1,721,075 research outputs found
Two medium size cavitation tunnel hydro-acoustic benchmark experiment comparisons as part of a round robin test campaign
Full text linkThe increase in marine transportation in the last decades has resulted in a rise of the different emissions linked to it. Amongst various detrimental emissions of shipping activities, underwater noise is known to affect ambient noise levels and hence threaten the life (or even survivability) of marine mammals. This concern brought the underwater radiated noise (URN) to the attention of regulators, considering the possible need to limits for commercial ships, thus resulting in a surge of interest. As being the main contributor to the URN of ships, the accurate prediction of propeller cavitation and hence associated noise in the design stage is crucial for achieving reductions in terms of emitted sound pressure levels. Whilst computational methods are developing at an exponential pace and so are prospective tools for the future, model scale experiments still represent the most reliable and largely adopted approach for the prediction of propeller radiated noise. Despite the importance of model tests, being the only reliable tool for cavitation noise prediction, a benchmark data is non-existent for facilities all over the world to compare and correlate their noise measurements. Within this framework, this paper presents the first comparison of a round robin test campaign amongst the Noise CoP (Community of Practice) of Hydro Testing Forum (HTF). Based on the extensive experiments conducted first at Emerson Cavitation Tunnel (ECT), a reduced test matrix is proposed to the forum members. University of Genoa (UNIGE) is the first member to complete the tests specified in the reduced matrix and this paper presents comparisons between the test results of ECT and UNIGE cavitation tunnel in terms of measured sound pressure levels, propeller open water performance, cavitation observations and cavitation inception characteristics. Moreover, in order to shed a light on the issue of propeller cavitation noise measurements, a series of investigations are carried out by UNIGE scrutinizing the effect of hydrophone position, oxygen content, propeller shaft revolution rate, sensitivity to thrust coefficient and cavitation number
An experimental study to detect cavitation erosion for different coated surfaces
Full text linkThis paper presents an experimental study to investigate the cavitation damage caused by bubble collapses in different surfaces coated by different methods. A cavitation air jet rig was constructed adapting similar specifications given in ASTM G134 Standard Test Method for cavitating liquid jet and the cavitation erosion tests were performed using this air jet rig. The tests were carried out under specified conditions in bubbly flow for the sample surfaces of CU1 (nickel-aluminium-bronze) alloy and CU3 (manganese-bronze) alloy in the cavitation test rig, which was setup for this study at ITU. The samples were coated by acrylic paint using different techniques such as dipping, spraying, brushing and acrylic paint by pen. One set of samples was left uncoated as the reference. Flow rate of the air and water, and stand-off distance of the samples were investigated and optimized. The tests were performed by intervals of 4 hours. Cavitation erosion on the surface of the samples was examined using a Reflective Light Microscope (RLM). Complementary experimental investigations, considering different test durations and coating techniques were conducted in the cavitation test rig. Results indicated a strong influence of the exposure time on the damage rate of the samples. On the other hand, it has been observed that the effect of stand-off distance is crucial on the development of cavitation erosion. The ultimate goal of the experimental study performed is try to explore similarity of the cavitation erosion formation to the erosion tests at cavitation tunnels for propellers. This will enable the replication of the propeller material and paint combination as an erosive indicator in a simpler setup
Physically plausible propeller noise prediction via recursive corrections leveraging prior knowledge and experimental data
Full text linkFor propeller-driven vessels, cavitation is the most dominant noise source producing both structure-borne and radiated noise impacting wildlife, passenger comfort, and underwater warfare. Physically plausible and accurate predictions of the underwater radiated noise at design stage, i.e., for previously untested geometries and operating conditions, are fundamental for designing silent and efficient propellers. State-of-the-art predictive models are based on physical, data-driven, and hybrid approaches. Physical models (PMs) meet the need for physically plausible predictions but are either too computationally demanding or not accurate enough at design stage. Data-driven models (DDMs) are computationally inexpensive ad accurate on average but sometimes produce physically implausible results. Hybrid models (HMs) combine PMs and DDMs trying to take advantage of their strengths while limiting their weaknesses but state-of-the-art hybridisation strategies do not actually blend them, failing to achieve the HMs full potential. In this work, for the first time, we propose a novel HM that recursively correct a state-of-the-art PM by means of a DDM which simultaneously exploits the prior physical knowledge in the definition of its feature set and the data coming from a vast experimental campaign at the Emerson Cavitation Tunnel on the Meridian standard propeller series behind different severities of the axial wake. Results in different extrapolating conditions, i.e., extrapolation with respect to propeller rotational speed, wakefield, and geometry, will support our proposal both in terms of accuracy and physical plausibility.Ship Design, Production and Operation
Ship performance monitoring dedicated to biofouling analysis: development on a small size research catamaran
No full textThis paper provides a description of the deterministic ship performance monitoring system developed and installed on Newcastle University's Research Vessel, The Princess Royal, for the estimation of the effect of hull and propeller fouling on the vessel's performance. The study revolves around the principle of data normalisation, both in its theoretical and practical aspects. A procedure for correcting weather and operational disturbances is introduced that takes into account plausible resources limitations. According to the needs emphasised by the normalisation process, the required onboard measurement system is described as it was implemented on the research vessel. A robust method to prepare the raw data for the analysis and suitable for all ship types and sizes is then proposed. A performance analysis method is finally defined using four different indicators of the vessel hydrodynamic performance. On-board measurements are presented and analysed according to the proposed methodology. Results show an increase in resistance of over 20% with extensive shell fouling and prove the effectiveness of the used method.</p
Fouling effect on the resistance of different ship types
Full text linkConsidering that the ship hydrodynamic behaviours differ by the ship types and dimensions, the effect of biofouling on ship resistance can also vary with different ships. In this study, Unsteady Reynolds Averaged Navier-Stokes (URANS) based towed ship models were developed to simulate the roughness effect of biofouling on ship resistance. A container ship (KCS) and a tanker (KVLCC2), representing slender and a full hull forms, were modelled with various scale factors and speeds. The CFD simulations were conducted with several fouling conditions by embedding the roughness function of barnacles into the wall-function of the CFD model (i.e. modified wall-function approach). The fouling effects on the resistance components, form factors, wake fractions and the flow characteristics were investigated from the simulations. Significant differences were observed varying with the hull types, lengths (scales) and speeds of the ships and it was concluded that these differences are dominated by two parameters; relative roughness height and the roughness Reynolds number
Computational Prediction of Propeller Cavitation Noise: Paper 161
Full text linkThe potential impact of ships underwater radiated noise (URN) on marine fauna
has become an important issue. The most dominant noise source on a propeller-driven vessel
is propeller cavitation, and the accurate prediction of its noise signature is fundamental for
the design process. In this work, we investigate the potential of using low-computational-cost
methods for the prediction of URN from cavitating marine propellers that can be conveniently
implemented within the design process. We compare computational and experimental results
on a subset of the Meridian standard propeller series, behind different severities of axial wake,
for a total of 432 experiments
Predicting the Effect of Hull Roughness on Ship Resistance Using a Fully Turbulent Flow Channel
Full text linkThe consequences of poor hull surface conditions on fuel consumption and emissions are well-known. However, their rationales are yet to be thoroughly understood. The present study investigates the hydrodynamics of fouling control coatings and mimicked biofouling. Novel experimental roughness function data were developed from the “young” fully turbulent flow channel facility of the University of Strathclyde. Different surfaces, including a novel hard foul-release coating, were tested. Finally, the performance of a benchmark full-scale containership was predicted using Granville’s similarity law scaling calculations. Interestingly, the numerical predictions showed that the novel hard foul-release coating tested had better hydrodynamic performance than the smooth case. A maximum 3.79% decrease in the effective power requirements was observed. Eventually, the results confirmed the practicality of flow channel experiments in combination with numerical-based methods to investigate hull roughness effects on ship resistance and powering. The present study can also serve as a valuable guide for future experimental campaigns using the fully turbulent flow channel facility of the University of Strathclyde
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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