三维小空化数空泡流数值方法

采用均质平衡流空泡模型,引入基于求解液体质量份数输运方程的空泡流模型,提出新的压力密度耦合的压缩性方法和控制气液两相转换的源项,求解汽水混合介质的RANS方程和带低雷诺数修正的k-ε模型,实现了小空化数(σ=0.2~0.01)下水下航行体的空泡流数值模拟.得到清晰的空泡形态特征与内部结构,以及空泡长度和最大直径随空化数的变化规律,给出航行体运动阻力系数与空化数之间的变化关系.将计算结果与解析结果及实验数据进行比较表明,该方法保证了极小空化数下自然空泡流计算的稳定性与收敛性,提高了空泡形态特性的预报精度.     

空化紊流流动的数值计算模型及其验证

本文介绍了一种空化紊流流动的数值计算模型,并计算了射流放水阀内部的空化流动现象。该模型基于均相平衡模型和液相与汽相传输方程,基本方程采用N-S方程,空化模型采用Kunz等提出的汽液质量转换模型。紊流封闭采用标准的κ-ε素流模型;稳定流动计算采用扩展的SIMPLE压力修正方法,非稳定计算采用PISO算法。为评价该数值模型,计算了绕射流放水阀的空化流动,并与实验结果进行了对比,计算结果与实验结果取得较好的一致,说明该流动模型和计算方法是可靠的。     

低温泵内空化流动演化规律的数值仿真

为了研究低温泵内的空化流动对泵效率的影响,采用SST k-ω湍流模型和Zwart空化模型对泵内空化流动进行数值模拟分析。结果表明:随着泵入口压力不断降低,叶轮入口处空泡越来越明显;空泡较明显的叶轮流道出口处形成较集中的高温区;当泵入口压力较高时,不同介质温度的泵效率均略有下降且斜率相近;当泵入口压力较低时,不同温度下液氮对应的泵效率曲线斜率有一定差异,温度越高,效率下降越快。这些研究的发现可以为低温泵性能的改善提供一定的参考。     

非定常粘性空化流动模型及其数值计算

基于气液两相当地均相介质模型,本文给出了一种模拟非定常粘性空化流动的计算模型。认为空化绕流流场中流动介质是一种当地均匀的气液混相物。控制方程采用了应用两相流模型的N-S方程。基于液相和气相的状态方程推导了混合介质密度的表达式。为了保证数值计算的稳定性,控制方程的数值求解采用了TVD-MacCormack格式。为了评价计算模型的可靠性,分别计算了绕台阶和管道水锤的空化流动,所得结果是合理的,说明该方法可以用于空化流动的数值计算。     

声-流耦合空化场观测与量化表征探索*

该文利用搭建的高速摄影和空化噪声同步观测的声-流耦合空化实验平台,观察分析了声-流耦合场中空化云的演化规律及相应的空化噪声特征。通过引入空化状态变量,给出了空化强度的一种新的明确表述,并提出了一种基于高速摄影图像分析来测量和表征空化状态变量及空化强度的方法。利用该方法进一步对声-流耦合空化时间演化周期性和空间强度分布进行了定量计算。结果表明,声-流耦合空化强度和作用范围相比单独声空化和单独水力空化有显著的提升。     

混流式水轮机内部流场的三维空化湍流计算

本文通过理论分析,引入三维混合流体完整空化湍流模型,确定了混流式水轮机内部三维空化湍流计算的方法.用数值模拟手段对混流式水轮机的内部空化流场进行了计算,采用了全三维全流道的湍流计算方法,基于RNG的k-ε湍流模型和SIMPLEC算法,从蜗壳进口到尾水管出口,包含所有流道在内的整体一次完成定常.根据得到的三维空化流场数据,对转轮内部流场进行了流场分析,求得了计算临界空化系数,和试验值进行了比较;模拟了特征工况下的尾水管涡带,和试验数据进行了比较.     

超音速汽液两相流激波的理论研究

建立了超音速汽液两相流激波过程的数学模型,得到了激波前后的流动关系,并对激波后的流动规律进行了初步分析,建立了两相扩压段的优化模型。研究结果表明,在激波后,流体压力上升,速度下降,与单相气体的正激波特性相似;其对应的音速与汽液两相流体的空泡率、压力及液相密度等因素有关。     

双泡耦合声空化动力学过程模拟

为了对双泡耦合的声空化过程进行模拟,本文从流体动力学控制方程和流体体积分数模型出发,在Fluent软件中构建双泡耦合超声空化三维有限元仿真模型,对超声波驱动下流体中双泡耦合声空化动力学过程进行数值模拟,并通过对空化气泡周围声场的变化进行分析研究双泡耦合声空化的非线性动力学特性.结果显示:在超声波驱动下,球形气泡先缓慢扩张,扩张到最大半径后迅速收缩直至溃灭;耦合双气泡间存在相互作用力,使得空化气泡的扩张受到抑制、气泡收缩时间增长;空化气泡在收缩阶段的能量转换能力增强,相比单气泡声空化,耦合双气泡溃灭时气泡内部的压强更大.本文分析结果将为超声空化泡群的动力学过程模拟提供参考.     

翼型空化绕流特征和流激特性的关联分析

基于DES湍流模型和Schnerr-Sauer空化模型对空化初生和片状空化阶段791翼型非定常绕流进行了数值模拟,着重分析了不同空化阶段翼型绕流的流动特征、压力脉动特性及两者间的关联关系。研究表明:翼型表面空穴的非稳态演变与翼型动力特性变化相互映射;在空化初生阶段,主空穴的周期性变化和翼型展向外缘处空泡团的周期性脱落是此阶段流激振动的主要激励;在片状空化阶段,主空穴和U型空穴结构的周期性变化是此阶段的主要激励;空泡脱落处的压力脉动幅值远大于无空穴区域和空穴发生区域。研究结论为翼型空化绕流诱发振动机理研究的深入提供了一定的参考.     

涡流二极管空化流动的数值计算

为了研究涡流二极管内空化现象的机理特性及对其性能的影响,我们假设流体为气液混相均质,并考虑不可凝结气相,采用基于组分输运方程,求解了涡流二极管全流道内气液混相均质流的雷诺平均N-S方程以及气相组分输运方程。数值计算结果显示了空化形成时涡流二极管入口、出口及旋流腔内的流场形态,研究表明:涡流二极管空化现象主要发生在流体切向进入时旋流腔和中心管的中心部位;空化流是由于液体在中心旋流场低压条件下汽化,同时不可凝结气体由于亨利定律在旋转流场形成的压力梯度下而发生的输运效应综合形成的;空化流由于强旋涡的原因对涡流二极管的性能产生明显的影响。上述结论对涡流二极管的设计及其指导工程应用具有重要的价值。     

Cavitation erosion by shockwave self-focusing of a single bubble

The ability of cavitation bubbles to effectively focus energy is made responsible for cavitation erosion, traumatic brain injury, and even for catalyse chemical reactions. Yet, the mechanism through which material is eroded remains vague, and the extremely fast and localized dynamics that lead to material damage has not been resolved. Here, we reveal the decisive mechanism that leads to energy focusing during the non-spherical collapse of cavitation bubbles and eventually results to the erosion of hardened metals. We show that a single cavitation bubble at ambient pressure close to a metal surface causes erosion only if a non-axisymmetric energy self-focusing is at play. The bubble during its collapse emits shockwaves that under certain conditions converge to a single point where the remaining gas phase is driven to a shockwave-intensified collapse. We resolve the conditions under which this self-focusing enhances the collapse and damages the solid. High-speed imaging of bubble and shock wave dynamics at sub-picosecond exposure times is correlated to the shockwaves recorded with large bandwidth hydrophones. The material damage from several metallic materials is detected in situ and quantified ex-situ via scanning electron microscopy and confocal profilometry. With this knowledge, approaches to mitigate cavitation erosion or to even enhance the energy focusing are within reach.     

液体薄层中的超声空化*

液体薄层中的超声空化,因其边界及所处空间的特殊性,而呈现出非常独特的空化结构和演化行为,在超声清洗、超声钎焊、表面处理、近场声悬浮、超声化学等领域都有所应用。该文梳理了近几年该课题组在液体薄层中的超声空化研究中的一些成果,力图揭示液体薄层内空泡、空化云、空化场的运动和分布规律,及其产生、发展和演化过程,以期对液体薄层中的超声空化行为有一个相对清晰和完整的认识。     

声空化泡对声传播的屏蔽特性*

该文介绍了声空化液体中声波被反常吸收的现象,即驱动声压越大,吸收越强,远场声压越低。研究给出其物理机理是高声压导致强空化,空化泡吸收驱动能量辐射高次谐波,高频声波更易被液体吸收,最终形成更低的远场声压。为了克服空化屏蔽,改善声空化的均匀性,提出了改变工作液体的空化阈值的思路。并就简单的双层液体系统进行了计算和实验,结果证实这种思路的正确性。     

The issue of cavitation number value in studies of water treatment by hydrodynamic cavitation

Within the last years there has been a substantial increase in reports of utilization of hydrodynamic cavitation in various applications. It has came to our attention that many times the results are poorly repeatable with the main reason being that the researchers put significant emphasis on the value of the cavitation number when describing the conditions at which their device operates.In the present paper we firstly point to the fact that the cavitation number cannot be used as a single parameter that gives the cavitation condition and that large inconsistencies in the reports exist. Then we show experiments where the influences of the geometry, the flow velocity, the medium temperature and quality on the size, dynamics and aggressiveness of cavitation were assessed. Finally we show that there are significant inconsistencies in the definition of the cavitation number itself.In conclusions we propose a number of parameters, which should accompany any report on the utilization of hydrodynamic cavitation, to make it repeatable and to enable faster progress of science and technology development.     

Applications of sonochemistry in Russian food processing industry

In food industry, conventional methodologies such as grinding, mixing, and heat treatment are used for food processing and preservation. These processes have been well studied for many centuries and used in the conversion of raw food materials to consumable food products. This report is dedicated to the application of a cost-efficient method of energy transfer caused by acoustic cavitation effects in food processing, overall, having significant impacts on the development of relatively new area of food processing such as food sonochemistry.     

超声空化现象影响因素的实验研究

超声空化在许多不同的学科和工业生产中有着广泛的应用。超声空化的应用与声场的分布及空化的机理密切相关,精准地反映空化场和空化机理是超声空化技术实际应用的关键。该文通过分析采集的声信号和金属箔膜空蚀法对空化区域随液位发生变化的现象进行研究,并利用Matlab对金属箔膜空蚀程度量化。实验发现,超声波会在液面与实验箱体底部形成驻波场。在某一液体温度下,随着液位高度的变化,超声空化现象的出现具有周期性。并且,在同一液位下,当超声功率改变时,空化区域强度分布情况随之改变。小功率时各空化区域空化强度分布均匀,当功率增大到一定时,会出现空化屏蔽现象。该研究为超声清洗设备的改良提供了借鉴,对进一步认识和利用超声空化效应具有重要意义。     

Sound field measurement in a double layer cavitation cluster by rugged miniature needle hydrophones

During multi-bubble cavitation the bubbles tend to organize themselves into clusters and thus the understanding of properties and dynamics of clustering is essential for controlling technical applications of cavitation. Sound field measurements are a potential technique to provide valuable experimental information about the status of cavitation clouds. Using purpose-made, rugged, wide band, and small-sized needle hydrophones, sound field measurements in bubble clusters were performed and time-dependent sound pressure waveforms were acquired and analyzed in the frequency domain up to 20 MHz. The cavitation clusters were synchronously observed by an electron multiplying charge-coupled device (EMCCD) camera and the relation between the sound field measurements and cluster behaviour was investigated. Depending on the driving power, three ranges could be identified and characteristic properties were assigned. At low power settings no transient and no or very low stable cavitation activity can be observed. The medium range is characterized by strong pressure peaks and various bubble cluster forms. At high power a stable double layer was observed which grew with further increasing power and became quite dynamic. The sound field was irregular and the fundamental at driving frequency decreased. Between the bubble clouds completely different sound field properties were found in comparison to those in the cloud where the cavitation activity is high. In between the sound field pressure amplitude was quite small and no collapses were detected.     

In vitro sonodynamic cytotoxicity in regulated cavitation conditions

Sonodynamic toxicity has always been linked to the cavitation phenomenon. In this work, sonodynamic effect with Photofrin^® was evaluated with a new ultrasound device: a regulated cavitation generator. In this way, acoustic intensity was substituted with cavitation level as ultrasound parameter. Photofrin^® potentiated significantly the cavitation cytotoxicity even for low setpoints where no inertial cavitation appeared. Therefore sonodynamic mechanism was principally mechanical, facilitated by the Photofrin^® insertion in cellular cytoplasmic membranes. This assertion was also supported by the fact that sonodynamic cytotoxicity was independent from the Photofrin^® presence or absence in the extracellular medium. Reproducible sonodynamic efficiency was perfectly obtained with this new regulated cavitation generator.     

Cavitation intensity of water under practical ultrasonic cleaning conditions

When measuring cavitation during cooling of thermally degassed water cavitation maxima are frequently observed at various temperatures. Relations between this phenomenon and frequency and power of ultrasounds as well as air content in water have been examined. It was found out that the secondary water regassing with air is the reason.