Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
24th Symposium on Naval Hydrodynamics Fukuoka, JAPAN, ~13 July 2002 Phase-Averaged PTV Measurements of Propeller Wake Sang Joon Lee, Bu Geun Paik and Choung Mook Lee (Pohang University of Science arid Technology, Korea) ABSTRACT The objective of present paper is to apply an adaptive hybrid 2-frame PTV (Particle Tracking Velocimet~y) technique for measuring the flow characteristics of turbulent wake behind a marine propeller with 5 blades. Compared to the conventional PIV method, he hybrid PTV technique increases the spatial resolution and measurement accuracy significantly, while reducing the computation time. For each of four different blade phases of 0°, 18°, 36° and 54°, four hundred instantaneous velocity fields were measured. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the region ranged from the trailing edge to two propeller diameter(D) downstream location. The phas~averaged mean velocity fields show that the trailing vorticity and the viscous wake are formed by the boundary layers developed on the blade surfaces. The vorticity contours at each phase angle show that the tip vortices are produced periodically. The slipstream contraction occurs in the near-wake region up to about x = 0.5D downstream. Thereafter the unstable oscillation occurs due to the separation of tip vortex from the wake sheet behind the maximum contraction point. As the tip vortex evolves downstream, its strength is reduced due to turbulent diffusion, viscous dissipation and active mixing between tip vortices and adjacent wake flow. The technique presented here can be readily extended to investigate the nominal and effective wake distribution as well as the details of the flow field of fore and aft of a rotating propeller behind a ship model. INTRODUCTION A marine propeller is the main source of noise. hull vibration, and cavitation at high speed. In order to increase the propulsion efficiency, the geometry of propeller should be optimally designed. To achieve this objective, an accurate wake analysis based on detailed experimental results is required. In general, a modern propeller blade has a complicated geometry and makes the wake behind a propeller more complex. Kerwin (1978) predicted the steady and unsteady marine propeller performance by numerical lifting surface theory. However, the lifting surface theory cannot give accurate prediction of the flow around the leading edge and the blade tips. Lee (1987) and Kim et al. (1993) used the potentiaLbased panel method to improve the prediction of the steady performance of a propeller. A further improvement was made by Cho and Lee (2000) by applying the gasoline higher order panel method to obtain more accurate numerical results. It is well known that any numerical attempts based on either potential or viscous flow model would not yield a satisfactory prediction of the formation and trajectory of tip or trailing vortices without an adequate wake sheet modeling. In the conventional numerical methods, the velocity field around a propeller blade is calculated from the position of wake sheet assumed in the previous iteration step, and a new position of wake sheet satisfying the boundary condition is determined and the process is repeated until a numerical convergence is obtained. To avoid such a tedious repetition of calculation, some numerical investigations used a linear or non-linear wake sheet model which was obtained from the experimental data . Hoshino (1989) computed the flow field around a propeller using the wake contraction and trailing vortex sheet model. However, he did not consider the viscosity, hub effects and velocity field influenced by the wake. Although the actual trailing vortex sheet in wake has a finite thickness, it has been assumed as a thin filament in the numerical analysis resulting in a decrease in the reliability of the prediction of the trailing and tip vortices. It is very important to accurately predict the tip vortex which can be the major source of energy loss in propulsion, hull vibration and noise. The position of tip vortex could be estimated using conventional numerical methods; however, the prediction of the strength or thickness of tip vortex requires much more accurate information which can be obtained only from reliable experiment focused on the wake and tip vortex behavior. The wake of a propeller has been measured with point-wise experimental techniques such as Pitot tube and LDV(Laser Doppler Velocimetry). Therefore, most previous investigations neasured flow velocities at
discrete points by scanning the flow field with an array of measurement probes. Stella et al. (1998) measured the axial velocity component of a propeller wake, and Chesnakas and Jessup (1998) investigated the tip vortex flow using LDV. Unfortunately, these point-wise methods take substantially long time to get the phase- averaged velocity field information. Recently, velocity field measurement techniques have been applied to measure the flow around a marine propeller. The PIV(Particle Image Velocimetry) technique does not interfere with the flow structure and also does not require much time, compared to the LDV method, to measure the velocity field in a reasonably large area. Controni et al. (2000) investigated the near- wake of a marine propeller in a cavitation tunnel using PIV method. Their results show a good spatial resolution compared with conventional LDV data. In the present study, an adaptive hybrid PTV(Particle Tracking Velocimetry) method(Kim and Lee (2002)) is used to investigate the near-wake of a marine propeller in detail. To demonstrate the effectiveness of the method, a simple case of measuring the wake of a propeller model in an open-water channel was chosen. Once the reliability of the method is affirmed, it s anticipated to apply the method to more complex flow problems in the ship stern region. This velocity field measurement technique requires a preliminary PIV routine to determine the local match parameters needed for the 2-frame PTV algorithm(Baek and Lee, 1996). The original 2-frame PTV based on match probability determines the match parameters globally for the whole flow field. The adaptive hybrid scheme by utilizing the advantages of both PIV and PTV techniques recovers significantly more velocity vectors while reducing computation time and errors. In addition, the adaptive hybrid PTV shows the superior spatial resolution, compared with the conventional PIV technique. The several hundreds instantaneous velocity fields were measured at four different phases of the propeller blade and they were phas~averaged to investigate the spatial evolution of the vertical structure and turbulence statistics for the propeller wake. DIAL APPARATUS ANI) MliTHOD The hybrid 2-frame PTV system consists of a Nd:YAG laser, a high-resolution CCD camera, a synchronizer, motor controller and an IBM PC as shown in figure 1. The dual-head Nd:YAG laser has a pulse width of about 7 ns with a pulse energy of 125 mJ for each head. The CCD camera can capture a couple of particle images at a time and has the spatial resolution of 2048 x 2048 pixels. A thin laser light . . . . ... . . sneer was used tO Illuminate the measurement planes and the scattered particle images were captured by the CCD camera for velocity field measurements. The Circulating Water Channel Sen,~motor and Encoder 2.2D I_ 1< >1 1 1\ L85D ~ ~ ~ X 1 _- / Kodak 2K'dK ~ / ~~ ~ Optics: r 1 1 IBMPC | Grabber | Dual-Head I ~ |-Controller l Nd:Yag ~ - Low pass filter Laser - Synchronizer Figure 1: Schematic diagram of experimental set-us _ __ CCD came ra and laser were synchronized with angular position of the propeller blade. Particle centroids were detected from the captured particle images in the preprocessing of the hybrid PTV. The post-processing routine includes particle tracking, data validation and interpolation to get the instantaneous velocity field on the regularly spaced grid points at each section. Details are described in Kim and Lee (2002) about the velocity field measurement technique. The circulating water channel where the propeller wake was measured has a test section size of 120~ x 30W x 2(F in centimeter. Figure 2 shows the propeller KP505 for the 3600TEU container vessel tested in present study. The KP505 propeller of 54mm in diameter has 5 blades with a design advance coefficient J of 0.72. The diameter of the propeller shaft is 7 mm. Velocity field measurements were carried out at three advance coefficients of J = 0.59, 0.72 and 0.88 to examine the influence of propeller loading. The free r/R StOE ELEVATION PROJECTED BLADE £XPANOEO BLADE P/O . to moo _ U.'DU - ' eco = . you coo , O. .~ , u an d~ .. I, o, _ ~C- 1 \ ': ~ Ll ~ ~ ~_O ~ FR PRINCTPAI PART lCULARS DIAMETER(MM) psalm SCAI~.E RATIO 4 000 (P/O} mCon : O .950 PP0P TYPE ~ FED Ar/Ao O. BOO SECTION NACA66 Mug RAT lo o. HO DRAWING SCALE 2,rJ00 NO OF B`,ADES 5 COMN4FNTS Figure 2: Propeller geometry
stream velocity was 32.5 cm/s and the Reynolds number based on the propeller diameter is about 18000. no .n ~ n R .1 ~ _ no -0.6 -na . ._ 0 0.2 _ ~ . . 0.4 0.6 0.8 1 X/D (a)J=0.88 XID (b) J = 0.72 -1 .2 2 0 0.2 0.4 0.6 0.8 1 X /D (b)J=0.59 Figure 3: Instantaneous velocity field subtracted by Uo at o= 0° The laser light sheet was illuminated from the bottom of water channel and the field of view was 11.8 x 11.8 in centimeter. Silver coated hollow glass beads with mean diameter of 10 ,um were used as seeding particle. A servo-motor attached with an encoder was used to derive the propeller, and the support strut was installed to prevent transfer of the surge or vibration of the propeller shaft with 0.1 3D diameter to the working fluid. The propeller was driven from downstream to avoid the effect of wake generated by the supporting struts. The center of propeller wake could not be measured because the propeller shaft was located at the downstream. The encoder mounted on the servo-motor generates trigger signals to synchronize laser and CCD camera with an accuracy of 0.36° for each selected angular position. The encoder signals were filtered with a low pass filter to get a clear trigger signal. The time interval between two consecutive particle images As set to 300 ,us, for which the propeller rotates 0.9°. The velocity field measurements were carried out at four different phases(~= 0°, 18°, 36°, 54°) with the angular interval between two adjacent measurement phases of 18°, and the corresponding elapse time was about 6 ms. A total of 400 instantaneous velocity fields was obtained for each measurement phase using the adaptive hybrid 2- frame PTV method. The turbulence characteristics such as turbulence intensity and Reynolds shear stress distribution were obtained by ensemble-averaging the instantaneous velocity fields. RESllTIS AND DISCUSSION The abscissa and ordinate of all results are represented in the plane of X and Y axes, normalized by the propeller diameter D. The positive X-axis is in the direction of the main flow and the positive Y-axis is directed vertically upward. The propeller is placed at X = 0 and the propeller shaft is placed at Y = 0. Figure 3 shows the instantaneous velocity field subtracted by Uo=32.5 cm/s at the phase angle of o= 0°. The periodic wake sheets and tip vortices in the clockwise rotation are shed successively from the blade tips with a regular interval are clearly seen at J = 0.59 and 0.72. In figure 3(a) counter-clockwise rotating vortices can be observed. These vortices do not appear to be associated with the tip vortices but are diagnoised to be generated from the shear interaction between the slipstream of propeller and the free stream. The contour plots of phase-averaged axial velocity at 0 = 0~ are shown in figure 4. The axial velocity component has relatively small values in the blade tip and propeller shaft. The viscous wake indicating the defects of axial velocity component appears in the
near-wake region due to the merging of two boundary not 06 (c)J=o.59 Figure 4: Contour of phase-averaged axial velocity layers developed on both sides of propeller blade 414 Q4 ED (a)J=0.88 o~i Q5 1 1 125 1.5 1.75 In (b)J=0.72 025 05 Q75 1 125 15 1.75 (c) J = 0.59 Figure 5: Contour of phas~averaged vorticity at ¢
no -0.6 _ ___ ~ -0.8 _ ~ _ -1.2 Figure 6: Trajectories of tip vortices for four blade angles at J = 0.72 -0.2 t -0.4 -0.6( -0.8 -1 .2 C. ~~+ . 47~-~ Figure 7: Variation of trajectory of tip vortices at three propeller loadings at o= 0° surfaces. As the advance coefficient(J) decreases (increasing propeller loading), the magnitude of the axial velocity component is increased within the slipstream of the propeller wake. The slipstream fluctuates unstably in the downstream region behind X = 0.5D for the advance coefficient of J = 0.59 and 0.72. This fluctuation seems to be originated mainly from the separation of tip vortex from the wake region. For the case of light loading(J = 0.88), the slipstream oscillates just from the trailing edge of propeller blade. On the average, the axial velocity has minimum values around 0.7R of the propeller blade span where R is the propeller radius. This agrees well with the propeller design which assigns the maximum loading around 0.7R. Figure 5 shows the contours of phase-averaged vorticity at ~ = 0° for three advance coefficients. Tip vortex generated from the pressure difference between upper and lower surfaces of a propeller blade is rolled up near the blade tip and forms vortex sheets. Tip vortices evolve downstream with a regular spacing periodically. The trailing vortices called as the potential wake are originated from the trailing edge of the propeller blade. Tip vortex has a concentric circles shape, while, the trailing vortices have the curled shape ~ 1 {2 ~4 {16 o . ~ _ ~ 0:~; 05 Y./D (a)J=0.88 of _ X,D (c) J = 0.59 L" Tu 20 Q1 5, 19 Q144 18 Q1~ 17 Q133 16 Q128 15 Q122 14 Q116 1 3 Q1 11 12 Q1Di 11 Q1Q) 1 0 Q094 9 Q088 8 QOe3 7 Q077 6 Q072 5 QOe6 4 Q063 3 QO$ 2 Q04 1 Q043 _ Let Tu 20 Q1~ 19 Q1 44 Q1 39 17 Q1 33 16 Q1 28 15 Q1 i2 14 Q1 16 13 Q1 11 12 Q1 Q5 11 Q1 Q1 10 Q094 9 Q088 8 Q083 7 Q077 6 Q072 5 GOES 4 COO 3 QO$ 2 Q04 1 Q043 Lad Tu 20 Q1 ~ 19 Q144 18 Q13) 17 Q133 16 Q128 15 Q122 14 Q116 1 3 Q1 11 12 Q1Q5 11 Q1Q, 1 0 Q094 9 Q088 8 Q083 7 Q077 6 Q072 5 QOe6 4 QOeO 3 QO$ 2 Q04 _ 1 Q043 Figure 8: Spatial distribution of axial turbulence intensity at o= 0°
~2 ~4 Q6 Q2 ~4 ~6 l . ~ 1' ... I .'t. 1. t.'I I' . 51 ~ .. ~ It . t. I.. 0:~ 05 075 1 1.~ 15 1J~ YJD (a)J =0.88 B _ Ski 05 075 1 1.;5 ID (b)J=0.72 15 1.75 Led ~ 20 OlCo 19 aos6 1 8 GOSH 1 7 aom 1 6 Q084 1 s aoeo 1 4 0076 13 ao72 12 ao6s 11 0 - 10 Doe g Dose 8 QOq' 7 Q04} 6 Go44 5 Q040 4 O 3 ao32 2 Got _ 1 Goal L - l 1;, 20 Q1CD 19 QO$ 1 8 Doe. 1 7 QO`B 1 6 Q084 1 5 QO8O 1 4 QO~i 13 Q02 1 2 QOEB 1 1 Q064 1 0 QOeD 9 QO$ 8 QO~ 7 Q04} 6 Q044 5 QO~ 4 QO:B 3 Qo32 2 Goal _ 1 Qo24 't I''''I ' ''' I ''' 'I''' 'I It ~ ·1 · ·\ · 1. OF QS 0.75 1 1Z; 15 1.75 ED (c)J=o.59 Figure 9: Spatial distribution of vertical turbulence intensity at o= 0° toward the propeller shaft from the tip. The vorticity in the tip region increases significantly as the propeller loading increases. The peak values of the first tip vorticity are -7.1, -3.9 and-2.3 sec~i for J = 0.59, 0.72 and 0.88, respectively. This indicates the fact that the propeller of heavy loading loses more energy due to tip vorices than that of light loading. For the small loading at J = 0.88, the vortices generated by the shear interaction between the slipstream of the propeller and free stream show relatively greater intensity of 2.7 sec~i than those of tip vortices whose maximum intensity is -2.3 sect, which indicates that J=0.88 is not a good operational condition for KP505 propeller. From this, it can be judged that it is desirable to operate the propeller at the design loading to get the optimized propulsion efficiency. The trailing vortices are composed of two vorticity layers with a positive or negative sign, which are developed on the blade surfaces and the vortex exists between these two layers. The tip vortex has a strong asymmetry shape in the initial wake region up to X = 0.25D at J=0.59, 0.72. However, for the advance coefficient of J=0.88, the tip vortex shows a weak asymmetry. The asymmetry of tip vortex was caused by the interaction between tip vortex and wake sheet. As the flow goes downstream at J = 0.59 and 0.72, the asymmetry turns to symmetry as the tip vortices are separated from the wake. Figure 6 shows the trajectories of tip vortex cores for four phases at the advance coefficient of J = 0.72. Each tip vortex has a nearly constant trajectory in the initial region O < X < 0.5D and then suddenly contracted from the slipstream at about X=0.8D. After the large contraction, tip vortices start to oscillate. The traces of tip vortex cores for 0 = 00 at three propeller loadings are shown in figure 7. The tip vortices are formed at regular intervals for all propeller loadings tested in this study. Up to the downstream location of X=0.8D, as the propeller loading decreases(J increases) the tip vortices moves slightly lord Tv outward due to shortage of a axial momentum. In the :D Q1~ it aOg~ region of X > 0.8D, the fluctuation of the trajectory of ,7 QO~ tip vortices at J = 0.88 appears slightly smaller than 345 X76 that at the other loadings. The separation of the tip '2 Co678 vortex from the wake, viscous dissipation and turbulent to Q064 diffusion seem to result in the decay of the trailing and 8 Y52 tip vortices. 7 oo4 Turbulence intensity distribution was obtained from 5 ~O30 the statistical analysis of the fluctuation velocity fields. 2 ~O238 Figure 8 and 9 show the turbulence intensity distributions of axial and vertical velocity components ( ~IUO,~IUo ), respectively. The turbulence intensity for the axial velocity component is increased significantly, as the propeller loading increases. The axial turbulence intensity has local maximum values along the trace of tip vortices. This indicates that the turbulece intensity for axial velocity component is stronger near the tip vortices than the other wake region. The turbulence intensity of vertical velocity component also has large values near the tip vortices as shown in figure 9. This may have resulted from the active interaction between the tip vortices and the wake sheet. However, the magnitudes of the vertical
~4 ma, _ no . . ~ . . . . I . . , o of 61 O:~i 05 075 1 1.Zi Y0D (a)J=0.88 e 42 ~4 It · 1~1 - of as 075 1 1.~5 15 1.75 Y7D (b) J= 0.72 _ 4B '. ~ 1, .., 1.... 1.... 1 .~..1 · .. 025 05 O~ 1 1~ 15 1. ID (C)J=0.59 Figure 10: Spatial distribution of Reynolds shear stress at ¢= 0° turbulence intensity at the design and higher propeller loadings do not vary much. The interaction between the tip vortices and the wake sheet transports the turbulence intensity of axial and vertical velocity components to the far downstream region. The propeller wake displays isotropic turbulent structure up to the range of X = 2D with large fluctuations of axial and vertical velocity components. Kiya and Sasaki(1983) mentioned that the maximum turbulence intensity occurs usually at the core of shear layer for a given flow. The shear layer start to oscillate from the downstream location of X = 0.7D due to the separation of the tip vortex from the wake sheet. The effect of propeller shaft on the wake structure seems to be small because the vertical turbulence intensity distribution is well matched with the vorticity contour and the turbulence intensity produced from the rotating propeller shaft is not so high. The Reynolds shear stress(u'v'/UO2 ) has large values within the shear layer developed from the propeller blade surface as shown in figure 10 within the wake region observed in the present experiment, the Reynolds shear stress becomes larger as the wake goes downstream and the advance coefficient J increases. The separation of tip vortices from the slipstream expands the region having large values of Reynolds shear stress. The tip vortex is mixed with near-wake flow for the cases of J = 0.59 and 0.72. Due to active mixing between tip vortices and wake flow, the turbulent shear stress increases at the downstream location. CONCLUSION The propeller wake in an open water condition was investigated using an adaptive hybrid PTV technique, and instantaneous velocity fields were measured at four different blade phases of 0°, 18°, 36° and 54°. The vis cous wake indicating the loss of axial velocity component is related to the merging of boundary layers of a propeller blade. The slipstream starts to oscillate after the rapid contraction at about X=0.7D at the design and higher loading conditions. Periodic tip vortices are formed due to pressure difference between two surfaces of propeller blade and go downstream with a regular interval. The magnitude of counter-clockwise rotating vortex increased as the propeller loading decreases. The vorticity value and the asymmetry of tip vortices are increased as the increase of propeller loading. The broadening and contraction of the gap between trailing vortices and tip vortex as the propeller wake goes downstream were attributed to the following factors; the separation of tip vortices from the wake, the interaction between tip vortex and wake sheet, turbulent diffusion and viscous dissipation. With increasing propeller loading, the magnitude of axial and vertical velocity component is increased and the turbulence structure becomes an isotropic one. After the region of slipstream contraction, the shear layer oscillates unstably. This results from the separation of tip vortex and the active interaction with the near-wake flow.
ACKNOliVLEDGMENT The present work is supported by National Research Laboratory Program of Ministry of Science and Technology(MOST) of Korea. RICE Back, S.J. and Lee, S.J., "A New Two-Frame Particle Tracking Algorithm Using Match Probability," Excrements in Fluids, Vol.22, 1996, pp. 23-32 Chesnaks C., Jessup S., "Experimental Characterisation of Propeller Tip Flow," 22nd SYmoosium on Naval Hydrodynamics, Washington D.C.~ 1998* oD.156-169. Cho, C.H. and --on or ~ Lee, C.S., "Numerical Experimentationof a 2-D Splice Higher Order Panel Method,' Journal of the SocietY of Naval Architects of Korea, Vol.37, No.3, 2000, pp.27-36. Cotroni, A., Di, Felice F., Romano, G.P. and Elefante,M., "Investigation of the Near Wake of a Propeller Using Particle Image Velocimetry," Experiments in Fluids, Vol.29, 2()00, pp.s227-236. Hoshino, T., "Hydrodynamic Analysis of Propellers in Steady Flow Using a Surface Panel Method," Journal of the Societv of Naval Architects of Janan, Vol.166, 1989, pp. 79-92. Kerwin, J.E. and Lee, C.S., "Prediction of Steady and Unsteady Marine Propeller Performance by Numerical Lifting Surface Theory," Trans. S NAME Vol.86, 1978, pp.218-253. Kim H.B. and Lee, S.J., "Performance Improvement of Two-frame Particle Tracking Velocimetr~y Using a Hybrid Adaptive Scheme," Measunnent Science & Technology, Vol.13, 2002, ppS7~582. Kim, Y.G., Lee, J.T., Lee, C.S., and Sub, J.C., "Prediction of Steady Performance of a Propeller by Using a Potential-Based Panel Method," Trans. of the SocietY of Naval Architects of Korea, Vol. 30 No. 1 , , 1993, pp.73-86. Kiya, M. and Sasaki, K., "Structure of a Turbulent Separation Bubble," Journal of Fluid Mechanics, Vol.137, 1983,pp.83-113.- Lee, J.T., "A Potential-based Panel Method for the Analysis of Marine Propellers in Steady Flow," Ph.D. Thesis, Department of Oc can Engineering, M.I.T., Cambridge, Mass., 1987. Stella, A., Guj, G., Di, Felice F. and Elefante, M., "Propeller Wake Evolution Analysis by LDV," 22nd Symposium on Naval Hydrodynamics, Washington D.C., 1998, pp. 171
discrete points by scanning the flow field with an array of measurement probes. Stella et al. (1998) measured the axial velocity component of a propeller wake, and Chesnakas and Jessup (1998) investigated the tip vortex flow using LDV. Unfortunately, these point-wise methods take substantially long time to get the phase- averaged velocity field information. Recently, velocity field measurement techniques have been applied to measure the flow around a marine propeller. The PIV(Particle Image Velocimetry) technique does not interfere with the flow structure and also does not require much time, compared to the LDV method, to measure the velocity field in a reasonably large area. Controni et al. (2000) investigated the near- wake of a marine propeller in a cavitation tunnel using PIV method. Their results show a good spatial resolution compared with conventional LDV data. In the present study, an adaptive hybrid PTV(Particle Tracking Velocimetry) method(Kim and Lee (2002)) is used to investigate the near-wake of a marine propeller in detail. To demonstrate the effectiveness of the method, a simple case of measuring the wake of a propeller model in an open-water channel was chosen. Once the reliability of the method is affirmed, it s anticipated to apply the method to more complex flow problems in the ship stern region. This velocity field measurement technique requires a preliminary PIV routine to determine the local match parameters needed for the 2-frame PTV algorithm(Baek and Lee, 1996). The original 2-frame PTV based on match probability determines the match parameters globally for the whole flow field. The adaptive hybrid scheme by utilizing the advantages of both PIV and PTV techniques recovers significantly more velocity vectors while reducing computation time and errors. In addition, the adaptive hybrid PTV shows the superior spatial resolution, compared with the conventional PIV technique. The several hundreds instantaneous velocity fields were measured at four different phases of the propeller blade and they were phas~averaged to investigate the spatial evolution of the vertical structure and turbulence statistics for the propeller wake. DIAL APPARATUS ANI) MliTHOD The hybrid 2-frame PTV system consists of a Nd:YAG laser, a high-resolution CCD camera, a synchronizer, motor controller and an IBM PC as shown in figure 1. The dual-head Nd:YAG laser has a pulse width of about 7 ns with a pulse energy of 125 mJ for each head. The CCD camera can capture a couple of particle images at a time and has the spatial resolution of 2048 x 2048 pixels. A thin laser light . . . . ... . . sneer was used tO Illuminate the measurement planes and the scattered particle images were captured by the CCD camera for velocity field measurements. The Circulating Water Channel Sen,~motor and Encoder 2.2D I_ 1< >1 1 1\ L85D ~ ~ ~ X 1 _- / Kodak 2K'dK ~ / ~~ ~ Optics: r 1 1 IBMPC | Grabber | Dual-Head I ~ |-Controller l Nd:Yag ~ - Low pass filter Laser - Synchronizer Figure 1: Schematic diagram of experimental set-us _ __ CCD came ra and laser were synchronized with angular position of the propeller blade. Particle centroids were detected from the captured particle images in the preprocessing of the hybrid PTV. The post-processing routine includes particle tracking, data validation and interpolation to get the instantaneous velocity field on the regularly spaced grid points at each section. Details are described in Kim and Lee (2002) about the velocity field measurement technique. The circulating water channel where the propeller wake was measured has a test section size of 120~ x 30W x 2(F in centimeter. Figure 2 shows the propeller KP505 for the 3600TEU container vessel tested in present study. The KP505 propeller of 54mm in diameter has 5 blades with a design advance coefficient J of 0.72. The diameter of the propeller shaft is 7 mm. Velocity field measurements were carried out at three advance coefficients of J = 0.59, 0.72 and 0.88 to examine the influence of propeller loading. The free r/R StOE ELEVATION PROJECTED BLADE £XPANOEO BLADE P/O . to moo _ U.'DU - ' eco = . you coo , O. .~ , u an d~ .. I, o, _ ~C- 1 \ ': ~ Ll ~ ~ ~_O ~ FR PRINCTPAI PART lCULARS DIAMETER(MM) psalm SCAI~.E RATIO 4 000 (P/O} mCon : O .950 PP0P TYPE ~ FED Ar/Ao O. BOO SECTION NACA66 Mug RAT lo o. HO DRAWING SCALE 2,rJ00 NO OF B`,ADES 5 COMN4FNTS Figure 2: Propeller geometry
DISCUSSION F. Di Felice INSEAN (Italian Ship Model Basin), Italy Authors performed the analysis of a propeller wake at very low Reynolds number by using PTV and phase averaged technique. Analysis extends far downstream the contraction including the vortex breakdown region. The images are analyzed using an Hybrid algorithm in which, in a first pass, classical PIV cross- correlation algorithm is applied to obtain the main parameters to be used for the PTV analysis. This approach allows an higher spatial resolution as shown also by Keane et al (1995) because a velocity vector is obtained for each particle simultaneously detected on both PIV images, even if randomly spaced data are obtained. Authors show instantaneous velocity field obtained interpolating the randomly spaced data over a regular grid. The choice of the sampling frequency is very critical because a dense re- sampling grid will introduce too much interpolated data while on the other hand a low density grid will loose many information. Could the authors explain how they managed such problem especially looking at the statistical computation, which can be affected by the presence of interpolated data? Looking at the result, figure 6 and 7 show the location of the tip vortices. A separation of the tip vortex trajectories downstream x/D= 1 is apparent. This is a typical behavior of the propeller vortex breakdown process that is caused by the interaction of the tip vortex with the wake of the previous blade. In fact, the actual blade wake, traveling at higher speed with respect the tip vortex, overtakes and interacts with the previous blade tip vortex as observed also at higher Reynolds number (Di Felice et al, 2000~. Did the authors observed such type of blade to blade interaction? REFERENCES Measurement Science and Technology, vol 6, 1995, pp 754-768. Symphosium on Naval Hydrodinamics, Vat de Ruil (F), 2000. AUTHORS' REPLY (1) Using the grey level intensity of particle image, the global thresholding algorithm was applied to extract particle centroids. Thereafter the particle tracking routine is applied. After validating all PIV data, error vectors in the velocity field are replaced with the vectors estimated using the AGW(Adaptive Gaussian Window) method for neighborhood correct vectors (Spedding and Rignot(1993), Agui and Jimenez(1987~. The AGW method is a simple convolution of the known data (uk,vk) with an adaptive Gaussian window ok. The weighting coefficients ok are adjusted so that their sum is always equal to unity, independent of the particle location. As the velocity vectors extracted from the present hybrid PTV algorithm has random particle locations, the interpolation scheme is required to obtain velocity vectors on regular spaced grid points and future calculation of turbulence statistics. In the interpolation procedure, the number of grids has to be determined in advance. The proper number of grids for interpolation depends on the number of particles and the spatial resolution of camera. (2) The separation of tip vortex from the wake sheet results from the interaction between blade wake and tip vortex. The previous experiment (Di Felice et al, 2000), the blade wake overtakes and interacts with the previous blade tip vortex at high Reynolds number. On the other hand, we measured the propeller wake at relatively small Reynolds number. For low Reynolds number flow, the viscous effect is usually a little overestimated than the actual one. However, we also observed the interaction between tip vortex and blade wake in the separation and the oscillation of tip vortices. The contour plots of axial velocity (figure 4) show that higher axial velocity at the inner radius makes the blade wake bend and go faster than the tip vortex. The Keane R D, Adrian R J. Zhang Y. "Super- vorticity evolution (figure 5) also shows that the resolution Particle Image Velocimetry", blade wake goes downstream faster than the tip vortex. Therefore, the blade wake travelling at higher speed with respect to the tip vortex overtakes and interacts with the previous blade tip vortex in our experiment. Di Felice F. Romano G P. Elefante M "Propeller wake Analysis by means of PIV", 23th
REFERENCES Agui J. C. and Jimenez J., "On the Performance of Particle Tracking", Journal of Fluid Mechanics, vol. 185, 1987, pp.447-468. Di Felice F., Romano G. P., Elefante M., "Propeller wake Analysis by means of PIV", 23th Symposium on Naval Hydrodynamics, Val de Ruil (F), 2000. Spedding G.R. and Rignot E.J.M., "Performance Analysis of Grid Interpolation Techinques for Fluid Flows", Exp. Fluids, vol 15, 1993, pp. 417- 430.
