自然通气降低导管螺旋桨空泡噪声试验和理论分析

我们已在文献中报导了通过导管24个周向分布径向孔,向螺旋桨叶梢部位人工低压充气(进口压力为2～3个大气压),可降低导管螺旋桨噪声达20分贝左右,当螺旋桨背面出现泡状空泡的强噪声源时,降低空泡噪声效果尤为显著,可高达30分贝左右。 本文继续研究,利用螺旋桨盘面上部的负压,采用自然通气,在导管上开一个孔、四个孔、八个孔向螺旋桨叶梢部位小气量通气降噪的试验和理论分析。试验证实:自然小气量通气在频率2～40千赫范围内可降低螺旋桨空泡噪声约4～12分贝左右。并从理论上分析了自然小气量通气后,可使空泡崩溃延缓;降低崩溃速度;降低辐射声能和声压;延长崩溃时间;结果可降低空泡噪声最高达15分贝左右,这与试验结果相当一致。虽然降噪幅度不及人工低压充气的降噪幅度大,但自然通气的最大优点是无需外加气源,因此更具有实用价值。试验还证实了不论是人工低压充气还是自然通气,不仅是降噪的非常有效的措施,而且也是抗剥蚀、减振的有力措施。

THEORETICAL ANALYSIS AND EXPERIMENTAL INVESTIGATION ON REDUCTION OF THE CAVITATION NOISE OF DUCTED PROPELLER BY NATURAL AIR INJECTION

In a paper entitled "An Investigation on Reduction of the cavitation Noise of Ducted Propeller by Air Injection", the results relating to air injection into the region of blade tip through the outlets on the inner surface of . the nozzle with lower overpressure (the entrance pressure is about 2 to 3 atm. ) have been reported.It is remarkable that the reduction of propeller cavitation noise amounts to 20 dB and the noise reduction will be greater than 30 dB once the strong noise source such as bubble cavitation occurs.The theoretical analysis and the experimental investigation on reduction of the cavitation noise on a ducted propeller by natural ventilation under conditions of the negative pressure over the propeller disc are carried out in the present paper. A noise reduction of 4 dB to 12 dB in the frequency range of 2kHz to 40kIIz is achieved. The theoretical analysis indicates that more than 15 dB noise reduction, which is in quite agreement with the experiments, will be attained due to the following causes: attenuation of the collapsing process, drop of the collapsing Velocity, reduction of the acoustical rediation energy and increment of the collapsing duration. It is advantageous and practical that the air supply is needed no more in this alternative. In addition, the experimental results show that both artificial and natural air injection are not only efficient for reducing cavitation noise but also significant for preventing vibration and erosion.