博士 / 國立成功大學 / 水利及海洋工程學系 / 104 / Experimental Study on the Reduction of Underwater Ship Propeller Noises by Using Bubble Screen Techniques
Yen-Tsen Lin
Ching-Jer Huang
Department of Hydraulic and Ocean Engineering
National Cheng Kung University
SUMMARY
The purpose of this experimental study is to design an optimal bubble screen for reducing the underwater propeller noises of surface vessels.The experiments were carried out in a towing tank. A propeller was installed in a ship model to simulate the noises produced by a surface vessel and hydrophones were used to monitor the radiated sounds. The bubble screen is generated by passing the compressed air through a long bubble emitter belt. By measuring the attenuated sounds behind the bubble screen, the optimal bubble size and air volume fraction for reducing the radiated sounds can be fixed. Furthermore, the calibration experiments of the bubble screen were implemented in a water tank to estimate the bubble populations and gas void fraction while the bubble emitter belt with different orifice sizes was pumped into variable air pressures and flow rates.
KEY WORDS: optimal bubble screen, propeller, surface vessel, towing tank, bubble
emitter belt, attenuated sounds.
INTRODUCTION
The sound speed in the bubbly flow differs significantly from the standard sound velocity in the sea water and the speed is frequency dependent. Propeller noise is produced by a purely hydrodynamic mechanism such as cavitation at the tips of the blades or cavitation on the blades themselves, or by mechanical vibration of the blades. Usually flow noise, propeller noise and mechanical noise will be concealed. Propeller noises are commonly reduced by using the variable pitch, skew, and the appropriate number of propeller blades. The spontaneous noises generated by the ship propeller are the main signal source of reconnaissance.
The noises generated by the propeller are mostly in the low frequency range （less than 500 Hz）. Accordingly, the low-frequency sound generated by the propeller becomes an important clue for the underwater vehicle to search for the sea surface vessels. It has been known for a long time that the speed of sound propagating in the water is affected by the presence of the gas bubbles. The propagation of sound through bubbly liquid has been thoroughly investigated both theoretically and experimentally. It was revealed that when the frequency of the incident sound coincides with the resonant frequency of the bubble in the bubbly flow, a small amount of sound will penetrate through the bubbly flow.
Figure 1.3 shows that two air emitter belts were installed to reduce the noise of the ship. FR 253 air emitter belt is fitted to the ship’s propeller, while FR177 belt is fitted to the external hull in the vicinity of the propulsion plant. Air bubbles are employed to mask potential target, or to provide alternate targets.
For the best use of the air emitter belt, the effect of belt control parameters, such as the air pressure, flow rate, and the size of orifice on the bubble size and air-volume fraction of the bubbly flow must be tested. Furthermore, by applying the air bubble screen technique for the reduction of the self-noises of the surface vessels, effect of bubble size and volume fraction on the reduction of noises produced by a propeller installed to a ship under way must also investigated. Based on the above-mentioned experimental studies, this work aims to design an optimal bubble screen for reducing the underwater propeller noises of surface vessels.
MATERIALS AND METHODS
In this study, the experiments are divided into three main parts. In the first part of the experiments, surface ships use air bubbles with a bubble curtain to weaken the main and auxiliary mechanical noise through the hull of radiative transfer to the sound of water, shelter air curtain with the cast bubble screen control mechanism and influence factors include: flow, pressure and gas screen with a pore size such as the three main control parameters. In this study, experiment to experiment according to different control parameters in order to find out the relationship between the bubble screen and associated control parameters in the water tank of Laboratory for fiber-optic sensing and underwater acoustics at the Department of Hydraulic and Ocean Engineering, National Cheng Kung University（NCKU） , Taiwan.
In the second part of the experiments, underwater noises generated by the propellers shown in Fig.3.13 and Fig.4.2 were measured. Effects of the bubble screen on the reduction of sound transmission were then studied by deploying a bubble curtain at the rear side of the propeller. These experiments were carried out in the towing tank of the Department of Systems and Naval Mechatronic Engineering, NCKU.
In the third part of the experiments, the ship model is equipped with two air bubble emitting belts. The underwater noises generated by the installed propeller in a moving ship hull with or without the emission of air bubble will be measured and compared. Effect of the air flow rate and bubble size on the reduction of sound transmission through the bubbly flow will be systematically investigated.
RESULT AND DISCUSSION
The effect of different air flows for high-speed propeller, by the bubble curtain noise reduction. As shown in Figure 4.20. The black line represents the location of underwater ambient noise for the experimental flume（not including the propeller and bubble）. The Green line represents the sound spectrum of the propeller in shallow water（0.15 m depth）, and high speed（rpm = 488）. The main radiation of propeller is approximated to 0.6 ~ 6 KHz. The blue and red lines, represents the sound spectrum of the propeller noise through the bubble curtain by 1.0 and 20.0（L / min）of air flow. In fact, the bubble screen generates a noise reduction effect of roughly 10 dB in the frequency range of 0.6 ~ 1.1 KHz.
CONCLUSION
（1） By the research results, the use of the principle of the bubble screen to reduce or change the propeller noise is concrete and feasible.
（2） The range of radiated noise frequency for four-leaf propeller between 600 to 6000 Hz. Bubble screen generated by Bubble stone can attenuate low-frequency radiation of propeller noise between 600 to 1100 Hz and attenuate cavitation noise about 700 Hz significantly, the result was match its natural resonance theory bubble frequency of the incident sound wave frequency equal happen.
（3） Bubble curtain control parameters include: flow, pressure and gas screen with pore sizes. Its impact factors are the volume fraction of bubbles (bubble cover ratio), the bubble curtain width (thickness).
（4） Gas volume fraction in bubble curtain is proportional to the gas flow. When the greater the volume fraction , the incident sound waves more difficult to penetrate.
（5） The relationship between bubble size and pore size is enlarged 8-13 times.
（6） Pore size is the direct effect factor for the bubble size of bubble screen, it’s the most critical parameter of the reducing noise frequency.
（7） CO2 really helps reduce the life cycle of the bubble, bubble and reduce ship stern flow generating interaction time.
| Identifer | oai:union.ndltd.org:TW/104NCKU5083090 |
| Date | January 2016 |
| Creators | Yen-TsenLin, 林彥岑 |
| Contributors | Ching-Jer Huang, Tai-Wen Hsu, 黃清哲, 許泰文 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 138 |
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