adv rm v6.2 lect 02a srf

8/2/2019 Adv Rm v6.2 Lect 02a SRF

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Frame (SRF) Model

Frame (SRF) Model

Advanced FLUENT CFD TrainingAdvanced FLUENT CFD Training

2008 ANSYS, Inc. All rights reserved. ANSYS, Inc. Proprietar


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Fluent User Services Center

www.fluentusers.com

Advanced FLUENT Training v6.3

Rotating Machinery July 2008

Outline

ntro uct on to o e ng

Navier-Stokes Equations for a Rotating Reference Frame Relative and Absolution Velocity Formulations

SRF Problem Setup

Solver

Physical Models

Material Properties

Boundary Conditions

Solver Settings Initialization

Troubleshooting SRF Problems

2-22008 ANSYS, Inc. All rights reserved. ANSYS, Inc. Proprietar

Appendix


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Introduction to SRF Modeling

any pro ems w c nvo ve rotat ng components can e mo e e us ng a

single moving reference frame. Why use a rotating reference frame?

A flow field which is unsteady with respect to the stationary frame becomes

steady with respect to the rotating frame.

Steady-state problems are easier to solve...

mp er s

Lower computational cost

Easier to postprocess and analyze

e w scuss ssues re a e o mo e ng n s sec on, u many

concepts (e.g. solver settings, physical models, etc.) will also apply to MRF,

MPM, and SMM



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Characteristics of SRF Models

ov ng rame s assoc ate w t a s ng e u oma n.

In FLUENT, you may divide this domain up into several connected fluid zones,but each fluid zone must have the same moving reference frame speed and axis

Domain rotates with a constant, prescribed rotational speed about a specifiedax s o ro a on

No translation considered (though this may be included)

FLUENT only provides for a constant rotational speed in the user interface

cce era ng rames o re erence can e mp emen e roug user e ne

functions



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Characteristics of SRF Models

oma n typ ca y cons sts o

Inlets and outlets Walls

Rotationally periodic boundaries

Boundaries which move with the fluid domain may assume any shape.

Boundaries which are stationary (with respect to the laboratory or fixed

frame must be surfaces of revolution.

Rotationally periodic boundaries require spatial periodicity of all boundary



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Illustration of a Typical SRF ModelShroud/casing

Fluid domain

sur ace

Rotatingx

frameAxis of rotation

Hub surface

z


Blade surface


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Stationary Walls in SRF Models

Stationary walla e

Rotor

Wron !


Wall with baffles nota surface

of revolution!


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Rotating Reference Frames

e norma y escr e u mot ons w t respect to an a so ute or

inertial reference frame We can define a rotating reference frame as a reference frame which is

sp nn ng w t a prescr e or entat on an spee w t respect to an

inertial reference frame

The motion of the reference frame gives rise to additional accelerations

Non-inertial reference frame

The velocity of the fluid can defined with respect to either the absolute or

Absolute velocity - Fluid velocity with respect to the stationary (absolute)

reference frame

Relative velocity - Fluid velocity with respect to the rotating reference


frame


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The Velocity Triangle

e re at ons p etween t e a so ute an re at ve ve oc t es s g ven y

UWVrrr

+= velocityabsolute=Vr

rUrrr

velocityrelative=Wr

In turbomachinery, this relationship can be illustrated using the laws of

vector addition. This is known as the Velocity Triangle

Wr

Ur


Vr


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Navier-Stokes Equations (Rotating Reference Frame)

wo erent ormu at ons are use n

Relative Velocity Formulation (RVF) Obtained by transforming the stationary frame N-S equations to a rotating reference frame

Uses the relative velocit as the de endent variable in the momentum e uations

Uses the relative total internal energy as the dependent variable in the energy equation

Available for the Segregated Solver only!

Absolute Velocity Formulation (AVF)

Derived from the relative velocity formulation

Uses the absolute velocity as the dependent variable in the momentum equations

Uses the absolute total internal energy as the dependent variable in the energy equation

Available for all solvers Se re ated and Cou led NOTE: RVF and AVF are equivalent forms of the N-S equations!

Identical solutions should be obtained from either formulation with equivalent boundary

conditions



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Relative Velocity Formulation

(Continuity)0=+

Wt

r

(Momentum)

( ) rWWWt

W

r

rrrrrrrr

+++

)2(

brp ++=

&rrrr

etr b trtr

=r


TermSourceGenerationHeat=Q&


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(Relative total internal energy)( )22

tr 2

1

UWee +=

Viscous stress += 2T

WWWvr rrr

(Rothalpy)2

22 UWpehtr

+

+=



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RVF Accelerations Due to Rotating Frame

or o s an centr uga acce erat ons are treate as source terms n t e

momentum equationsrrrrr

Coriolis

acceleration

Centrifugal

acceleration



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Absolute Velocity Formulation

(Continuity)0=+ Wt

r

V rrrrrr

omentumbp

t++=++

(Energy)( ) ( ) Qb &rrrrr+++=+

VFVUpTkhWt

et

t



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(Relative total internal energy)2t21Vee +=

+= 2T

VVVrrr

3

(Total enthalpy)2

2Vpehtr ++=


TermSourceGenerationHeatForcesBody

==

QFb&


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AVF Accelerations Due to Rotating Frame

rrrrrrr=

or o s an centr uga acce erat ons re uce to a s ng e term

Coriolis

acceleration

Centripetal

acceleration



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Scalar Equations

ener c sca ar transport equat on n a mov ng re erence rame

( ) ( )+=+

SWt

r

tcoefficiendiffusionscalar

iablescalar var

=

=

The use of the relative velocity in the convective term implies that, for

=

- ,

This form is employed for turbulence models, species and phase transport,


e c.


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Summary of SRF Equations

wo orms o t e av er- to es equat ons can e app e to pro ems

(AVF and RVF) RVF only available for the pressure-based solver

Scalar transport equations can be transformed to moving frame by modified

convection term

Source terms may require modification depending on dependent variables

required (e.g. production term in turbulence model equation may need relative

velocity gradients)

Appropriate boundary conditions complete the problem specification

Inlet / outlet flow boundaries, walls, periodics, etc.



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SRF Problem Setup

e w ocus on aspects o mo e setup rect y re ate to pro ems

Topics SRF geometries (2D, 3D)

Solver Choices

Physical Models

Material Properties

Boundary Conditions

Solver Settings

Initialization



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SRF Geometries (2D)

p anar geometr es

Geometries rotate about axis normal to xy plane with specified origin (periodicboundaries are permitted)

ax symmetr c, ax symmetr c w t sw r

Geometries rotate about the x axis

y

x


Planar Axisymmetric

x


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SRF Geometries (3D)

ee to e ne ot

rotational axis origin anddirection for the fluid

Rotationally periodic

boundaries permitted

Origin

Axis of

r


ro a on


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Choice of Solver for SRF Models

ame cons erat ons or genera

flow field modeling apply to SRFsolver choice

egrega e o ver: ncompress e,

low speed compressible flows.

Fans

Pumps

Coupled Solvers: high speed

com ressible flows where Machnumber is above 0.3

High-pressure axial compressors

Turbines


Turbochargers


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Velocity Formulation Recommendations

se w en n ow comes rom a

stationary domain

Absolute total pressure, total temperature,

Use RVF with closed domains (all surfaces

are moving) or if inflow comes from a

rotating domain

Relative total pressure, relative total

temperature or relative velocities are known

in this case

As noted previously, RVF and AVF are

equivalent, and therefore either can be used

successfully for many problems


disappear with suitable mesh refinement.


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Disk Cavity Example

urpose ompare so u ons o a ne us ng ree eren pro emformulations

Case 1 - Stationary frame, moving walls

- ,

Case 3 - SRF, AVF

Disk cavity air flow study based on the experiments of Pincombe, 1981= = =

Solutions obtained for following conditions : Cw = Q/b = 1092,Re

= b2/ = 105

All cases use the same mesh 20576 uad cells 2D se re ated solver(axisymmetric with swirl), incompressible flow, RKE turbulence model,second order discretization.

Additional cases were computed using a fine mesh (82,304 cells) to examine the


mes n epen ence o e so u ons


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Disk Cavity Mesh

Bot wa s rotate

Inlet tube


InletAxis


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Disk Cavity Stream Function

Case 1 Case 2 Case 3

Separated flow


Nearly identical flow patterns observed for all three cases.


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Radial Velocity Profile (r/b = 0.633)



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Disk Cavity Results

x a orce an moment resu ts

Case Mesh Axial Force (103 N) Torque (103 Nm)

Coarse 6.431 7.231a onary

Fine 6.141 7.435

2 (RVF)Coarse

Fine

6.681

6.156

7.195

7.444

3 (AVF)Coarse

Fine

6.449

6.089

7.241

7.446

onc us ons

All three numerical approaches yield essentially the same results

Closer agreement is obtained through mesh refinement


esu ts emonstrate t e equ va ence o stat onary, , ormu at ons


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S i C


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Heat Transfer

,

SRF zone

Note: BCs for stationary walls must be circumferentially uniform

,

distributions relative to the rotating frame

Conduction and radiation models can also be enabled with SRF models o e: or con uc ng so s w c are con a ne n a mov ng re erence

frame, you shouldNOTactivate the Moving Reference Frame option!

Reason MRF option activates convection terms in the solid, which arent

relevant to SRF modelin

( ) ( ) ( )+=+ s STkCTVCTr

Solid convection term


velocitysolid=sVr

Fl t U S i C t


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DPM and Pathline Modeling

ou can use an

pathline models for SRFproblems

art c e pat s are compute

in the relative frame

If you want to see particlepat s n t e a so ute rame,

you can select this option in

the Pathlines panel.

o e a par c es mov ng n

absolute frame may hit wall

surfaces, since the rotation of


Particle injection at fan blade tips

Fl t U S i C td d FLUENT i i


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Other Physical Models

VOF, ASMM, Eulerian (Fluent 6), Cavitation models are all

compatible with SRF (and MRF, Sliding Mesh) modeling in Fluent

Exam les: mixin tanks cavitatin um s flows

Real Gas Model

Can model specific fluids using non-ideal gas equation of state-

Two options are available (Fluent 6.1)

NIST Library (REFPROP) - available fluids include: carbon

, , , , , ,of refrigerants (e.g. R11, R134a)

User-Defined Function user can write custom real gas propertylibrary


UDF source code available for Redlich-Kwong equation of state

Fl t U S i C tAd d FLUENT T i i 6 3


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Boundary Conditions For SRF Models

u

Inlet BCs Pressure Inlet

Shroud

Velocity Inlet

Mass Flow Inlet

Outlet BCs Blade

Outle

Pressure Outlet

Non-reflecting BCs

Mass flow outlet

Inlet

Walls

Periodics

Conformal

Hub


Non-conformal Axial Pump IGV

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Fluid Zone BCs

se u to se ect rotat ona ax s

origin and direction vector for rotatingreference frame

o e: a rec on vec ors s ou e

unit vectors, but FLUENT will

normalize them if they are not

Motion Type for SRF

Enter rotational speed

Can use negative value to reverse

sense of rotation




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Velocity Inlets

se or ncompress e, m ycompressible flows when inlet

velocity is known Can specify absolute or relative

velocities using Reference Frameoption

Can specify vector components orma nitude and direction in Cartesianor Cylindrical coordinates

For 2D, axisymmetric with swirl and3D problems you can specify

velocitntialuser tan e

inin,

=

+= V

VV


locityangular veuserin

n,

=



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Flow Direction

ow rect on s e ne as

the velocity vector normalizedby its magnitude Polar)-al(Cylindric

)(Cartesian

aarr

zzyyxx

d

d

++=

++=

nput a ows artes an or

Cylindrical-Polar coordinate

forms

Frame)(AbsoluteV

Vdabs

r

r

r

=

ote t at t e ow rect ons

differ in absolute and relative

frames!

Frame)(RelativeW

drel r=

e oc y r ang e ru e


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Pressure Inlets

ressure n ets can e use w t e t er ncompress e or compress e ows

Definitions of total pressure and total temperature depend on velocityformulation and compressibility:

Incompressible, AVF+=+=

2

22

2

2

1

p

ttC

VTTVpp

Neglected for

incompressible flow

Incompressible, RVF+=+= 2 2

2 p

trtrC

TTWpp

Incompressible, AVF

+=

+=

1

22

11

21

21 tt MTTMpp


Incompress e, RVF

+= += 2121 rtrrtr TTpp



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Pressure Inlets

pec y appropr ate tota pressure

and total temperature

If inlet flow is supersonic, you

such that desired the Mach number

corresponds toptotal/pstatic

Specify flow direction vector

Frame of flow direction depends

on velocity formulation!

If using AVF: absd

Specify other scalar BCs as

appropriate (energy, turbulence,

species, etc.)

re




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Mass Flow Inlets

ass ow n ets can e use w t

incompressible or compressible

flows

flux

Same flow direction options as

Velocity Inlet

Specify other scalar BCs as

appropriate (energy, turbulence,

species, etc.)




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Flow Direction and Mass Flow Inlets

ow o you eterm ne t e ve oc ty magn tu e now ng t e mass ow rate

and flow direction?

dVV=r&

r

)( ndVnVA

Vn ===

V

tV

d)( ndA

mV

=

&

n

areafaceboundary

rateflowmass

=

=

A

m& NOTE: For relative frame, substitute

relative velocity and direction (W) for


velocitytangentialvelocitynormal

==tn

VV absolute velocity (V)



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Pressure Outlets

pec y stat c pressure constant or pro e at t e out et.

Can employ a simple radial equilibrium assumption which computes a radialpressure variation from

The specified pressure is then

R

V

R

p =

assumed to be the hub static

pressure.

Appropriate for axial compressors

an ur nes, w ere e ow sparallel to rotational axis.

You must also specify appropriate


.



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Backflow

ac ow reverse ow at a oun ary typ ca y occurs w en t e stat c

pressure in a cell adjacent to a pressure boundary (Pc) forces the flow in adirection opposite to what is intended

or a ressure n e b < c : oun ary o a pressure s assume o e a

static pressure for the purposes of determining the flow velocity

Backflow scalars (temperature, species, etc.) are obtained from the solution b c

total pressure for the purposes of determining the flow velocity

Backflow scalars (temperature, species, etc.) are prescribed in the GUI panel

P < P P > P c


cb

Pressure Inlet

c b

Pressure Outlet

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Backflow

or a pressure n e , e ow rec on comes rom e ve oc y componen scalculated in the cell

For a pressure outlet, there are three methods for determining the direction of reversedflow:

Normal to boundary

From adjacent cell

Prescribed direction vector,

relative to the boundary in the absolute frame if AVF is used

relative to the boundary in the relative frame if RVF is used

Recommendations

As some backflow may occur during the solution process, prescribe reasonable values forall backflow quantities

Try to minimize (or eliminate) backflow by extending your boundaries further upstream ordownstream




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Target Mass Flow BC

or some pro ems t e n et mass ow rate an n et tota pressure an

temperature are known, and the exit pressure is unknown Example: fans and compressors for which the pressure rise is unknown

Fluent can address this situation with the target mass flow outlet BC

How it works

User sets the exit BC to a pressure outlet

Desired mass flow rate is prescribed

As calculation proceeds, exit pressure is adjusted automatically to achieve desired

mass flow rate




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Target Mass Flow Methods

wo met o s are ava a e or comput ng t e pressure or a prescr e mass

flow Method 1 (best suited for turbomachinery applications)

ass ow rate at t e out et oun ary s compute an compare to t e es re

mass flow rate

A pressure increment is determined based on the required change in mass flowrate the basic behavior is to:

Increase the exit pressure if computed mass flow > desired mass flow

Decrease the exit pressure if computed mass flow < desired mass flow

Method 2 (best suited for incompressible flows)

Same basic algorithm except the pressure increment is determined from a

linearized form of Bernoullis equation




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Target Mass Flow Outlet Setup

o act vate target mass ow

outlet , simply enable the Targetmass-flow rate option in pressure

desired mass flow rate

ou can spec y t e mass ow

outlet method in the text interface:

- -mass-flow-rate-settings>

enable? set method verbosity?




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Target Mass Flow Outlet Example

Convergence HistoryMass flow outlet BCapplied at compressor

outlet

Target mass flow:

. g s


Eckardt centrifugal compressor



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Non-Reflecting Boundary Conditions

tan ar pressure s or compress e ow x spec c ow var a es at

the boundary (e.g. static pressure at an outflow boundary)

Result: pressure waves incident on the boundary will reflect in an unphysical

Can lead to local errors and convergence degradation

Effects are more pronounced if the boundary is close to the blade (e.g. truncated,

Non-reflecting boundary conditions (NRBCs) permit waves to pass through

the boundaries without spurious reflections

Turbo-specific NRBCs

General NRBCs




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www.fluentusers.comRotating Machinery July 2008

Turbo-Specific NRBCs

ur o-spec c s are ase on t e axer- es stea y-state ormu at on

NRBCs are available for pressure inlets and pressure outlets only All pressure boundaries will be affected (cannot selectively activate NRBCs for

specific boundaries)

can coexist with other BCs (e.g. mass flow inlet)

NRBCs require the use of the steady-state, coupled solver Mesh requirements

The mesh at the pressure inlet/outlet boundaries must be a structured quad (2D)

or hex (3D) mesh

Note that away from the boundaries, any mesh type is permissible (e.g. hybrid tri-

quad mesh is permitted in 2D)

You can use NRBCs with mixing planes!




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Using the Turbo-Specific NRBCs

ur o- pec c s areactivated using the TUI

select enable? and answeres

/define/boundary-conditions/non-reflecting-

bc/turbo-specific-nrbc>enable? show-status

NRBC Controls (under the set/menu in the TUI)

discretization option to


bc/turbo-specific-nrbc> enable

enable non-reflecting b.c.'s [no] yes

permit first or second orderreconstruction at boundary faces

under-relaxation sets

the under-relaxation factor for

- - -

bc/turbo-specific-nrbc>

enable? set/

initialize show-status

NRBCs (0 < URF < 1)

verbosity option to enableprinting of debugging messages


bc/turbo-specific-nrbc>




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Turbo-Specific NRBC Example: 2D Turbine Vane

mp e vane c or = . m

Compressible flow, ideal gas (air)

Boundary conditions

n e o a pressure = . a m

Inlet total temperature = 300 K

Inlet turbulence intensity = 1%= .

Hybrid quad-tri mesh used (quad block at inlet)

Solutions on two meshes compared

- - Short mesh - mesh truncated near trailing edge - NRBCs required

Compare solutions on long mesh with solutions on the short mesh with andwithout NRBCs




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2D Vane Long Mesh




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2D Vane Short Mesh

Truncated downstream


boundary


R i M hi J l 2008


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2D Vane Long Mesh

Shock wave at

vane trailing edge



R t ti M hi J l 2008


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2D Vane Short Mesh NBRCs Off

onstant pressure

results in incorrect shock

location





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2D Vane Short Mesh NRBCs Activated

Non-reflecting boundary

conditions permit shockwave to pass t roug t e

boundary - shock location

is correctly predicted!





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General NRBC Formulation

new genera ormu at on as een eve ope or

Uses general characteristics-based algorithms from the literature Applies to pressure outlets only

Benefits

Can be used for both steady-state and unsteady flows

No geometry or mesh restrictions

Limitations

Can only be used with the coupled-explicit or coupled-implicit solvers (no

segregated solver implementation at this time)





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General NRBC User Interface

pt on se ecte n pressure out et pane

Permits selected enabling of the NRBCs (unlike Turbo-Specific NRBCs). Two options available

Pressure at Infinity

Assumes specified pressure

is defined downstream of-

Example: rocket nozzle

Average Boundary Pressure

is an average pressure at the

outlet boundary

Example: turbine blade row


ex t oun ary




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General NRBCs: 2D Stator Vane

Constant Pressure BC Non-Reflecting Pressure BC


Contours of Static Pressure (atm)




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Wall Boundary Conditions

a s en orce zero normavelocity at all wall surfaces

no slip (zero velocity) forviscous flows

For moving reference frames,you can specify the wallmotion in either the absolute

or relative frames

Recommended specificationof wall BCs for all movingreference frame problems

lab frame) use zeroRotational speed, Absolute

For moving surfaces, usezero Rotational s eed,


Relative to Adjacent CellZone




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Conformal Periodic Boundary Conditions

on orma per o c s requ re t at t e oun ary ace mes

element match one-for-one on the periodic boundary.

Rotationally periodic BCs rely on the rotational axis

.

Rotationally periodic boundaries can be used in SRF

problems to reduce mesh size, provided that both the

geometry and flow are periodic.

If you are using themake-periodic TUI command,

make sure you set the rotational axis

in the Fluid BC anel first beforecreating the periodic boundaries.

Once the periodic BCs have

been set, perform a grid check to see


correct.


fl




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Non-conformal Periodic Boundary Conditions

perm ts t e use o non-con orma rotat ona y per o c s.

Non-conformal periodics do not require a matching mesh on the boundaries. Coupling of the periodic zones is accomplished using the same algorithms

employed in non-conformal interfaces.

Setting up a non-conformal periodic BC is performed in the TUI:

define/grid-interfaces/make-periodic.

Shadow zone [()] interface.5

Rotational periodic? (if no, translational) [yes] yes

Rotation angle (deg) [0] 90

Periodic zones

Create periodic zone? [yes] yes

grid-interface name [] int1

er o c ang e



fl t




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Conformal vs. Non-Conformal Periodic Boundaries

Conformal

-



fl t




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Pressure-Based Solver Settings

ressure- e oc y oup ng e o

Coupled

Recommended (provides robust, fast convergence) Requires twice the memory relative to other schemes

SIMPLE, SIMPLEC, PISO

Use when computer memory is an issue (large mesh)

Pressure Interpolation

For highly swirling flows, use PRESTO! scheme

Other equations - use second order discretization

Can start with first order for stability, especially for problems with high rotational

spee s Compressible flows with Pressure-based solvers

May need to under-relax Density (0.1 is recommended)

Can also run with energy equation initially turned OFF enable the energy equation


after establishing a reasonable, isothermal flow field.


fl t




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Density-Based Solver Settings

mp c t sc eme s recommen e un ess computer memory s an ssue

Flux type options Roe-FDS baseline methods

AUSM can provide enhanced accuracy for strong shocks

Use first order discretization to begin your calculation, then switch to second

order when the solution is close to convergence

Use default Courant numbers as a start (1 for explicit solver, 5 for implicit

solver)

For coupled-explicit solver

Use 4 levels of FAS MultiGrid for most problems

helps propagate solution more rapidly through the domain

Use more levels of you have a very large mesh



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www.fluentusers.comg y y

Initialization

oo n a za on o e so u on s o en e ey o o a n ng rap anrobust convergence of turbomachinery problems

Less of an issue for

Fixed flow rate provides stability to the calculation

Problems with favorable pressure gradients (e.g. turbines)

Less propensity for reverse flow at boundaries Often critical for

Compressible flows with adverse pressure gradients (e.g. compressors, diffusers)

Adverse pressure gradient leads to reverse flows, solution instability

Use grid interpolation to patch a coarse mesh solution onto a fine mesh.

Use Full Multigrid (FMG) initialization technique.



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Solution Interpolation

roce ure

Run coarse grid version of your

model

r e a a o n erpo a on e

Set up fine mesh model

Read interpolation file to initialize

Advantages

Can be applied to nearly any

, Easy to use

Disadvantage


equ res eve opmen o coarse mes

model


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Full Multigrid (FMG) Initialization

n t a zat on opt on n uses t e u u t gr a gor t m n

FLUENTs Coupled Explicit solver to generate a system of coarse meshes

Solves the inviscid equations on coarsest mesh, interpolates to next finest, and so

Inviscid solution used as initial condition for subsequent full Navier-Stokes

calculation

Benefits

Convenient for user (no separate meshes or solutions required)

ny so ver can e use segrega e , coup e -exp c or mp c

Permits very large Courant numbers for coupledimplicit solver



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FMG Interface (TUI) FMG controls can be set in TUI usin

solve/initialize/set-fmg-

initialization

Permits setting of convergence

,

on each coarse grid level, Courant

number, verbosity

Once FMG arameters set the

initialization can be started using the

text command

solve/initialize/fmg-

n t a zat on



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FMG Initialization Eckardt Rotor

With FMG

Initialization

Without FMG

Initialization

~


,

Number of iterations reduced by a factorof 5 using FMG initialization!


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Troubleshooting SRF Problems

pro ems may e cu t to so ve ecause o arge ow gra ents

resulting from the rotation of the fluid domain

May need to use lower under-relaxations than default

Some things to consider for troublesome cases

Make sure the mesh quality is good (max cell skewness < 0.9 0.95)

Use FMG initialization for hard-to-start problems

Reduce under-relaxation factors and/or Courant numbers

Consider running the problem as a transient calculation

Can provide more robust convergence versus the standard steady-state approach

Use first order discretization in time and about 2-3 time steps per iteration

Run until steady-state is achieved



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Summary

mo e ng s t e s mp est mo e ng approac or rotat ng mac nery

Applications typically involve a single passage of a rotating machine (e.g.

single compressor blade row)

Some example applications are provided in Appendix A

FLUENT provides two formulations of the Navier-Stokes equations for

rotating reference frames




which are compatible with SRF modeling.


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SRF Examples

SRF Examples

3D propeller fan

3D cavitating centrifugal pump

3D propeller fan

3D cavitating centrifugal pump

2D axisymmetric flow in a labyrinth seal

3D flow in a transonic axial compressor blade row

2D axisymmetric flow in a labyrinth seal

3D flow in a transonic axial compressor blade row

2008 ANSYS, Inc. All rights reserved. ANSYS, Inc. Proprietar


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Propeller Fan

mo e o a our a e prope er an

Results compared with data from open literature:

, . . an ang, . ., umer ca nves ga on o e ua er ormanceCharacteristics of a Small Propeller Fan Using Viscous Flow Calculations,Computers and Fluids, 28 (1999), pp. 815-823.

Solutions obtained for a range of flow rates at 2000 rpm.

Numerical model Mesh size = 269265 cells (tets + wedges)

Segregated solver with moving reference frame

Incompressible flow (air)


Realizable k model with non-equilibrium wall functions


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Propeller Fan - Mesh



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Comparison to Data: Head Coefficient



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Comparison to Data: Power Coefficient



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Fan Flow Field: Flow Coefficient = 0.1Significant flow reversalu stream of fan face

Static pressure contoursdisplayed on fan surfaces



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Fan Flow Field: Flow Coefficient = 0.35

Strong radial outflow



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Fan Flow Field: Flow Coefficient = 0.5Strong axial outflow



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Cavitating Centrifugal Pump

was use to s mu ate ow n a centr uga pump w t cav tat on

effects

Geometry based on pump design reported in paper by Hoffman et al. (2001):o man, ., o e , ., r e r c s, ., osyna ., m ar es an eome r ca

Effects on Rotating Cavitation in Two Scaled Centrifugal Pumps, Proc. 4th

International Symposium on Cavitation, Pasedena, CA, June 2001.

TFA pump design used in the present study

Impeller diameter = 278 mm

Number of blades = 5

Speed = 2160 rpm

Single blade passage modeled with rotationally-periodic boundaries

Mesh Type: Hex mesh, 284,955 cells



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Pump Geometry

diffuser

impeller


inlet tube


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Pump Model Mesh

Single blade

passage



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Physical / Numerical Models

tea y-state so ut ons, segregate so ver

Incompressible flow, SRF, AVF

Multiphase cavitation model enabled

Mixture model used

Primary phase = water (density = 1000 kg/m3)

Pvap = 2620 Pa; surface tension = 0.0717 N/m,

Non-condensible gas = 1.510-5

Secondary phase = water vapor (density = 0.01927 kg/m3,

viscosity = 8.810-6Ns/m2)


Realizible k turbulence model with standard wall functions


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Case Studies

on-cav tat ng cases

Model run over a range of flowrates (100 275 m3/hr) to obtain non-cavitating

pump curve

ump ea r se pressure r se pre c e an compare o non-cav a ng a a

Cavitating Cases Flow rate fixed at design flow (210 m3/hr)

Exit pressure initial set to 600 kPa to ensure non-cavitating flow

Exit pressure decreased in 50 kPa increments to gradually develop cavitating

con t ons

Predicted head rise vs NPSH compared with data

NPSH =PinletPvapor



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Non-Cavitating Flow Comparison with Data



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Midspan Pressure Non-Cavitating Flow

Design Flow rate

03.0=



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Midspan Relative Velocity Non-Cavitating Flow

Design Flow rate

03.0=



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Cavitating Flow Comparison to Data



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Cavitating Midspan Pressure

Exit Pressure: 500 kPa



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Cavitating Vapor Volume Fraction


Cavitation

inception



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Significant

cavitation on

pressure side



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Illustration of Cavitation Induced Separation

Separation bubble

downstream of

vapor cavity

Exit Pressure

300 kPa



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Labyrinth Seal

ax symmetr c mo e o ve-toot a yr nt sea

Results compared with experimental data of Millward and Edwards (ASME

94-GT-56)

Numerical model Steady state, incompressible flow

2D axisymmetric with swirl and viscous dissipation

Se re ated solver

Realizable k turbulence model

Solutions calculated over a range of rotational speeds (3000 13000 rpm)



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Labyrinth Seal - Mesh

InletOutlet

-


straight seal


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Labyrinth Seal - Stream FunctionPR = 1.5 , N = 13000 rpm

Seal leakage flow



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Labyrinth Seal - Total Temperature



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Total Temperature

Windage heating due

to viscous dissipation



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Comparison with Data



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Transonic Axial Compressor

ranson c compressor rotor otor

36 blades

Design conditions

17188 rpm, PR = 2.1, mass flow = 20.2 kg/s

Numerical model

steady-state, compressible flow

coupled implicit solver

mesh: ~90,000 hex cells

standard KE turbulence model inlet TU=3.5%

inlet profiles from test data

back pressure varied to obtain speed line



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Rotor 37 - Mesh



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Comparison with Data - Pressure Ratio

Choked mass flow

predicted: 20.80 kg/s

data: 20.93 kg/s



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Pressure Ratio (Choked Flow)



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Relative Mach Number (Choked Flow)



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Pressure Ratio (94.3% Relative Mass Flow)



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Relative Mach Number (94.3% Relative Mass Flow)



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Centrifugal Compressor

c ar t otor otor- geometry

20 blades

Design conditions

14000 r m PR = 2.0 mass flow = 5.31 k /s

Numerical model

- , Coupled implicit solver

Mesh: 199,480 hex cells


-

Inlet profile from 2D axisymmetric solution


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Centrifugal Compressor Mesh



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Centrifugal Compressor Surface Pressure



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Centrifugal Compressor esu ts es gn on t on

Mass flow Pressure Efficiency

Fluent 5.31 2.08 88.8

Test Data 5.284 2.066 89.2

adv rm v6.2 lect 02a srf

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