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    10-1 ANSYS, Inc. Proprietary

    © 2009 ANSYS, Inc. All rights reserved. April 2, 2009

    Inventory !002"00

    Chapter 10

    Transient Flow Modeling

    Introductory FLUENTTrainingSharif Uniersity of Technology

    Lecturer! Ehsan Saadati

    ehsan"saadati#g$ail"co$

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    Transient Flow Modeling 

    10-2 ANSYS, Inc. Proprietary

    © 2009 ANSYS, Inc. All rights reserved. April 2, 2009

    Inventory !002"00

    Training Manual Motiation

    • Nearly all flows in nature are transient% –  Steady&state assu$ption is possi'le if we!

    • Ignore transient fluctuations• E$ploy ense$'le(ti$e&aeraging to re$oe unsteadiness )this is what is done

    in $odeling tur'ulence*

    • In CF+, steady&state $ethods are preferred

     – Lower co$putational cost

     –  Easier to postprocess and analy-e

    • Many applications re.uire resolution of transient flow! –  /erodyna$ics )aircraft, land ehicles,etc"* orte shedding

     – 

    2otating Machinery rotor(stator interaction, stall, surge –  Multiphase Flows free surfaces, 'u''le dyna$ics

     –  +efor$ing +o$ains in&cylinder co$'ustion, store separation

     –  transient 3eat Transfer transient heating and cooling

     –  Many $ore"

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    Transient Flow Modeling 

    10-# ANSYS, Inc. Proprietary

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    Inventory !002"00

    Training Manual 4rigins of Transient Flow

    • Naturally occurring transients

     –  transient flow due to growth of insta'ilities within the fluid or a non&e.uili'riu$

    initial fluid state

     –  Ea$ples! natural conection flows, tur'ulent eddies of all scales, fluid waes

    )graity waes, shoc5 waes*

    • Forced transients

     –  Ti$e&dependent 'oundary conditions, source ter$s drie the transient flow field

     –  Ea$ples! pulsing flow in a no--le, rotor&stator interaction in a tur'ine stage

    6elin&3el$holt- Cloud Insta'ility 7uoyant 7o Falling Into a 8ool of 9ater  

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    Transient Flow Modeling 

    10-$ ANSYS, Inc. Proprietary

    © 2009 ANSYS, Inc. All rights reserved. April 2, 2009

    Inventory !002"00

    Training Manual Transient CF+ /nalysis

    • Si$ulate a transient flow field oer a specified ti$e period

     –  Solution $ay approach!

    • Steady&state solution Flow aria'les stop changing with ti$e

    • Ti$e&periodic solution Flow eria'les fluctuate with repeating pattern

     –  :our goal $ay also 'e si$ply to analy-e the flow oer a prescri'ed ti$e

    interal"

    • Free surface flows

    • Moing shoc5 waes• Etc"

    • Etract .uantities of interest

     –  Natural fre.uencies )e"g" Strouhal Nu$'er*

     – Ti$e&aeraged and(or 2MS alues

     –  Ti$e&related para$eters )e"g" ti$e re.uired to cool a hot solid, residence

    ti$e of a pollutant*

     –  Spectral data fast Fourier transfor$ )FFT*

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    Transient Flow Modeling 

    10-% ANSYS, Inc. Proprietary

    © 2009 ANSYS, Inc. All rights reserved. April 2, 2009

    Inventory !002"00

    Training Manual Transient Flow Modeling 9or5flow

    • Ena'le the transient soler"

    • Set up physical $odels and 'oundary conditions as usual" –  Transient 'oundary conditions are possi'le you can use either a U+F or

    profile to acco$plish this"

    • 8rescri'e initial conditions

     – 7est to use a physically realistic initial condition, such as a steadysolution"

    • /ssign soler settings and configure solution $onitors"

    Configure ani$ations and data output(sa$pling options

    • Select ti$e step and $a iterations per ti$e step

    • 8rescri'e the nu$'er of ti$e steps"

    • 2un the calculations )Iterate*

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    Transient Flow Modeling 

    10-" ANSYS, Inc. Proprietary

    © 2009 ANSYS, Inc. All rights reserved. April 2, 2009

    Inventory !002"00

    Training Manual Ena'ling the Transient Soler 

    • To ena'le the transient soler, select the Transient 'utton on the ;eneral 

    pro'le$ setup for$!

    • 7efore perfor$ing iterations, you will need to set so$e additional controls"

     –  Soler settings

     –  /ni$ations

     –  +ata eport ( /utosae options

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    Transient Flow Modeling 

    10-& ANSYS, Inc. Proprietary

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    Inventory !002"00

    Training Manual Selecting the Transient Ti$e Step Si-e

    • Ti$e step si-e, t, is set in the 2un Calculation for$" –   t $ust 'e s$all enough to resole ti$e&dependent features< $a5e surethe conergence is reached within the nu$'er of Ma Iterations per Ti$eStep

     –  The order or $agnitude of an appropriate ti$e step si-e can 'e esti$atedas!

     –  Ti$e step si-e esti$ate can also 'e chosen so that the transientcharacteristics of the flow can 'e resoled )e"g" flow within a 5nownperiod of fluctuations*

    • To iterate without adancing in ti$e, specify -ero ti$e steps" Thiswill instruct the soler to conerge the current ti$e step only"

    • The 8IS4 sche$e $ay aid in accelerating conergence for $anytransient flows )set in the Solution Methods for$*"

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    Transient Flow Modeling 

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    Training Manual Transient Flow Modeling 4ptions

    • /daptie Ti$e Stepping

     –  /uto$atically ad=usts ti$e&step si-e

    'ased on local truncation error analysis

     –  Custo$i-ation possi'le ia U+F

    • Ti$e&aeraged statistics

     –  8articularly useful for LES tur'ulence

    calculations

    • For the density&'ased soler, the Courant

    nu$'er defines!

     –  The glo'al ti$e step si-e for density&'ased

    eplicit soler"

     –  The pseudo ti$e step si-e for density&

    'ased i$plicit soler 

    • 2eal ti$e step si-e $ust still 'e defined in

    the Iterate panel

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    Transient Flow Modeling 

    10-9 ANSYS, Inc. Proprietary

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    Training Manual Transient Flow Modeling /ni$ations

    • :ou $ust set up any ani$ations 7EF42E perfor$ing iterations"

     –  /ni$ation fra$es are written(stored on&the&fly during calculations"

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    Transient Flow Modeling 

    10-10 ANSYS, Inc. Proprietary

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    Training Manual Creating /ni$ations /lternate Method

    • /nother $ethod in FLUENT is

    aaila'le which $a5es use of the

    Eecute Co$$ands feature"

    • Tet co$$ands or $acros can 'e

    defined which are eecuted 'y the

    soler at prescri'ed iteration or

    ti$e step interals"

    This approach is ery useful increating high&.uality ani$ations

    of CF+ results"

     –  / co$$and is defined which

    generates an ani$ation fra$e

    )contour plot, ector plot, etc"*

    and then writes that fra$e to ahard copy file"

     –  Third&party software can then

    'e used to lin5 the hard copy

    files into an ani$ation file

    )/>I, M8;, ;IF, etc"*

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    Transient Flow Modeling 

    10-11 ANSYS, Inc. Proprietary

    © 2009 ANSYS, Inc. All rights reserved. April 2, 2009

    Inventory !002"00

    Training Manual 8erfor$ing Iterations

    • The $ost co$$on ti$e adance$entsche$e is the iteratie sche$e"

     – The soler conerges the current ti$estep and then adances ti$e"

     –  Ti$e is adanced when MaIterations(Ti$e Step is reached orconergence criteria are satisfied"

     –  Ti$e steps are conerged se.uentiallyuntil the Nu$'er of Ti$e Steps is

    reached"

    • Solution initiali-ation defines the initialcondition and it $ust 'e realistic" –  Sets 'oth the initial $ass of fluid in the

    do$ain and the initial state of the flow

    field"

    • Non&iteratie Ti$e /dance$ent )NIT/*is aaila'le for faster co$putationti$e"

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    Transient Flow Modeling 

    10-12 ANSYS, Inc. Proprietary

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    Training Manual Conergence 7ehaior 

    • 2esidual plots for transient si$ulations are not always indicatie of a

    conerged solution"

    • / residual plot for a si$ple transient calculation is shown here"

    • :ou should select the ti$e step si-e such that the residuals reduce

    'y around three orders of $agnitude within one ti$e step"

     –  This will ensure accurate resolution of transient 'ehaior"

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    Transient Flow Modeling 

    10-1# ANSYS, Inc. Proprietary

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    Training Manual Tips for Success in Transient Flow Modeling

    • Use 8IS4 sche$e for 8ressure&>elocity Coupling this sche$e

    proides faster conergence for transient flows than the standard

    SIM8LE approach"

    • Select the ti$e step si-e so that the solution conerges no$inally

    three orders of $agnitude for each ti$e step )of course, conergence

    'ehaior is pro'le$&specific*"

    • Select the nu$'er of iterations per ti$e step to 'e around ?0 it is

    'etter to reduce the ti$e step si-e than to do too $any iterations per

    ti$e step"

    • 2e$e$'er that accurate initial conditions are =ust as i$portant as

    'oundary conditions for transient pro'le$s initial condition should

    always 'e physically realistic%

    • Configure any ani$ations you wish to see 'efore running the

    calculations"