numerical study using fluent of the separation and

8/18/2019 Numerical Study Using FLUENT of the Separation And

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NUMERICAL STUDY USING FLUENT OF

THE SEPARATION AND REATTACHMENT

POINTS FOR LAMINAR FLOW IN A

BACKWARDS-FACING STEP

by

Abhiroop Das(BME IV Roll-001011201105 Sec B-2-2)

Final year project submitted to SirHimadri Chatterjee of

Mechanical Engineering department of

Jadavpur University


2/24

What is a ac!"ardFacing Step#

• A backward facing step in 3D has beenshown in the following gure.Basically auid (air in our case) enters through the

inlet and encounters a step leading to owseparation and reattachment.


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Why is the study of ac!"ardfacing step important#

o aid in study of ow in channels withre!ersals."or e#ample

$eat e#changers

Ducts for industrial use.

%icroelectronic circuit boards.

"low around buildings.

&ombustors and afterburners insome engines.


4/24

')"low in ducts


5/24

) "low around buildings

3)Afterburners


6/24

What "ill "e be doing#

he aim of my proect is numerically studyof the separation and reattachment pointsfor laminar ow in a Backward "acing *tep

using Ansys '3 and compare the resultswith the e#perimental research conductedby Armaly et al. on the same topic.

"or comparision purposes the geometry

used by us is the e#act replica of thatadopted by Armaly et al and the geometryhas been specied below.


7/24

$he %eometry Used

• h+,nlet height- e +inlet length - o+outletlength- s+ step height - $+/utlet height+h 0 s

• 1#pansion ratio+$2h- e+eynolds4o.+u.h25(from this e6uation we calculated the inlet!elocity u)

• u+inlet !elocity - 5+kinematic !iscosity


8/24

he !alues of dimensions and parameters used

&ESC'()$(*

+

S,M*- &(ME+S(*+.m/

$eight of inlet

channelh 5.2

$eight of outlet

channelH 10.1

*tep height s 4.9,nlet channel length 0e 200/utlet channel

length0s 500)arame

ters

Symbol

s

1alues &imensions

Density 7 '.8 9g2m:3Dynamic

!iscosity

; '.?#'

@:3

9g2ms

1#pansio

nratio($2h

1 '.>?


9/24

Some previous research.by2rmaly/ on ac!"ard facing step

• "low can be considered laminar for e'@@ and

turbulent for e C @@• "or laminar ow multiple separation and reattachment

points were obtained .As the eynolds numberincreases from Eero the rst separation occurs on thebottom wall at a distance of x1.4e#t the second regionof separation occurs between x4 and x5 on the topwall.As the eynolds number increases a thirdseparation region occurs between x2 and x on thebottom wall.heoretically-recirculation Eones will

continue to de!elop as the eynolds number increasesand the ow remains laminar but this has not beenobser!ed e#perimentally and the ow e!entuallybecomes turbulent

• "or turbulent ows only single reattachment point is

obser!ed and the reattachment length is fairly


10/24

For laminar 3o"multiple reattachment

and separation

For turbulent 3o"single reattachment a

nd separation


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Methodology used in 2nsys

• he eynolds number is !aried from '@@ to

'@@@ in steps of '@@ and the separation andreattachment points are determined from theminimum !alues of skin friction coeFcent.

• *kin friction coeFcient is dened by

• and changes it sign at the

separation and reattachment points.

04+distance of rst separation point atbottom wallG 56+distance of secondseparation point at upper wallG 57+distanceof second reattachment point at upper wall


12/24

• he following boundary conditions wereusedH no slip walls- Eero gauge pressureoutlet- and constant normal inlet !elocity

that does not !ary along the inlet height.

• A coupled pressure and !elocity algorithm

has been used for laminar ows-whichsol!es the continuity and momentume6uations in a simultaneous fashion.&oupled algorithm does not pro!ide

solution accuracy impto!ement o!ersegregated sol!ers -rather it pro!idesimpro!ement in stability and ability tocon!erge.


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• A con!ergence criterion of '# '@:8

is used for continuity- # !elocity andy I !elocity. All solutions con!ergedwith second order pressure and thirdorder %J*&K(%onotone Jpstream&entered *hemes for &onser!ationKaws) momentum interpolationschemes for laminar ow.

• Ade6uate grid independence issatised with 6uadrilateral mesh of'33@ nodes consisting of ''=@@

elements.


14/24

*bservations and discussion

he plot below shows a closer !iew for

e'@@@ to compare present results withthe e#perimental data of Armaly et al.

@ '@@ @@ 3@@ ?@@ 8@@ @@ @@ '@@@@

?

=

'@

'

'?

'

'=

@

?

#'2s(presentscheme)

#'2s(Armaly)

#?2s(present

scheme)#?2s(Armaly)

#82s(presentscheme)

'eynolds number

08S


15/24

• Le can obser!e from the plot that

good agreement has been found withthe e#perimental !alues - althoughthe present !alues are slightly lowerand no ow separation has beenobser!ed for e !alues less than '@@.


16/24

E9ect of mesh re:nement on results

o demonstrate grid independence - the gridrenement was adapted for multiple times andthe summary is pro!ided in the able belowH

he third adaptation of mesh pro!idesreasonably accurate results and !ery littlechanges are obser!ed o!er further renementand thus the third one has been used for allows.

+o; of

elemen

ts

048S 068S 078S

3@'8' '@.=3 =.?? '>.8>8' ''.' >.? '>.=

'33@ ''.?@ >. @.'3


17/24

Comparision "ith some other authors

• he able below summariEes the

!alues for separation andreattachment points at e+=@@determined by !arious authors.2utho

r

048S 068S 078S .07. @.'3 '@.8'

Kima ''.>< >.8' @.?@ '@.=>

Armalyet al.

'?.@@ ''.'' '>.33 =.

Nu

and

*tella

'.@8 >.


18/24

he results of the present study are

lower than the a!erage of !aluesobtained by !arious authors -but arestill within the data range . he

largest diOerence between presentresults and a!erage literature !alueis the lower reattachment point-'2*+''.?- while the a!erage !alue

from !arious authors is '.


19/24

2+S,S F-UE+$ )-*$S8C*+$*U'SF*' 12'(*US )2'2ME$E'S

Contours of velocitymagnitude for 'e=>??


20/24

Contours of stream function for

'e=>??


21/24

A plot of skin friction coeFcient !s # positions fore+=@@ for !arious locations which was used to

obtain the reattachment points is gi!en in thefollowing "igure


22/24

he !alues for separation and reattachment pointsobtained in this study compare well with published data .

he present !alues begin to diOer from e#perimentaldata at eynolds number eC ?@@ where 3D eOects i.e

side separation become important .A general trend in thelaminar results of this analysis is slightly lower !alues for

separation and reattachment points than compared withother numerical studies . his diOerence with present

results for laminar ow can be attributed to the range of

methods and grid used to perform numerical calculations. Jnsteady methods iterated o!er a large time span are

typically used for laminar case because of con!ergenceissuesG howe!er- this study used a steady state methodwith a coupled pressure!elocity algorithm.

C*+C-US(

*+


23/24

'.&omputational uid dynamicsA practicalapproach Piyuan u -$eng Qeoh -&hao6uin lee

. 1#perimental and theoretical in!estigation

of backwardfacing step ow IB" Armaly-".

Dursts-P& Mereira

3."luid mechanicsDr A9 Pain

?.www.google.co.in

'EFE'E+C

ES


24/24

numerical study using fluent of the separation and

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