cfx12 12 moving_zones

12-1ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.

April 28, 2009Inventory #002598

Chapter 12

Moving Zones

Introduction to CFX

Moving Zones



Training ManualTopics

• Domain Motion– Rotating Fluid Domains

• Single Frame of Reference

• Multiple Frames of Reference– Frame Change Models– Pitch Change

• Mesh Defomation

• Appendix– Moving Solid Domains

• Translation

• Rotation

Moving Zones



Training ManualOverview of Moving Zones

• Many engineering problems involve flows through domains which contain moving components

• Rotational motion:– Flow though propellers, axial turbine blades, radial pump impellers, etc.

• Translational motion:– Train moving in a tunnel, longitudinal sloshing of fluid in a tank, etc.

• General rigid-body motion– Boat hulls, etc.

• Boundary deformations– Flap valves, flexible pipes, blood vessels, etc.

Moving Zones



Training ManualOverview of Moving Zones

• There are several modeling approaches for moving domains:– For rotational motion the equations of fluid flow can be solved in a

rotating reference frame• Additional acceleration terms are added to the momentum equations• Solutions become steady with respect to the rotating reference frame• Can couple with stationary domains through interfaces• Some limitations apply, as discussed below

– Translational motion can be solved by moving the mesh as a whole• Interfaces can be used to allow domains to slide past each other

– More general rigid-body motion can be solved using a 6-DOF solver combined with mesh motion, or the immersed solid approach

– If the domain boundaries deform, we can solve the equations using mesh motion techniques

• Domain position and shape are functions of time• Mesh deformed using smoothing, dynamic mesh loading or remeshing

Moving Zones



Training Manual

x

Domain

y

Moving Reference Frame – flow solved in a rotating coordinate system

Mesh Deformation – Domain changes shape as a function of time

Rotating Reference Frames vs. Mesh Motion

Moving Zones



Training ManualRotating Reference Frames

• Why use a rotating reference frame?– 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...• Simpler BCs• Low computational cost• Easier to post-process and analyze

– NOTE: 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 rotationally-periodic boundaries for efficiency (reduced domain size)

Moving Zones



Training ManualSingle Frame of Reference (SFR)

• Single Frame of Reference assumes all domains rotate with a constant speed with respect to a single specified axis

• Wall boundaries must conform to the following requirements:– Walls which are stationary in the local reference frame (i.e. rotating walls when

viewed from a stationary reference frame) may assume any shape– Walls which are counter-rotating in the local reference frame (i.e. stationary walls

when viewed from a stationary reference frame) must be surfaces of revolution

stationary wall

rotor

baffleCorrect

Wrong! The wall with baffles is not a surfaceof revolution! This case requires a rotating domain placed around the rotor while the stationary wall and baffles remain in a stationary domain

Moving Zones



Training ManualSingle Versus Multiple Frames of Reference

Domain interface with change in reference frame

Single Component(blower wheel blade passage)

Multiple Component(blower wheel + casing)

• When domains rotate at different rates or when stationary walls do not form surfaces of revolution Multiple Frames of Reference (MFR) are needed (not available with the CFD-Flow product)

Stationary Domain

Rotating domain

Moving Zones



Training ManualMultiple Frames of Reference (MFR)

• Many rotating machinery problems involve stationary components which cannot be described by surfaces of revolution (SRF not valid)

• Systems like these can be solved by dividing the domain into multiple domains – some domains will be rotating, others stationary

• The domains communicate across one or more interfaces and a change in reference frame occurs at the interface

• The way in which the interface is treated leads to one of the following approaches, known as Frame Change Models:

– Frozen Rotor Steady-state solution• Rotational interaction between reference frames is not accounted for – i.e. the

stationary domain “sees” the rotating components at a fixed position

– Stage Steady-state solution• Interaction between reference frames are approximated through circumferential

averaging at domain interfaces

– Transient Rotor Stator (TRS) Transient solution• Accurately models the relative motion between moving and stationary domains

at the expense of more CPU time

Moving Zones



Training Manual

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

– Steady-state solution

rotationPitch change is accounted for

Frozen Rotor Frame Change Model

Moving Zones



Training Manual

• 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

Frozen Rotor Frame Change Model

Moving Zones



Training Manual

• 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

Moving Zones



Training Manual

• 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

– You should obtain an approximate solution using a Frozen Rotor interface and then restart with a Stage interface to obtain the best results

Stage Frame Change Model

Moving Zones



Training Manual

• 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

Transient Rotor Stator Frame Change Model

Moving Zones



Training ManualPitch 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

Moving Zones



Training ManualPitch Change

• When using the Frozen Rotor 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 or compressing it to fit onto side 2

– Some stretching / compressing 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 this is not an accurate approach

Moving Zones



Training Manual

• 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

• When a pitch change occurs the two sides do not need to line up physically, they just need to sweep out the same surface of revolution

Pitch Change

Flow passes through the overlapping portion as expected. Flow entering the interface at point 1 is transformed and appears on the other side at point 2.

Moving Zones



Training Manual

• 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

Pitch Change

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

• E.g. the surface of constant r

• Interfaces that contain surfaces of both constant r and z should be split, otherwise the intersection will fail

Moving Zones



Training ManualSetup 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”

Moving Zones



Training ManualNavier-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

stationaryframe

rotatingframe

axis ofrotation

r

CFD domain

or R

Moving Zones



Training ManualThe 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

Moving Zones



Training ManualComparison of Formulations

ˆ2 rWx

pwW

t

wvrxx

x

• Relative Velocity Formulation: x-momentum equation

ˆVx

pvW

t

vvxx

x

• Absolute Velocity Formulation: x-momentum equation

Coriolis acceleration Centripetal acceleration

Coriolis + Centripetal accelerations

Moving Zones



Training ManualMesh 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

Moving Zones




• 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 and User Functions can be

used

– Sequential loading of pre-defined meshes• Requires User Subroutines

– Coupled motion• For example, two-way Fluid Structure

Interaction using coupling of CFX with ANSYS

Moving Zones




• Setup 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 that are moving

Moving Zones




• See the following tutorials for information on other mesh motion types:

– “Fluid Structure Interaction and Mesh Deformation “

– Oscillating Plate with Two-Way Fluid-Structure Interaction”

• Dynamic remeshing is also available

– Advanced topic– Used for significant mesh

deformation, where addition/subtraction of elements is required to maintain element quality, e.g. Internal Combustion Engine

– Uses Multi-configuration setup– Dynamic remeshing occurs in ICEM

CFD using a range of meshing algorithms



Chapter 12 Appendix

Moving Solid Zones

Introduction to CFX

Moving Zones



Training ManualMotion 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 never 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

Moving Zones



Training ManualMotion in Solid Domains

• 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

– Rotational Motion• For example, a brake rotor

which is heated by brake pads

q’’’

q’’=0

q’’=0

q’’=0

Tin = Tspec

Moving Zones



Training Manual

ES SThU

t

h

)() (

Motion in Solid Domains

• 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

Moving Zones



Training ManualTranslating Solid Domains

• Enable Solid Motion

• Specify Solid Motion in terms of Cartesian Velocity Components

The solid should extend completely through the domain for this approach to be valid

Moving Zones



Training ManualRotating 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

The solid should be defined by a surface of revolution for this approach to be valid. For example a solid brake disk is OK, a vented brake disk is not.

Moving Zones



Training ManualRotating Solid Domains

– Rotating Domain Method• Set the Domain Motion to “Rotating”• Do not set any Solid Motion• Without using Transient Rotor Stator

interfaces, this method puts the mesh coordinates in the relative frame, but does not account for rotational motion of the solid energy

– Useful only for visualization of rotating components which are solved in the stationary frame of reference

• 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”

This is the most general approach and could be used to model a vented brake disk.

cfx12 12 moving_zones

Documents

moving zones introduction

reference frame flow

stationary frame

translational motion

domain boundaries

stationary domains

functions of time mesh

equations of fluid flow