Moving Zones in ansys cfx tutorial

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Chapter 12
Moving Zones
Introduction to CFX
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Moving Zones
Topics
Training Manual
• 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
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Moving Zones
Overview of Moving Zones
Training Manual
• 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.
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Moving Zones
Overview of Moving Zones
Training Manual
• 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
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Moving Zones
Rotating Reference Frames vs. Mesh Motion
Training Manual
y
Moving Reference
Frame – flow
solved in a rotating
coordinate system
x
Domain
Mesh Deformation
– Domain changes
shape as a function
of time
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Moving Zones
Rotating Reference Frames
Training Manual
• 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)
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Single Frame of Reference (SFR)
Training Manual
• 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
baffle
Correct
Wrong!
rotor
The wall with baffles
is not a surface
of revolution! This
case requires a
rotating domain
placed around the
rotor while the
stationary wall and
baffles remain in a
stationary domain
stationary wall
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Moving Zones
Single Versus Multiple Frames of Reference
Training Manual
• 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
Domain interface
with change in
reference frame
Rotating domain
Single Component
(blower wheel blade passage)
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(blower wheel + casing)
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Moving Zones
Multiple Frames of Reference (MFR)
Training Manual
• 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
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Moving Zones
Frozen Rotor Frame Change Model
Training Manual
• Frozen Rotor: Components have a fixed relative position, but the
appropriate frame transformation and pitch change is made
– Steady-state solution
Pitch change is
accounted for
rotation
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Moving Zones
Frozen Rotor Frame Change Model
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
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Moving Zones
Stage Frame Change Model
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
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Moving Zones
Stage Frame Change Model
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
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Moving Zones
Transient Rotor Stator Frame Change Model
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
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Moving Zones
Pitch Change
Training Manual
• 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
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Moving Zones
Pitch Change
Training Manual
• 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
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Moving Zones
Pitch Change
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
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.
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Moving Zones
Pitch Change
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
– Axial Direction: used when the
axial variation is > radial variation
Split interfaces when they have both regions
of constant z and regions of constant r
• 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
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Moving Zones
Setup Guidelines
Training Manual
• 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”
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Moving Zones
Navier-Stokes Equations: Rotating Reference Frames
Training Manual
• 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
y
y
CFD domain

ro
rotating
frame
z
– Rotational source terms appear
in momentum equations
stationary
frame
z
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r
12-20
R
x


x
axis of
rotation
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Moving Zones
The Velocity Triangle
Training Manual
• 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  Absolute Velocity

W  Relative Velocity

W

V
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
U
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Moving Zones
Comparison of Formulations
Training Manual
• Relative Velocity Formulation: x-momentum equation



    

  wx
p
   W wx      vrx   2   W      r  ˆ
t
x
Coriolis acceleration
Centripetal acceleration
• Absolute Velocity Formulation: x-momentum equation

 

vx
p
   Wvx      vx     V  ˆ
t
x


Coriolis + Centripetal accelerations
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Moving Zones
Mesh Deformation
Training Manual
• 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
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Moving Zones
Mesh Deformation
Training Manual
• 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
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Moving Zones
Mesh Deformation
Training Manual
• 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
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Moving Zones
Mesh Deformation
Training Manual
• 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
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Chapter 12 Appendix
Moving Solid Zones
Introduction to CFX
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Moving Zones
Motion in Solid Domains
Training Manual
• 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
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Moving Zones
Motion in Solid Domains
• Solid Domain Motion can be
classified into two areas:
Training Manual
q’’=0
Tin = Tspec
– Translational Motion
• For example, a process where
a solid moves continuously in
a linear direction while
cooling
q’’=0
q’’=0
q’’’
– Rotational Motion
• For example, a brake rotor
which is heated by brake pads
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Moving Zones
Motion in Solid Domains
Training Manual
• 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
 h 
   (  U S h )    (  T )  S E
t
Solid Velocity
• Note that the solid is never physically moved when using this
approach, there is only an additional advection term added to the
energy equation
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Moving Zones
Translating Solid Domains
Training Manual
• 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
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Moving Zones
Rotating Solid Domains
Training Manual
• 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.
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Moving Zones
Rotating Solid Domains
Training Manual
– 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.
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