Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Propeller Wake Analysis by Means of PIV
Propeller Wake Analysis by Means of PIV
Pages 493-510
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From page 493... ...
he hyd odynamic md geomehiccl mve tigation of 6he wake md its evolution hcs been pomted o t he bkde viscous wake, developmg from th blcde surfae bo mdary Icyers, the hailmg vo tex sheets, due to 6he radicl g adient of 6he bo md circulation, ad the velocity fluctuation distributions me identffied md discussed he near wake geomet y is descobed th ough 6he bendmg of the bkde wake sheets, 6he slipsheam conhation md 6he tip vortex trajectory md viscous i te~ations ~ the near field 6he effects of tmbulent dfffusion md isc ms dissipation, which c mse c rapid spaebroadenmg of 6he velocity g cdients m 6he t~ailmg edge wake, me clso examined in 6he far wake th d velopment of the slipsheam instability md 6he brekdown of the hub md tip vortices are outlmed IN7.~0DUCPION he experimentcl inve tigation of the propeller wake holds a impo t mt role for the desigm md 6he pe formance crurlysis of ship propulsion ~ modern desigm, to reduce propeller-mduced hull vibrcti ms, efficiency decay md noise generction, due to cavitation, 6he~e is c contmoous hend towards m increcsed complexity of 6he bkde geometry his compl :xity is primarily due to the low cspect mtio a d to th sk w of marine propellers, which cmse shong thee-dimensiom~l effects 7berefore, 6here is c rising i te~est on detailed data of the velocity flow feld aro md 6he bkdes md in the wake he kmowledge of th velocity field m 6he wake em~bles to check locclly th desigm requi ements For example, this c m be done by comparing the mecsmed bkde section d cg coefficients ad bo md cl coktion wi6h 6hose provided by desigm dish~butions Kobayashi 1982, Koyamc 1986 md Jesmp 1989) Moreover, the kmowledge of 6he position of the t~aili g vo t :x sheets is necessary to evaluate 6he atual wake-induced velocity feld aro md 6he blades md to determme 6he propeller pe fommances Velocity mecsurements are clso c tool for 6he development md the validation of m mericcl codes md flow modeling Mo t cunent m mericcl methods for propeller mvestigations are based on potenticl flow 6heories md simplffied wake models More complex md ref ned m odels of viscous flows Kobayashi 1981, Jessup 1989, Amdt md Mcines 1994)
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From page 494... ...
Wake characteristics and tip vortex spatial fluctuations, leading to the breakdown farther downstream, are pointed out considering three adjacent windows starting from the blade trailing edge to about 1.5 propeller diameters downstream. The propeller used in the present experiment is the same investigated by Cenedese et al.
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From page 495... ...
, ~ 7Light sheet plane Flow direction ~Tip- `3 \ Vortices ~ sections ~ Investigation Camera windows Figure 3: Measurement planes IMAGE ANALYSIS The acquired images were analysed using an algorithm in which the window off-set correlation method has been implemented (Westerweel 19974. Furthermore a recursive processing method is used by implementing a hierarchical approach in which the sampling grid is continually refined and also the size of the interrogation windows is reduced during the iterations.
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From page 496... ...
in 6he following, th mam 6sressments r~scess6 y to qualffy 6he present results 6 e leported The uncertainty on docity measurements by meams of 6 PIV system is mamly due to th error on p6 ticle di pl6 ments evaluation which cam be consideled less thm 1/106 of 6 pi el for the plesent imag arulysis61goribm Intemmsof slocityism6he order of I m/s Perk lockmg enors, mainly due to p6 ticle image siYs, has been Iv5duced, 6s much 64 possible, by using im rge defccusmg techmiques E rcrs due to noiYs were impo tamt only m flow egiom where light reflecti ms fi om the c6 ibtmg hub or fiom the bkde smf6 e wsre plesent E sn if enor~sous w~ctors 6 e elimim~ted 6md repkced by mte polrti m durmg po t-processing, sometimes spurious w~ctors 6re validated 6md sifect 6he stati tics This effect is relevamt especi611y for the second order statistics
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From page 497... ...
0=~;3 · 60 556 ~ _7 ~ sO In the tip vortex core and near to the blade (where the highest velocity gradient are encountered) , only a few data are available to compute statistics which result in low accuracy estimation especially for the second order statistics PROPELLER WAKE ANALYSIS An example of an instantaneous flow field obtained in the first measurement window for a revolution angle O=0° is shown in figure 7.
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From page 498... ...
The blade wake almost disappears within one diameter downstream, whereas a strong deformation, due to the higher axial velocity at the inner radius bends the blade wake. The strong acceleration of the radial velocity component near the hub reveals the strong roll up process of the hub vortex (which is cavitating just outside the measurement area)
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From page 499... ...
for th measured velocity components for q=0°, while figure 14 shows th evolution of av for O=20°,40°,60°,80° Even if She confidence of She statistical e timator is limited, due to th fact that has been evaluated only own 65 samples, some importmt features of She wake cm be recombed The turbulent wake released by She bhde is quickly dissipated md diffused d wnsheam The same process of wake defommation md broadening, due to the action of She tip vortices md of the hub vo tex, observed m th vorticity plots, is also seen m She turbulence level distributions Newx6heless, new i formation are obtained by 6 is second order statistics: - The effect of hub vo tex roll up m She wake is why importmt md et x/R=2 the t lad wake, ii king She tip vortex to She hub vo tex, almost d6scppe s especially for the at, di tribution - Turbulence diffusion from She hub vortex occurs m the l ongitudincl evo hit ion e pec id ly for c. es expected due to the hub vo tex orientation - Some small scale turbulence, more evid nt m She c, distribution, generated probably et the lecdi g edge of the bade, is quickly dissipated downstream withm one prop llemadius - The effect of noise in She images, due to She cavitating hub is pouted o t by the intense spikes in She turbulence level di tribution - Velocity fluctuations et She tip vortex core increase while this is convected d wnsheam This aspect is rented to She vortex breckdow in tability which effects She velocity fluctuations th ough paticl o sc il lit ions of th c m e Fur6herm ore , the pattem of the turbulence dishdbution suggests that the tip vortex oscillation occurs m some preferential direcriom The tip vortex fluctuations, lecdmg to She t ret down, c m be pointed out also by evaluating She stmdard deviation of the spatial fluctuations of tip vo tex core wish reap cl to She metn et c given longit dirul position The result, given m figure 15, shows that the cmplit de of She spatial fluctuation is mcrecsi g dow sheam md that She amplitude of h msverscl fluctuations has c higher g owth mte m respect to the lo git dmal This reach is similar to chose obtained by Cot om et cl (1999)
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From page 500... ...
of the 16th Symposium on Naval Hydrodynamics, 1986. Landgrebe, A.J., Johnson, B.V., "Measurement of model helicopter rotor flow velocities with a laser Doppler Velocimeter", Journal of American Helicopter S ociety, 1974.
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From page 501... ...
Westerweel J., "Efficient detection of spurious vectors in particle image velocimetry data", Experiments in Fluids, Vol.16, pp.236-247., 1994 Westerweel J., "Fundamentals of Digital Particle Image Velocimetry", Meas. Science and Technology Vol.8, pp.1379-1392., 1997 A
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From page 502... ...
1 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 1 -n 0.5 O -0 .5 -1 __ 1 1 1 111 1 1 111 1 1 111 1 1 1 11 1 1 111 1 1 111 1 1 1 11 1 1 1 11 1 1 111 1 1 111 1 1 1 11 1 1 1 11 1 1 111 1 1 111 1 1 1 1 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 x/R Figure 9: Modulus of the inplane velocity with streamlines .
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From page 504... |
From page 506... ...
x/R* .OQ4663 0.004 0.002 1 0.5 -0 .5 -1 my/ o 0 0.5 1 1.5 2 x/R 2.5 3 Figure 15: Tip vortex spatial fluctuation with respect the mean position B lade 2 T ip vortex tra rectories Blade 1 / Q rat (~ (~ ~~ Blade 3 Blade 4 -0.88 -0.9 Van ' ' ~~ Con.
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From page 507... ...
· 1 1 1 ~ ~ Figure 17: Flow visualitation in incipient cavitation of the propeller vortex system dispersion and hub vortex deformation
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From page 508... ...
1 0.1 rIR In figure is shown the comparison of the vorticity field for the above higher and lower advance coefficients for the same propeller revolution angle. At higher J due to the higher pitch of the wake, the interaction of the actual blade wake with the tip vortex of the previous one is shifted downstream.
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From page 509... ...
it is not strictly p cessary to average over different propeller revolutions, unless this was requu ed by the user Problems arise in re pect to the spatial resolution of the sy tem when high frame pttes are r quired (es m She case of large flow velocity) It should also be noticed that, with the FIV measurement sy tem employed in the paper, cldhough the velocity time histo y is not derived, the velocity field is almost in d mtaneous indeed, the time Interval betw en the two loser exposures (used to compute the cross-correlation fun non)
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From page 510... ...
1 X w~ ~w Figure Velocity vectors in the near watt legion of ~ water chonnst recorded at time intervals equal to 1/2511 s.
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