The Specialist Committee on Wake Fields Final Reports and Recommendations to the 25th ITTC

The recommended actions of 25th ITTC Specialist Committee on Wake-Fields, as stated above are focused on two main areas, the review of the numerical prediction and experimental measurement (methods) of wakefields and the review and development of ITTC procedures

The Specialist Committee on Detailed Flow Measurements. Final Report and Recommendations to the 26th ITTC

The scope of this report is to review up-todate measurement systems and methods available for flow-field and wave-field measurements and describe applications of Particle Image Velocimetry (PIV), stereoscopic PIV (SPIV), Laser Doppler Velocimetry (LDV),Particle Tracking Velocimetry (PTV),holography, and other emergent methods, for the measurements of flow separation, wake,vortex strength, etc, for ship hydrodynamics problems. Furthermore, practical issues related to the application of these measurement techniques, especially PIV and SPIV, in largescale tow tank facilities and cavitation tunnels will be discussed, with recommendations for future work for the ITTC in these areas

Propeller tip and hub vortex dynamics in the interaction with a rudder

In the present paper, the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed in its wake are addressed; in particular, following the works by Felli et al. (Exp Fluids 6(1):1-11, 2006a, Exp Fluids 46(1):147-1641, 2009a, Proceedings of the 8th international symposium on particle image velocimetry: Piv09, Melbourne, 2009b), the attention is focused on the analysis of the evolution, instability, breakdown and recovering mechanisms of the propeller tip and hub vortices during the interaction with the rudder. To investigate these mechanisms in detail, a wide experimental activity consisting in time-resolved visualizations, velocity measurements by particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) along horizontal chordwise, vertical chordwise and transversal sections of the wake have been performed in the Cavitation Tunnel of the Italian Navy. Collected data allows to investigate the major flow features that distinguish the flow field around a rudder operating in the wake of a propeller, as, for example, the spiral breakdown of the vortex filaments, the rejoining mechanism of the tip vortices behind the rudder and the mechanisms governing the different spanwise misalignment of the vortex filaments in the pressure and suction sides of the appendage

Investigation of the flow field around a propeller-rudder configuration: on-surface pressure measurements and velocity-pressure-phase-locked correlations

The present paper deals with the problem of the propeller induced perturbation on the rudder . The study aims at providing insights on the key mechanisms governing the complex interaction between the propeller wake structures and the rudder. In this regard, a wide experimental activity that concerned PIV and LDV velocity measurements and wall-pressure-measurements on the two faces of the rudder was performed in a cavitation tunnel. The major flow features that distinguish the flow field around a rudder operating in the race of a propeller, were highlighted, such as the complex dynamics of the propeller tip votices and the re-storing mechanism of the tip vortex downstream of the rudder. Wall-pressure signals were Fourier decomposed and, then, reconstructed isolating the contributions of the more energetic harmonics when both the propeller phase and the rudder deflection change

Acoustic signature reduction, modulation and control

In the present study the problem of the ship signature is dealt with considering the main perturbation sources that have a relevance on the propeller induced noise and vibrations. Specifically the following aspects that cause major effect on the hydroacoustic and propulsive performance of a marine propeller will be discussed: (i) the effect of the inflow, (ii) the contribution of the wake evolution and breakdown mechanisms, (iii) the propeller-rudder/propeller-strut interaction in conventional and podded propulsors. The results presented in the paper are part of research activities carried out by INSEAN, most of them supported by the Italian Navy. In this framework INSEAN has developed and implemented a number of advanced experimental and numerical tools, pioneering sometimes, which have aimed at probing into the complex problem of the hydrodynamic and hydroacoustic performance of a marine propeller

Analysis of the propeller wake by pressure and velocity correlation

In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non cavitating regime is reported. Velocity measurements were performed in phase with the propeller angle by using 2D Particle Image Velocimetry (2D-PIV). Measurements were carried out arranging the light sheet along the mid longitudinal plane of the propeller, to investigate the evolution of the axial and the radial velocity components, from the blade trailing edge up to 2 diameters downstream. The pressure measurements were performed at four radial and eight longitudinal positions downstream the propeller model. Measurements of the pressure field were performed at different advance ratios of the propeller. Pressure data, processed by using slotting techniques, allowed to reconstruct the evolution of the pressure field in phase with the reference blade position. In addition, the correlation of the velocity and pressure signals was performed. The analysis demonstrated that, within the near wake, the tip vortices passage is the most important contribution in generating the pressure field in the propeller flow. The incoming vortex breakdown process causes a strong deformation of the hub vortex far downstream the slipstream contraction. This process contributes to the pressure generation at the shaft rate frequency

Mechanisms of evolution of the propeller wake in the transition and far fields

In the present study the mechanisms of evolution of propeller tip and hub vortices in the transitional region and the far field are investigated experimentally. The experiments involved detailed time-resolved visualizations and velocimetry measurements and were aimed at examining the effect of the spiral-to-spiral distance on the mechanisms of wake evolution and instability transition. In this regard, three propellers having the same blade geometry but different number of blades were considered. The study outlined dependence of the wake instability on the spiralto- spiral distance and, in particular, a streamwise displacement of the transition region at the increasing inter-spiral distance. Furthermore, a multi-step grouping mechanism among tip vortices was highlighted and discussed. It is shown that such a phenomenon is driven by the mutual inductance between adjacent spirals whose characteristics change by changing the number of blades

Analysis of the propeller wake evolution by pressure and velocity phase measurements

In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non-cavitating regime is reported. Particle image velocimetry measurements were performed in phase with the propeller angle, to investigate the evolution of the axial and the radial velocity components, from the blade trailing edge up to two diameters downstream. In phase pressure measurements were performed at four radial and eight longitudinal positions downstream the propeller model at different advance ratios. Pressure data, processed by using slotting techniques, allowed reconstructing the evolution of the pressure field in phase with the reference blade position. In addition, the correlation of the velocity and pressure signals was performed. The analysis demonstrated that, within the near wake, the tip vortices passage is the most important contribution in generating the pressure field in the propeller flow. The incoming vortex breakdown process causes a strong deformation of the hub vortex far downstream of the slipstream contraction. This process contributes to the pressure generation at the shaft rate frequency

A novel approach for the isolation of the sound and pseudo-sound contributions from near-field pressure fluctuation measurements: analysis of the hydroacoustic and hydrodynamic perturbation in a propeller-rudder system

The main scope of the present work is to investigate the mechanisms underlying the hydroacoustic and hydrodynamic perturbations in a rudder operating in the wake of a free running marine propeller. The study consisted of detailed near-field pressure fluctuation measurements which were acquired on the face and back surfaces of the rudder, at different deflection angles. To this aim, a novel wavelet-filtering procedure was applied to separate and analyze distinctly the acoustic and hydrodynamic components of the recorded near-field pressure signals. The filtering procedure undertakes the separation of intermittent pressure peaks induced by the passage of eddy structures, interpreted as pseudo-sound, from homogenous background fluctuations, interpreted as sound. The use of wavelet in the filtering procedure allows to overcome the limitations of the earlier attempts based on frequency (wave number) band-pass filtering, retrieving the overall frequency content of both the acoustic and the hydrodynamic components and returning them as independent signals in the time domain. Acoustic and hydrodynamic pressure distributions were decomposed harmonically and compared to the corresponding topologies of the vorticity field, derived from earlier LDV measurements performed by Felli and Falchi (Exp Fluids 51(5):1385-1402, 2011). The study highlighted that the acoustic perturbation is mainly correlated with the unsteady load variations of the rudder and to the shear layer fluctuations of the propeller streamtube. Conversely, the dynamics of the propeller tip and hub vortices underlies the hydrodynamic perturbation

Coherent and turbulent process analysis of the effects of a longitudinal vortex on boundary layer detachment on a {NACA0015} foil

In the present paper, interactions mechanism of the propeller hub vortex with the rudder installed in its wake are studied. This configuration corresponds to the field of marine propulsion allowing specific problem such as cavitation inception, modification of the propulsive performances and induced vibrations. To better understand the complex mechanisms due to the impact of the propeller-rudder interactions decided to put more emphasis onto configuration where the hub vortex is generated by an elliptical 3D foil and is located upstream the 2D NACA0015 foil at high incidences at Reynolds number 5 105. The physical mechanism involved in the interactions between a single longitudinal vortex and boundary layer detachment was studied using time resolved stereoscopic PIV techniques. Particular attention was paid to the detachment at 25° incidence and a detailed cartography of the mean and turbulent properties of the wake is proposed. The proper orthogonal decomposition (POD) analysis was applied in order to highlight the unsteady flow aspects using phase averaging based on the first POD coefficients to characterize the turbulent and coherent process in the near wake of the rudder