fluent12-lecture10-transient (1).ppsx

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

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Chapter 10

Transient Flow Modeling

Introductory FLUENTTrainingSharif Uniersity of Technology

Lecturer! Ehsan Saadati

ehsan"saadati#g$ail"co$


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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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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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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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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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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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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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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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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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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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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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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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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"

fluent12-lecture10-transient (1).ppsx

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