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To explore the reason for the bubble sweep-down phenomenon of research vessels and its effect on the position of the stern sonar of a research vessel, the use of a fairing was investigated as a defoaming appendage. The separation vortex turbulence model was selected for simulation, and the coupled Eulerian-Lagrangian method was adopted to study the characteristics of the bubble sweep-down motion, captured using a discrete element model. The interaction between the bubbles, water, air, and hull was defined via a multiphase interaction method. The bubble point position and bubble layer were calculated separately. The spatial movement characteristics of the bubbles were extracted from bubble trajectories. It was demonstrated that the bubble sweep-down phenomenon is closely related to the distribution of the bow pressure field and that the bubble motion characteristics is related to the speed and initial bubble position. When the initial bubble position is between the water surface and the ship bottom, the impact on the middle of the ship bottom is greater and increases further with increasing speed. A deflector forces the bubbles to both sides through physical shielding, strengthening the local vortex structure and keeping bubbles away from the middle of the ship bottom.

Abstract The bubble sweep-down phenomenon around oceanographic research vessels generates acoustic disturbances. A specific experimental protocol in a wave and current circulating tank is used to study this phenomenon around a 1/30 scale ship model in a configuration with current, waves and ship motions. 3D visualizations of the bubble clouds and Stereo-PIV (Particle Image Velocimetry) measurements results, both obtained in the bow vicinity of the ship model, are presented in this paper. The behaviour of the breaking wave with bubble generation is studied and the spatial evolution of the bubble clouds is investigated. It is demonstrated that the bubble cloud generation is mainly conditioned by the ascending vertical velocity of the flow near the bow. It is observed that the bubble clouds are propagated in the flow direction with a horizontal expansion which is non negligeable, reaching a close position to the hull. The overall results provide new elements for the study and the understanding of the phenomenon compared with previous 2D studies. Highlights • Air entrainment is studied around a 1/30 Scale Ship Model. • 3D bubble cloud characteristics are determined. • PIV measurements and POD analysis allow to study the air bubble generation. [ABSTRACT FROM AUTHOR]

The bubble sweep-down phenomenon around the oceanographic research vessels generates acoustic perturbations. A specific experimental protocol has been developed in a wave and circulating tank to study this phenomenon. This protocol is used to carry out trials on three different ship models in order to study the influence of the bow geometry on the bubble generation. For different test configurations, bubble clouds are described and compared in terms of area, maximal depth and vertical velocity to highlight bubble cloud dynamics surrounding the three ship models. The relation between the hydrodynamic flow field and the bubble generation is studied by means of Particle Image Velocimetry (PIV) measurements to study the phenomenon by the use of phase averaged velocity fields. The overall results enable us to characterize the bubble sweep-down phenomenon from the air bubble generation and propagation to the frequency of occurrence and the clouds behaviour. [ABSTRACT FROM AUTHOR]

Oceanographic research vessels are fitted with acoustic sonar transducers at the bottom keel region. For a ship underway, atmospheric air gets mixed naturally with the surface water in the presence of wind and waves. Bubbles get entrapped in the region within the draft of the vessel and flow in the stream past the vessel. Literature records show that the bubbles are formed in the upper regions (made worse by the pitching motion) below the surface. When they flow immediately below and in the region of the sonar transducer resulting in bubble sweep-down phenomenon, they directly interfere with the acoustic transmission and deteriorate the functioning of the sonar transducer. Degradation of performance of the acoustic transducer seriously limits the mission capability of the vessel. This is a major concern and there is no complete remedy as of date, for the avoidance of the bubble formation in the flow stream. This paper describes a hydrodynamic re-design approach for the hull geometry in the forward region and the creation of an effective bubble diverter bow. A new modified bow form is investigated to help in deflecting the stream lines away from the location of the sonar transducer. The strategy in the approach here is to design the bow region to control hydrodynamic flow such that the bubbles-entrapped water of the upper surface layers is strategically diverted to flow side-ways of the hull or at the bottom side-ways well away from the location of the sonar transducer. Numerical flow simulations for the developed hull form using CFD tools demonstrate that the streamlines can be effectively thus diverted without degradation of the performance of the sonar transducer. The strategy for the hull form design is evolved by parametric variation of the side-shape using computer aided surface generation tools. The parameters influencing drag as well as diversion of the streamlines are the length parameter of the bubble diverter bow at side, the cross-sectional area parameter at a pre-defined section at the forward and the wetted surface parameter of the bubble diverter bow. The beneficial effect of the bubble diverter bow is to be weighed against increased hull resistance. The validation studies include simulation of the flow and drag assessment and comparison from towing tank tests as well. Three hull forms are created and the streamlines traces are studied in these cases respectively. The results demonstrate that a minor re-design of the forward sides of the hull form can drastically minimize the bubble streamline interference at the sonar transducer without penalty on the resistance. A major breakthrough is offered in the mitigation of bubble sweep down by the new design. [ABSTRACT FROM AUTHOR]

Vessels will encounter the phenomenon of bubble sweep-down when sailing in the ocean, which seriously affects the accuracy of underwater detection and resource exploration. In addition, the installation of defoaming attachments will affect the resistance performance of the ship. It is significant for a scientific research vessel to study the mechanism of bubble sweep-down and propose a bubble suppression scheme considering the ship's resistance characteristics. Detached-Eddy Simulation (DES) Shear Stress Transfer (SST) k - ω model combined with the Discrete Element Method (DEM) is used to simulate the process of microbubbles sweeping down to the ship's bottom under the influence of the flow at the bow of the scientific research vessel, and the deflection effect of the fairing on the bubbles, to carry out a mechanical analysis of the bubble sweep-down phenomenon. The results show that the bubble movement is closely related to the characteristics of the bow flow, the bubble's trajectory was related to the vessel speed and the bubble starting position, and the bubble at the bottom of the bow was easier to sweep down to the bottom of the ship. The fairing was a defoaming attachment with excellent resistance performance, which can physically block the air bubbles and strengthen the bottom vortex structure to change the bubbles' trajectory. Exploring the bubble sweep-down mechanism and the suppression scheme can better serve the design of the hull shape and defoaming attachment of scientific research ships. • Studied the mechanism of bubble sweep-down used DES mothed. • Propose a bubble suppression scheme considering the ship's resistance characteristics. • Used the Euler-Lagrange coupling method to simulate the bubble sweep-down phenomenon. • Bubble's trajectory was related to the speed and the bubble starting position. • The fairing was a defoaming attachment with excellent resistance performance. [ABSTRACT FROM AUTHOR]

In this second part, images acquired from the specific trials developed in order to study the phenomenon of bubble sweep-down are analysed. A post-processing method has been developed to analyse the two air entrainment mechanisms described in the first part, for several test configurations. Bubble clouds are described in terms of depth, area and velocity for both vortex shedding and breaking wave bubble clouds. A parametric study is also performed to calculate the influence of each test parameter on the frequency of bubble generation. It is demonstrated that the occurrence of bubble clouds is proportional to the wave height, with a considerable influence of the phase shift between waves and motions. The overall results provide new elements for the understanding and the study of the phenomenon, with the final objective of obtaining a reliable tool that facilitates the design of research vessels. [ABSTRACT FROM AUTHOR]

Bubble sweep-down is a significant issue for the oceanographic vessels, which affect the acoustic surveys. Experimental trials, carried out in the Ifremer wave and current circulating tank on a 1/30 model of the Pourquoi pas? , are presented. The results demonstrate that this kind of experimental facility is well suited to study the phenomenon of bubble sweep-down encountered around the bow of a ship under specific conditions. From these results, two kinds of bubble clouds formation have been observed and analysed: bubble clouds generated by vortex shedding and breaking waves. The vortex shedding bubble clouds appear randomly in all the configurations tested, even without waves or motions. This phenomenon is due to the interaction between the turbulent flow and the specific bow shape of the Pourquoi pas? . On the other hand, the breaking wave clouds appear in the presence of relative motions between the free surface and the bow ship and more significantly under wave sollicitations. A complementary paper presents a parametric study carried out to quantify the influence of the test conditions. [ABSTRACT FROM AUTHOR]

Bubble sweep-down on oceanographic vessels generates acoustic perturbations. We propose in this work to characterize the sub-surface bubbles occurrence conditions from acoustic data analysis acquired during surveys in relatively shallow water with the IFREMER research vessels Thalassa and Pourquoi Pas?. The methodology of data analysis used in this work allows us to characterize the sailing conditions influence on bubble sweep-down occurrence. The correlation between sailing conditions and acoustic perturbations tends to demonstrate that the presence of bubbles under the hull is clearly related to the wind speed and natural aeration, and that surface bubbles are advected differently in the water column by the two vessels. [ABSTRACT FROM AUTHOR]

The bubble sweep-down phenomenon around oceanographic research vessels generates acoustic disturbances. The phenomenon can be reproduced on a 1/30 scale ship model in a wave and current circulating flume tank taking into account ship motions. Optical probe can then be used to measure locally the size and the velocity of the bubbles generated by breaking waves. A comparison of the obtained results with more classical results obtained from bubble detection by image tracking is discussed. The two methods provide complementary results for bubble characteristics. However, bubble detection by optical probe has the advantage of knowing the bubble position in the three dimensions of space. The overall results of this paper provide new elements for the study and the understanding of the bubble sweep-down phenomenon in addition to previous 2D and 3D studies, where the dynamics of bubble clouds have been characterized. • Bubble sweep-down phenomenon is studied around a 1/30 scale ship model. • Breaking wave bubble measurements by optical probe were conducted. • Bubble velocities and diameters were determined. • Our findings have important implications for marine research vessels. [ABSTRACT FROM AUTHOR]

To investigate the bubble sweep-down phenomenon of a scientific research ship from the perspective of its flow mechanism, the flow-field characteristics of the bow were measured in a towing-tank environment. First, the silk-thread method was used to visualise the streamline characteristics of the bow. Next, the flow field of the bow was measured using multiple two-section 2-dimensional computed tomography scans with stereoscopic particle image velocimetry. The flow-field information of different sections at different speeds (e.g., velocity distribution, vector characteristics, and streamline distribution) were then obtained. A 3-dimensional (3D) three-component (3C) space reconstruction of the time-averaged flow field in the bow was thus realised. From the visualisation results, the flow field in the bow generally exhibits a downward movement trend. The flow structure of the bow flow field was found to be closely related to the wave-making change of the bow. On the other hand, the cross-sectional flow characteristics and 3D reconstruction results showed that with the bubble sweep-down, some bubbles flowed along the bilge and amassed with other bubbles, breaking through the geometric inflection point of the bilge. Compressed by the bilge vortex, they slid backward and affected the operation of acoustic equipment. • Flow visualisation experiment was carried out from macro and micro perspectives. • Bow flow field will bring bubbles into the bottom of the ship. • Downward swept flow field was more stable and prominent at high speeds. • A set of three-dimensional/three-component space flow field test methods is used. [ABSTRACT FROM AUTHOR]

To explore the reason for the bubble sweep-down phenomenon of research vessels and its effect on the position of the stern sonar of a research vessel, the use of a fairing was investigated as a defoaming appendage. The separation vortex turbulence model was selected for simulation, and the coupled Eulerian-Lagrangian method was adopted to study the characteristics of the bubble sweep-down motion, captured using a discrete element model. The interaction between the bubbles, water, air, and hull was defined via a multiphase interaction method. The bubble point position and bubble layer were calculated separately. The spatial movement characteristics of the bubbles were extracted from bubble trajectories. It was demonstrated that the bubble sweep-down phenomenon is closely related to the distribution of the bow pressure field and that the bubble motion characteristics is related to the speed and initial bubble position. When the initial bubble position is between the water surface and the ship bottom, the impact on the middle of the ship bottom is greater and increases further with increasing speed. A deflector forces the bubbles to both sides through physical shielding, strengthening the local vortex structure and keeping bubbles away from the middle of the ship bottom. [ABSTRACT FROM AUTHOR]