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When a research ship sails at a high speed, there is relative motion between the ship and fluid. The ship is slammed by the fluid. To reduce the direct impact of the fluid, sonar is installed in the moonpool, and acoustic detection equipment is installed along the research ship bottom behind the moonpool. However, during high-speed sailing, a large number of bubbles form in the moonpool. Some bubbles escape from the moonpool and flow backward along the bottom of the ship. When a large number of bubbles are around the sonar and acoustic detection equipment, the equipment malfunctions. However, there have been few studies on bubble formation in the moonpool with sonar and distribution along the ship bottom behind the moonpool. Therefore, a related model was developed and prototype tests were carried out in this study. The appropriate similarity criteria were selected and verified to ensure the reliability of the experiment. Considering the influences of speed, sonar, moonpool shape, and draft, the reason and mechanism of bubble formation in a sonar moonpool were studied. An artificial ventilation method was used to simulate a real navigation environment. Because the bubbles are in a bright state under laser irradiation, the bubbles can be used as tracer particles. A high-speed camera captured illuminated bubbles. The distribution mechanism of bubbles along the ship bottom behind the moonpool was investigated using particle image velocimetry under the influence of the moonpool shape and sailing speed. The model experimental results agreed well with those of the prototype test. The air sucked into the water was the dominant factor in bubble formation in the moonpool. The bubbles were distributed in a W shape under the ship bottom.

Abstract Compared with the square and circle moonpools, the rectangular moonpool with a large aspect ratio is more conducive to install the equipment. To reduce fluid motion in the rectangular moonpool with a large aspect ratio, a recess is installed in the moonpool. However, the fluid motion in a recessing type moonpool with a large aspect ratio has been rarely studied. In this study, a series of experiments are carried out to investigate on the hydrodynamic characteristics of fluid in the recessing type moonpool with a large aspect ratio. To facilitate the monitoring of the fluid motions, experiments are carried out in a transparent wave channel with the model made up of transparent acrylic. According to the experiment results, there are some complicated fluid motions in the moonpool. Under the resonance condition, the fluid moves violently in the moonpool. To reduce the fluid motion in the recessing type moonpool, three damping devices including the positive grid of flaps, the negative grid of flaps, and the grid of baffles are proposed. The reduction effect of the three damping devices is investigated experimentally. The damping devices have good reduction effects at most time. The smaller the incident wave period, the better is the reduction effect. Introduction The moonpools run vertically through the hull of ships or marine structures, providing a sheltered working environment under harsh ocean conditions. The fluid motion in the moonpool is similar to that in the slit between the ships or rectangular bodies (Faltinsen et al. 2007; Mavrakos & Chatjigeorgiou 2009; Ikeda et al. 2012; Chen et al. 2014; Heo et al. 2014; Zhang & Bandyk 2014; Faltinsen & Timokha 2015; Yu et al. 2017; Gao et al. 2019d). Two dominant types of fluid motions, the piston motion and the sloshing motion, are discussed (Fukuda 1977; Molin 2001; McIver 2005; Kristiansen & Faltinsen 2012; Zhou & Zhang 2013).

Some research vessels set moonpools for sonars installation. The moonpool provides a relatively good working environment for sonar in a harsh marine environment. At the same time, other acoustic detection equipment are installed along the ship bottom behind the moonpool. Due to the exist of the sonar with a variable cross-section, the number of bubbles generated in the moonpool increases. The bubbles generated in the moonpool expel and flow along the vessel bottom to the stern, which leads to the sweepdown effect. The performance of the sonar and other acoustic detection equipment degrades by the bubbles around. However, the research on reducing bubble generation in the moonpool and sweepdown effects is rare. So in present paper the reduction effects of damping devices such as the flange and choke deck on the bubble generation in the moonpool are investigated experimentally. Then, in order to reduce the sweepdown effects of bubbles on the ship bottom, three other damping devices which are double flaps, diversion channel and wedge are investigated. It can be seen that through reducing the area of bubble leakage in the moonpool the distribution position of bubbles can be effectively restricted and the width and thickness of the bubbles reduce.

For some research vessels, a sonar is installed in the moonpool, and some acoustic detection equipment are installed on the ship bottom behind the moonpool, which helps to avoid the impact of the high-speed flow. The moonpool causes the ship bottom discontinued, forming a particular shear layer oscillation. The shear layer oscillation affects the bubble generation and motion in and behind the moonpool. The sonar and acoustic equipment will malfunction when surrounded by many bubbles. However, there is almost no research on the shear layer oscillation near the moonpool. So, in this paper, by measuring the pressure near the moonpool and monitoring the fluid motion in the moonpool and bubbles’ distribution along the ship bottom, the shear layer oscillation near the moonpool is studied experimentally under the action of the incident current and wave. Furthermore, the effects of the sonar and the moonpool shape are investigated. It can be seen that the shear layer oscillation excites the fluid motion in the moonpool. The sonar forms a complicated boundary in the moonpool, resulting in the increase in the frequency of the shear layer oscillation. The shear layer propagates along the ship bottom in the form of the ship bottom wave. Clarifying the oscillating characteristics of the shear layer along the ship bottom with a moonpool is conducive to the design of moonpools in the research ships, and the detection instruments are arranged in the right place along the ship bottom, so as to make sure the detection instruments work properly and detect the marine environment more accurately.

The word embedding model embeds each word into a low-dimensional space by using the distribution information of unlabeled words in the corpus, such that the generalization ability of lexical features can be improved. However, the performance of word embedding models is limited by Out of Vocabulary (OOV) words because the relevant information of OOV words can’t be fully used to generate accurate word embedding. To effectively process OOV words, morphological structure information and context information should be considered. In view of the characteristics of Chinese, we propose a Fusion Multi-feature Encoder Based on Attention (FMEBA) for processing Chinese OOV words, in which we use the radical feature of characters, as well as use character-level Transformer Encoder to process character sequence information and context information. To test our model, we conducted experiments on a Chinese power plan professional dataset. Experimental results on the dataset shows compared with other models, our model achieved the best results. We conclude that our method is suitable for processing Chinese OOV words.

With the increase of the equipment size and the operating environment requirement, more and more rectangular moonpools with large aspect ratios are used to instead of the circular or square moonpools with small aspect ratios. However, the research on the rectangular moonpool with a large aspect ratio is few. Firstly, the fluid motion characteristic in the rectangular moonpool with a large aspect ratio is investigated experimentally in this paper. By tracking and quantizing the free surface motion, the moonpool has obvious shielding effects. Then the natural frequency of the piston motion in the moonpool is derived by theoretical methods, which agrees well with the experimental results. When the incident wave's period approximates the moonpool's natural oscillation period, the resonant motion occurs with some adverse effects. Finally, the reduction effects of four damping devices, the positive grid of flaps, negative grid of flaps, grid of baffles and vertical bulkhead, are investigated to reduce the adverse effects. The damping devices have good reduction effects coupling with some incentive effects. The grid of flaps effectively destroy the shear layer self-sustaining oscillations and generate vortices. The grid of baffles increases the fluid motion damping and forms the waterfall. The vertical bulkhead causes interlaced motion.