fluent12-lecture10-transient (1).ppsx
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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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Transient Flow Modeling
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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 $ore"
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Transient Flow Modeling
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Training Manual 4rigins of Transient Flow
• Naturally occurring transients
– transient flow due to growth of insta'ilities within the fluid or a non&e.uili'riu$
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$holt- Cloud Insta'ility 7uoyant 7o Falling Into a 8ool of 9ater
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Transient Flow Modeling
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Training Manual Transient CF+ /nalysis
• Si$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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Transient Flow Modeling
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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$$and is defined which
generates an ani$ation fra$e
)contour plot, ector plot, etc"*
and then writes that fra$e to ahard copy file"
– Third&party software can then
'e used to lin5 the hard copy
files into an ani$ation file
)/>I, M8;, ;IF, etc"*
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Transient Flow Modeling
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Training Manual 8erfor$ing Iterations
• The $ost co$$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"
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