Underlying mechanisms of propeller wake interaction with a wing

Abstrac

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

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

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

Recent advances in the theoretical & experimental analysis of naval unit hydroacoustic performance

The present paper reviews the experimental and theoretical techniques currently used at CNR-INSEAN for hydroacoustic applications. The theoretical procedure used for hydroacoustic analyses is based upon the Lighthill\u27s acoustic analogy and concerns a hybrid hydrodynamic and hydroacoustic solver. Experimental hydroacoustics regards simultaneous velocity-pressure/visualization-pressure measurements and cross-correlation and conditional techniques. Furthermore, special signal processing techniques are used for the sake of removing unwanted noise contributions and separating the sound and pseudo-sound contributions. Examples of hydroacoustic analyses undertaken by aforementioned theoretical and experimental tools are documented in the paper

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

Noise measurements techniques and hydrodynamic aspects related to cavitation noise

In the framework of the European project Hydro Testing Alliance (HTA), INSEAN is leader of the subtask 1.1. and Task 3 of the Joint Research Project (JRP) 10. Specifically, the topics of the tasks are as follows: - Task 1.1: "Hydrophone set-up, data acquisition and processing". Under the task "Noise measurement at model scale", this subtask concerns a review of typical test setup and specification of instrumentation, transducers, data acquisition hardware and procedures. These aspects are dealt with is two sections, organized as follows: a) Review of transducers and measurement set-ups of participants, including limitations and experiences; b) Review of either the setup for noise measurements using noise cancellation techniques and signal processing techniques for noise cancellation to reduce the facility background noise. In a) some hydrophone and pressure transducer models, as typically used for noise measurement in the JRP10 organizations, are reviewed and compared. Section b) is focused on techniques which aims at correcting the contribution of the "unwanted noise sources" (e.g. background noise of the facility, possible reverberation of the test section, noise from the ship model mechanical transmission). - Task 3: " Hydrodynamic aspects influencing cavitation noise". This task involves a review and analysis of the flow topologies involved in the acoustic emission in both cavitating and non-cavitating conditions. Specifically, the task concentrates on two aspects mainly: a) Hydrodynamic aspects influencing the non-cavitation noise; b) Hydrodynamic aspects influencing the cavitation noise

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

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