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lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE UNSTEADY FLOW AROUND A PROPELLER 511 Experimental and Numerical Investigation of the Unsteady Flow around a Propeller P.G.Esposito, F.Salvatore, F.Di Felice, G.Ingenito (Istituto Nazionale per Studi ed Esperienze di Architettura Navale, Italy), G.Caprino (Centre per gli Studi di Tecnica Navale, Italy) ABSTRACT In the present paper the analysis of the flow around the propeller of a twin screw vessel is performed both experimentally by LDV measurements and numerically by a potential flow solver. The velocity fields obtained through the experimental survey have been used to assess the accuracy of the solver for the application to modern propellers, featuring rather extreme geometries and large pitch variations. The results show that, in spite of the approximations introduced, the solver is able to capture the main features of the flow. The combined use of numerical and experimental tools reveals itself as a fundamental step for a deep comprehension of phenomena taking place in modern ship units. INTRODUCTION Large fast vessels represent a severe challenge to ship designer in the quest for outstanding calm-water performance. The hydrodynamic optimization of the propeller being an essential step of this pursue, the need of reliable and effective tools for the analysis of propeller flow is much felt. The typical configuration is a twin screw controllable pitch propeller with quite a long shaft, due to the gradual rise of the keel in the aftbody, and a set of shaft brackets with a L arrangement so that the vertical bracket is in the shaft wake. The request of large thrust and the constraint on the maximum diameter posed by tip clearance naturally lead to high expanded area ratios; rather extreme skew distributions are adopted to decrease induced pressure pulses, while large pitch variations allow to avoid excessive loading at the propeller tip. Time honored computer codes based on lifting surface theory are inadequate to deal with the new propulsive configurations and are currently in the process to be replaced by panel methods, which provide a fully three dimensional model of blade geometry. On the other hand, the complexity of the unsteady phenomena involved requires a thorough validation of such codes in operating conditions. To this end, high quality experimental data of the flow around the propeller are needed. A joint effort within the framework of the national research program has been undertaken by INSEAN and CETENA to achieve a better physical understanding of the unsteady effects on propeller performance through model tests in the circulating water channel and the application of an unsteady panel code, described in Esposito and Giordani (2000). Flow field survey has been performed upstream and downstream the propeller by using Laser Doppler Velocimetry (LDV). LDV is a key technique for investigating the complex flow around propellers and many studies are available for uniform inflow, see e. g., Min (1978), Kobayashi (1982), Cenedese et al. (1985), Jessup (1989), and for axisymmetric inflow by Hayun and Patel (1991). However, very few studies of propellers operating in non uniform inflow are available, since the experimental analysis is particularly onerous, requiring several days of facility occupancy and a huge amount of data to be processed. The experimental technique adopted for the present measurements allows the reconstruction of the three dimensional velocity field in phase with the propeller. The high level of detail reached by these measurements represents a powerful tool for the analysis of the complex phenomena taking place in propeller-hull-appendage interaction. In addition to the experimental investigations, a theoretical analysis of the flow field around the propeller has been performed by using a Boundary Ele the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. Figure 9: Downstream axial velocity component (Configuration B). EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE UNSTEADY FLOW AROUND A PROPELLER 521

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lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE UNSTEADY FLOW AROUND A PROPELLER 523 measurement plane is extremely close to the trailing edges of the brackets; thus the velocity defect of the bracket wakes is stronger and confined in a thinner region than on the propeller disk plane, where the nominal wake used in the computations is measured. Further, this effect is enhanced by the low intensity of the perturbation induced by the propeller. Pressure and performance For the sake of completeness, some additional results obtained by the theoretical analysis are presented. Specifically, Fig 12 shows history of thrust coefficient KT and torque coefficient KQ for both configurations A and B. It may be observed that the stronger shaft wake of configuration B determines higher fluctuations for both KT and KQ than in the case configuration A results. For bracket configuration A, pressure coefficient distributions on the blade surfaces at angular locations θ=0°, 30°, and 60° is given by Fig. 13. CONCLUDING REMARKS An experimental and theoretical analysis of the flow field around a four-bladed propeller installed on a twin screw vessel has been presented. Flow field survey upstream and downstream the propeller has been performed by using LDV. Particular attention is payed to set up a procedure suitable for measurements of unsteady velocity fields as is the case of propellers in the wake of a hull. Phase sampling is performed by means of a Tracking Trigger Technique, by which velocity samples are arranged into angular slots according to the propeller position at measurement time. The procedure provides a satisfactory compromise between statistical requirements and angular resolution. The results of the measurement campaign demonstrate the capability of the present LDV phase sampling technique to provide an accurate description of both hull and propeller wakes. The analysis of measured velocity fields is helpful to highlight some important features of the interactions among wakes of the hull appendages and to investigate the flow field in the propeller wake region. In addition, experimental results are used to assess the validity of a theoretical approach based on a boundary element method for the analysis of propellers in non uniform flows. Comparisons with measured velocity fields show that the present theoretical approach provides reasonable predictions of the most relevant flow features in the propeller wake region. In particular, the wake alignment procedure is fully adequate to simulate the trailing vorticity convection process, in which viscosity has a minor influence. Better agreement between theoretical predictions and experimental results could be obtained by taking into account the streamwise variation of the hull wake used in the computations. ACKNOWLEDGMENTS The present work was supported by the Ministero dei Trasporti e della Navigazione in the frame of INSEAN and CETENA Research Programs 1997–99. REFERENCES Campbell, R., 1973, Foundations of Fluid Flow Theory, Addison-Wesley. Cenedese, A., Accardo, L., and Milone, R., 1985, “Phase sampling technique in the analysis of a propeller wake,” in Proc. of the International Conference on Laser Anemometry Advances and Application. Esposito, P.G. and Giordani, A., 2000, “Numerical analysis of non cavitating propeller in uniform and non uniform flow conditions,” in Proc. of the 9th IMAM Congress, pp. B21–B28. Greeley, D.S. and Kerwin, J.E., 1982, “Numerical methods for propeller design and analysis in steady flow,” SNAME Transactions, vol. 90, pp. 416–453. Hayun, B.S. and Patel, V.C., 1991, “Measurements in the flow around a marine propeller at the stern of an axisymmetric body,” Experiments in Fluids, vol. 11, pp. 105–117. Hoshino, T., 1993, “Hydrodynamic analysis of propellers in unsteady flow using a surface panel method,” Journal of The Sociey of Naval Architects of Japan, vol. 174, pp. 71–87. Huang, T.T. and Groves, N.C., 1980, “Effective wake: Theory and experiment,” in Proc. of the Thirteenth Symposium on Naval Hydrodynamics, pp. 651–673. Jessup, S.D., 1989, An Experimental Investigation of Viscous Aspects of Propeller Blade Flow, Ph.D. thesis, The Catholic University of America, Washington, D.C. Kinnas, S., Hsin, C., and Keenan, D., 1990, “A potential based panel method for the unsteady flow around open and ducted propellers,” in Proc. of the Eighteenth Symposium on Naval Hydrodynamics, pp. 677–685. Kobayashi, S., 1982, “Propeller wake survey by laser doppler velocimeter,” in Proc. of the 4th International Symposium on Application of Laser Doppler Anemometry to Fluid Mechanics. the authoritative version for attribution.

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lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE UNSTEADY FLOW AROUND A PROPELLER 524 Koyama, K., 1992, “Application of a panel method to the unsteady hydrodynamic analysis of marine propellers,” in Proc. of the Nineteenth Symposium on Naval Hydrodynamics, pp. 817–836. Min, K.S., 1978, Numerical and Experimental Methods for Prediction of Field Point Velocities Around Propeller Blades, Tech. Rep. 78–12, MIT Department of Ocean Engineering. Morino, L., Chen, L.-T., and Suciu, E., 1975, “Steady and oscillatory subsonic and supersonic aerodynamics around complex configurations,” AIAA Journal, vol. 13, pp. 368–374. Pyo, S. and Kinnas, S., 1997, “Propeller wake sheet roll-up modeling in three dimensions,” Journal of Ship Research, vol. 41, pp. 81–92. Stella, A., Guj, G., Di Felice, F., Elefante, M., and Matera, F., 1998, “Propeller flow field analysis by means of LDV phase sampling techniques,” in Proc. of the 3th International Symposium on Cavitation, pp. 171–188. Stella, A., Guj, G., and F.Di Felice, 2000, “Propeller wake flowfield analysis by means of LDV phase sampling techniques,” Experiments in Fluids, vol. 28, pp. 1–10. Figure 12: History of propeller loads during a revolution: ∆, configuration A; , configuration B. the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. Figure 13: Pressure coefficient on blade. EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE UNSTEADY FLOW AROUND A PROPELLER 525

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lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE UNSTEADY FLOW AROUND A PROPELLER 526 DISCUSSION T.Hoshino Mitsubishi Heavy Industries, Ltd. Japan The authors should be congratulated for their efforts to measure the complicated flow fields around a propeller operating in non-uniform flow by LDV and develop a panel method with a flow-aligned wake. I have the following questions. (1) Did you compare the calculated flow fields between with flow-aligned wake and without flow-alignment procedure? (2) Where is the remaining wake (far wake) with prescribed pitch starting from? (3) How many calculations are needed to obtain a final convergent solution? AUTHOR'S REPLY The authors wish to thank Dr. Hoshino for submitting his comments and discussion to the paper. In response to his questions the following remarks can be made: 1. Some comparisons concerning the effects of wake alignment have been presented in a previous paper by Esposito and Giordani [1], where the Seiun Maru HSP propellers has been analysed. In Figure 1, the tip vortex line of the prescribed wake is compared to the flow aligned wake. Of course the different position of the wake in the two cases has a strong impact on the flow field on downstream planar sections orthogonal to the axis. Some effects are also apparent in the loads as shown in Figure 2. 2. In the present computation two wake turns are used, with wake alignment limited to the first turn. 3. Our experience showed that the number of iterations needed to reach wake alignment convergence is slightly greater than the number of streamwise elements of the free portion. 1.1. REFERENCES 1. Esposito, P.G., Giordani, A. “Numerical Analysis of Non Cavitating Propeller in Uniform and Non Uniform Flow Condition”, Proceedings of the IMAM 2000 Conference, Ischia, Italy, Apr. 2000, pp. B-21:28. 2. Hoshino, T., “Hydrodynamic Analysis of Propellers in Unsteady Flow Using a Surface Panel Methos”, Journal of the Society of Naval Architects of Japan, Vol. 174, 1993, pp. 71:87. Figure 1: Propeller free wake geometry in axysimmetric flow. Figure 2: Single blade contribution to thrust and torque coefficients along a revolution. the authoritative version for attribution.