Noise as an indicator of cavitation in a centrifugal pump

One of the sources of instability in a centrifugal pump is cavitation within the pump. Cavitation of a centrifugal pump is the result of insufficient net positive suction head (NPSH) and can occur within the entire range of operating conditions. Cavitation may cause three different and undesirable effects: (1) a drop in head-capacity and efficiency curves, (2) damage to the impeller by pitting and erosion, and (3) structure vibration and resulting noise. Therefore, the cavitation process must be prevented by all means. To prevent the onset of cavitation we have to detect the beginning of the cavitation process in the pump. To detect the beginning of the cavitation process, the emitted noise can be used, among other possibilities. Specifically, a noise spectra structure can be used to detect the beginning of cavitation and its development. Experiments have shown that there is a discrete frequency tone, at 147 Hz, which is strongly dependent on the cavitation process and its development. Therefore, noise spectra can also be used to determine the NPSH required or the critical value, representing the upper limit of the permissible pump operation without cavitation.     

Detection of cavitation in operation of kinetic pumps. Use of discrete frequency tone in audible spectra

Safe operation of kinetic pumps, as liquid movers, can be threatened by cavitation phenomenon in, amongst others. Cavitation is the Achilles’ heel of kinetic pumps. It can cause deterioration of the hydraulic performance, damage of the pump by pitting and material erosion, and structure vibration and noise. Cavitation can appear within the entire range of operating conditions, therefore it must by all means be prevented. To prevent cavitation in a pump we have to know the beginning and development of the cavitation in the pump. For this purpose, the emitted noise in the audible range can be used, amongst other possibilities. Experiments have shown that there is a discrete frequency tone within the audible noise spectra, which is in strong correlation with development of the cavitation process in the pump. Therefore, the discrete frequency tone can be separated from the noise spectra of a cavitating pump and used to detect the incipient of cavitation and its development as well as to prevent the onset of the cavitation process in the pump, by means of initiating an alarm, shutdown, or control signal via an electrical control system.     

空化流动诱导离心泵低频振动的实验研究

空化流动严重影响泵安全稳定运行,为充分认识泵内空化发展程度及其诱导的低频振动特性,设计了一台离心式模型泵作为研究对象,同时采用高速摄影及振动加速度测试手段,实现了叶轮内部空化流动的可视化及对应的泵体低频振动信号提取,分析了空化泡形态随空化发展的演化规律,对比了空化发生前后泵体低频振动频谱特性,探讨了泵体总振级水平、离散频率下振动加速度幅值随空化发展程度的变化,提出振动临界汽蚀余量可作为空化程度的另一判据,叶频时的振动加速度幅值变化亦可作为泵内空化程度的表征。     

Propeller sheet cavitation noise source modeling and inversion

Propeller sheet cavitation is the main contributor to high level of noise and vibration in the after body of a ship. Full measurement of the cavitation-induced hull pressure over the entire surface of the affected area is desired but not practical. Therefore, using a few measurements on the outer hull above the propeller in a cavitation tunnel, empirical or semi-empirical techniques based on physical model have been used to predict the hull-induced pressure (or hull-induced force). In this paper, with the analytic source model for sheet cavitation, a multi-parameter inversion scheme to find the positions of noise sources and their strengths is suggested. The inversion is posed as a nonlinear optimization problem, which is solved by the optimization algorithm based on the adaptive simplex simulated annealing algorithm. Then, the resulting hull pressure can be modeled with boundary element method from the inverted cavitation noise sources. The suggested approach is applied to the hull pressure data measured in a cavitation tunnel of the Samsung Heavy Industry. Two monopole sources are adequate to model the propeller sheet cavitation noise. The inverted source information is reasonable with the cavitation dynamics of the propeller and the modeled hull pressure shows good agreement with cavitation tunnel experimental data.     

Investigation of cavitation noise using Eulerian-Lagrangian multiscale modeling

We have employed the large eddy simulation (LES) approach to investigate the cavitation noise characteristics of an unsteady cavitating flow around a NACA66 (National Advisory Committee for Aeronautics) hydrofoil by employing an Eulerian-Lagrangian based multiscale cavitation model. A volume of fluid (VOF) method simulates the large cavity, whereas a Lagrangian discrete bubble model (DBM) tracks the small bubbles. Meanwhile, noise is determined using the Ffowcs Williams-Hawkings equation (FW-H). Eulerian-Lagrangian analysis has shown that, in comparison to VOF, it is more effective in revealing microscopic characteristics of unsteady cavitating flows, including microscale bubbles, that are unresolvable around the cloud cavity, and their impact on the flow field. It is also evident that its evolution of cavitation features on the hydrofoil is more consistent with the experimental observations. The frequency of the maximum sound pressure level corresponds to the frequency of the main cavity shedding for the noise characteristics. Using the Eulerian-Lagrangian method to predict the noise signal, results show that the cavitation noise, generated by discrete bubbles due to their collapse, is mainly composed of high-frequency signals. In addition, the frequency of cavitation noise induced by discrete microbubbles is around 10 kHz. A typical characteristic of cavitation noise, including two intense pulses during the collapsing of the cloud cavity, is described, as well as the mechanisms that underlie these phenomena. The findings of this work provide for a fundamental understanding of cavitation and serve as a valuable reference for the design and intensification of hydrodynamic cavitation reactors.     

转速对油泵空化性能影响及其换算研究

离心泵输送粘性流体时的空化余量,目前还没有成熟的理论计算方法.本文通过实验研究了转速、流量和输送介质的粘度对离心油泵空化性能的影响.实验结果表明:泵的临界空化余量和空化余量换算系数与输送介质的粘度和流量有关.随着粘度的增加,泵的临界空化余量和空化余量换算系数增加;随着流量的增加,泵的临界空化余量增加,而空化余量换算系数减小.     

Experimental study of the cavitation noise and vibration induced by the choked flow in a Venturi reactor

In this paper, the cavitation performance and corresponding pressure pulsation, noise and vibration induced by the choked cavitating flow in a Venturi reactor are investigated experimentally under different cavitation conditions by using high-speed camera and high frequency sensors. Based on the instantaneous continuous cavitation images, the Proper Orthogonal Decomposition (POD), a tool to analyze the large-scale cavitation flow structure, is applied to investigate the choked cavitating flow dynamics. The POD results show that two mechanisms, re-entrant jet flow mechanism and shock wave mechanism, govern the shedding and collapse of cavitation cloud at different pressure ratios. These mechanisms contribute to the variation of pressure pulsation, noise and vibration at different pressure ratios. The pressure pulsation spectrum behaves differently in various cavitation regions induced by the choked cavitating flow. Due to the existence of low pressure in re-entrant region, the influence of high frequency fluctuation on pressure pulsation caused by re-entrant flow is small. Moreover, with the increase of pressure ratio, the induced noise and vibration intensity decreases gradually, then increases and reaches a maximum value. Finally, it drops to a low and stable level. Despite different inlet pressures, the intensity of cavitation noise and vibration reaches the maximum value at the same pressure ratio. Specifically, the FFT analysis of noise and vibration signals indicates that low frequency component prevails at small pressure ratio owing to the re-entrant jet mechanism, while high frequency component prevails at large pressure ratio owing to the shock wave mechanism. The relationship between the choked cavitation dynamics and the induced pressure pulsation, noise and vibration in the Venturi reactor is highlighted. The results can provide guidance for the optimal operation condition of the Venturi reactor for cavitation applications such as water treatment.     

不均匀伴流场中螺旋桨空化的黏性流数值模拟和低频噪声预报

阐述了采用空化黏流CFD瞬态模拟和脉冲球形气泡辐射噪声理论相结合的思路在螺旋桨空化低频噪声谱预报上的应用方法。预报了全附体SUBOFF潜艇标称伴流条件下的NSRDC4383五叶大侧斜桨和某七叶大侧斜桨的片空化低频噪声谱,分析了桨叶负载和空化程度对线谱成分及其谱源级的影响。空化模拟时采用作者提出的且可信性经过验证的改进Sauer空化模型和修正SST湍流模型。噪声谱预报时空化体积由空化特征长度求取,较空泡表面球形等价假设更加合理。计算表明,七叶桨较五叶桨的确具有负载小、空化初生延迟、空化低频线谱噪声低的特征。在相同的基于航速的空化数下,非均匀进流与桨叶相互作用会明显增加线谱成分及其谱源级。在伴流、空化数和转速一定时,随着负载减小,推力、力矩和桨叶空化面积均会减小,但空化体积加速度幅度却变大,离散线谱噪声级增加且由奇次谐频为主转变为以偶次谐频为主;当仅减小空化程度时,谐频线谱成分明显被抑制,且1 kHz频率处谱源级减小2.54 dB。较完整地构建了螺旋桨空化水动力和噪声性能评估的数值平台,可用于指导艇尾低噪声桨的数值设计。      

An acoustic backscattering technique for the detection of transient cavitation produced by microsecond pulses of ultrasound

An acoustic backscattering technique for detecting transient cavitation produced by 10-microseconds-long pulses of 757-kHz ultrasound is described. The system employs 10-microseconds-long, 30-MHz center frequency tone bursts that scatter from cavitation microbubbles. Experiments were performed with suspensions of hydrophobic polystyrene spheres in ultraclean water. Transient cavitation threshold pressures measured with the active cavitation detector (ACD) were always less than or equal to those measured using a passive acoustic detection scheme. The measured cavitation thresholds decreased with increasing dissolved gas content and increasing suspended particle concentration. Results also show that ultrasonic irradiation of the polystyrene sphere suspensions by the ACD lowered the threshold pressure measured with the passive detector. A possible mechanism through which suspensions of hydrophobic particles might nucleate bubbles is presented.     

离心油泵输送粘性流体空蚀性能实验研究

本文通过实验研究了离心油泵输送粘性流体时的空蚀性能和油泵的空蚀性能与输送介质粘度的关系,研究了油泵的性能参数(流量、扬程、功率和效率)随进口真空度的变化规律,并与输送清水时的空蚀性能进行了对比,研究了进口真空度对油泵水力性能的影响。离心油泵的空蚀性能随输送介质的不同存在很大差异。实验结果表明:在相同的流量工况下,输送粘性流体比输送清水具有较高的临界空蚀余量。     

The acoustic emissions of cavitation bubbles in stretched vortices

Pairs of unequal strength, counter-rotating vortices were produced in order to examine the inception, dynamics, and acoustic emission of cavitation bubbles in rapidly stretching vortices. The acoustic signatures of these cavitation bubbles were characterized during their inception, growth, and collapse. Growing and collapsing bubbles often produced a sharp, broadband, pop sound. The spectrum of these bubbles, and the peak resonant frequency can generally be related to quiescent flow bubble dynamics and corresponding resonant frequencies. However, some elongated cavitation bubbles produced a short tonal burst, or chirp, with frequencies on the order of a few kilohertz. Theses frequencies are too low to be related to resonant frequencies of a bubble in a quiescent flow. Instead, the frequency content of the acoustic signal during bubble inception and growth is related to the volumetric oscillations of the bubble while it interacted with vortical flow that surrounds the bubble (i.e., the resonant frequency of the vortex-bubble system). A relationship was determined between the observed peak frequency of the oscillations, the highly stretched vortex properties, and the water nuclei content. It was found that different cavitation spectra could relate to different flow and fluid properties and therefore would not scale in the same manner.     

混响场中脉冲超声空化效应的研究

本文实验研究了不同脉冲宽度的８２０ｋＨｚ超声波的空化效应，结果得出，空化致自基产量随脉冲宽度及声强呈现规律性变化，尤其观察到了“脉冲空化峰”现象，本文从液面受迫振动角度出混响场中“脉冲空化峰”现象的机制。     

Experimental research into time–frequency characteristics of cavitation noise using wavelet scalogram

The cavitation has become the main cause of the damage to the hydraulic machine. Cavitation detection is very important to guarantee the safe running of the hydraulic machine. The sound, especially the audible sound, based methods are becoming attractive due to their simplicity and logicality in the application. However, the cavitation noise is easy to be contaminated by the background noise. In order to understand the characteristics of the cavitation noise deeply, using the wavelet scalogram analysis, this paper presents an experimental study to investigate the time–frequency characteristics of the cavitation noise of various cavitation states and the relation between the cavitation noise and the cavitation process. In addition, the method of parameters optimization for the wavelet is used to improve the transform performance of the wavelet scalogram. The results show that: the cavitation noise is composed of the components with wide band frequency and obvious impulse feature; but the cavitation noise of different cavitation stages has different time–frequency characteristics and compositions; in addition, the cavitation noise can be distinguished from the background noise because they have totally different frequency characteristic. This study validates that the cavitation noise can be used to detect the cavitation state and monitor the cavitation process.     

Anechoic chamber qualification: traverse method,inverse square law analysis method,and nature of test signal

Qualification of anechoic chambers is intended to demonstrate that the chamber supports the intended free-field environment within some permissible tolerance bounds. Key qualification issues include the method used to obtain traverse data, the analysis method for the data, and the use of pure tone or broadband noise as the chamber excitation signal. This paper evaluates the relative merits of continuous versus discrete traverses, of fixed versus optimal reference analysis of the traverse data, and of the use of pure tone versus broadband signals. The current practice of using widely space discrete sampling along a traverse is shown to inadequately sample the complexity of the sound field extant with pure tone traverses, but is suitable for broadband traverses. Continuous traverses, with spatial resolution on the order of 15% of the wavelength at the frequency of interest, are shown to be necessary to fully resolve the spatial complexity of pure tone qualifications. The use of an optimal reference method for computing the deviations from inverse square law is shown to significantly improve the apparent performance of the chamber for pure tone qualifications. Finally, the use of broadband noise as the test signal, as compared to pure tone traverses over the same span, is demonstrated to be a marginal indicator of chamber performance.     

Detecting incipient cavitation by assessing low frequency acceleration and analysis of CMC

Cavitation is a common phenomenon in a fluid circuit especially wherever local pressure is lower than fluid saturated pressure. The cavitation negatively affects a fluid system and structure in different ways: i.e. erosion, flow rate reduction, noise and vibration. In order to diminish cavitation, adding some nanomaterials seem to be applicable in different ways. This research aims at assessing the effects of CMC (Carboxy methyl cellulose) additives on incipient cavitation by analyzing the resultant change in low frequency acceleration. Furthermore, this study attempts to examine the accuracy of low frequency acceleration for detecting incipient cavitation.     

伴流场中对转桨空化初生的判定与辐射噪声预报和校验

采用空化多相流瞬态模拟和边界元数值声学计算相结合的混合方法,预报了全附体假尾后对转桨在初生空化状态下的线谱和宽带谱噪声,分析了初生空化状态下对转桨噪声谱级相对于无空化状态的增量,提出了同时从流场与声场角度判定对转桨空化初生的四个充分条件。预报值与空泡水筒噪声测量值进行了比较。多相流瞬态模拟包括非定常雷诺时均模拟、尺度适应模拟和分离涡模拟三种方法。计算结果表明,在大范围进速系数范围内预报对转桨敞水性能曲线与测量值吻合很好。尺度适应模拟(SAS)和分离涡模拟(DES)在捕捉对转桨空化脉动压力时精度相当,均能满足非定常负载噪声预报的精度要求;雷诺时均模拟(URAN S)仅对于低频负载噪声来说基本适用。空泡体积脉动诱导线谱噪声在800 Hz处的预报误差小于4 dB;在800 Hz^3 kHz频段内,预报得到1/3oct中心频率处谱级的平均误差小于1.5 dB,总声级预报误差小于2.4 dB。空化初生的充分条件为:空化面积与桨盘面面积的比值小于2%、积分力和叶梢截面压力系数分布较无空化状态基本不变且中高频段噪声谱级增加8~10 dB。较好地解决了伴流场中对转桨空化初生判定和初生空化状态下辐射噪声预报的两个技术难题,可直接服务于高速、低噪声鱼雷的设计研发。      

脉冲超声激励下SonoVue微泡的瞬态空化特性

将SonoVue微泡从临床疾病诊断拓展至治疗引起了诸多研究人员的兴趣。为了平衡治疗效率和生物安全性,深入理解声学参数和SonoVue微泡瞬态空化的关系至关重要。本研究自行制备仿体容器放置SonoVue微泡,使用1 MHz发射换能器激励其产生空化效应,另一个7.5 MHz的聚焦换能器接收声信号,经放大及高速数据采集后送上位机处理。通过深入分析信号的时频域特征,我们提出以宽带信号的能量及其随时间变化曲线的半高宽来表征瞬态空化的剂量(ICD)和相对持续时间(ICP),并确定:瞬态空化的发生和ICD依赖于峰值负声压,但ICP随峰值负声压的增加而减小;脉冲重复频率和脉冲持续时间都和ICD及ICP正相关;且脉冲持续时间的影响较大。这些结果有望为SonoVue微泡的治疗应用提供理论支持。      

Numerical investigation of cavitation generated by an ultrasonic dental scaler tip vibrating in a compressible liquid

Bacterial biofilm accumulation around dental implants is a significant problem leading to peri-implant diseases and implant failure. Cavitation occurring in the cooling water around ultrasonic scaler tips can be used as a novel solution to remove debris without any surface damage. However, current clinically available instruments provide insufficient cavitation around the activated tip surface. To solve this problem a critical understanding of the vibro-acoustic behaviour of the scaler tip and the associated cavitation dynamics is necessary. In this research, we carried out a numerical study for an ultrasound dental scaler with a curved shape tip vibrating in water, using ABAQUS based on the finite element method. We simulated the three-dimensional, nonlinear and transient interaction between the vibration and deformation of the scaler tip, the water flow around the scaler and the cavitation formation and dynamics. The numerical model was well validated with the experiments and there was excellent agreement for displacement at the free end of the scaler. A systematic parametric study has been carried out for the cavitation volume around the scaler tip in terms of the frequency, amplitude and power of the tip vibration. The numerical results indicate that the amount of cavitation around the scaler tip increases with the frequency and amplitude of the vibration. However, if the frequency is far from the natural frequency, the cavitation volume around the free end decreases due to reduced free end vibration amplitude.     

Strategies for the representation of a tone in background noise in the temporal aspects of the discharge patterns of auditory-nerve fibers

The responses of populations of auditory-nerve fibers to both a 1.0-kHz tone, and 1.0-kHz tone in broadband noise, have been measured. Period histograms were generated from fiber spike trains and discrete Fourier transforms (DFTs) with a resolution of 125 Hz were computed from each histogram. Sample mean and sample variance statistics were generated for period histograms of response and for temporal response measures derived from discrete Fourier transforms. It is demonstrated how the statistical properties of auditory-nerve fiber response determine the strategy for the estimation and discrimination of particular stimulus components. When the tone is presented alone, the entire population of auditory-nerve fibers provides statistically reliable estimates of the 1.0-kHz tone. Upon addition of the broadband noise stimulus only those units with characteristic frequencies which are close in frequency to the 1.0-kHz stimulus provide spectral estimates which have high signal-to-noise ratios (mean-squared-to-variance ratios). Estimates of the 1.0-kHz-tone stimulus derived from auditory-nerve fibers with characteristic frequencies which are far from the 1.0-kHz stimulus are statistically unreliable. Based on the responses of the population of auditory-nerve fibers, the strategy for estimating the 1.0-kHz-tone stimulus is to derive estimates of the 1.0-kHz stimulus from the subpopulation of neurons with characteristic frequencies close to the 1.0-kHz stimulus. It is concluded that neurons which are tuned close to 1.0-kHz provide the central nervous system (CNS) with the most salient information about the 1.0-kHz stimulus in the presence of the broadband background.     

上下并联轴流风机室外机气动声学预测模型

本文基于CFD模拟方法,分析了空调器室外机上下并联轴流风机系统噪声源分布,建立了室外机气动声学预测方法.研究发现,上下并联轴流风机系统由宽频和离散频率噪声组成,宽频噪声是影响室外机噪声总声压级的重要因素.涡声分析表明,涡脱落噪声是宽频噪声的主要影响因素.基于CFD的叶片尾缘涡脱落噪声预测方法计算得到宽频声压误差为2 dB,考虑离散频率影响时,室外机A计权总声压级预测误差小于2 dBA.基于CFD的点源时域预测模型,捕捉到了上下并联轴流风机系统的离散频率噪声峰值,且在上下叶轮前二阶谐波处预测值与实验值吻合较好.