Proceedings of 25th ITTC – Volume II 535 The Specialist Committee on Wake Fields Final Reports and Recommendations to the 25th ITTC 1. GENERAL • Rome, Italy, September 2006 at INSEAN before the ONR Symposium. 1.1 Membership and Meetings • Tokyo, Japan, April 2007 at the The members of the Specialist Committee National Maritime Research Institute on Wake-Fields of the 25th ITTC are: (NMRI). • Dr. Jin Kim (Chairman) • Bethesda, Maryland, U.S.A., August Maritime & Ocean Engineering 2007 at the Naval Surface Warfare Research Institute (MOERI), KORDI Center Carderock Division (NSWC- Daejeon, Korea CD). • Dr. Thomas C. Fu (Secretary) • Gdansk, Poland, January 2008 at the Naval Surface Warfare Center Ship Design and Research Center Carderock Division (NSWC-CD) (CTO-SA). W. Bethesda, Maryland, U.S.A. 1.2 Tasks • Dr. Tomasz Bugalski Ship Design and Research Center The recommendations for the work of the (CTO-SA), Gdansk, Poland Specialist Committee on Wake-Fields as given by the 24th ITTC were as follows: • Dr. Munehiko Hinatsu National Maritime Research Institute • Conduct a survey of numerical methods (NMRI) for prediction of wake-fields at model Tokyo, Japan and full scale. • Dr. Fabio Di Felice • Review the experimental methods of INSEAN Italian Ship Model Basin determining the velocity distribution in Rome, Italy the wake. Five committee meetings have been held • Develop procedures for measuring the during the work period: velocity distribution in the wake at model and full scale. • Daejeon, Korea, January 2006 at the Maritime & Ocean Engineering • Review and update the existing Research Institute (MOERI)/KORDI. guidelines for the simulation of the wake-field for cavitation testing. The Specialists Committee on Wake Fields 536 2. INTRODUCTION Review of these procedures was made to standardize the procedures for generating and The recommended actions of 25th ITTC assessing simulated wakes and these Specialist Committee on Wake-Fields, as stated modifications have been submitted. above are focused on two main areas, the review of the numerical prediction and Development of wake-field measurement experimental measurement (methods) of wake- procedures utilizing 5-Hole Pitot Tubes, Laser fields and the review and development of ITTC Doppler Velocimetry (LDV), and Particle procedures. Since the Committee on Wake- Image Velocimetry (PIV) for both model and Fields is a Specialists Committee the literature full scale were all considered by the committee. survey of numerical and experimental methods The committee assessed the current status of was not constrained to the period since the 24th full-scale wake-field measurements and ITTC Meeting, and the emphasis of these tasks recommended to the Advisory Committee that was on completeness. Much attention was paid procedures for full-scale measurements not be to the numerical prediction of wakes at full- pursued, because these types of measurements scale, as this has been a major research topic in are far from routine. Similarly, PIV, while recent years. The review of experimental widely utilized to measure velocity fields, is methods includes efforts at both model and full not commonly utilized to measure nominal scales and also focused not only on wake fields at the propeller plane, because of measurements for simulated wake verification issues related to optical access (obstruction). but also for Computational Fluid Dynamics Additionally, PIV is typically utilized as a (CFD) validation and basic research. This research tool and the committee felt it was committee’s tasks related to the review of premature to establish procedures for its use. existing ITTC procedures and the development For these two reasons, the committee of procedures, and was, as expected, more recommended that PIV procedures not be focused, concentrating on the generation, use, developed, though guidelines for the use of and verification (measurement) of simulated PIV are provided in Section 7.0 of this report. wakes. Procedures for nominal wake field By reviewing all of the ITTC Procedures measurements utilizing 5-Hole Pitot tubes and for references to wake-fields, it was found that LDV were developed and submitted to the only four existing procedures refer to wake- Advisory Committee. Additionally, guidance fields. These procedures are: for generating simulated wake-fields for model scale testing is given in Section 6.0. • Procedure 7.5-02-03-03.1: Model-Scale Cavitation Test Cavitation Induced What follows are sections on the review of Pressure the numerical predictions (Section 3), of experimental methods of wake-field • Procedure 7.5-02-03-03.3: Fluctuations measurements (Section 4), followed by section Model Scale Experiments 5 on the developed procedures on the use of 5- hole Pitot Tubes and LDV to measure nominal • Procedure 7.5-02-03-03.5: Cavitation wakes, and finally sections on the reviewed Induced Erosion on Propellers, Rudders procedures (Section 6), guidance on the use of and Appendages Model Scale PIV (Section 7), and the committee’s Experiments recommendations (Section 8). • Procedure 7.5-02-03-03.6: Podded Propulsor Model-Scale Cavitation Test Proceedings of 25th ITTC – Volume II 537 3. SURVEY OF NUMERICAL a challenge in itself, the extreme grid density at METHODS FOR PREDICTION OF the hull surface causing problems with WAKE FIELDS AT MODEL AND convergence or stability. However, most codes FULL SCALE passed the test and could be further extended and applied. Developments were done on modeling the propeller action, comparing 3.1 Introduction turbulence models and including the wave pattern. An internal EFFORT workshop Since the 24th ITTC, a number of advances compared results and stimulated improvements in numerical methods have occurred leading to of the accuracy and applicability of several improved predictions of the wake-field. This codes. All model and full-scale experimental report includes a survey of the papers published data, for the two new ships and five existing on both global and regional conferences, ones, were then used to study the achievable journals, benchmarkings and other types of level of accuracy of the predicted wake fields scientific activities. Further open results of at full-scale, to decide on the best turbulence EFFORT and LEADING EDGE European models and to find out desired extensions of Union joint research projects were taken into the methods. Finally, the codes were used in account. case studies for practical applications, in cooperation with the industrial participants. December 2005 saw the successful conclusion of the three-year European Full- Several of the CFD codes were found to scale FlOw Research and Technology predict the full-scale flow relatively accurately, (EFFORT) project, a 5th Framework EU provided fine enough grids were used. Already project. Principal objective was to validate some further improvement has been achieved CFD predictions of viscous flow and wake by a changing turbulence models; while for a field of ships by comparison with experimental case like the dredger, incorporation of propeller data for full-scale. Such data being scarce, new ducts is a desired step. The full-scale measurements of wake-fields of two ships at predictions for practical projects, already full scale have been done. Full-scale flow frequently made, thus may play an even larger measurements were conducted by MARIN, for role soon. EFFORT certainly lived up to its ‘Nawigator XXI’, a single screw research name; ambitious goals were set at the start and vessel of the Maritime University of Szczecin, not all could be achieved. But definitely an and ‘Uilenspiegel’, a hopper dredger of important contribution has been made to the Dredging International; a twin-screw vessel practical use of CFD for prediction of the full- with bossings, exposed shaft hoses, shaft scale viscous flow and propeller action. support struts and ducted propellers. A dedicated LDV system was used, operating The demand of ship owners for efficient through windows flush-mounted in the hull. As and quiet operation of ships at relatively high usual, these measurements were not without speed or at high power is a challenge for the problems, due to environmental conditions and shipbuilding industry. The propeller plays other complications; but the data obtained form herein a significant role and is, for instance, a a valuable addition to existing material. Parallel critical element in the control of vibrations and model experiments were carried out, including noise on board. There is, however, a notable Pitot measurements by HUT, PIV lack of tools in predicting and scaling the measurements by CTO and wave pattern development and evolution of the vortex at the measurements using a laser sheet technique by tip of the propeller blades. The LEADING NTUA. On the CFD front, RANS EDGE project aimed at providing such tools to computations were requested for full-scale industry (shipyards, model basins, propeller Reynolds numbers. For some methods this was designers and manufacturers). The Specialists Committee on Wake Fields 538 To that end, the knowledge of and cavitation inception tests, but also flow field experience with computational codes and measurements with 3D Particle Image techniques (RANS codes) at several Velocimetry (PIV), a technique which has been universities and institutes in Europe has been applied in ship model basins in the past only brought together and applied to ship propellers. incidentally, but recently is increasing in use. The results obtained give information about the Based on the results of measurements and origin, the strength and the structure of the computations an attempt at blade shape non-cavitating tip vortex at model and at full optimization has been made for all three scale. Complementary experiments have been propellers. The new designs have subsequently carried out, among others with the latest PIV been investigated numerically. techniques. Transfer of the tool to industry has been given support via the execution of an The LEADING EDGE project has shown optimization process, led by industry, in which that viscous flow analysis for ship propellers is the results of the new tool were related with feasible and useful.