ON ACOUSTIC PERFORMANCE OF MEASURING STRUCTURE FOR CAVITATION NOISE

The acoustic performance of the structure for measuring cavitation noise is theoretically analysed, and twojudging criteria of the acoustic performance of STW (sound transmission window) are presented in this paper. Oneis of impedance matching between working liquid and STW: ρ_1C_1=ρ_2C_2. The other is of maximum frequency(or minimum wave length) of cavitation noise spectrum: f_(max)≤c/60D(orλ_(min)/D≥60). If either of the criteria is metreliable measured results of cavitation noise will be obtained. The theoretical results are verified through experi-ments. According to the criteria, the paper analyses the acoustic performance of routinely used plexiglass STW. Theresults show that in the research of clear water cavitation the impedance ratio, 2.18, does not satisfy ρ_1C_1=ρ_2C_2,and f_(max)≤C/60D hardly holds for actual cavitation noise spectrum. Therefore, plexiglass STW is not good enoughfor sound transmission. From this point of view, polyethylene is better sound transmission material than plexi-glass.     

决定我国某大型水电站水轮机直径的方案问题

本文第一部分明确了在水电站设计工作中被长期混乱了的两种计算水头的意义。确切指出“水输机平均计算水头”H_r 和 H′_r,指出它既不等于 H_ρ,也不等于 H_α,而是当水轮机直径修正为标准值时与η=f(NH)图上水轮机限制出力缐转折点相应的水头。第二部分说明了增加(Q′1)min 和(Q′1)_α的影响。作者推导出设计水轮机时的最优单元流量(Q′01)_P=(2g~(1/2)πf((ηopδ)/(2)(tanα_esinα_e)/(cosβi))~(1/2)从而论证了修割叶片以增加单元流量的方法,同时总结了一系列增加单元流量的途径。并且指出增加单元流量对水轮机空蚀性能的影响。推导出能说明影响空蚀系数的普遍公式:σokp=(2ηor)/z((Q′01)_pH_p~(1/2))/(ωlf)+k~2+ηor-1这一公式也可以说明水轮机空蚀系数测定试验之所以要求在原始设计水头下进行的基本原因。第三部分是根据作者1957年2月份出席我国某大型水电站初步设计审查会议时所获得的资料。对某厂提出要求减少水轮机直径的各个方案进行了评述。并且指出选用 PC-211型水轮机的可能性。希望承制这顶水轮机的苏联列宁格勒斯大林金属工厂给予注意。但是作者指出设计既能满足系统运行条件的要求又能减少直径的水轮机新品种对该电站来说仍是十分必要的,本文已指出了方向。最近该电站的正常高水位已经更改,那么新水轮机的设计已提到日程上来了,关于新品种问题作者将另文讨论。本文希望廓清两种计算水头的混乱概念进而指出在满足系统工作条件下减少水轮机直径以获得较高经济效益的途径。从而评述了决定我国某大型水电站水轮机直径的几个方案。     

水吸收及距离对减压试验的影响

根据水吸收及距离引起的声波衰减计算公式,对水吸收及距离可能对减压实验中空化噪声强度测量结果造成的影响作了初步的理论探讨。研究表明,水吸收及距离产生的声衰减在一定距离范围内对空化初生的判别并无影响,超过一定距离范围则水听器监测到的空化初生判别结果不再准确。     

NUMERICAL PREDICTION OF BLADE FREQUENCY NOISE OF CAVITA- TING PROPELLER

The blade frequency noise of a cavitating propeller in a uniform flow is analyzed in the time domain. The unsteady loading (of a dipole source) and the sheet cavity volume (of a monopole source) on the propeller surface are calculated by a potential-based surface panel method. Then the time-dependent pressure and the cavity volume data are used as the input for the Fowcs Williams-Hawkings formulation to predict the acoustics pressure. The integration of the noise source is performed over the true blade surface rather than the ideal blade surface without thickness. The noise characteristics of the cavitating propeller are discussed. With the sheet cavitation, the thickness (cavitation) noise is larger than the loading noise and is the dominant noise source.The noise directivity is not as clear as that of the noise under a non-cavitation condition. The cavitation noise is attenuated mores lowly than the non-cavitation noise.     

小浪底工程1号孔板泄洪洞水流空化原型试验研究

在库水位为210.18～210.28 m之间时,对小浪底水利枢纽工程1号孔板泄洪洞泄洪情况下多级孔板水流空化特性进行了原型试验。对弧形工作闸门在连续开启过程中各级孔板水流噪声的频谱分析发现,闸门孔口相对开度e/a为0.9时,第二级和第三级孔板处水流噪声声压级突然增加10 dB以上,表明开始出现空化水流,相应三级孔板的水流空化数分别为5.85、5.65和5.03,该结果与考虑缩尺影响修正后的模型试验研究结果基本一致。     

Application of signal processing techniques to the detection of tip vortex cavitation noise in marine propeller

The tip vortex cavitation and its relevant noise has been the subject of extensive researches up to now. In most cases of experimental approaches, the accurate and objective decision of cavitation inception is primary, which is the main topic of this paper. Although the conventional power spectrum is normally adopted as a signal processing tool for the analysis of cavitation noise, a faithful exploration cannot be made especially for the cavitation inception. Alternatively, the periodic occurrence of bursting noise induced from tip vortex cavitation gives a diagnostic proof that the repeating frequency of the bursting contents can be exploited as an indication of the inception. This study, hence, employed the Short-Time Fourier Transform (STFT) analysis and the Detection of Envelope Modulation On Noise (DEMON) spectrum analysis, both which are appropriate for finding such a repeating frequency. Through the acoustical measurement in a water tunnel, the two signal processing techniques show a satisfactory result in detecting the inception of tip vortex cavitation.     

掺气气泡对减压实验测量结果的影响

根据气泡的声学特性,对掺气水流中气泡可能对减压实验中声压级差和空化噪声强度测量结果造成的影响作了初步的理论探讨。研究表明,气泡对声波的吸收对空化初生的判别并无影响,对空化噪声强度的影响不可忽略。     

三门峡大坝双层泄水孔原型噪声观测与底孔改建设计

三门峡大坝泄流排沙孔原为施工导流底孔，其体型不满足黄河高含沙水流的要求，泄水后其斜门槽、工作门槽导轨等部位发生严重破坏。为了查明破坏机理，提出合理的改建措施，于１９８９年对双层泄水孔进行了原型噪声观测。本文介绍了噪声测试水流空化的原理、方法及成果，对照水工模型试验及底孔破坏情况进行了分析，对改建方案进行了论证。此次原型噪声观测为水工建筑物水流空化情况测试和判断提供了新的途径，底孔改建为改善水力条件     

A HIGH-SPEED PHOTOGRAPHIC STUDY OF ULTRASONIC CAVITATION NEAR RIGID BOUNDARY

This article investigated an existing steady pattern of collapse and rebound （disintegration and aggregation） of cavitation bubbles near rigid boundary in acoustic field. A deformation process of cavitation bubble was accomplished in two acoustic cycles, namely, a spherical bubble collapsed towards the boundary to its minimum volume and then rebounded and grew into a toroidal bubble （or two individual bubbles） in one acoustic cycle, and the toroidal bubble （or two individual bubbles） collapsed towards the center of ring to its minimum volume, and then rebounded into a spherical bubble in the next acoustic cycle. Inertia force plays a key role in the transition between these two states. The microjet produced during the collapse of spherical bubble and the shock wave produced during the collapse of toroidal bubble （or two individual bubbles） impacts the boundary alternately. A cavitation bubble operating in this pattena can thus be an effective corrosion mechanism of rigid boundary.     

基于声波探测的水轮机空化分析方法研究与应用

结合水轮机空化特点,对轴流转桨式水轮机中主要发声故障模式进行了研究,提出了相应特征提取方法;此外,对轴流转桨式水轮机空化影响因素进行了分析,在上述基础上,提出了水轮机运行工况分布统计法以及空化声波信号与机组运行工况关联分析法。工况统计、工况关联分析已在葛洲坝电厂水轮机空化状态监测中得到成功应用,并取得了良好的效果。     

空泡噪声的数值研究

空泡噪声是一个非线性动边界的声辐问题，作者针对这个特点，提出了一种新的数值方法-混合边界元法，本文应用混合边界元法对无界流场、自由面或固壁面附近空泡溃灭过程辐射的噪声进行计算，得到了空泡在不同情况下溃灭辐射声音的特征，结果显示混合边界元方法在这类动边界声辐射问题上，能体现出声音传播的延迟效应，体现出液体压缩性对声压峰值的影响，作者认为混合边界法在处理这类动边界声辐射问题上有着很好的应用前景。     

URANS COMPUTATION OF CAVITATING FLOWS AROUND SKEWED PROPELLERS

Cavitating flows around skewed propellers are investigated numerically by means of the unsteady Reynolds Averaged Navier-Stokes (RANS) Equation method. The standard k-ε turbulence and the modified Z-G-B cavitation models are employed. A measured nominal wake is used for the inlet velocity boundary condition. Predicted cavitating evolution processes and tip cavity patterns are compared with experimental observations. In addition, the influence of the skew angles on the cavitation and unsteadiness performances of propellers operating in a non-uniform wake is also studied. Results show that the modified Z-G-B cavitation model performs better to simulate the cavitating flow cases studied in this paper. Comparisons demonstrate that the skewed propeller with a skew angle of 20 o is the best choice for a given stern wake with a assigned thrust and the minimum force fluctuations.