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Mitigation of shipping noise is a topical issue in marine engineering because of the dramatic increase in the levels of anthropogenic underwater noise and its impact on marine life. In recent years, hydro-acoustic research has focussed on the development of reliable methods for predicting underwater radiated noise (URN) due to cavitation, which is...
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... is possible to identify the main features of this theoretical cavitation noise spectra, as illustrated in Fig. 6. The analysis of the main characteristics of this schematic spectrum may provide a more in-depth insight into the physics of the problem. The features that are more directly related to the dynamics of cavitation are the central frequency, amplitude, width and power content of the spectral hump; and the energy content and decay ratio of ...
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... a practical point of view, a schematic representation based on piecewise linear functions, as shown in the example in Fig. 6, is used to define the main characteristics of spectra for further analyses. In this work, the analysis is focused on the following ...
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... available in the literature [21]. The reasons for the spread in the data within different participants and operational conditions are not clearly understood. The analysis of the decay ratio at a given frequency allows comparing the characteristics of the high frequency noise spectrum only partially. As clearly visible in the examples shown in Fig. 6, this frequency (i.e. 10 kHz) is otherwise lower or higher than the frequency that divides the two spectral parts with different decay slopes of the schematized ...
Citations
... The validation of the methods themselves, especially when side effects like cavitation and pressure pulses are accounted, is one of these. Also, the renovated interest in benchmark campaigns on propellers performance (Tani et al., 2019a;Tani et al., 2019b), even if not strictly related to the role of the laminar boundary layer on propeller characteristics, points out the need of reliable numerical tools capable of accounting for these phenomena. Similarly, ...
In this paper, after the successful applications to open water propeller performance estimations, the influence of transition sensitive and modified mass transfer models tuned to account for the laminar flow in the prediction of the cavitation inception of marine propulsors is investigated from the point of view of the unsteady functioning and induced pressure pulses. The VP1304 (also known as PPTC) test case, for which dedicated data were collected during several workshops, is considered first. After preliminary analyses using RANS, also Detached Eddy Simulations (DES) are included to better account for the vortex dynamics and its influence on pressure pulses. Similarly to what observed in uniform inflow, results show a better agreement with the available measurements of propeller performances and confirm the reliability of the proposed approaches for unsteady, non-cavitating, model scale propeller predictions. The overall cavitation pattern is improved too by the application of the transition sensitive correction to the mass transfer model, but the complex dynamics of bubble cavitation observed in experiments prevents quantitatively better predictions in terms of thrust/torque breakdown and induced pressure pulses levels regardless the use of RANS or DES methods.
... A round robin test programme for an open water propeller setup was undertaken among the "HydroTesting Forum" [27]. The aim of this was to better understand the accuracy and reliability of underwater radiated noise measurements at 7 different institutes, by UNIGE, SSPA and CRN-IRM, corrected for the effects of reverberation using a measured transfer function following the ITTC guidelines [28], where as MARIN computed the Lloyds Mirror effect using the method presented in [29]. ...
Sound generated by marine vessels and structures, radiates into the marine environment. It is of great importance regarding the acoustic signature of naval vessels, and can have a dramatic impact on the behaviour and well-being of marine fauna. Additionally there is a need to make autonomous and remotely operated underwater vehicles as quiet as possible such that they can record the ocean soundscape, and not influence the behaviour of the marine life which they may be researching. Towing tanks offer a practical and controlled environment to measure and optimise the noise radiated by a ship at model scale. Additionally most underwater robotic vehicles are small enough to operate in a towing tank. However, sound measurements in towing tanks suffer from the effects of reverberation. This means sources acoustical properties measured in the tank can be significantly different to a measurement made in free field conditions. Current methods to overcome this method involve taking a measurement close to the source such the direct field is dominant over the reverberant field. However the measurement of reverberation distance is achieved with classical room acoustic equations which use an assumption of a diffuse field in their derivation. It is shown in this thesis, that the acoustic field inside a towing tank is not diffuse, and as such these classical room acoustic equations are not applicable to a towing tank. Additionally, limiting measurements to be within the direct field may cause the measurement to be in the near field of the source for low frequencies. In order to characterise the acoustic environment of a towing tank, and understand its impact on the acoustical properties of a source, an expression for the exact acoustical field inside a rectangular room with the boundaries and geometry of a towing tank is derived. To improve the computational cost of numerically evaluating this expression, an approximate form is derived, known commonly as the image source model. A spherical wave reflection coefficient is derived to improve the accuracy of the image source model. Impulse response measurements are undertaken in the Boldrewood towing tank, which are used to calibrate the damping in the image source model, through the tuning of the shear and compressional wave speeds in the plane wave reflection coefficient of the concrete wall of the tank. This method has produced an indirect way of measuring the shear and compressional wave speeds in the concrete walls of a water tank. A non-dimensional coefficient relating the power radiated by a point monopole source in a towing tank, to the surface averaged mean square pressure over the side wall of the tank is defined. This power coefficient can be used to predict the radiated power of a source in a tank, through measurement of the mean square pressure on the wall of the tank. The calibrated image source model is used to build a dataset of source radiated power and surface averaged mean square pressure for a large number of source locations. This enables the power coefficient to become statistically valid, and enable a prediction of power to be made with the measurement of surface averaged mean square pressure, with a quantifiable random error. Using the statistical analysis it is found that only 15 hydrophones are required to achieve a 98% probability of being within 3dB of the tanks mean value. This is a significant reduction in the number of hydrophone measurement locations used to measure power using the method of volume averaged mean square pressure in a diffuse field. The radiated power of an omnidirectional ITC1032 transducer is measured and validated against manufacturers specifications.
... From a completely different point of view, also the renovated interest in benchmark campaigns on propellers performance (Tani et al. 2019a(Tani et al. , 2019b, even if not strictly related to the role of the laminar boundary layer on propeller characteristics, pointed out the need of reliable numerical tools capable of accounting for boundary layer transition. In most of the cases, indeed, the thrust rather than the advance coefficient is used to identify functioning conditions since the advance ratio can be easily influenced by many issues of a small/medium cavitation tunnels (confinement, disturbed inflow) when systematic campaign (and numerical comparisons) in terms of cavitation and its side effects are considered (Barkmann et al. 2011;Heinke and Luebke 2011). ...
The influence of the laminar-to-turbulent boundary layer transition on model scale propellers characteristics is investigated using the γ-Reθ model. OpenFOAM and StarCCM+ RANSE solvers are used for the investigation, which is carried out for two test cases. Available measurements at a sufficiently low rate of shaft revolution (i.e. Reynolds number)are used to assess the capabilities of the
model in improving model scale predictions. This first part of the study is focused on fully wetted conditions, considering the influence of grid resolution and inflow turbulent intensity on the transition
onset predicted by the model. The influence of the laminar-to-turbulent transition on the inception and development of cavitation, and the capabilities of predicting such phenomena using a modified
mass transfer model, are discussed in the second part of the study (Gaggero, 2020. Influence of Laminar-to-Turbulent transition on model scale propeller performances. Part II: cavitating conditions.
Ship and Offshore Structures).
The present study includes experimental and numerical investigations on propeller cavitation phenomena, particularly the tip vortex cavitation, for the test cases conducted with the model propeller of the Newcastle University research vessel, “The Princess Royal”. These test cases were recommended recently as the benchmark data for cavitation observations and noise measurements by the ITTC, and have been tested in a major round-robin campaign by eight cavitation tunnel/basin facilities around the world. This round-robin test campaign included the Newcastle Emerson Cavitation Tunnel where the first set of tests was conducted.
This study presents further systematic measurements conducted in the Shanghai Jiao Tong University (SJTU) cavitation tunnel with this benchmark propeller in the open water conditions, including the cavitation observations, and tests for the tip vortex cavitation inception and desinence. Also, the study includes computational fluid dynamics (CFD) investigation to discuss the results of the experiments, especially the tip vortex cavitation, in a comparative manner. To simulate tip vortex cavitation, the newly developed adaptive mesh approach MARCS (Mesh Adaption Refinement Approach for Cavitation Simulations) was further enhanced. The results of the CFD computations using this approach showed good agreement with the results of the tunnel tests for the open water propeller performance characteristics, cavitation observations and tip vortex cavitation pattern, including its extent.
