数值模拟两相汽蚀流动的新模型和算法

提出了数值模拟两相汽蚀流动现象的新汽蚀模型和算法.提出的汽蚀模型和算法耦合了考虑紊流粘性效应的Reynolds-Averaged Navier-Stokes方程求解方法,可以自动模拟空泡起始点、空泡长度和汽蚀空泡形状.在流场计算和界面修正的迭代计算过程中,跟踪并得到液相/气相界面.数值模拟了圆锥形圆柱体和半球形圆柱体在不同汽蚀系数下的汽蚀流动现象,得到了与实验值完全吻合的数值模拟结果,并且与已发表的数值研究结果进行了比较.计算结果表明提出的汽蚀模型和算法能够有效地模拟汽蚀流动中的气泡界面和气泡长度,汽蚀模型和算法的正确性和实用性得到了相应的验证.     

空化数对离心泵水力性能影响的数值研究

采用发展的Reynolds-Averaged Navier-Stokes方程和液相/气相界面跟踪方法的单相空化模型数值求解技术,开展了空化数对离心泵的水力性能影响特性的数值研究。在两种空化数下,数值预测的离心泵水力性能与试验数据具有很好的一致性,验证了数值预测方法的可靠性和实用性。研究了不同空化数下离心泵水力性能能头的变化规律和离心泵叶轮内附着空化空泡的形状变化特性。研究结果表明,离心泵内空化空泡尺寸随着空化数的降低而增加,离心叶轮附着空化空泡尺寸的增加降低了离心泵的能头,使得离心泵水力性能下降。     

变螺距高速诱导轮的汽蚀特性

为明确前置变螺距诱导轮对离心泵汽蚀性能的影响,对前置变螺距的离心泵全流道进行了数值计算。计算时基于SIMPLEC算法,采用了雷诺时均N-S控制方程和修正了的k-ε湍流模型。通过计算,得出了前置变螺距诱导轮使离心泵叶轮的整体压力得到提高的结论;得到了诱导轮与离心泵叶轮易发生汽蚀的关键位置;得到了各工况下汽蚀余量的性能曲线图。通过不带诱导轮和前置诱导轮的离心泵外特性实验,揭示了前置变螺距诱导轮提高离心泵汽蚀性能的特性;通过实验与计算结果的比较,得出了在设计点处可以用数值计算结果来预测实验结果的结论。     

激光熔蚀金属平板的三维有限体积法数值分析

为了研究气流条件下强激光对金属靶的熔蚀效应,采用有限体积方法建立了数值模型,并开发了三维Fortran计算程序。综合考虑强激光与材料耦合规律、光束能量空间分布、材料高温热物理性能以及熔蚀界面移动等关键影响因素,模拟了激光辐照下金属靶板升温、熔化和剥蚀的复杂物理过程。最后,将计算结果与试验数据进行了比较,验证了计算模型和程序的有效性。结果表明,计算模型能够反映强激光熔蚀金属平板的基本规律,熔蚀深度和后表面温度计算值与试验吻合较好,并且自编计算程序简单高效。     

冷氦气瓶放气过程建模及模型有效性验证

采用系统仿真软件AMESim,通过构建冷氦气瓶放气过程中的内、外侧换热模型及气瓶金属壁径向导热模型,数值研究了冷氦气瓶放气过程中的压力、温度特性,并采用冷氦增压系统的低温试验数据开展了仿真模型的有效性验证,得到了以下结论:所提出的计算模型相比绝热放气模型更能反映实际的放气过程,尤其对于放气过程的末期,绝热放气模型的气瓶内介质温度已远远偏离真实过程,而当前模型则很好的与试验数据相一致。     

基于电场数值加权的跨介质元胞共形新技术

针对时域有限差分法处理跨介质元胞现有共形技术中存在误差大、平行方向未被包含等问题, 提出一种利用电场数值加权法来进行跨介质元胞共形计算的新算法. 该算法不再对介电常数ε做加权平均处理, 而是根据中值定理对时域有限差分法真正的求解分量电场强度进行权值选择, 并将权值乘入积分路径中. 该方法同时考虑了跨介质安培环路元胞与跨介质法拉第环路元胞对结果带来的不连续影响, 而且对介质界面与元胞中场分量的各种位置关系均能应用, 通用性强、精度高且易于实现. 利用介质填充圆波导作为数值模型来进行理论数值计算和仿真验证, 比较不同共形方法的二维TE模式的特征根与理论值的偏差以及由共形技术所带来的各向相异性. 数值结果表明, 本文所提出算法求得的特征根最接近理论值, 而且造成的各向相异性更小, 从而验证了该算法在处理跨介质元胞时的有效性. 关键词： 时域有限差分 电场数值加权法 二维TE模式 跨介质元胞     

低温液氧箱体自由界面晃动动态响应

采用计算流体力学(CFD)技术数值研究低温液氧贮箱在外部晃动激励下箱内气液界面动态响应。计算中详细考虑了外部漏热以及气液相间相变对箱体热力耦合过程的影响,通过用户自定义程序将外部正弦激励加载到低温贮箱壁面作为动量边界,采用流体体积(VOF)方法精确捕捉晃动过程气液界面波动变化。通过与相关晃动实验结果对比,验证了本文所构建数值模型的有效性。基于所构建数值模型,对流体晃动进行数值模拟,获得了外部正弦激励下,箱体内部气液相分布以及界面形状变化;通过设置动态监测点,分析了气液界面晃动动态响应。结果表明,流体晃动对低温贮箱内部气液界面动态响应具有较大影响。为抑制流体大幅波动,需采取合适的防晃措施。     

三维溶质枝晶生长数值模拟

建立了在低Péclet数条件下三维溶质枝晶生长的数值模拟模型.该模型采用Zhu和Stefanescu 提出的溶质平衡方法,即根据固/液界面的平衡浓度和实际浓度之差计算固/液界面演化的驱动力.界面的平衡浓度由界面温度和曲率所确定,实际浓度通过采用有限差分法对溶质扩散控制方程进行数值求解而获得.该方法能够合理定量地描述枝晶从初始的非稳态到稳态的生长过程,并且具有较高的计算效率.为了描述具有不同晶体学取向的三维枝晶生长,提出了一种权值平均曲率算法用于计算固/液界面的曲率,在权值平均曲率的算法中耦合了界面能各向异性的因素.该算法简单易实现,并易于从二维推广到三维系统.为了对模型进行验证,将模拟的枝晶尖端稳态生长数据和理论模型的预测结果进行了比较.结果表明,模拟的Al-2wt%Cu合金枝晶尖端稳态生长速率和半径随过冷度的变化接近于Lipton-Glicksman-Kurz解析模型的预测结果.模拟分析了稳态枝晶尖端的形貌,发现三维枝晶尖端是非轴对称的,以四次对称的方式偏离旋转抛物面.最后,应用所建立的模型模拟出具有发达分枝和不同晶体学取向的三维等轴多枝晶生长形貌. 关键词： 微观组织模拟 溶质枝晶生长 权值平均曲率 三维     

退火铝合金中Portevin-Le Chatelier效应的数值模拟研究

通过综合考虑沉淀动力学和动态应变时效机理，建立了一套具有明确物理内涵的唯象本构模型.并在对不同加载应变率下的单轴拉伸实验的数值模拟中，得到了3种类型的Portevin-Le Chatelier效应应力曲线.计算结果与实验数据的较好吻合，验证了该模型的有效性. 关键词： Portevin-Le Chatelier效应 动态应变时效 数值模拟     

声品质成对比较主观评价的数据检验及判据

针对声品质主观评价成对比较法试验结果统计分析中,采用Kendall一致性系数进行数据有效性判断的不足,从检验成对比较试验数据中存在的相同声事件、不同回放次序以及三角循环等三种误判出发,分析了评价试验的三角循环误判的发生情况,提出了三角循环误判次数的准确计算方法。在成对比较评价试验数据有效性检验中,综合考虑了三种误判情形,提出了判断数据有效性的计权一致性系数判据。最后,将数据检验方法和判据应用于车内噪声的声品质成对比较评价数据的分析,获得了分析结果与Bradley-Terry模型拟合结果较好的相关性,验证了判据的可靠性。     

基于汽相/液相界面跟踪方法的弯管内空化流动的数值预测

采用发展的基于Reynolds-Averaged Navier-Stokes方程和液相/气相界面跟踪方法的单相空化模型和数值迭代算法,数值预测了二维弯管内空化卒泡的形状和相应的压力系数分布.将发展的二维空化模型和算法推广到i维空化流动的数值模拟,预测了三维非对称弯管内的三维空泡形状和流动特性.数值预测三维非对称弯管内空化流动时的卒泡位置和相应压力系数分布符合理论分析结论.研究结果证明了所发展的空化模型和算法能够应用于三维空化流动时的数值预测.     

Numerical 3D flow simulation of ultrasonic horns with attached cavitation structures and assessment of flow aggressiveness and cavitation erosion sensitive wall zones

As a contribution to a better understanding of cavitation erosion mechanisms, a compressible inviscid finite volume flow solver with barotropic homogeneous liquid–vapor mixture cavitation model is applied to ultrasonic horn set-ups with and without stationary specimen, that exhibit attached cavitation at the horn tip. Void collapses and shock waves, which are closely related to cavitation erosion, are resolved. The computational results are compared to hydrophone, shadowgraphy and erosion test data. At the horn tip, vapor volume and topology, subharmonic oscillation frequency as well as the amplitude of propagating pressure waves are in good agreement with experimental data. For the evaluation of flow aggressiveness and the assessment of erosion sensitive wall zones, statistical analyses of wall loads and of the multiplicity of distinct collapses in wall-adjacent flow regions are applied to the horn tip and the stationary specimen. An a posteriori projection of load collectives, i.e. cumulative collapse rate vs. collapse pressure, onto a reference grid eliminates the grid dependency effectively for attached cavitation at the horn tip, whereas a significant grid dependency remains at the stationary specimen. The load collectives show an exponential decrease towards higher collapse pressures. Erosion sensitive wall zones are well predicted for both, horn tip and stationary specimen, and load profiles are in good qualitative agreement with measured topography profiles of eroded duplex stainless steel samples after long-term runs. For the considered amplitude and gap width according to ASTM G32-10 standard, the analysis of load collectives reveals that the distinctive erosive ring shape at the horn tip can be attributed to frequent breakdown and re-development of a small portion of the tip-attached cavity. This partial breakdown of the attached cavity repeats at each driving cycle and is associated with relatively moderate collapse peak pressures, whereas the stationary specimen is rather unfrequently stressed at the end of each subharmonic oscillation cycle by the violent collapse of the complete cavity.     

Modeling cavitation in a rapidly changing pressure field – Application to a small ultrasonic horn

Ultrasonic horn transducers are frequently used in applications of acoustic cavitation in liquids. It has been observed that if the horn tip is sufficiently small and driven at high amplitude, cavitation is very strong, and the tip can be covered entirely by the gas/vapor phase for longer time intervals. A peculiar dynamics of the attached cavity can emerge with expansion and collapse at a self-generated frequency in the subharmonic range, i.e. below the acoustic driving frequency. The term “acoustic supercavitation” was proposed for this type of cavitation Žnidarčič et al. (2014) [1].We tested several established hydrodynamic cavitation models on this problem, but none of them was able to correctly predict the flow features. As a specific characteristic of such acoustic cavitation problems lies in the rapidly changing driving pressures, we present an improved approach to cavitation modeling, which does not neglect the second derivatives in the Rayleigh–Plesset equation. Comparison with measurements of acoustic supercavitation at an ultrasonic horn of 20 kHz frequency revealed a good agreement in terms of cavity dynamics, cavity volume and emitted pressure pulsations.The newly developed cavitation model is particularly suited for simulation of cavitating flow in highly fluctuating driving pressure fields.     

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.     

Optical breakdown in a liquid: The slow phase of the dynamics of cavity collapse and a noncontact technique for pressure measurement in a liquid

It is demonstrated that, in the case of optically probing the breakdown region, a pulsed signal is formed due to the acoustic pulses produced by the breakdown and collapse of the cavitation cavity. The measured dependence of the delay time of the second pulse on the hydrostatic pressure in the liquid and the pressure of saturated vapor agrees well with the theoretical dependence corresponding to the hydrodynamic model of the cavity. On the basis of the optical cavitation effect, a technique is proposed for a noncontact measurement of hydrostatic pressure in a liquid enclosed in a sealed cell. The range of applicability of the technique is evaluated.     

Laser vaporisation of absorbing liquid under transparent cover

Vaporization of absorbing liquid (water) under a transparent solid cover upon exposure to nanosecond pulses of a holmium laser (λ = 2920 nm) is studied using acoustic and optical diagnostics. The features of the optical signal reflected from the liquid–cover interface suggest that a vapor cavity appears at a submicrometer distance from this interface and exists for about one hundred microseconds. An additional acoustic signal appearing after returning the light signal to the initial level is caused by known cavitation effects accompanying vapor cavity fracture and collapse in liquid.     

Some aspects of the design of sonochemical reactors

The magnitudes of collapse pressures and temperatures as well as the number of free radicals generated at the end of cavitation events are strongly dependent on the operating parameters of the equipment namely, intensity and frequency of irradiation along with the geometrical arrangement of the transducers and the liquid phase physicochemical properties, which affect the initial size of the nuclei and the nucleation process. In the present work, the effect of these parameters on the collapse pressure generated and the maximum size of the cavity during the cavitation phenomena have been studied using the bubble dynamics equation, which considers the compressibility of the medium and a single bubble in isolation. The different liquid phase properties considered include, liquid vapor pressure, viscosity, bulk liquid temperature, surface tension and nature of dissolved gases (polytropic constant of the gas). The theoretical predictions have been also compared with the experimental results observed in the literature qualitatively and some recommendations have been made for the selection of the operating parameters so as to achieve maximum benefits. The work presented here is novel in sense that no earlier studies have considered the compressibility of the liquid medium and tried to evaluate the effect of all the operating parameters on the cavitational activity.     

轴流式水力机械转轮中局限空化流动的面元奇点解法

轴流式水力机械转轮中局限空化流动的面元奇点解法刘四清，常近时（北京农业工程大学水利与建筑工程系北京１０００８３）关键词：空化，水力机械，转轮。ＳＯＬＵＴＩＯＮＴＯＰＡＲＴＩＡＬＣＡＶＩＴＡＴＩＯＮＦＬＯＷＩＮＩＭＰＥＬＬＥＲＯＦＡＸＩＡＬ－ＦＬＯＷＨ...     

基于高速摄像实验的开放腔体圆柱壳入水空泡流动研究

基于高速摄像方法,针对入水空泡流动特征和机理,进行了开放腔体圆柱壳垂直入水实验研究.通过对实验现象的观测,发现开放腔体圆柱壳入水运动会形成波动流动和云化流动两种流动方式,结合影像数据,分别描述了两种流动状态下的空泡形态特征,并获得了空泡波动参数的变化规律;对比不同入水速度实验,分析了入水速度对入水空泡流动方式和流动参数的影响;依据流体力学基本理论,分析了入水空泡波动和云化现象的形成机理.结果表明:随入水速度增加,入水空泡依次呈现波动和云化两种流动状态,波动频率与入水速度无关,闭合发生时间随入水速度增加而减小,与Froude数呈线性关系;入水导致开放空腔内部气体涨缩,引起开放端压力场和速度场周期性扰动,空泡截面扩展程度出现差异,形成空泡波动现象;空泡闭合后尾部形成回射流,回射流触及空泡壁面引起壁面流动转捩,形成空泡云化现象.