Impinging Single and Twin Circular Synthetic Jets ...

11TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY – PIV15 Santa Barbara, California, September 14-16, 2015

Impinging Single and Twin Circular Synthetic Jets Flow Field Castrillo G.1, Greco C.S.1, Crispo C.M.1, Astarita T.1 and Cardone G.1 1

Department of Industrial Engineering, University of Naples Federico II, Naples, Italy [email protected]

ABSTRACT The behavior of single and twin circular synthetic jets devices is experimentally investigated by using Particle Image Velocimetry (PIV) at a Reynolds number equal to 5,100 and a Strouhal number equal to 0.024. The twin synthetic jets are in phase opposition and different inter-axes distances (l) have been studied. Moreover, several nozzle-to-plate distances (H/D=2, 4, 6, 8 and 10) have been investigated. The twin synthetic jets show an interaction which causes higher timeaveraged axial velocities and fluctuations than the single synthetic jet case and lower jet width. The time-averaged turbulent fluctuations show that both the single synthetic jet and the twin synthetic jets have a region characterized by low values of turbulence (potential core-like region). The evolution of the mean and statistics quantities have been described through phase-averaged measurements. High turbulence is observed along the shear layer emanated by the nozzle edge and in the vortex ring core. Also the saddle point behavior has been investigated. 1. INTRODUCTION Synthetic jets are zero-net mass flux jets produced by the interaction of a train of vortices caused by periodic ejection and suction of fluid across an orifice or a nozzle. This periodic ejection and suction across the nozzle is generated by an oscillating membrane (such as a piston, a loudspeaker or a piezoelectric). These membranes create an oscillating pressure inside the cavity which allows the generation of the synthetic jets through the nozzle. Although they conserve the mass, they transfer a positive net momentum to the external ambient. Synthetic jets formation and evolution were initially investigated by Smith and Glezer [1]. When the ejection stroke begins, a pair of vortices is generated at the exit edges due to the flow separation. Subsequently, this vortex pair convects downstream, if the formation criterion is satisfied [2], and a steady turbulent jet is created [1]. ⁄ and the The operating parameters which govern the synthetic jets behavior are the Strouhal number, ⁄ , where Reynolds number, is the characteristic velocity of the jet, is the orifice diameter and is the actuation frequency. The characteristic velocity, according to [1], is defined as: (1) is the exit velocity on the jet axis. where 1/ is the phenomenon period and Synthetic jets have been used in many applications: flow control, thrust vectoring and heat transfer. As for continuous impinging jets [3, 4, 5, 6] also the heat transfer capabilities of synthetic jets have been investigated [7, 8]. In order to enhance their heat transfer rate several synthetic jet innovative configurations have been studied over the last years. Rylatt et al. [9] confined the impinging synthetic air jet in order to draw cold air from a remote location during the suction phase. This ducted configuration leads to achieve a heat transfer enhancement of 36% in the stagnation region. Chaudhari et al. [10] proposed a synthetic jet with the central orifice surrounded by multiple satellite orifices. The experiments, carried out at 1,000 < Re < 2,600 and 1 < H/D < 30, show a heat transfer coefficient higher (approximately 30%) than the conventional single orifice jet. A new generation of synthetic jet actuators consisting in two cavities sharing the same oscillating membrane (a piezoelectric) was proposed by Luo et al. [11]. A slide block was used to separate the two exit slots at an appropriate distance. They obtained, through a numerical simulation, a device which not only doubles the function of the existing synthetic jet with a single diaphragm but also resolves their problems of pressure loading and energy inefficiency. In the near field, they found [12], through PIV measurements, a flow characterized by a “self-support” phenomenon between the two synthetic jets while, in the far field, the two jets merge into a single and more stable synthetic jet. A double slot synthetic jets device was investigated by Persoons et al. [13]. Both PIV and IR thermography heat transfer measurements were carried out, finding a 90% enhancement of the maximum and overall cooling rate, compared to a single

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11TH INTERN NATIONAL SY YMPOSIUM ON N PARTICLE IIMAGE VELO OCIMETRY – P PIV15 Santa Barbara,, California, Sepptember 14-16, 2015

jet, for a phaase shift equaal to 120° at jet-to-surface spacing of 112 diameters. Lasance et all. [14] replaceed the classiccal circular singlle jet configurration with a ddouble circularr configurationn. The double configurationn is found to bbe advantageouus because of nooise reductionn [15] and imprrovement of hheat transfer peerformances [16]. The free flow w field and thee heat transferr performancees of a twin ciircular configuuration, compared to a singgle synthetic jeet, was investigaated by Greco et al. [17, 18]]. Their PIV experiments [18], carried outt using a devicce composed bby two adjaceent synthetic jetss with a 180° pphase shift, result in higher streamwise veelocity compoonent and loweer jet width. Moreover, M a heeat transfer enhaancement is obbtained in the ttwin configuraation characterrized by the tw wo adjacent syynthetic jets [117]. The aim of tthe present woork is to inveestigation the impinging floow field of cirrcular single synthetic s jets (SSJ) and tw win synthetic jetss (TSJ). Experiments are carried ouut setting the jet axes distannce equal to 1.1 and 3 nozzlee diameters annd varying thee nozzle to plaate ween 2 and 10 nozzle diam meters. The innvestigation iss performed ffor a fixed vaalue of the Reeynolds numbber distance betw (5100) and Strouhal numbeer (0.024). 2. EXPERIIMENTAL A APPARATUS S A scketch o of the twin circular air synthetic jetts (TSJ) device is represeented in Figgure 1. A lou udspeaker, th he CIARE®HS2 250, whose diameter is 27 70 mm, splits the cavity in n two sub‐cav vities with a volume Vol equal e to 2 dm m3. Two nozzless, having a len ngth L = 210 mm and an in nner diameteer (D) of 21 m mm, are attach hed to both th he sub‐cavitiees. The distancee between theeir axes, l, is vvaried duringg the tests: deefining the dim mensionless p parameter Ʃ= =l/D, Ʃ = 1.1an nd Ʃ = 3 configu urations are aanalyzed. Thee distance l = 1.1 D corresp ponds to the condition forr which the tw wo nozzles arre adjacent. Th he single syntthetic jet (SSJJ) device is obtained subsstituting one of the straigh ht pipes with h a bended on ne (checking th hrough the pressure transd ducers that no o pressure vaariations are iinduced in the sub‐cavitiess). The nozzle-too-plate distannce (H/D) is aaccurately set through a lineear slide on w which the syntthetic jet is loocated. A rotaary table allows to t rotate the im mpinging platee in order to seet the plate itself perpendicuular to the jet axis. a The two sub--cavities are ddesigned in ordder to have thee same resonaance frequencyy. Indeed, the nozzle lengthhs and diameteers are equal for both the syntthetic jets and the same cavvity volume is achieved by filling f the uppper and the bottom sub-caviity wo using particuular geometricaal items. A coomplete devicee characterizattion is reportedd in Greco at al. [18]. It behhaves like a tw degrees of freedom mass-sspring damperr system [19] with two resoonance frequenncies (experim mentally evaluated) equal too 4 Hz, respectivelly. Hz and 210 H

. Figure 1

Twin synthetic jets devicee.

The instantanneous velocityy fields are meeasured througgh the PIV tecchnique, as shhown in Figuree 2. The planee containing thhe two jet axes is i illuminated by using the QUANTEL Q E EVERGREEN laser (Nd-YA AG, 200 mJ/puulse). The timee delay betweeen the two laserr pulses is sett equal to 65 µs. The pulsee duration is of 5 ns and thhe sheet thickkness is of abbout 1 mm. Thhe synthetic jetss impinge on a glass plate (400 mm widde, 400 mm llong and 10 mm m thick) whhich is big enoough to negleect boundary efffects. Air is seeeded by olive oil droplets haaving a nominnal diameter off about 1 µm. The acquisitiion system is ccomposed by a camera (AN NDOR-Zyla 5.5 Mpixel sCM MOS) equippeed with a 50 m mm focal lenggth lens. The relative aperturee f# is set to 11. The pixel array a size is 22560 x 2160 ppixels and eacch pixel depth is 16 bits. Thhe resulting resoolution for thee digital imagees is 10.84 pixeels/mm in the illumination pplane. The louudspeaker is suupplied with a 4 Hz sinusoidaal input signal generated byy a signal geneerator (DIGIL LENT Analog Discovery™)). In order to perform phasseaveraged PIV V measuremennts, the phenom menon has beeen sampled att a frequency f = 3.87 Hz (according to [[17]) so to havve

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30 phases (eaach one everyy 12°). A totall of 12,000 doouble frame im mages are acqquired to obtaiin reliable meean velocity annd turbulent stattistics. The vectors fields are obbtained processsing images with multiple pass algoriithm and winndow deformaation, using thhe Blackman weighting winddows, accordinng to Astaritaa and Cardonee [20] and Astarita [21, 22,, 23]. The finnal interrogatioon window size is 24 x 24 pixxels with 75% overlap. The resulting r vectoor pitch is equual to 0.55 mm m (39.2 vectorss/D). The uncertainnty in displaccement, relatedd to the interrrogation area size, is the m main contributiion to uncertaainty in velociity measurementts. In these expperiments, acccording to [200] and [21, 22, 23], for the uused particles ((with diameterr approximateely equal to 2 piixels) and for a signal to nooise ratio ≥ 5, the uncertainnty was found to be almost everywhere ssmaller than 00.1 pixels providding a velocityy uncertainty oof 2% in the jett.

Figure 2

Twin synthetic jets devicee.

3. TRIPLE DECOMPOSITION ANA ALYSIS Data are anaalyzed througgh the applicaation of the triiple decompo osition ([24]aand [25]): , , ′ , where u1 = uu, u2 = v, u3 = w and , , . Ui is the time-avveraged veloccity componennt, organized conntribution to tthe velocity annd u’i is the turrbulent velocitty fluctuation.. The time-aveeraged velocityy is defined ass: lim m → , while the phaase-averaged vvelocity is: ∑ , lim → , where N is thhe number of tthe instantaneoous flow fieldss at same phasse and n is a nnatural numberr. Therefore, thhe phase-correllated organizeed contributionn is: , , and the turbuulent velocity fluctuations f arre evaluated ass: , , ,

(2) is the pphase-correlateed

(33) (44)

(5) (66)

4. RESULT TS The single syynthetic jet feeatures and tthe influence of inter‐axes distance in the twin synth hetic jets are analyzed at R Re = 5100 and SSr = 0.024. On nly the resultts for the sin ngle and twin synthetic jetts with Σ = 1.1 are reporteed because th he twin configu uration with Σ Σ = 3 shows aa behaviour ssimilar to two o superimpossed separate iimpinging syn nthetic jets (aas also found fo or the free configuration b by Greco et al.., [17]). Because of thheir symmetryy with respectt to the yz plaane, all the tim me-averaged m measurements and the SSJ phase-averageed ones are obtaained averaginng the x0 maps in order to rreduce the meaasurement noiise. Moreover,, for the sake of brevity, onlyy the maps forr H/D = 2 andd 6 are shownn: these two coonfigurations have been chhosen because the jet featurres and behaviouur are highly ddifferent for H/D H/ ≤ 4 and forr H/D > 4.

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4.1 TIME-A AVERAGED MEASUREM MENTS FOR R SINGLE SY YNTHETIC JJET (SSJ) In Figure 3 the t SSJ axial and radial tim me-averaged vvelocity compponents are shhown. The flow w field morphhology stronggly depends on thhe nozzle-to-pplate distance. For H/D = 2 tthe flow field is affected byy the presence of the impingging plate whicch influences it up to about 1 D upstream m. For this reaason, at this nozzle-to-plate distance, thee centerline veelocity attains a maximum veelocity smallerr than the onee reached by tthe higher H/D D cases. The region r (nearbyy the plate), characterized c bby high radial vvelocity valuees, shows an axial a extensioon which, starrting from thee axis, abrupttly increases, then decreasees, reaching a m minimum at x/D /D = 1.2, and finally slightlyy increases. C Completely diffferent is the bbehavior at hiigher nozzle-ttoplate distancees. Indeed, at H/D = 6, a m mild increase oof the axial exxtension can be b observed att the beginninng and any loccal minimum cann’t be detectedd. The axial annd radial velocity behavior near the imppinging plate (at a distancee of 0.05 D ffrom the impiinging plate) is depicted in F Figure 4. At H/ H/D = 2, the axxial componennt shows a locaal minimum onn the jet axis aand a maximuum at x/D = -00.5 (as found in [26]). This double-peak d pprofile is ascriibed to the addverse pressurre gradient caaused by the iimpinging plaate presence. Ass x/D increases, some oscilllations can bee observed. Suuch oscillationns are caused by the presennce of a countter rotating vorteex ring on thee plate (see phhase-averaged measurementts). Such a couunter rotating vortex ring iss created by thhe passage overr the impinginng plate of thhe sweeping vortex v ring. A At H/D = 4, the inner peakk is barely evvident, the axiial velocity valuue on the jet axxis is increasedd and the oscilllations are stiill present. At H/D ≥ 6 the maximum m is loocated on the jjet axis (bell-shaaped profile) aand it decreasees as the nozzzle-to-plate disstance increasses. Moreover,, for H/D > 6 the oscillationns are not visiblle. The radial coomponent, at H/D = 2, inccreases from tthe stagnation point to x/D = 0.75 wheree its value is about 0.8. Thhis strong peak iis strictly relatted to the doubble peak profille of the axial velocity. At hhigher x/D, soome oscillationns are present in the same possitions where the axial onees are observeed. As the noozzle-to-plate distance increeases several features can be b observed: thee peak decreasses (apart from m 4 ≤ H/D ≤ 6 where it remaains constant),, it shifts towaards greater x position p and thhe lateral oscillaations disappear. Figure 5 show ws the mean-ssquared axial and radial phhase-correlatedd organized coontributions too velocity. Theey resemble thhe time-averageed axial and rradial componnents, respecttively. Howevver, the axial phase correlaated componeent is ten tim mes higher than the t radial onee. This can bee explained coonsidering thaat these contriibutions are ddue to the exteernal sinusoiddal force that perrturbs the flow w field acting mainly in the axial directioon. This also cclarifies why thhe highest vallues of the axiial component arre on the nozzzle exit sectionn.

Figu ure 3

Axiaal and radial tim me-averaged velocity v compponent for H/D D = 2 (top) andd H/D = 6 (boottom).

Figure 6 show ws the mean-ssquared turbullent contributions to the veloocity componeents. It is worrth noting that a region of loow turbulence, similar to the ppotential core one for continnuous jets, cann be observedd between the two shear layyers (where higgh turbulent valuues are attaineed). Such highh values alongg the shear layeers are due to two main conntributions: thee passage of thhe vortex ring, w whose core haas high turbuleent values, andd the shear layyers generatedd between the trailing jet annd the quiesceent ambient. At short nozzle-tto-plate distannce, the two shhear layers doo not merge, differently d froom the configuurations at H//D higher than 4. The radiall component shows s high vvalues along tthe shear layeers but its maaximum is atttained near thhe impinging plate. These maaps are very sim milar to those presented by Narayanan et al. [27] and Roux R et al. [288] for impinginng

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continuous jeets. However, although in thhe region not iinfluenced by the plate the aaxial componeent values are about twice thhe radial compoonent ones (as observed in [27] [ for an im mpinging contiinuous jets sloot), near the w wall they are abbout 40% of thhe radial compoonent ones (forr H/D lower thhan 4), differeently from the circular continnuous jets where Knowles eet al. [29] founnd the axial turbbulent componnent equal to aabout the 60% of the radial tturbulent one.

Figure 4

Time-averaaged axial (lefft) and radial ((right) velocityy components near the impinnging plate foor all the H/D configurationns.

Figure 5

Mean-squared axial (left) and radial (rigght) phase-corrrelated organiized contributtion to the veloocity maps forr H/D = 2 (ttop) and H/D = 6 (bottom).

Figure 6

Mean-squareed axial (left) aand radial (rigght) turbulent vvelocity mapss for H/D = 2 (top) ( and H/D D = 6 (bottom)).

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Figure 7-a) shows s the behaavior of the m mean-squared pphase-correlatted organized contribution to t the velocityy near the platte. At H/D = 2, the axial com mponent has ttwo peaks loccated at x/D = -0.5 and -2, approximatelly, while a loccal minimum is present on thhe jet axis. At H H/D = 4 the m minimum valuee increases whhile the inner maximum m is oonly barely vissible and shifteed through the jet axis. On the other hand, the outer maxximum increasses and movess to x/D = -2.11. As H/D risees, the profile is bell shaped nnear the jet axxis and the outter peak decreeases. Even thhe radial compponent shows two peaks at x/D = 0.75 annd 1.8, at H/D = 2. As H/D inncreases, the fiirst peak decreeases while thee second one tends t to coalessce with the fiirst.

a) b) Figure 7 a) Mean--squared axiall (left) and raddial (right) phaase-correlated organized conntribution to thhe velocity b) Mean-squarred axial (left)) and radial (riight) turbulentt componentts near the imppinging plate ffor all H/D connfigurations; b vvelocity compoonents near the impinging pplate for all H/D /D configuratioons. In Figure 7-bb) the mean-sqquared axial and a radial turbbulent componnents over thee wall, for all tthe nozzle-to--plate distancees, are reported. At H/D = 2 thhe axial compoonent presentss a local minim mum in the staagnation pointt because the ttwo shear layeers do not have enough space to merge. Ann inner peak, due to the sheear layers influence, is locaated at x/D = -0.65. An outter The positions of peak is obserrved at x/D equual to -1.85:it can be ascribed to the preseence of the coounter rotatingg vortex ring. T the maximum m axial fluctuuations corresspond to thosse found by O’Donovan [[30] and Rouux at al. [27] for impinginng continuous jeets. At H/D = 4 the minimuum, still locatted on the jet axis, increases; moreover, tthe inner peakk moves towaard the stagnationn point, increaasing its valuee, and the outeer one moves toward x/D = -2.2. The seccond maximum m has a smalller value than thhat related to H H/D = 2 configguration becauuse the counteer rotating vorttex is less eneergetic. As thee nozzle-to-plaate distance incrreases to H/D = 6, the inneer peak has aalmost arrivedd to the stagnaation point, beecause the shhear layers havve completely m merged, whilee the second ppeak (at x/D = 2.2) is redduced becausee the counter rotating vorttex ring is onnly partially geneerated. In thiss configurationn Roux et al. [28] have nott detected the outer maximuum for impingging continuouus jets, most proobably because in their casee the counter rootating vortexx ring is created by the passaage of a vortexx ring generateed by the Kelvinn-Helmholtz innstability. Thiis is less energgetic than the primary vorteex created duriing the expulssion stoke of thhe synthetic jet. At H/D = 8, a maximum iis observed onn the jet axis w while the outeer one is barelly appreciable. At the higheest tested nozzlee-to-plate disttance, the staagnation valuee decreases aand the outerr peak completely disappears. The radiial component, aat H/D = 2, shhows a plateauu from the axis to x/D = 0.55. Then, it rapiidly increases and reaches its highest valuue at x/D = 1.6. An additionall peak is locateed at x/D = 2 (as reported inn [30]). As H//D increases, tthe stagnation value increases and the plateau disappears; on the other hand, the peaaks merge in a single one wiith a smaller vvalue and movves to about x//D meter if the nnozzle-to-platee distance is hhigher than thhe = 2.1. Its poosition seems not to be inffluenced by thhe H/D param potential coree-like region length, differenntly to the pheenomenology characterizingg the continuouus jets. In Figure 8, tthe centerline axial velocityy and the jet w width (defined as in [1]), for all the tested nozzle-to-platte distances, aare reported. It iss evident that tthe plate influuences the flow w field up to aabout 1 D upsttream (in accoordance with [330]). Moreoveer, at H/D = 2 aand 4, the maxximum value, located approoximately at z/D z = 3.5, is nnot reached. T The presence oof the plate alsso causes a jet w width increasinng. Such effeccts are more visible v in the sshorter configuurations becauuse for the higgher H/D values the jet itself iis already spreead when it is approaching tthe impinging plate.

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Figurre 8

Time-aaveraged centeerline axial veelocity (left) annd jet width (rright) for all thhe H/D configuurations.

4.2 TIME-A AVERAGED MEASUREM MENTS FOR R TWIN SYNT THETIC JET T (TSJ) The time-aveeraged axial annd radial velocity maps are reported in Fiigure 9. The m maximum axiaal velocity vallue, for both thhe configurationns, occurs alonng x/D = 0 annd it is higher than that attaiined by the SS SJ case. Also for the radial component, thhe values are hiigher than thoose observed for the SSJ cconfiguration bbecause of thhe presence off two wall jetts. Furthermorre, varying the nnozzle-to-platee distance, thee same consideerations aboutt the maps moorphology, as ffor SSJ configguration, can be b done.

Figure 9

Time-avveraged axial ((left) and radiaal (right) veloccity maps for H H/D = 2 (top)) and H/D = 6 (bottom).

The time-aveeraged axial annd radial compponents, near tthe plate, are shown s in Figuure 10. Considdering that the nozzle axes aare located at x/D D = ±0.55, ass for the SSJ ccase, for the sm maller distancce, a minimum m is located neear the axis (aabout at x/D = 0.6) while tw wo peaks aree present at x/ x/D = 0 and -0.9. These positions p do nnot correspondd to those evvaluated simpply superimposinng two shiftedd SSJ. This is due to the interaction of thee two 180° phhase-shifted jeets that causess jets deflectioon. This effect iss more evidentt as H/D increeases. Some osscillations are barely evidennt but they disaappeared as H/D H increases sso that a unique peak is locateed at x/D = 0. Therefore, at H H/D=4, the m maximum value is reached allong the jet axxis (x/D=-0.555), as occurs for the single jet flow field. /D = 1.2. Somee oscillations are present alsso For the shortest configurattion, the radiall component ddisplays a peakk located at x/D in this case bbut they have of lower ampplitude than those characteriizing the SSJ device. As H//D increases, tthe peak moves outward and the oscillationns disappear. A As previously said, these osscillations are due to the couunter rotating vortex ring buut, TSJ), they are ddumped becauuse of the pressence of two w wall jets sweepping the impinnging plate. in this configguration (i.e. T

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The mean-sqquared axial annd radial phasee-correlated orrganized contrributions to the velocity com mponents are sshown in Figuure 11. They aree different from m those previiously seen foor SSJ case beecause of the destructive innterference of the two shifteed sinusoidal foorces. Indeed, the axial com mponent show ws smaller valuues around x/D /D = 0 and a lower axial extension e of thhe region wheree / is hiigher than 1.

Figure 100

Time-averaaged axial (lefft) and radial ((right) velocityy components near the impinnging plate foor all the H/D configurationns.

For the radiaal component, the interferennce acts constrructively for --0.55 ≤ x/D ≤ 0.55. This is evident in thee H/D = 2 maap: near the platee the two walll jets, movingg in opposite direction, d invoolving high vaalues of / . Far from the jet axis, thhe values are sm maller than thhose related too the SSJ casee because the two wall jets have the sam me direction. A At H/D = 6 thhe influence of the t interferencce is less visibble.

Figure 11

Mean-squared axial (left) and radial (rigght) phase-corrrelated organiized contributtion to the veloocity maps forr H/D = 2 (ttop) and H/D = 6 (bottom).

Figure 12 reepresents the mean-squaredd axial and raadial turbulentt velocity maaps related to the H/D = 2 and H/D = 6 configurationns. The axial ccomponent, allong the shear layers, is highher than that evaluated e in thhe SSJ case, eespecially in thhe region near thhe axis. Moreoover, it is posssible to noticee that the two shear s layers merge m at about z/D = 2, diffeerently from thhe SSJ case whhere it occurs at higher z/D D. This is duue to a deflection of the tw wo jets towarrds the centerr caused by thhe interaction. Inndeed the poteential core-likke region has thhe same extennsion, as visiblle in the phasee averaged measurements. The mean-sqquared axial phhase-correlatedd organized velocity compoonent, near thee plate, for all the H/D valuues are shown in Figure 13-a. A At the shortesst nozzle-to-plate distance, tw wo peaks are ppresent: the fiirst is located at a x/D = 0 whiile the second is at about x/D = -1. The firsst is lower thann the other beecause of the ddestructive intterference. A m minimum is appproximately at x/D = -0.45 while some oscillations aree detectable att x/D higher thhan 1. Their aamplitude is loower than thoose observed for f M the oscillations aare the SSJ case.. As H/D increeases, the firstt peak disappeears and the seecond one mooves inward. Moreover,

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damped. Thee mean-squareed radial phasee-correlated orrganized veloccity componennt, at H/D = 22, has a first m maximum in thhe stagnation pooint and a second one at x/D /D = 1.2 whilee a minimum is located at xx/D = 0.9. At higher radial positions som me oscillations are a visible. Ass H/D increasees, the first peak initially deecreases and suuccessively diisappears; the oscillations aare damped.

Figure 12

Mean-squareed axial (left) and radial (rigght) turbulent vvelocity mapss for H/D = 2 ((top) and H/D D = 6 (bottom).

b) a) Figure 13 a) Mean--squared axiall (left) and raddial (right) phaase-correlated organized conntribution to thhe velocity componentts near the imppinging plate ffor all H/D connfigurations; bb) Mean-squarred axial (left)) and radial (riight) turbulentt vvelocity compoonents over thhe impinging pplate for all H/D /D configuratioons. The mean-sqquared axial aand radial turrbulent velociity componennts, over the plate, p for all the H/D connfigurations, aare reported in F Figure 13-b. T The axial compponent, in thee shortest H/D D case, has thrree peaks, resppectively at: xx/D = -0.3, -11.2 and -2.3. Com mparing to thee results previiously shown,, the inner peaak of the SSJ case is relatedd to the first ppeak of the TS SJ, whereas the S SSJ outer peak is connectedd to the TSJ second and thee third one. Att x/D higher thhan -2.3, the ccurves show thhe same trend bbut with higherr values than tthose of the S SSJ case. As H H/D increases up to 6, the thhird peak decrreases while thhe H/D = 6, the m maximum valuue occurs at thhe first and secoond merge, inccreasing their value and shiifting through x/D = 0. At H stagnation pooint. It is twice that evaluaated using thee SSJ device. Moreover, ddifferently to the t SSJ case, the outer peaak disappears. F For H/D > 6, thhe fluctuationns decrease. Axxial fluctuationns due to turbulence, at x/D D = 0, are everr higher than thhe axial phase-ccorrelated orgaanized contribbution to veloccity. The radiaal component, at H/D = 2, has higher vaalues than those shown in the SSJ case and there is not a plateau for x//D < 0.5. Twoo peaks are eveen present butt they are locaated at x/D = 11.8 and 2.2, whose relative disstance is the saame observed in the SSJ results. At H/D = 4, the two peeaks merge in x/D = 2 and, as the nozzle-too-plate distance increases, thhis maximum shifts outwardd and decreasees, disappearinng at H/D = 110. The value at x/D = 0, insteead, increases as H/D increaases up to 8, aand then starts decreasing. In Figure 14 the centerlinee axial velocityy and the jet w width, for all thhe nozzle-to-pplate distancess, are reported. As for the SS SJ D = 2 and H/D D = 4 configurrations do not reach the maxximum value w which is, insteead, reached byy the other ones case, the H/D at approximaately 4 D. Wiith respect to the SSJ casee, the TSJ attaains a higher centerline veelocity but a llower jet widdth because of thhe interaction bbetween the tw wo jets.

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Figurre 14

Time-aaveraged centeerline axial veelocity (left) annd jet width (rright) for all thhe H/D configuurations.

4.3 PHASE A AVERAGED M MEASUREME ENTS FOR SIN NGLE SYNTH HETIC JET (SS SJ) In Figure 15 the SSJ flow field, at H/D = 2, for differrent phases, iss depicted. Assuming the beeginning of the jet ejection as wn. The veloccity vector arrrows are placeed the referencee phase (φ=0°°), only eight phases ranginng from 0° to 168° are show every 0.08 D D.

Figuree 15

Phase-aaveraged axiall (left) and raddial (right) veloocity maps at H/D = 2.

At φ=0° the jjet starts beingg issued from the nozzle andd, at φ=24°, thhe vortex ring is fully formeed. At φ=48° the t jet impinges over the platee and the form mation of a coounter rotatingg vortex ring is observed at x/D = -2.3, appproximately. Moreover, tw wo regions of hiigh radial com mponent valuues are presennt: one aroundd x/D = 0.75 and the otheer in the regioon between thhe impinging plate and the voortex ring. As φ increases, thhe vortex ring moves outwaard, according to the externaal region of higgh radial velocitty values. For 48° ≤ φ ≤ 1200° the effect ddue to the platee presence is cclearly visiblee considering tthe morphologgy of the axial component. IIndeed the strrong adverse pressure graddient causes aan axial veloccity profile, approaching a thhe minimum on thhe jet axis. Foor φ > 96°, thhe vortex ring is out from thhe impinging pllate, characterrized by two ppeaks with a m measurementt zone and the axial and radiial velocities aare decreasingg. In Figure 16 the mean-squuared axial turrbulent fluctuaations are reprresented. At φ = 0° their vaalues are almoost zero. At φ = 24° no zero values v of fluctuations are vvisible in frontt of the jet whhit the highestt values of thee radial compoonent located in

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the vortex rinng core regionn. At φ = 48°, both componeents attain thee highest valuee between the sweeping vorrtex ring and thhe counter rotatting one. For higher φ valuues, the radial fluctuations are located inn the vortex riing core and along the sheear layers. No zeero radial flucttuation values are also visible along the im mpinging platee. It is interestting to notice that, t for 48° ≤ φ ≤ 120° no flluctuations aree in the regioon between thhe two shear layers l (-0.5 < x/D < 0.5). This behaviorr resembles thhe potential coree behavior of ccontinuous jetts. In Figures 17-18 the sam me velocity com mponents are shown at H//D = 6. Conssidering the fiirst three phasses of the axiial mponent, it is ppossible to see how the shear layers evolve generating thhe vortex ringg. At φ = 72° the t jet impinges velocity com on the plate aand a small coounter rotating vortex is deteected.

Figurre 16

Phase--averaged meaan–squared axxial (left) and radial r (right) tturbulent veloccity maps at H H/D = 2.

Differently fr from the shorteest nozzle-to-pplate case, thee axial velocityy profile, apprroaching the im mpinging plate, shows a belllshaped distribbution. At φ = 96°, the vorrtex is sweepiing over the plate p and the hhighest valuess of the radial component aare located in thee wall jet regioon. As φ increases, the radiaal and the axiaal values decreease. Figure 18 shoows the turbuulent componeents. For 0° < φ < 72° highh axial fluctuattions are visibble along the sshear layers annd near the fronnt boundary off the vortex ring. As φ increases, high vaalues are also present in thee vortex ring core. c The radiial turbulence shhows its maxim mum along thhe shear layer and near the vvortex ring coore, as well. The T two shear layers merge at about z/D = 4, showing a triangular reggion of low tuurbulence (poteential core-likke region). At φ = 96° the aaxial turbulencce has its maxim mum along thhe shear layeers and in thee vortex ring core but, diffferently from H/D = 2, thhe zone of higgh turbulence neear the wall, att about x/D = 2, shows lower values. Thee radial turbuleence still show ws its maxima along the sheear layers and innside the vorteex ring core. F Furthermore, a region of higgh values, spannning from thhe shear layerss and the vorteex ring, near thee impinging pllate can be obbserved. At φ > 96° the turbbulence valuess start decreasing and the voortex ring is oout of the measurrement regionn. Figure 19 shoows the saddlee point positioon at φ = 300°. In this configguration (i.e. S SSJ) the saddle point reachees the maximuum excursion jusst touching thee impinging pplate. In general, such a behhavior is very relevant for a device whichh is supposed to work as an ellectronic cooleer. Indeed, in tthis condition, during the suuction phase, tthe device couuld ingest air ccoming from thhe hot plate whiich has to be cooled. This bbehavior coulld deteriorate the heat transsfer performannces of the eleectronic coolinng device. Such a behavior dooes not occur aat higher nozzle-to-plate disstance.

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Figu ure 17 Phase-averaged axiaal (left) and raddial (right) vellocity maps att H/D=6.

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Figuree 18

Phaase-averaged m mean-squared axial (left) andd radial (right)) turbulent vellocity maps att H/D = 6.

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Figuree 19

Saddle point (white sspot) position w with streamlinnes for SSJ at H H/D = 2.

4.4 PHASE‐A AVERAGED M MEASUREME ENTS FOR TW WIN SYNTHE ETIC JET (TSJJ) The phase-avveraged axiall and radial ccomponents, at H/D = 2, for the TSJ case are repported in Figuure 20 and 221, respectively. At φ = 0° thhe right jet staarts coming ouut but, at φ = 24°, the vortex ring is nott fully formedd because of thhe suction of thhe left jet which deflects thhe right one toowards x/D = 0. Indeed, att φ = 72°, thee maximum aaxial velocity is located alongg the z axis annd not along tthe jet axis. T The maximum m radial valuess are located iin two regionss: where the jjet rotates and w where the vorteex ring is sweeeping the imppinging plate. As the phase increases, thee interaction bbetween the tw wo jets decreasess and both thee radial and thee axial componnents become lower. The mean-sqquared axial annd radial turbuulence componnents for TSJ aare depicted inn Figure 22 annd 23, respectiively. At φ = 00° no zero valuees of axial andd radial turbullence are still present due too the trailing ppart of the leftt synthetic jet. At φ = 24° thhe axial turbulennce attains higgh values on tthe front of thhe vortex ringg while the raddial componennt maximum iis located in thhe vortex ring core and betweeen the two jets. At φ = 48°° the axial turbbulence showss high values along the inteernal shear layyer which are hiigher than thee external onee. This is cauused by the innteraction of the t two 180° phase-shift jeets. Indeed, thhe extension off the turbulencce of the inteernal shear laayer arrives unntil the nozzlle exit, differeently from thhe external onne. s phase of Moreover, thhe axial turbullence of the leeft vortex ringg footprint is llower than thee right one beccause of the suction the left synthhetic jet. Thee same considderations can be drawn forr the radial tuurbulence. Addditionally, a region of higgh turbulence, cconnecting the left nozzle exxit and the inteernal shear layyer, is presentt (above all forr the radial coomponent). Att φ = 72° the turrbulence alongg the internal shear layer inncreases and iss still higher tthan the externnal one. The aaxial turbulencce attains also hhigh value in the vortex corre and in the zone near thee plate betweeen the vortex ring r and the sshear layer. Thhe radial compoonent acts as thhe axial one, apart from thee region near the vortex rinng. Here, the m maximum is aattained near thhe plate, where the counter rootating vortex is located. Att φ = 96° the behavior of tuurbulence is thhe same: indeeed, maxima aare located alongg the shear layyer, in the ringg vortex centeer (on the vorttex ring left siide, near the im mpinging platte, for the radiial component) and a near the im mpinging platte. For φ > 966° the values start s decreasing. In all phasees it is possiblle to see that, as previously ddescribed, the right syntheetic jet is defflected towardds the x/D = 0 axis. Furrthermore, eveen in this TS SJ configurationn, a potential ccore-like regioon is detectablee. In Figure 24 the saddle poiint, during thee right synthetiic jet ejection phase, is depiicted. At φ = 224° a first sadddle point (whiite spot) is obserrvable near the left synthetiic jet nozzle exxit. The saddle point is not located along the left synthhetic jet axis bbut on its left sidde because thee ejection strooke of the righht synthetic jett begins. At φ = 48° two saaddle points ccan be detecteed. The first onee continues mooving towardss higher radiaal position (beecause of the formation f andd convection oof the right rinng vortex) and a second one ccreated between the left vorrtex ring footpprint and the ssuction of the left synthetic jet. At φ = 722° the first sadddle point is alm most on the eddge of the meaasurement reggion while the second one iss slightly movving towards thhe impinging pllate. For φ = 96° the first ssaddle point iss out of the fiield of view, while w the secoond one attainns its maximuum excursion disstance from thhe nozzle exit.. For φ > 96° the latter sadddle point returrns to the nozzzle exit. Diffeerently from thhe SSJ case, thiss configurationn creates a fluuid dynamic coondition that ddoes not allow w having suctioon of air comiing from the hhot plate. This iss due to the right impingiing synthetic jjet that confinnes the field affected by tthe suction phhase of the leeft synthetic jet, which can inggest air (durinng its suction phase) p only frrom its left sidde and from thhe right nozzlee (in this planee). SJ case, shouldd not deterioraate the heat traansfer rate. Such a condition, differenttly from the SS maps related to the TSJ ddevice are nott reported herre. For the sakee of brevity, tthe H/D = 6 phase-averagged velocity m However, thee same differeences that havee been observved for a SSJ, comparing thee shortest connfiguration witth respect to thhe highest one, are also preseent in the TSJJ configuratioon (i.e. shear llayers merge, bell shaped axial a profile aapproaching thhe impinging plate, the absencce of radial velocity peak eetc.).

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Figure 220 Phase-aveeraged axial vvelocity maps at a H/D = 2.

Figure 221

Phase-aveeraged axial vvelocity maps at a H/D = 2.

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Figuree 22

Phase-avveraged meann-squared axial turbulent vellocity maps att H/D = 2.

Figure 23

Phase-avveraged mean--squared radiaal turbulent velocity maps att H/D = 2.

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Figure 24

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Saddle ppoints (white spots) s positionn with streamliines for TSJ att H/D = 2.

CONCL LUSIONS

In this workk, the single aand twin circuular impinginng synthetic jeets have beenn experimentaally investigated at Reynoldds number and Strouhal num mber equal too 5,100 and 00.024, respecttively. The tim me-averaged behavior of aall the velociity components hhas been reporrted and discuussed. Their diistributions, neear the impingging plate, havve been describbed for both thhe synthetic jet configurationns and for all the nozzle-to--plate distancees. At low noozzle-to-plate distance (H/D D < 4) the axiial mpinging platte, shows a doouble peak witth a minimum m on the jet axiis. Instead, at high nozzle-ttovelocity proffile, near the im plate distancee (H/D > 6), thhe axial velociity profile is bbell-shaped. The turbulennce distributionn shows a regiion characterizzed by low vaalues, resemblling the potenttial core regionn of continuouus f jets. Compariing the two syynthetic jet connfigurations, a higher centeerline velocity and a smallerr jet width hass been found for the twin casse. Moreover, a higher turbbulence levell, due to the strong interacction betweenn the two synnthetic jets, aare observed. Suuch an interacttion causes thhe deflection oof the synthettic jet towardss the x/D = 0 during the exxpulsion strokke. Also the phaase-correlated velocity fieldd is affected byy this interacttion. As matteer of fact, the 180° phase-shift generatess a destructive innteraction whhich causes a reduction of the phase-corrrelated veloccity values in the twin connfiguration wiith respect to thee single one. The evolutionn of the flow field for bothh configuratioons has been eexplained throough the phasseaveraged meeasurements. During D the sucction phase, thhe single syntthetic jet has a saddle poinnt that touchess the impinginng plate differenntly from the twin t configuraation. Indeed, in this latter case, c the interraction betweeen the two 1800° phase-shifteed synthetic jetss causes a geeneration of two saddle ppoints which do not touchh the impinginng plate. Thiis is a relevaant observation ffor a device whhich is suppossed to be used as an electronnic cooling sysstem.

REFERENC CES [1] Smith BL and Glezer A “T The formation ennd evolution of synthetic s jets” Phhys.Fluids, 10(9)) (1998) pp.2281-2297 [2] Holman R R, Utturkar Y, Mittal R, Smith BL L and Cattafestaa L “Formation C Criterion for Synnthetic Jets” AIA AA Journal 43(100) (2005) pp.211102116 [3] Jambunathhan K, Lai E, Mossand M MA andd Button BL “A rreview of heat trransfer data for single circular jeet impingement”” Int. J. Heat Fluuid Fl. 13 (1992) ppp.106-115 [4] Meola C, de Luca L and C Carlomagno GM M “Azimuthal innstability in an im mpinging jet: addiabatic wall tem mperature distribuution” Exp. Fluiids 18(5) (1995) ppp.303-310 [5] Meola C, de Luca L and C Carlomagno GM M “Influence of shear layer dynaamics on impinggment heat transsfer” Exp. Therm m. Fluid Sci. 13((1) (1996) pp.29-377

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11TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY – PIV15 Santa Barbara, California, September 14-16, 2015

[6] Martin H “Heat and mass transfer between impinging gas jets and solid surfaces” Adv. Heat Tran. 13 (1977) pp.1-60 [7] Chaudhari M, Puranik B and Agrawal A “Heat transfer characteristics of synthetic jet impingement cooling” Int. J. Heat Mass Tran., 53 (2010) pp.1057-1069 [8] Valiorgue P, Persoons T, McGuinn A and Murray DB “Heat transfer mechanisms in an impinging synthetic jet for small jet-to-surface spacing” Exp. Therm. Fluid Sci., 33 (2009) pp.597-603 [9] Rylatt DI and O’Donovam TS “Heat transfer enhancement to a confined impinging synthetic air jet” 51 (2013) pp.468-475 [10] Chaudhari M, Puranik B and Agrawal A “Multiple orifice synthetic jet for improvement in impingement heat transfer” Int. J. Heat Mass Transfer 54 (2011) pp.2056-2065 [11] Luo ZB, Xia ZX and Liu B “New Generation of Synthetic Jet Actuators” AIAA Journal 44(10) (2006) pp. 2418-2420 [12] Luo Z.B., Deng X., Wang L., Xia Z.X., 2011, Experimental technique based on delay phase angle and PIV measurements of a dual synthetic jets actuator, Proceedings of the 2011 Symposium on Piezoelectricity, Acoustic Waves and Device Applications, 1-5, Shenzhen, China. [13] Persoons T. O’Donovan TS and Murray DB “Heat transfer in adjacent interacting impinging synthetic jets” (2009) Proceedings of 2009 ASME summer heat transfer conference, 1-8, San Francisco, California. [14] Lasance CJM and Aarts RM “Synthetic jet cooling part I: overview of heat transfer and acoustic” in: 24th Annual IEEE Semiconductor Thermal Measurement and Management Symposium IEEE (2008) pp.20–35 [15] Russell DA, Titlow JP and Bemmen YJ “Acoustic monopoles, dipoles, and quadrupoles: an experiment revisited” Am. J. Phys. 67 (1999) pp.660–664 [16] Lasance CJM, Aarts RM and Ouweltjes O “Synthetic jet cooling part II: experimental results of an acoustic dipole cooler” in: 24thAnnual IEEE Semiconductor Thermal Measurement and Management Symposium IEEE(2008) pp.26–31 [17] Greco CS, Ianiro A, Cardone G “Time and phase averaged heat transfer in single and twin circular synthetic impinging air jets” International Journal of Heat Transfer 73 (2014) pp.776-778 [18] Greco CS, Ianiro A, Astarita T, Cardone G “On the near field of single and twin circular synthetic air jets” Int. J. Heat Fluid Fl. 44 (2013) pp.41-55 [19] de Luca L, Girfoglio M, Coppola G “Modeling and Experimental Validation of the Frequency Response of Synthetic Jet Actuators” AIAA Journal, Vol. 52, No. 8 (2014), pp. 1733-1748 [20] Astarita T and Cardone G “Analysis of interpolation schemes for image deformation methods in PIV” Exp. Fluids 38 (2005) pp.233-243 [21] Astarita T “Analysis of interpolation schemes for image deformation methods in PIV: effect of noise on the accuracy and spatial resolution” Exp. Fluids 40 (2006) pp.977-987 [22] Astarita T “Analysis of weighting windows for image deformation methods in PIV” Exp. Fluids 43 (2007) pp.859-872 [23] Astarita T “Analysis of velocity interpolation schemes for image deformation methods in PIV” Exp. Fluids 45 (2008) pp.257-266 [24] Hussain AKMF., Reynolds WC “The mechanics of an organized wave in turbulent shear flow”, J. Fluid Mech., 41(02) (1970)pp.241-258 [25] Kitsios V, Cordier L, Bonnet JP, Ooi A, Soria J “Development of a nonlinear eddy-viscosity closure for the triple-decomposition stability analysis of a turbulent channel” J. Fluid Mech. 664 (2010)pp.74-107 [26] Rohlfs W, Haustein HD, Garbrecht O, Kneer R, “Insights into local heat transfer of a submerged impinging jet: Influence of local flow acceleration and vortex-wall interaction” Int. J. Heat Mass Tran. 55 (2012) pp.7728-7736 [27] Narayanan V, Seyed-Yagoobi J, Page RH. “An experimental study of fluid mechanics and heat transfer in an impinging slot jet flow” Int. J. Heat Mass Tran. 47(8) (2004) pp.1827-1845 [28] Roux S, Fénot M, Lalizel G, Brizzi LE, Dorignac E “Experimental investigation of the flow and heat transfer of an impinging jet under acoustic excitation” Int. J. Heat Mass Tran. 54(15) (2011) pp.3277-3290 [29] Knowles K, Myszko M “Turbulence measurements in radial wall-jets” Exp. Therm Fluid Sci. 17(1) (1998) pp.71-78 [30] O’Donovan TS “ Fluid flow and heat transfer of an impinging air jet” Ph.D. Thesis (2005)

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