本研究主要目的是經由實驗研究設計出船舶降噪用之氣泡幕，以達到最佳俥葉噪音的降噪效果。降噪用氣泡幕是應用操作簡單的水中聲學技術來降低船舶俥葉所產生之水下噪音。
實驗研究在拖航水槽中進行，主要分成兩個階段進行。第一階段量測俥葉噪音並進而研究氣泡幕對俥葉噪音的降噪效果。第二階段將俥葉安裝於可移動船模來模擬實際情形，量測及比較俥葉轉動造成的艉流區其有無使用氣泡幕之差異。船模及俥葉均是基於相同福祿數運動動力近似所設計之實驗及物理實體模型。氣泡是將空氣壓縮經由長條型氣泡帶產生。藉由改變氣泡帶孔徑、空氣壓力及流速來調整氣泡流體的氣泡大小及體積分率。使用水下麥克風來監測輻射噪音。可量測在不同氣泡大小及體積分率下氣泡幕後方噪音衰減的特性。
在另一個水槽進行率定實驗，來估算氣泡流體的氣泡分佈及體積分率。另外，使用二氧化碳替代氣體產生氣泡幕，比較聲音通過一般空氣氣泡及二氧化碳氣泡幕之差異。
由本研究實驗結果得知，運用氣泡幕原理來減低或改變螺槳噪音是具體可行的。四葉螺槳輻射噪音頻率範圍為600至6000 Hz。多孔隙氣泡石所產生的氣泡可顯著衰減600至1100 Hz的螺槳低頻輻射噪音及空蝕現象所產生700 Hz噪音，符合氣泡其自然共振頻率恰巧等同入射聲波的頻率之理論。
氣泡幕控制參數包含：氣體施放流量、氣體施放壓力及氣幕帶孔徑大小。其影響因子為氣泡體積分率(氣泡包覆率)、氣泡幕寬度(厚度)。氣體流量與氣泡幕的氣體體積分率成正比，體積分率愈大入射聲波越不易穿透；氣泡大小與孔徑大小的關係為放大8至13倍，氣幕帶孔徑直接影響施放氣泡粒徑，直接決定降噪頻率，為最關鍵之參數；可溶性氣體（如CO2）確有助於減少氣泡生命週期，降低氣泡與船艦艉流產生交互作用的時間。
艦船氣泡帶存在最佳巡弋速率，本研究所設計之船模換算派里級實船最佳巡弋速率船速分別為12節及20節。氣泡幕運用不僅在於降噪效果，可依所欲管制的特定頻率，操作氣泡幕氣泡大小及體積分率，以獲得主頻強度增揚或改變頻率特徵之效果。

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.

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