2,017 research outputs found

    Data-Driven and Hybrid Methods for Naval Applications

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    The goal of this PhD thesis is to study, design and develop data analysis methods for naval applications. Data analysis is improving our ways to understand complex phenomena by profitably taking advantage of the information laying behind a collection of data. In fact, by adopting algorithms coming from the world of statistics and machine learning it is possible to extract valuable information, without requiring specific domain knowledge of the system generating the data. The application of such methods to marine contexts opens new research scenarios, since typical naval problems can now be solved with higher accuracy rates with respect to more classical techniques, based on the physical equations governing the naval system. During this study, some major naval problems have been addressed adopting state-of-the-art and novel data analysis techniques: condition-based maintenance, consisting in assets monitoring, maintenance planning, and real-time anomaly detection; energy and consumption monitoring, in order to reduce vessel consumption and gas emissions; system safety for maneuvering control and collision avoidance; components design, in order to detect possible defects at design stage. A review of the state-of-the-art of data analysis and machine learning techniques together with the preliminary results of the application of such methods to the aforementioned problems show a growing interest in these research topics and that effective data-driven solutions can be applied to the naval context. Moreover, for some applications, data-driven models have been used in conjunction with domain-dependent methods, modelling physical phenomena, in order to exploit both mechanistic knowledge of the system and available measurements. These hybrid methods are proved to provide more accurate and interpretable results with respect to both the pure physical or data-driven approaches taken singularly, thus showing that in the naval context it is possible to offer new valuable methodologies by either providing novel statistical methods or improving the state-of-the-art ones

    Assessment of RANS turbulence models and Zwart cavitation model empirical coefficients for the simulation of unsteady cloud cavitation

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    The numerical simulation of unsteady cavitation flows is sensitive to the selected models and associated parameters. Consequently, three Reynolds Average Navier-Stokes (RANS) turbulence models and the Zwart cavitation model were selected to assess their performance for the simulation of cloud cavitation on 2D hydrofoils. The experimental cavitation tests from a NACA65012 hydrofoil at different hydrodynamic conditions were used as a reference to tune the modeling parameters and the experimental tests from a NACA0015 were finally used to validate them. The effects of near wall grid refinement, time step, iterations and mesh elements were also investigated. The results indicate that the Shear Stress Transport (SST) model is sensitive to near wall grid resolution which should be fine enough. Moreover, the cavitation morphology and dynamic behavior are sensitive to the selection of the Zwart empirical vaporization, Fv, and condensation, Fc, coefficients. Therefore, a multiple linear regression approach with the single objective of predicting the shedding frequency was carried out that permitted to find the range of coefficient values giving the most accurate results. In addition, it was observed that they provided a better prediction of the vapor volume fraction and of the instantaneous pressure pulse generated by the main cloud cavity collapse.Postprint (published version

    Cavitation dynamics and underwater radiated noise signature of a ship with a cavitating propeller

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    The paper presents SSPA’s work in the EU project AQUO to predict underwater radiated noise (URN) generated by a coastal tanker with a cavitating propeller. A CFD method, consisting of a multi-phase Delayed Detached Eddy Simulation (DDES) and a Ffowcs Williams-Hawkings (FWH) acoustic analogy, is applied to predict the cavitation, pressure pulses and radiated noise for the ship at model and full scale. In comparison with the data obtained from the model test and full scale measurement, it is found that the predicted sheet cavity correlates quite well with the observed ones in the experiment and sea trial. Some success is made in predicting the collapse and rebound of tip vortex cavitation (TVC) at model scale, yet the extension of TVC is under-predicted.The predicted pressure pulses agree reasonably well with the measured ones at the first three harmonics, deviation becomes larger at higher harmonics.The tonal noise has fairly good agreement with the measured signal at both scales up to 5th harmonics. The simulation however under-predicts part of broadband noise that is caused by the TVC, mainly due to an under-resolution of the flow in the tip region and the propeller wake. The agreement with the data for the model scale case is slightly better than that for the full scale case

    Numerical prediction of propeller induced hull pressure pulses and noise

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    An operating marine propeller is one of the major sources inducing hull pressure pulses, onboard noise and vibration as well as underwater radiated noise. There are rising concerns of environmental impacts and comfort and welfare of passengers and crews due to these negative effects. Cavitation is a significant source of these effects, but it is typically inevitable if only the hydrodynamic efficiency of the propeller is optimized. To reduce the noise and the pressure pulses caused by the cavitation, a trade-off of the hydrodynamic efficiency should be made to design and optimize a propeller that possess both high hydrodynamic performance and low noise and hull pressure pulse generation. More accurate predictions are needed to identify the best trade-off between a high efficiency propeller design and a low pressure pulse and noise one.The study focuses on the numerical prediction of hull pressure pulses and radiated underwater noise using viscous CFD including the opensource package OpenFOAM and commercial package Star-CCM+. Numerical predictions are performed regarding different experimental configurations for determining hull pressure pulses and ship noise, including propellers mounted on inclined shafts and propellers operating behind ship hulls, under different scales and scaling laws with different operating conditions and Reynolds numbers.Non-cavitating propeller induced pressure pulses are generally lower in levels and rich in blade passing frequency comparing to cavitating conditions, with blade tip clearance as a major impact factor. For cavitating conditions the rate of cavity growth/shrinkage is found to play the dominating role generating pressure fluctuations. For certain model scale configurations, numerical predictions with ordinary approaches predict massive sheet cavity on propeller blades leading to pressure pulse prediction discrepancies comparing to experimental observations and measurements. These can be significantly improved by a developed bridged model considering laminar to turbulence transition. Tip vortex cavitation bursting is a common phenomenon found on propellers operating behind the ship hull and generating significant levels of pressure pulses. The phenomenon is numerically predicted with investigations of its generation mechanisms in relation to the propeller inflow, convex shaped sheet cavitation closure line and traveling re-entrant jet underneath the sheet cavity.Propeller induced noise prediction was studied using approaches focused on the FWH (Ffowcs Williams-Hawkings) acoustic analogy with incompressible input on permeable/porous data surface (PDS). \ua0Studies show this combination between incompressible input and FWH acoustic analogy can be erroneous, though using certain PDS placements and closer receivers the error can be reduced

    A review on the turbulence modelling strategy for ship hydrodynamic simulations

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    Ship operations are accompanied by turbulent regimes that play a significant role in the hydrodynamic characteristics. With the ongoing development of computational technologies, it is now feasible to numerically simulate turbulent ship flows with a high degree of detail. Turbulent simulations, however, tend to be computationally expensive and require a trade off between computational costs and fidelity. Whilst a range of turbulence modelling strategies is available in Computational Fluid Dynamics, there is a lack of up-to-date recommendations on their suitability for different ship-flow simulation scenarios. Addressing this gap, the present work reviews the state-of-the-art of turbulence modelling for ship hydrodynamic applications. As a result, this paper introduces the most known turbulence modelling approaches used in the field, followed by a thorough discussion of their applicabilities and limitations. Furthermore, this paper provides recommendations for the selection of turbulence modelling strategies versus various ship simulation scenarios, such as resistance prediction, ship flow modelling, self-propulsion, and cavitation analyses. It is expected that the present paper will provide decision-making support by helping CFD users minimise the time spent on trial and error, as well as providing valuable insights to promote the advancement of turbulence modelling

    Ship propeller side effects: pressure pulses and radiated noise

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    The present paper deals with the side effects of propellers cavitation, i.e. pressure pulses and radiated noise. These effects are gaining more and more importance for commercial ships for different reasons. Pressure pulses significantly affect comfort onboard, thus their reduction is of utmost importance for all ships carrying passengers. As regards the underwater radiated noise, in the last decade interest has shifted from navy applications to commercial ships, due to the concern for the rising background noise in the oceans. The propellers, generating noise directly in water, represent one of the main contributions to the overall underwater noise emitted from ships. Due to the complexity of the mechanisms of propeller noise generation, different complementary strategies have to be followed to properly analyze the problem, ranging from induced pressure pulses to broadband noise and cavitation. In the present work, part of the activities carried out in the framework of the collaborative EU FP7 project AQUO (Achieve QUieter Oceans by shipping noise footprint reduction, www.aquo.eu) are reported. The paper presents the investigations carried out on a specific test case represented by a single screw research vessel, which is analyzed with three different strategies: numerical calculations, model scale investigations and full scale measurement

    The influence of turbulence modelling techniques on the predicted cavitation behaviour on a NACA0009 foil

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    The work presented here forms part of a project focusing on the development of cost-effective measures of classifying the noise levels from ship propellers with the use of numerical techniques available in OpenFOAM software. It is also related to the on-going research within the Faculty of Engineering and the Environment at the University of Southampton, looking at underwater noise of tidal turbines. Ultimately, the aim of the complete study is to enable the assessment of the environmental impact of a ship on the marine ecosystems. In this work a set of results from numerical experiments applied to the NACA0009 foil is presented in the context of quantifying the noise levels produced by a cavitating body in a uniform flow. The simulations utilise both URANS and LES methods and provide a means of characterising the differences between the observed flow patterns from the cavitation modelling point of view. In particular, the interactions of the cavitation phenomena with the turbulence, both modelled and resolved, are studied. Furthermore, an overview of how the considered cavitation models may be used for the purpose of noise prediction is give

    Multi-scale modelling of cavitation-induced pressure around the delft twist 11 hydrofoil

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    A hybrid Lagrangian-Eulerian cavitation model based on the Schnerr-Sauer mass-transfer formulation is developed and then applied to study the flow around the Delft Twist 11 hydrofoil. The model uses volume-of-fluid approach to resolve large cavities and uses an interface reconstruction algorithm to identify vapour structures smaller than a grid-related threshold. These are then transferred to a Lagrangian framework and convected as particles acting as point noise sources. The underlying volume-of-fluid (VOF) model is shown to be in qualitatively good agreement with the experiment although it is found to under-predict the extent of cavitation. The combined model shows a substantial improvement in the prediction of near-field pressure fluctuations by accounting for the broadband contribution of bubbles smaller than the Eulerian grid size. In the pressure fluctuation spectra this is seen as a plateau extending to over a kilohertz beyond the low-frequency harmonics associated with the shedding frequency

    Vocal fold vibratory and acoustic features in fatigued Karaoke singers

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    Session 3aMU - Musical Acoustics and Speech Communication: Singing Voice in Asian CulturesKaraoke is a popular singing entertainment particularly in Asia and is gaining more popularity in the rest of world. In Karaoke, an amateur singer sings with the background music and video (usually guided by the lyric captions on the video screen) played by Karaoke machine, using a microphone and an amplification system. As the Karaoke singers usually have no formal training, they may be more vulnerable to vocal fatigue as they may overuse and/or misuse their voices in the intensive and extensive singing activities. It is unclear whether vocal fatigue is accompanied by any vibration pattern or physiological changes of vocal folds. In this study, 20 participants aged from 18 to 23 years with normal voice were recruited to participate in an prolonged singing task, which induced vocal fatigue. High speed laryngscopic imaging and acoustic signals were recorded before and after the singing task. Images of /i/ phonation were quantitatively analyzed using the High Speed Video Processing (HSVP) program (Yiu, et al. 2010). It was found that the glottis became relatively narrower following fatigue, while the acoustic signals were not sensitive to measure change following fatigue. © 2012 Acoustical Society of Americapublished_or_final_versio
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