap regions

The Interface object

The Side 1 and Side 2

boundary conditions



Domain Interfaces Panel

• Domain Interfaces connect two sets of surfaces together - Side 1 and Side 2

• First select the combination of domains

• 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?

H

ow

?



Interface Models

• 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 (Additional Interface Models – see later)

• Rotational Periodicity – Simulates rotationally periodic geometries

• General Connection – For all other types of connections

– A Frame Change/Mixing Model and a Pitch Change apply where there are multiple frames of reference. These are discussed in the Moving Zones lecture



Additional Interface Models

On the Additional Interface Models tab you can:

• Represent source or sink of momentum, e.g. a fan or a porous wall, by specifying a pressure change between side 1 and side 2 or a mass flow rate

– With a translational periodic interface this feature can be used to set up fully-developed flow in a short section of duct

• Set up infinitely thin walls by selecting the appropriate wall boundary condition option for Mass and Momentum

- Can model heat conduction across wall (see Heat Transfer lecture)



Additional Interface Models contd…


On the Additional Interface Models tab you can:

• Create a switch, using Conditional Connection Control, to determine whether the interface is open to flow or closed, i.e. a wall

• Provide a logical expression, e.g. a function of time or pressure difference

• Either define when the interface is open or when its state changes


Mesh Connection Method

• 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 used for Fluid – Solid, Fluid - Porous 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 from 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

• Automatic

– This is generally the recommended option when 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 cases only the GGI option will be available

• e.g. when there is a change of reference frame across the interface

– 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



Automatic Domain Interfaces

• CFX-Pre automatically creates domain interfaces within a single mesh assembly

– To connect multiple domains within the assembly

– To connect 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 correct!

• Can disable automatic interface creation from Case Options > General in the Outline tree

• You always need to create interfaces manually between mesh assemblies



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Chapter 08-2 Moving Zones


Introduction • Many flow problems involve movement of

domains

• There are two approaches for modeling moving domains:

• Moving Frames of Reference

– Frame of reference is attached to the moving domain

– Governing equations are modified to account for moving frame

– Rotation or translation (translation in solid domains only)

• Mesh Motion /Mesh Deformation

– Domain position and shape are tracked with respect to a stationary reference frame

– Solutions are inherently unsteady although might approach equilibrium solution



x

y

Moving Frames of Reference vs. Mesh Motion

• Moving Frames of Reference – Domain moves with coordinate system

– To follow the motion of the body topology of the mesh does not need to be updated

– Rotation / translation of the moving domain

• Mesh Motion – Domain can change shape as a function of time

– To follow the motion of the body, topology of the mesh might need to be updated

– Smoothing / remeshing of the domain



Rotating Equipment – Moving Frames of Reference

• Why use a rotating frame of reference?

– Flow field which is unsteady when viewed in a stationary frame can become steady when viewed in a rotating frame

– Steady-state problems are easier to solve

– Additional acceleration terms are added to the momentum equations

• Simpler BCs

• Low computational cost

• Easier to post-process and analyze

Limitation: You may still have unsteadiness in the rotating frame due to

turbulence, circumferentially non-uniform variations in flow, separation, etc.

Example: vortex shedding from fan blade trailing edge

• Can employ rotational-periodic boundaries for efficiency (reduced domain size)

Centrifugal

Compressor

(single blade passage)



Single vs. Multiple Frames of Reference

stationary wall

MFR is necessary SFR is sufficient

• When domains rotate at different rates or when stationary walls do not form surfaces of revolution Multiple Frames of Reference (MFR) are needed

stationary wall

MFR is necessary

stationary wall

baffle



Multiple Frames of Reference

• Interface between a rotating zone and the adjacent stationary zone must be a surface of revolution with respect to the axis of rotation of the rotating zone

• Interfaces can be conformal or non-conformal but will always be treated as GGIs

Correct Interface shape is wrong!

Interface shape is not

a surface

of revolution



Moving Frames of Reference

Overview of Modeling Approaches:

• Single Reference Frame (SFR) – Entire computational domain is referred to a single moving reference frame

steady-state

• Frozen Rotor (MFR) – Fixed relative positions

– Suitable when through flow speed >> machine speed at interface, e.g. a full pump with a volute

– Interaction effects are ignored

steady-state

• Stage (MFR) – Circumferential averaging of fluxes in bands at the domain interface

– Incurs one-time mixing loss – suitable when relative motion at interface is large enough to mix out any upstream velocity profile

– Accounts for time-averaged interaction effects

steady-state



Moving Frames of Reference

Overview of Modeling Approaches (continued):

• Transient rotor-stator (MFR) – Simulates the transient relative motion between the components on each side of the

interface

– Unsteady problem – captures transient interaction effects but computationally expensive

• Transient Blade Row (TBR) – Removes problem of unequal pitch so that it is possible to model just one or two passages

per row rather than the full 360°, so reducing the computational cost. Methods are:

• Profile Transformation

• Time Transformation

• Fourier Transformation

– Covered in an advanced turbo-machinery course



Pitch Change

When the full 360o of rotating and stationary components are modeled the two sides of the interface completely overlap

• However, often rotational periodicity is used to reduce the problem size

• Pitch change is used to account for differences in rotational symmetry between the cut-down domains, e.g. a rotating component with 113 blades connected to a downstream stator containing 60 vanes

The Pitch Change can be specified in several ways

• None

• Automatic (ratio of the two areas)

• Value (explicitly provide pitch ratio)

• Specified Pitch Angles (specify Pitch angle on both sides)

– Select this method when possible

The two sides do not need to line up physically


Pitch Change

When using the Frozen Rotor frame-change model the pitch change should be close to 1 for best accuracy

• Pitch change is accounted for by taking the side 1 profile and stretching it to fit on to side 2

• Some stretching is acceptable, although not ideal

For the Stage model the pitch ratio does not need to be close to 1 since the profile is averaged in the circumferential direction

• Very large pitch changes, e.g. 10, can cause problems

For the Transient Rotor Stator model the pitch change should be exactly 1, which may mean rotational periodicity cannot be used

• If the pitch change is not 1, the profile is stretched as with Frozen Rotor, but there will be some loss of information, e.g. that relating to frequencies.


Intersection An intersection algorithm is used to find the overlapping parts of each mesh

face at the interface

The intersection can be performed in physical space, in the radial direction or in the axial direction

• Physical Space: used when Pitch Change = None

– No correction is made for rotational offset or pitch change

• Radial Direction: used when the radial variation is > axial variation

– E.g. the surface of constant z below

Split interfaces when they have both regions

of constant z and regions of constant r

• Axial Direction: used when the axial variation is > radial variation

– E.g. the surface of constant r

By default CFX determines automatically whether the radial or axial direction is used.

Interfaces that contain surfaces of both constant r and z should be split. Otherwise the intersection will fail


Mesh Deformation

• Mesh Deformation can be applied in simulations where boundaries or objects are moved

• The solver calculates nodal displacements of these regions and adjusts the surrounding mesh to accommodate them

• Examples of deforming meshes include

– Automotive piston moving inside a cylinder

– A flap moving on an airplane wing

– A valve opening and closing

– An artery expanding and contracting



Mesh Deformation

• Internal node positions can be automatically calculated based on user specified boundary / object motion

• Automatic Smoothing

• Boundary / object motion can be moved based on:

– Specified displacement

• Expressions or User Functions can be used

– Coupled motion

• For example, two-way Fluid Structure Interaction using coupling of CFX with ANSYS

– Sequential loading of pre-defined meshes

• Requires User Subroutines



Mesh Deformation

Guidelines when using Expressions / Functions to describe Mesh Motion:

• Under Basic Settings for the Domain, set “Mesh Deformation” to “Regions of Motion Specified”

• Create expressions to describe either nodal displacements or physical nodal locations

• Apply Mesh Motion equations at boundaries/subdomains that are moving



Mesh Deformation

• Remeshing is also available

– Advanced topic

– Used to maintain mesh quality when there is significant deformation, e.g. Internal Combustion Engine

• IC Engine model also uses multi-configuration setup

– Remeshing can employ ICEM CFD or ANSYS Meshing

• See the following tutorials for information on other mesh motion types: – Modeling a Ball Check Valve Using Mesh deformation and the CFX Rigid Body Solver

– Oscillating Plate with Two-Way Fluid-Structure Interaction



Summary • Five different approaches may be used to model flows in moving zones

– Single (Rotating) Reference Frame Model (SFR)

– Multiple Frames of Reference (MFR)

• Frozen Rotor

• Stage

• Transient Rotor Stator

– Mesh Motion (Consider our Advanced Training)

• SFR, Frozen Rotor and Stage methods are primarily steady-state approaches while Transient Rotor Stator is inherently unsteady

• Enabling these models, involves in part, changing the stationary fluid zones to either Moving Frames of Reference or Moving Mesh

• Most physical models are compatible with moving reference frames or moving meshes (e.g. multiphase, combustion, heat transfer, etc.)



Workshop

• Workshop 06 – Axial Fan Stage (MFR)

– Using ANSYS CFX outside ANSYS Workbench

– Working with rotating domains

– Basic setup using multiple frames of reference

– Turbo-specific post-processing

OR

• Workshop 07 – Centrifugal Pump (SFR)

– Working with rotating domains

– Modelling cavitation (multiphase flow)

Results files are provided for Workshop 07



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Appendix


Porous 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


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Appendix: Multiple Frames of Reference (MFR)


Frozen Rotor Frame Change Model

Frozen Rotor: Components have a fixed relative position, but the appropriate frame transformation and pitch change is made

• Steady-state solution

rotation

Pitch change is

accounted for


Frozen Rotor Frame Change Model

Frozen Rotor Usage: • The quasi-steady approximation involved becomes small when the through flow speed is

large relative to the machine speed at the interface

• This model requires the least amount of computational effort of the three frame change models

• Transient effects at the frame change interface are not modeled

• Pitch ratio should be close to one to minimize scaling of profiles – A discussion on pitch change will follow

Profile scaled down

Pitch ratio is ~2


Stage Frame Change Model

Stage: Performs a circumferential averaging of the fluxes through bands on the interface

• Accounts for time-averaged interactions, but not transient interactions

• Steady-state solution

upstream downstream


Stage Frame Change Model

Stage Usage:

• Allows steady-state predictions to be obtained for multi-stage machines

• Incurs a one-time mixing loss equivalent to assuming that the physical mixing supplied by the relative motion between components is sufficiently large to cause any upstream velocity profile to mix out prior to entering the downstream machine component

• The Stage model requires more computational effort than the Frozen Rotor model to solve but not as much as the Transient Rotor-Stator model


Transient Rotor Stator Frame Change Model

Transient Rotor-Stator:

• Predicts the true transient interaction of the flow between a stator and rotor passage

• The transient relative motion between the components on each side of the interface is simulated

• The principle disadvantage of this method is that the computer resources required may be large

• Initialise with converged solution from a Frozen Rotor or Stage model to minimise the number of timesteps needed to establish the flow


Pitch Change

When the full 360o of rotating and stationary components are modeled the two sides of the interface completely overlap

• However, often rotational periodicity is used to reduce the problem size

Example: A rotating component has 113 blades and is connected to a downstream stator containing 60 vanes

– Using rotationally periodicity, a single

rotor and stator blade could be

modeled

– This would lead to pitch change

at the interface of (360/113):

(360/60) = 0.53:1

– Alternatively two rotor blades could be

modeled with a single stator vane, as

shown to the right, giving a pitch ratio

of 2*(360/113):(360/60) = 1.06:1


Setup Guidelines

When Setting Boundary Conditions, consider whether the input quantities are relative to the Stationary frame, or the Rotating frame

Walls which are tangential to the direction of rotation in a rotating frame of reference can be set to be stationary in the absolute frame of reference by assigning a Wall Velocity which is counter-rotating

Use the Alternate Rotation Model if the bulk of the flow is axial (in the stationary frame) and would therefore have a high relative velocity when considered in the rotating frame

• E.g. the flow approaching a fan will be generally axial, with little swirl

• Used to avoid “false swirl”


Navier-Stokes Equations: Rotating Reference Frames

Equations can be solved in absolute or rotating (relative) reference frame

• Relative Velocity Formulation

– Obtained by transforming the stationary frame N-S equations to a rotating reference frame

– Uses the relative velocity as the dependent variable

– Default solution method when Domain Motion is “Rotating”

• Absolute Velocity Formulation

– Derived from the relative velocity formulation

– Uses the absolute velocity as the dependent variable

– Can be used by enabling “Alternate Rotation Model” in CFX-Pre setup

• Rotational source terms appear in momentum equations

x

y

z

z

y

x

stationary

frame

rotating

frame

axis of

rotation

r

CFD domain

or R


The Velocity Triangle

The relationship between the absolute and relative velocities is given by

In turbomachinery, this relationship can be illustrated using the laws of vector addition. This is known as the Velocity Triangle

V

W

U

Velocity Relative

Velocity Absolute

W

V


Comparison of Formulations

ˆ2 rW

x

pwW

t

wvrxx

x

• Relative Velocity Formulation: x-momentum equation

ˆV

x

pvW

t

vvxx

x

• Absolute Velocity Formulation: x-momentum equation

Coriolis acceleration Centripetal acceleration

Coriolis + Centripetal accelerations


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Appendix: Moving Solid Zones


Motion in Solid Domains

In most simulations it is not necessary to specify any motion in solid domains

Consider a rotating blade simulation in which the blade is included as a solid domain and heat transfer is solved through the blade

• Even though the fluid domain is solved in a rotating frame of reference, the mesh is not actually rotated in the solver. Therefore it will always line up with the solid

• The solid domain does not need to be placed in a rotating frame of reference since the heat transfer solution has no Coriolis or centripetal terms

Solid domain motion should be used when the advection of energy needs to be considered

• For example, a hot jet impinging on a rotating disk. To prevent a hot spot from forming, the advection of energy in the solid needs to be included



Solid Domain Motion can be classified into two areas:

• Translational Motion

– For example, a process where a solid moves continuously in a linear direction while cooling

– The solid must extend completely through the domain

• Rotational Motion

– For example, a brake rotor which is heated by brake pads

q’’’

q’’=0

q’’=0

q’’=0

Tin = Tspec


ES SThU

t

h

)() (


In Solid Domains, the conservation of the energy equation can account for heat transport due to motion of the solid, conduction and volumetric heat sources

Note that the solid is never physically moved when using this approach, there is only an additional advection term added to the energy equation

Solid Velocity


Translating Solid Domains

Enable Solid Motion

Specify Solid Motion in terms of Cartesian Velocity Components


Rotating Solid Domains

Rotating Solid Domains can be set up in one of two ways:

• Solid Motion Method

– Set the Domain Motion to Stationary

– On the Solid Models tab, set Solid Motion to Rotating

– The solid mesh does not physically rotate, but the model accounts for the rotational motion of the solid energy


Rotating Solid Domains

• Rotating Domain Method

– Set the Domain Motion to “Rotating”

– Do not set any Solid Motion

• To account for rotational motion of solid energy or a heat source which rotates with the solid, a transient simulation is required and a Transient Rotor Stator interface must be used

Note: “Solid Motion” as applied on the “Solid Models” tab is calculated relative to the

Domain Motion, so is not normally used if “Domain Motion” is “Rotating”

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