水下超声速气体射流回击现象的实验研究

对水下超声速气体射流的动力学行为进行了实验研究.通过流动可视化揭示了回击现象的演化过程,利用探针排获得了射流近场区的脉动压力分布,实验结果表明:在超声速喷管出口两倍直径处,射流形貌的变化导致气体中出现了大幅值压力脉冲.通过流场可视化与压力测量的同步校验,证实了喷口端面处回击事件与流场气相区中压力脉动之间相关性.     

水下欠膨胀高速气体射流的实验研究

采用实验途径研究了下水高速气体射流的动力学特性,研制了水下高速气体射流实验系统并发展了相应的测试手段。实验中,用插入式静压探针测量了射流轴线静压分布;用γ射线衰减法测量了径向空隙率分布,从而揭示了水下高速气体射流均压和掺混两个过程的基本规律。测量结果表明：水下高速气体射流在欠膨胀工况下运行时,近场将出现含有复杂波系结构的膨胀压缩区域,由于气水的掺混作用,水下欠膨胀气体射流均压化过程比空气中衰减得快。测量结果还表明,水下射流在近场区的混合层由气水两相占据,其流态从靠近气体侧的液滴流型过渡到靠近液体侧的气泡流型。     

环形喷管喷口气泡演化的实验研究

水下气泡的发展演化及气泡动力学行为是气液两相动力学的基础理论与水下射流应用的重要基础.环形喷管/喷口形成的气泡及气体射流具有其不同于圆孔实心射流的特殊表现与规律机制,随着同心筒破水发射等特殊应用的出现,环形喷口气体射流/泡流的基础现象观测和机制分析成为迫切的需求.基于环形喷管的设计和水下射流条件的分析,设计建立了一套环形喷管水箱实验系统,对水下环形喷管喷口气泡发展演化过程进行了初步的实验研究.为观测研究气体通过环形喷管气泡生长发展过程,在较低压力、较低流速下,采用高速摄影仪记录气泡生长及发展演化过程.结合对气泡发展演化过程的图像处理与分析,研究分析了环形喷口气泡形成区制、气泡生长过程形态发展特点、以及气泡形成时间及气泡体积变化特点.研究表明:在本实验气体流量范围内(50.8～237.3 dm~3/min),环形喷口气泡发展演化过程呈现较为明显的三周期区制,前泡尾流影响是环形气泡呈三周期区制的主要原因;不同周期内的气泡形成时间具有较稳定规律,并受到流量影响;气泡生长过程中有较为明显的下沉、回升特征;气泡表面张力、液体惯性与流动的共同作用,造成了典型的气泡顶部坍塌现象.     

脉冲液体射流泵能量平衡试验研究

根据脉冲液体射流泵主要流动部件的能量损失压力比,对其能量平衡进行试验,研究了主要过流部件的能量转化与脉冲频率之间的关系。与恒定液体射流泵试验数据进行对比,证明了脉冲频率、脉冲装置对脉冲液体射流泵的主要流动部件的能量损失压力比有一定的影响;且脉冲射流是提高射流泵传能及传质效率的有效途径。     

水下爆炸中气泡射流壁压特性实验研究

水下爆炸气泡射流载荷是中近场水下爆炸壁压载荷的重要组成部分, 将水下爆炸气泡射流简化为一段高速水柱来研究水下爆炸气泡射流载荷特性是研究水下爆炸气泡射流载荷的主要手段。本文基于腔内爆炸提出了一种新的高速水射流实验方法，并给出了实验装置设计、实验方法以及实验系统。基于实验系统，开展了不同工况下高速水射流的实验研究，研究了腔口位置、腔深对水射流形态的影响，并对水射流的形态形成因素进行了分析。使用压电型壁压传感器测得了水射流冲击壁压，给出了水射流冲击壁压的特性及其特点。实验结果表明:腔口位置与腔深是影响水射流端面形态的重要因素；生成的高速水射流冲击壁压峰值满足水锤理论。基于腔内爆炸的高速水射流实验方法能够应用于包括水下爆炸气泡射流在内的高速水射流形态、壁压特性的研究。     

圆柱形充液室中4股贴壁燃气射流扩展特性的实验研究

为了探索高温高压周向均布4股贴壁燃气射流在受限空间中的扩展特性，设计了贴壁燃气射流在圆柱形充液室内扩展的实验装置，借助数字高速录像系统，观察了4股贴壁燃气射流在充液室中的扩展过程，发现由Kelvin-Helmholtz不稳定性引起的表面不规则一直存在于整个射流扩展过程；通过处理拍摄记录的射流扩展序列图，获得不同时刻射流扩展的轴向和径向位移; 对比了不同破膜喷射压力和喷孔结构参数对4股贴壁燃气射流扩展过程的影响。实验结果表明:喷孔面积越大，贴壁射流初期轴向扩展速度越大，但由于径向扩展达到交汇的时间较早，湍流掺混和干涉强烈，衰减也越快；破膜喷射压力越高，射流径向扩展到达交汇的时间越短; 破膜喷射压力从12 MPa升高到20 MPa，射流轴向扩展速度大幅增加，气液湍流掺混效应增强。     

脉冲磨料射流中球泡溃灭特性研究及数值分析

从理论上研究了脉冲磨料射流中空化球泡的溃灭特性,建立了脉冲磨料射流中空化球泡的动力学方程,数值计算分析了磨料浓度、磨料粒径、射流振荡频率和幅值以及流场压力对脉冲磨料射流中空泡溃灭过程的影响规律。研究表明,脉冲磨料射流中磨料的存在将增大空泡溃灭历时,从而减弱射流的空蚀破坏能力,其中磨料的粒径、浓度和流场压力影响幅度较大。     

交变电场作用下单液滴蒸发的分子动力学模拟

利用分子动力学方法研究了正弦形式的交变电场对三维悬浮水滴在超临界氮气环境下蒸发特性的影响, 主要考虑了电场幅值和频率对液滴蒸发寿命和液滴瞬时蒸发速率的影响. 其中水滴由8000个水分子组成, 环境气体由27000个氮气分子组成. 首先利用分子动力学方法模拟计算了不同状态下水的物性参数以及亚临界条件下匀强电场对液滴蒸发特性的影响, 从而验证了分子模型和蒸发模型的正确性. 接着模拟了在不同幅值和频率的交变电场作用下水滴在氮气环境下的蒸发过程, 结果表明, 相比于无电场或匀强电场, 交变电场能够更显著地促进水滴的蒸发. 在频率一定时, 随着电场幅值的增大, 液滴的蒸发速率不断升高, 蒸发寿命不断下降, 且液滴的瞬时蒸发速率、液滴温度、水分子的排列结构等参数都会产生频率为所加电场二倍的振荡特性, 且电场幅值越大, 振荡幅值也越大. 而在电场幅值一定时, 随着频率的增大, 液滴蒸发寿命和速率并不是单调变化的, 而是在频率$f=5$GHz时, 分别达到一个极大值和极小值, 文中从液滴能量和分子排列结构两个方面解释了产生了这一现象的原因.      

可压缩旋转气体中超空化射流的热稳定性

液体射流热稳定性研究是对射流稳定性问题的更深层次的探讨,可以进一步加深对液体射流分裂与雾化机理的认识,具有重要的学术意义和工程应用价值. 基于射流稳定性理论,在同时考虑射流周围气体旋转、射流和周围气体可压缩性以及射流液体中含空化气泡的条件下,建立了描述可压缩旋转气体中超空化射流热稳定性的数学模型,并对数学模型及其求解方法进行了验证分析;在此基础上,分析了液体射流表面与周围气体间温差及射流内部温度梯度同时作用下对射流稳定性的影响;并进一步探讨了超空化射流的热稳定性. 结果表明,射流表面扰动波的最大扰动增长率、最不稳定频率以及最大扰动波数皆随气液温差的增大呈近似线性增大趋势;射流内部温度梯度的存在使得气液温差对射流的失稳作用更加显著;射流内部温度梯度会抑制超空化对射流稳定性的影响,但气液温差会在一定程度上促进超空化对射流的失稳作用.      

可压缩旋转气体中超空化射流的热稳定性

液体射流热稳定性研究是对射流稳定性问题的更深层次的探讨,可以进一步加深对液体射流分裂与雾化机理的认识,具有重要的学术意义和工程应用价值.基于射流稳定性理论,在同时考虑射流周围气体旋转、射流和周围气体可压缩性以及射流液体中含空化气泡的条件下,建立了描述可压缩旋转气体中超空化射流热稳定性的数学模型,并对数学模型及其求解方法进行了验证分析;在此基础上,分析了液体射流表面与周围气体间温差及射流内部温度梯度同时作用下对射流稳定性的影响;并进一步探讨了超空化射流的热稳定性.结果表明,射流表面扰动波的最大扰动增长率、最不稳定频率以及最大扰动波数皆随气液温差的增大呈近似线性增大趋势;射流内部温度梯度的存在使得气液温差对射流的失稳作用更加显著;射流内部温度梯度会抑制超空化对射流稳定性的影响,但气液温差会在一定程度上促进超空化对射流的失稳作用.     

Oscillation flow induced by underwater supersonic gas jets

This paper describes an experimental study on the oscillation flow characteristics of submerged supersonic gas jets issued from Laval nozzles. The flow pattern during the jet development and the jet expansion feedback phenomenon are studied using a high-speed camera and a pressure measurement system. The experimental results indicate that along the downstream distance, the jet has three flow regimes: (1) momentum jet; (2) buoyant jet; (3) plume. In the region near the nozzle exit a so-called bulge phenomenon is found. Bulging of the jet occurs many times before the more violent jet expansion feedback occurs. During the feedback process, the jet diameter can become several times that of the original one depending on the jet Mach number. The frequencies of the jet bulging and the jet expansion feedback are measured.     

Lagrangian-based investigation of multiphase flows by finite-time Lyapunov exponents

Multiphase flows are ubiquitous in our daily lifeand engineering applications.It is important to investigatethe flow structures to predict their dynamical behaviors effectively.Lagrangian coherent structures(LCS) defined bythe ridges of the finite-time Lyapunov exponent(FTLE) isutilized in this study to elucidate the multiphase interactionsin gaseous jets injected into water and time-dependent turbulent cavitation under the framework of Navier-Stokes flowcomputations.For the gaseous jets injected into water,the highlightedphenomena of the jet transportation can be observed by theLCS method,including expansion,bulge,necking/breaking,and back-attack.Besides,the observation of the LCS revealsthat the back-attack phenomenon arises from the fact that theinjected gas has difficulties to move toward downstream region after the necking/breaking.For the turbulent cavitatingflow,the ridge of the FTLE field can form a LCS to capturethe front and boundary of the re-entraint jet when the adverse pressure gradient is strong enough.It represents a barrier between particles trapped inside the circulation regionand those moving downstream.The results indicate that theFTLE field has the potential to identify the structures of multiphase flows,and the LCS can capture the interface/barrieror the vortex/circulation region.     

Effect of Condensation on the Length of Strongly Underexpanded Jets Exhausting Into a Rarefied Submerged Space

Exhaustion of supersonic argon and nitrogen jets through sonic and supersonic nozzles into a rarefied submerged space at high stagnation pressures is studied experimentally. The shapes and lengths of the jets are visualized by means of detecting radiation excited in the considered flow by an electron beam. Dependences of the geometric parameters of the jets on exhaustion and clusterization conditions at low Reynolds numbers based on the reference length of the jet are obtained. It is found that the coefficient of proportionality between the length of the first “barrel” of the supersonic jet and the degree of jet expansion increases with an increase in the stagnation pressure. Empirical dependences of the proportionality coefficient on the size of clusters formed in supersonic flows are derived for the first time.     

Ultrasonic jet in an external acoustic field

The interaction of supersonic jets with external acoustic waves is investigated in connection with the emission of sound of discrete frequency by the jets. A plausible physical scheme explaining the appearance and maintenance of the oscillations of supersonic jets with discrete frequency was proposed in [1]. A model problem of the effect of pressure perturbations of a given frequency, traveling along the surface of a two-dimensional jet is also investigated there. The results of the solution of this problem (in particular, the presence of critical frequencies at which the perturbations in the jet grow indefinitely in the direction of motion of the flow) substantiate the hypothesis that by virtue of its periodic (cellular) structure a supersonic jet has the properties of a resonator. In [1] the more general problem of interaction of a supersonic jet with an external acoustic field is also formulated, which is in complete correspondence with the physical scheme of the phenomena developed in that article. In the present work this problem is solved in its complete form for plane and cylindrical jets for symmetric and antisymmetric perturbations in an external acoustic field, and also in the presence of subsonic accompanying flow in the outer medium.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 105–113, March–April 1974.     

Local subsonic zones in supersonic jet flows

The results of the experimental investigation of supersonic turbulent jets with local subsonic zones of forward and reverse flow exhausting into the ambient atmosphere or an outer stream, either parallel or transverse to the jet, are presented. Some gasdynamic features of the flows containing these zones, which have not been adequately addressed in the literature, are analyzed. Thus, supersonic flows with back pressure, e.g., highly overexpanded and underexpanded jet flows, and those upstream and downstream of a jet on the leeward side of a cone in a supersonic gas stream, are studied. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 143–150, January–February, 1998.     

Visual observations of supersonic transverse jets

We present experimental results on penetration of round sonic and supersonic jets normal to a supersonic cross flow. It is found that penetration is strongly dependent on momentum ratio, weakly dependent on free-stream Mach number, and practically independent of jet Mach number, pressure ratio, and density ratio. The overall scaling of penetration is not very different from that established for subsonic jets. The flow is very unsteady, with propagating pressure waves seen emanating from the orifice of helium jets.     

Numerical Investigation of Multi-Jet Channel Flows

The results of a numerical simulation of the three-dimensional outflow of a system of circular supersonic turbulent jets into a cocurrent supersonic (or subsonic) air flow in a partially bounded region are given. Solutions are obtained by the splitting method using a matrix sweep of the parabolized Navier-Stokes equations. Assuming that the flow is nonseparated in the boundary layer, features of the three-dimensional structure of the jet system are investigated as functions of the pressure ratio number and the jet and cocurrent flow Mach numbers.     

Experimental Investigation of Nozzle Exit Reflector Effect on Supersonic Jet

The present study describes an experimental work to investigate the effect of a nozzle exit reflector on a supersonic jet that is discharged from a convergent–divergent nozzle with a design Mach number of 2.0. An annular reflector is installed at the nozzle exit and its diameter is varied. A high-quality spark schlieren optical system is used to visualize detailed jet structures with and without the reflector. Impact pressure measurement using a pitot probe is also carried out to quantify the reflector’s effect on the supersonic jet which is in the range from an over-expanded to a moderately under-expanded state. The results obtained show that for over-expanded jets, the reflector substantially increases the jet spreading rate and reduces the supersonic length of the jet, compared with moderately under-expanded jets. The reflector’s effect appears more significant in imperfectly expanded jets that have strong shock cell structures, but is negligible in correctly expanded jet.     

Flows of Supersonic Underexpanded Jets on the Range of Moderate Reynolds Numbers

The results of an experimental investigation of the gasdynamic structure of supersonic underexpanded air jets flowing out of a sonic nozzle into a low-pressure medium are presented. This setting of the experimentmakes it possible to achieve high values of the nozzle-to-ambient pressure ratio at moderate outflow Reynolds numbers characteristic of underexpanded jets issuing from micronozzles. The data on the supersonic core length, the laminar-turbulent transition location, and the jet flow characteristics are obtained. The results are compared with those obtained in microjets flowing out of sonic nozzles. Emphasis is placed on the earlier discovered effect of inverse transition of a turbulent jet into the laminar flow regime with increase in the Reynolds number.