Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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On the Flow Structure, Tip Leakage Cavitation Inception and Associated Noise
On the Flow Structure, Tip Leakage Cavitation Inception and Associated Noise
Pages 639-653
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From page 639... ...
was used to measure be flow, especially to ccptme the slender tip vortices where cavitation inception was observed Seventy imt mhneous realizations for the 0 6mm gap md 65 for She 1 4mm gap w re crurly:D:d to obtain di tobutions of circulation of She leakage vortex Prv experiments for She 2 6mm gap are presently mde way md only mstmtmeous samples are presented The vortex cme diameter was fo md to be 3-4 vector spacings Minim m p~essme coefhcients in th cores of th se vortices w re estimated using c h~nlnne model These coefhcients show d c why good cg cement with th mecsmed cavitation inception indices INTRODUCTION Cavitation occurs m liquid flows whey c m lens Bubbles, particles etc ) is ccptmed m c region where She pressure is low r or equal to the vapor pressure lAmdt 1981, Brenren 1995, Joseph 1998)
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From page 640... ...
PIV experiments were conducted using an inclined light sheet so that a cross-section of the tip leakage vortex could be captured. One purpose of these measurements is to estimate minimum pressure coefficients from the circulation and compare it to the experimental cavitation indices.
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From page 641... ...
camerz Kodak Ekbpro M Motion Analyzer, Model 1012) zt 2000fps was used to record cavibting bubbles m fhe tip leakage vortex These fiames w re coupled with the acelerometer signals, providmg z good conelation between fhe physical zppearance of the bubb le s du mg cz itat i on md fhe ace ler ometer signals A Dztz Trmshtion A-D board capable of sampling rates up to 1 33M a was used, md datz was aqui cd zt 250kHz/chumel usmg LzbView software Th acelerometer signals were arudy:osd usmg m mhouse code, so that cavitation events could be coumted (z sample m figme 5)
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From page 642... ...
rate of cavitation events (rc) for the three gap sizes, all at a 1° incidence angle.
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From page 643... ...
The slope of the 0.6mm gap is also quite different than the 1.4 and 2.6mm gaps. Since all experiments are performed with the same nuclei distribution, the substantial differences in event rate indicates that the probability of finding low-pressure regions for the 0.6mm case is significantly higher than the 1.4mm and 2.6mm gap sizes.
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From page 644... ...
bubbles of all sizes in the range shown appeared in all gap sizes. In figure 7a, the cavitation noise begins at frame 1495, where high amplitude noise is emitted by the bubble on the left as it grows gets distorted and fragmented along the vortex core (frame 14964.
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From page 645... ...
1 1 1 1 1 1 1 1 2 2.5 N: Figure 11: Vertical distance of the tip leakage vortex trajectory from the trailing edge (by)
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From page 646... ...
, highlighting the cross section of the tip vortex. Figure 10 shows an extended exposure image of the trajectory of the bubbly tip leakage vortex as seen in a side view for the three gap sizes.
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From page 647... ...
No significant differences in the core sizes were seen between the two gap sizes, although this statement is greatly affected by our coarse resolution. Increased resolution would clarify this point in addition to the 2.6 mm gap data.
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From page 648... ...
SUMMARY Tip leakage cavitation is studied in detail with three tip gap sizes of 0.6, 1.4 and 2.6mm (\ = 0.12, 0.28, 0.524. Cavitation inception indices and event rates are highest for the smallest gap size and decreases with increasing gap size.
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From page 649... ...
FhidsEngng, 111, 495-501 HUANG, N E et cl 1998 The empi iccl mode decomposition md the Hilbert spech m for non-linear md non- tatiorury time series crurly is Pnoc R Soc L~ndon 454, 903-995 HS AO, C T & PAULEY, L L 1998 N mericcl st dy of the tecdy- tate tip vortex flow over c fmitesp m hyd of oil Journol of Fhid Engineenng, 120 JOSEPH, D D 1998 Cavitation md 6he state of t~ess in c flowing liquid J
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From page 650... ...
We are grateful to Dr. Rood and the authors of the very interesting paper"On the Flow Structure, Tip Leakage Cavitation Inception and Associated Noise", for requesting a discussion.
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From page 651... ...
We have not looked et the effect of various imticl bubble si es on bubble dynamics duri g cavitati m But as indicated by Clhthme we Tree that various bubble si es will behave differently For very small bubbles the critical pressure leading to cc itation inception is d pend nt on bubble si e Larger bubbles will be deformed, disto ted Ed fragme ted by He local p~essme nonunfformities In the present study w use bubbles Nat r pure little tension to initiate cc itati m, but are still signifcmtly mcller thm He core Once cc itation starts He bubble grows substantially lecdmg to odd shapes, ficgmentation et
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From page 652... ...
A St zkz Pemmsyl mid State University, USA I welt to thmkfhe mthors for contobutmg this mo t i toasting paper to the Naval Hyd odynamics symposium The collection of detailed :' V flow field measurements, visual observations of individual bubble dynamics, Ed cone pondng t msient acoustic measurements are comprehensive Ed unprecedented It this small scale it is intere ting that She higher zmplit de noise peaks w re associated with bubble disto non Ed fizgmentation rath r th m shme or size charge In my opmion, She intended relevance of She subject Investigation to th "effect of gm si e on She flow stmctme Ed on the inception of tip leakage cz itation" is limited Ed somewhat misleading d e to the omission of :eserrl impo t mt features of tip leakage flows Pa t, She rektive moti m of She tip Ed end-wall is omitted in z pump, She relative motion zugme ts the leakage flow; m z t rhine She motion opposes it The so-called scraping vo tex conhibutes Toni it to the leakage vortex; Ed this co tobution is ce thinly z f notion of gap size Additionally, centrifugal lorces conh~bute to secondary flows, which c m interact with the leakage so tex at the tip Pmth rmore, She contemporary practice of unloadi g She blade tip is not z feat re of the test hyd of oil Rasher, th low r half of the sp m is linearly unload d fr m z con t mt Ifft coefficient along She upper half-sp m of She hyd of oil Given these omitted featmes of z tip leakage -I .., th tudy is more ~elevmt to She effect of confinement m tip Rolex cavitation mceptim Ed associated noise This is import mt distinction rektive to the observation of z cavitation inception Index minimum It some value of nommali ed clearance The an hors conectly point out that Boulon et al (1) do not observe z cavitation minimum, while Powell Ed Billet (2)
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From page 653... ...
indicating flat the flee stream mmclei are limo t eliminated Then w "flood" She tip region with bubbles (~lOOlrm diameter - size distribution md details are provid d in th papery et approximately 2500 bubbles/s The measur me ts w He made ju t up cream of the leading edge md had c field of vi w of about 3mm The images w re crurlyzed using c blob analysis sof wme Bubbles in good focus w re used for size dishibution All the Laces including those that me not exactly m focus were used for computing She bubble flmc Bubble with diameters of Worm Aqua little tension to initiate cavitation, Thus bubble size is not c critical issue m th present study Also al non of 2500 bubbles per second makes cavitation inception less sensitive to bubble populations How ver, being c se y t rbulent flow with large coherent struct res, i fr que t cavitation events Jess f m 2 per second) me c result of "extreme" flow conditions md not Hopi 91 of She flow Wish ins sing events per second c Rend c m be conectly identified Thus, She slope of She cavitation eve t rate curve is importmt in addition to its absolute values
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