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Tip vortices--single phase and cavitating flow phenomena

Green, Sheldon I. (1988) Tip vortices--single phase and cavitating flow phenomena. California Institute of Technology , Pasadena, CA. (Unpublished)

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Tip vortices occur wherever a lifting surface terminates in a fluid. An understanding of tip vortices is salient to the solution of many engineering problems, including lift induced drag tip inefficiency, the overturning of small planes flown into the tip wake of larger aircraft, and marine propellor tip cavitation. The tip vortex shed by several rectangular planform wings, fitted with three different tips, was studied in a water tunnel. Four techniques were employed to examine the tip vortex: (i) Surface flow visualization to reveal the early stages of vortex rollup. (ii) Double pulsed holography of buoyant, Lagrangian particle tracers for detailed tangential and axial velocity data around the vortex core. Holograms were also a source of instantaneous core structure information. (iii) Single pulse holography of air bubbles, of uniform, measured, original size. The size of the bubbles is related to the instantaneous local static pressure. The bubbles are driven by the centripetal pressure gradient forces into the vortex core, providing a means of measuring the average and transient vortex core pressure non-intrusively. (iv) Direct observation of vortex cavitation. These measurements are useful in their own right because of the considerable technological significance of tip vortex cavitation. In addition, many single phase tip flow characteristics have cavitating flow counterparts. The present study has shown that one chord downstream of the wing trailing edge virtually all the foil bound vorticity has rolled up into the trailing vortex. Armed with this knowledge one may a priori evaluate, in the near field, the tangential velocity distribution, the core axial velocity excess, and the core mean pressure. These predictions are in agreement with the experimental measurements. Three aspects of the core flow, first observed in the present study, remain analytically inexplicable: (i) The trend towards a Reynolds number dependent, axial velocity deficit with downstream distance. (ii) The unsteady core velocity, particularly immediately downstream of the foil. (iii) The vortex kinking which is coincident with highly unsteady axial core flow. As a first approximation, cavitation inception occurs when the core pressure is reduced to the vapour pressure. The large measured fluctuating core pressure explains the occurrence of inception at core pressures somewhat above p[v] and the dependence of sigma[i] on the dissolved air content. Modifying the tip geometry profoundly affects the trailing vortex. Installation of a ring wing tip can reduce the inception index relative to that of a normal rounded tip foil by a factor of three. The reduction was caused primarily by the redistribution, in the Trefftz plane, of the shed vorticity about a line and circle. Fortuitously, this redistribution caused most of the wing bound vorticity to be shed from the ring, decreasing the tip effect lift loss over the foil body.

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Additional Information:©1988 Sheldon Isaiah Green. This work was supported by the Naval Sea Systems Command General Hydromechanics Research Program administered by the David Taylor Naval Ship Research and Development Center under Contract No. N000167-85-K-0165. Report No. Eng. 183.17. The author would first and foremost like to express his gratitude for the guidance and support of his advisor, Professor A. J . Acosta. Without the perspicacity and pertinacity of Professor Acosta, the author would still doubtless be performing experiments. The research was funded by the ONR under contract number N000167-85-K-0165. The financial support of Caltech (including Cole and Powell Fellowships) is acknowledged. Caltech being the open institution that it is, the author was able to consult with many faculty members over the course of his stay. A partial list includes Professors: A. Leonard, C. Brennen, A. Roshko, M. Billet (Penn State), E. Hauptmann (UBC), and J . Katz (Johns Hopkins) . The author also owes a great debt to his professors at the University of Toronto, and to L. Pollock (Tulane) for her insightful suggestions about the thesis text. Many students have also helped the author. Thai Pham and Raza Akbar each spent a summer "SURFing" under the author's guidance. The author's immediate predecessor, Tim O'Hern, was instrumental in the crucial LTWT and holocamera familiarization period. Luca d'Agostino, Steve Ceccio, Norbert Amdt, Ron Franz, Hojin Ahn, Agnes Allard, and Regina Rieves have all served as sounding boards for ideas. A host of other friends made Caltech more enjoyable. The author is appreciative of the technical assistance given him by Joe Fontana and Rich Eastvedt of the Keck Hydraulics Laboratory, and by George Lundgren and other members of the Aero shop. Cecilia Lin drew many of the thesis figures. The numerical simulation conducted at NASA Ames could not have been done without the encouragement and aid of Dr. Dochan Kwak, Dr. Terri Holst, and Stuart Rogers. Finally, the author wishes to express (but cannot in words alone) his deepest thanks to his family, who have bolstered his morale when it flagged, and served as an anchor in the stormy academic arena.
Funding AgencyGrant Number
ONR Naval Sea Systems Command General Hydromechanics Research ProgramN000167-85-K-0165
Subject Keywords:Tip Vortices, Cavitation, Cavitation Inception, Holography, Pressure Fluctuations, Velocity Fluctuations, Vortex Core, Vortex Breakdown
Record Number:CaltechAUTHORS:20140509-144825502
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:45653
Deposited By: Kristin Buxton
Deposited On:12 May 2014 20:25
Last Modified:12 May 2014 20:25

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