Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Hydrofoil Turbulent Boundary Layer Separation at High Reynolds Numbers
Hydrofoil Turbulent Boundary Layer Separation at High Reynolds Numbers
Pages 314-329
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From page 314... ...
R Pankajakshan (Mississippi State University, USA) One of the mam hyd oacoustic noise sources from fully submerged li ting so faces is file un teady separated turbulent flow neat file so face's hailmg edge Hat produces pressure tract ations on file su face md unsteady oscillato y flow in the neat wake However, the t rbulent flow character tics near boundary layer sepmation are largely undocumented at the high Rey olds numbers typical of mmy hyd ody amic Implications This paper describes results fiom file Or t phase of m experimental ego t to identify md measure file dominmt flow features neat the tmiling edge of a hyd foil at chordbased Reynolds numbed mproachmg 108 The experiments are conducted at the US Navy's Large Cavitation Ch mnel with a tw dimensional test-section- pmnmg hyd of oil (2 I m chord, 3 Om par I atflow peedsfiom 0 5 to 16 m/s The foil section is a modified NACA 16 wish a flat pressure side md m mti-smgmg trading edge The results presented here cover file flu t phase of experiments md emphasi e LDV measured mem flow velocities md turbulence statistics from the sepamtmg boundary layer flow near the hyd foil's tmiling edge at Rey olds mmmbers fiom 6 to 60 million INTRODUCTION The flow at the hailing edge of liflmg su faces has received considerable attention md has been inw stigated by m my ho hers Desigmers of ship propulsory md conhol surfaces have ex mined flow over tw -dimensional hyd foils md al foils m order to understmd how modification of the hailmg edge geomet y rllnence.
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From page 315... ...
cm test tw -dimension i hyd of oils wifh a sp m of 3 05 met~ss, md cm a hieve cho d baed Reynolds numbers approa hmg 108 The pmpose of fLis paper is to pres:~t fLe results from fLe fu st pha e of a program to examme the trailmg edge flow of a hyd of oil al high Reynolds number A tw -dimensiona hyd foil wa consiru led for testing in the LCC Presented here a e La er Doppler Velocimeter (LDV) mea mements nea fLe hyd of oil leading md hailing edges md m the fa w ke These results a e compa ed to numenca simulaiions of the flow EXPERIMENTAL SET-UP AND UNCERTAINTY The expenments were conducted al fLe US Na y's Lage Cwitaiion Chmnel (LCC)
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From page 316... ...
file Imear regression ht of the calibration curve Disk speed unce tainty based on file mamfactur~s's pec is +0 040 rew lutions per second The di k radius is 100 mm with m unce tainty of +0 35 mm arising fiom the need to locate file disk center The unce tainty introduced by file linear regression hit is no greatens then +30 mm/s Based on these values, the maximum calibration bias in the LDV velocity measurement r mges fi om +1 4% at a flow peed of 3 m/s to +0 4% at IS 3 m/s This h mslates into a maximum fractional bias enor in the no malized mem velocities (dimensionless) of +0 02 at 3 m/s to +0 006 at IS 3 m/s Through file no malization velocity, bias also enters the no malized mem squares of file velocity Duct ations, but is limited to a Faction of +0 002 or less Precision :Tor is also present m file calibration, but 1000 LDV samples are t ken to produce each calibration cm e point, rendering this emu negligible In order to make proper use of facility time, a nommal sampling period of 0 6 mm pa coo dinate location was chosen for collection of all data sets Since the LDV data rate ch mged with survey location md flow speed, the collection period was conholled (whet necessary)
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From page 317... ...
Nearly complete data sets at all five stations were taken at four channel velocities: 3.0, 6.0, 12.0, and 18.3 m/s with the bulk of the data (2,500 coordinate locations) being taken within the trailing edge region (station 3~.
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From page 318... ...
Although measurements at the center plane (50% foil span) of the LCC are clearly preferred by symmetry, LDV measurements are easier and more time-efficient - for optical and mechanical reasons related to water opacity, valid LDV burst data rate, and foil deflection - when the LDV focal volume is closer to the test section windows.
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From page 319... ...
Trailing-edge LDV measurements of normalized streamwise mean square velocity fluctuations |
From page 320... ...
Trailing-edge LDV measurements of normalized vertical mean square velocity fluctuations |
From page 321... ...
Trailing-edge LDV measurements of normalized streamwise mean square velocity fluctuations |
From page 322... ...
Taken together these meanflow findings and those for the turbulence quantities, all suggest that the suction side boundary layer separates closer to the foil's trailing edge at the higher speed. Thus, a simple interpretation of the suction side flow emerges.
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From page 323... ...
~ 0 05 Although the flow near the trailing edge is more complicated than any simple free-shear flow, the turbulent fluctuation levels found in this study are in reasonable agreement with classical shear layer characteristics. For example, measurements in the attached pressure-side boundary layer upstream of the trailing edge nearly match classical results (provided in {,)
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From page 324... ...
Likewise the measured peak streamwise and vertical velocity fluctuations are approximately the same and this matches the expectation for twodimensional wake flows (Townsend 1976)
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From page 325... ...
As shown in Fig. 25, the point dis ribution was densely packed near the foil surface and in the trailing edge region; y + values near the surface are less than 0.5.
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From page 326... ...
Normalized mean streamwise velocity at x/C=0.992, 6.0 m/s
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From page 327... ...
26 through 33 should be considered preliminary. ACKNOWLEDGEMENTS The authors of this paper wish to the acknowledge the contributions of Paul Tortora and Ronnie Bladh of the University of Michigan; William Blake, Ken Edens, Bob Etter, Ted Farabee, Jon Gershfeld, Joe Gorski, Tom Mathews, David Schwartzenberg, Jim Valentine, Phil Yarnall, and the LCC crew from the Naval Surface Warfare Center Carderock Division; Lafe Taylor, Min-Yee Jaing, and David Whitfield from Mississippi State University; and Pat Purtell and Candace Wark from the Office of Naval Research.
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From page 328... ...
"Turbulent boundary layer separation," A mmal Review of Fluid Mech mics, Vol. 21, 205-234 Tow send, AA (1976)
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From page 329... ...
HOR'S REPLY A experimental procedure was devised to measure velocities close to She surface of the hyd of oil With She probe volume located m the area of mterest, She LDV heed was tilted to the minimum Ogle from horizontal et which all four LDV beams cleared the hyd of oil Since this Ogle is k own, its effect on the measured streamwise Ed t msverse velocities could be taken into account How ver, in all cases the Ogle effect was negligible in comparison with other sources of en or Ed so was disregarded She charmel flow was then set on condition so that the hyd of oil assumed its lift-loaded shape At this time the sharp tip of the hailing edge was located Ed used es the spatial reference pomt for the LDV mecsmements (7he point was located by observing She appearance Ed disappearance of the surface loser flare es the LDV probe volume was scarmed vertically across the tip of the trailing edge ) Using this relerence point with the hyd of oil's k own Ogle of attack Ed surface contour, my given paticl coordinate for the LDV date may be related to my point on the hyd of oil surface She LDV spatial coordinates were arranged m vertical columns rising from the hyd of oil surface he order to ensme that data was acquired es close to She surface es conditions allowed, each column beg m with c coordinate just inside the hyd of oil surface Ed marched outward fi om the su face et incr merits of 0 2 mm Probe volume diameter was 0 17 mm )
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