1,721,013 research outputs found
Experimental Analysis of the Influence of Ship Wake Scaling on Marine Propeller Radiated Noise and Pressure Pulses
No full textThe propeller performances and in particular the non-stationary cavitating behavior and its side effects, i.e. pressure pulses and radiated noise, are strongly dependent on the ship wake field. Many different predictive methods are available, however in many cases predictions are still performed on the basis of semi-empirical approaches, whose validity is doubtful. In par-allel to this, the ever increasing power of computational tools (namely RANS solvers) allows for a direct numerical prediction of the ship full scale wake. In the present paper, the effect of the adoption of two different full scale wakes obtained with semi-empirical methods and di-rect computations is evaluated experimentally by means of cavitation tunnel tests on a model propeller, focusing the attention not only on mechanical characteristics and cavitation extents but, specifically, on pressure pulses and radiated noise. The results of this study put into evi-dence the differences which may be expected using the two different scaling approaches
Experimental analysis of the influence of ship wake scaling on marine propeller radiated noise and pressure pulses
No full textThe propeller performances and in particular the non-stationary cavitating behavior and its side effects, i.e. pressure pulses and radiated noise, are strongly dependent on the ship wake field. Many different predictive methods are available, however in many cases predictions are still performed on the basis of semi-empirical approaches, whose validity is doubtful. In parallel to this, the ever increasing power of computational tools (namely RANS solvers) allows for a direct numerical prediction of the ship full scale wake. In the present paper, the effect of the adoption of two different full scale wakes obtained with semi-empirical methods and direct computations is evaluated experimentally by means of cavitation turmel tests on a model propeller, focusing the attention not only on mechanical characteristics and cavitation extents but, specifically, on pressure pulses and radiated noise. The results of this study put into evidence the differences which may be expected using the two different scaling approaches
Evaluation of methods to measure acoustic transfer functions in cavitation tunnels
No full textThe interest for cavitation noise studies and prediction is increasing in the field of naval architecture, mainly because of attention to environmental issues. Model scale tests in cavitation tunnels are commonly considered as one of the most effective tools for cavitation noise studies. However, despite being carried out since long time, model scale experiments still present many challenges. These are mainly related to scale effects on propeller hydrodynamics, cavitation behaviour and scale effects on noise generation and propagation. Besides these phenomena the effect of the confined environment in which tests are carried out may be of great importance. In present work, an acoustic characterization of the SSPA large cavitation tunnel section #3 is presented, with the aim to obtain suitable transfer functions in order to take into account (at least partially) this phenomenon. The acoustic characterisation is performed considering two different underwater transducers and different signals and post processing techniques. The obtained transfer functions are shown and discussed in order to analyse advantages and shortcomings of the different procedures and to generally identify main problems related to this kind of activity
Aspects of the measurement of the acoustic transfer function in a cavitation tunnel
No full textModel scale experiments represent one of the most effective, and probably the most commonly used, approaches for the study of cavitating propeller noise. Despite this, the acquirement of consistent data of propeller noise from tests in model scale facilities is still a challenging task. Data collected with such an approach are usually affected by several scale effects and facility related issues which make such experiments still very complex. Among others, the effect of the facility itself as acoustical surrounding must be carefully addressed in order to properly estimate propeller source levels in ideal free field conditions. This is currently carried out, when possible, by measuring facility transfer functions and applying such functions as a correction to measured noise spectra. The development of robust procedures for the measurement of these transfer functions is thus very important. In the present work, this problem is addressed by discussing in details techniques, merits and possible problems related to this topic
Experimental investigation of single blade loads by captive model tests in pure oblique flow
No full textManeuvering motion is a critical off-design condition experienced by the propeller during realistic operations. Failures of the propulsive system, loss of efficiency and modification of the propeller side effects (propeller-hull induced pressure and noise) are the undesired consequences of these working conditions. Free running model tests, still representing the primary approach for a reliable performance assessment, requires facilities and devices that are not commonly affordable; alternatively, rectilinear towing tank can be used for maneuvering investigations by static or dynamic tests and can be a valid alternative to investigate propeller performance in offdesign. On these basis, in this paper the propeller performance in maneuvering conditions is investigated by
means of oblique towing tests in case of a twin screw model equipped with a novel set-up for single blade loads measurements.In the experiments, the drift angle and the advance speed of the model varied systematically, to focus on the relation between propeller operating conditions and loads. Moreover, the averaged and periodic blade loads are compared, in terms of the equivalent drift angle, to the measurements obtained by free running model tests, in order to demonstrate the reliability of pure oblique flow tests for the preliminary quantification of the off-design loads developed by the propeller
Single screw ships radiated noise measurements in model and full scale
No full textIn this work the problem of underwater radiated noise emitted by cavitating propellers is addressed, considering the case of single screw ships. To this aim, a series of experimental results obtained in model and full scale is presented. The analysis of results reported allows to discuss the effect of the surrounding ambient in model scale measurements and the importance of a correct representation of the ship wake. A comparison of model and full scale measurements is also reported
Model scale investigation of the effect of different speed reduction strategies on cavitating propeller radiated noise
No full textThe continuous development of design tools and analysis procedures has allowed propeller performances to increase progressively during years following more and more stringent design requirements . High propeller efficiency and avoidance of erosive cavitation are considered nowadays basic requests for a well designed propeller. However this may not be enough to meet the requirements of high added value ships such as passengers ships, mega yachts, research vessels, naval vessels etc. These kinds of ship need a propulsion system not only with good efficiency but also with low levels of vibrations and noise transmitted inside the ship and with low underwater radiated noise. Moreover, the above mentioned ships often work in a rather wide range of operational condition. Actually, the possibility to operate at reduced speed regards almost all kinds of ships, due to the increasing attention to fuel consumption and
exhaust gas emissions, and the related tendency towards slow steaming. Speed reduction may be obtained in different ways. In
the case of ships equipped with CPPs, in principle it may be achieved reducing RPM at constant pitch or reducing pitch at constant RPM; the latter is usually preferred when a power take off is present. This results in working conditions far from design ones for the propeller; Cavitation and related radiated noise may increase significantly in similar situations, despite the speed reduction is felt to be generally beneficial for the purpose. In this work, the effect of different speed reduction strategies on cavitation and radiated noise is studied by means of cavitation tunnel tests of four CPPs including both single screw and twin screw configurations
Design, analysis and experimental characterization of a propeller in decelerating duct
No full textDespite having been applied since a long time, ducted propellers design still represents a challenge for the designer, especially when dealing with the less widespread decelerating duct, due to the complex evaluation of the duct effects and to the limited amount of available experimental information. In the paper an hybrid design approach is described. Moreover, a series of experimental results at towing tank and, especially, at cavitation tunnel are presented, in order to provide an insight into the particular functioning of this propulsor including cavitation behavior and a validation of the proposed design procedure. Particular attention is given to the development of the cavitating tip leakage vortex, which characterizes this propulsor functioning. To this aim, experimental observations are compared with numerical results obtained by means of a commercial RANSE solver, in order to check its applicability for the prediction of cavitation inception and extent
Propeller nozzles design using viscous codes and optimization algorithms
No full textMarine propellers design requirements are always more pressing and the application of unusual propulsive configurations, like ducted propellers with decelerating nozzles, may represent a valuable alternative to fulfil stringent design constraints. If accelerating duct configurations were realized mainly to increase the propeller efficiency in highly-loaded conditions, decelerating nozzles sustains the postponing of the cavitating phenomena that reflects into reduction of vibrations and radiated noise. The design of decelerating nozzle, unfortunately, is still challenging. No extensive systematic series are available and the design relies on few measurements. On the other hand, viscous flow solvers appear as reliable and accurate tools for the prediction of complex flow fields. Hence, in the present paper the opportunity to use CFD as a part of a design procedure based on optimization, by combining a parametric description of the geometry, the OpenFOAM solver and a genetic type algorithm in the ModeFrontier environment, is investigated. Design improvements for both accelerating and decelerating ducts are measured by comparing the performance of the optimized geometries with those of conventional shapes available in literature
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