PIV analysis and high-speed photographic observation of cavitating flow field behind circular multi-orifice plates

Based on a self-developed hydrodynamic cavitation device with different geometric parameters for circular multi-orifice plates,turbulence characteristics of cavitating flow behind multi-orifice plates,including the effects of orifice number and orifice layout on longitudinal velocity,turbulence intensity,and Reynolds stress,were measured with the particle image velocimetry(PIV)technique.Flow regimes of the cavitating flow were also observed with high-speed photography.The experimental results showed the following:(1)high-velocity multiple cavitating jets occurred behind the multi-orifice plates,and the cavitating flow fields were characterized by topological structures;(2)the longitudinal velocity at each cross-section exhibited a sawtooth-like distribution close to the multi-orifice plate,and each sawtooth indicated one jet issuing from one orifice;(3)there were similar magnitudes and forms for the longitudinal and vertical turbulence intensities at the same cross-section;(4)the variation in amplitude of Reynolds stress increased with an increase in orifice number;and(5)the cavitation clouds in the flow fields became denser with the increase in orifice number,and the clouds generated by the staggered layout of orifices were greater in number than those generated by the checkerboard-type one for the same orifice number.The experimental results can be used to analyze the mechanism of killing pathogenic microorganisms through hydrodynamic cavitation.

Analysis of droplet transfer of pulsed MIG welding based on electrical signal and high-speed photography

In order to study how welding parameters affect welding quality and droplet transfer, a synchronous acquisition and analysis system is established to acquire and analyze electrical signal and instantaneous images of droplet transfer simultaneously, which is based on a self-developed soft-switching inverter. On the one hand, welding current and voltage signals are acquired and analyzed by a self-developed dynamic wavelet analyzer. On the other hand, images are filtered and optimized after they are captured by high-speed camera. The results show that instantaneous waveforms and statistical data of electrical signal contribute to make an overall assessment of welding quality, and that optimized high-speed images allow a visual and clear observation of droplet transfer process. The analysis of both waveforms and images leads to a further research on droplet transfer mechanism and provides a basis for precise control of droplet transfer.

The effect of activating fluxes on the cathode spots in the activating TIG welding

The cathode spots are a common phenomenon in the TIG(tungsten inert gas)welding process.However,it is rarely observed in the activating TIG welding process.This research is mainly focused on the effect of activating flux on cathode spots in the activating TIG welding.The characteristics and behaviors of cathode spots were investigated in activating TIG welding by the high-speed camera and the spectrograph.Three kinds of oxide(TiO_(2),SiO_(2),MnO_(2))and two halide(MnCl_(2),CaF_(2))activating fluxes are used in the activating TIG welding process.The results show that differ from the TIG welding,the oxide activating flux increases the number of cathode spots and decreases the velocity.The effect is the opposite for the halide activating flux.Moreover,the number of spots no longer varies with the current except TiO2 activating flux.As the temperature of the weld pool surface increases the spot moves away from the center.But this rule is not valid when silica and manganese compounds activating fluxes are used.The variation of cathode spots is caused by the oxide film reformed and the distribution of weld slag.The formation mechanism of cathode spots might be the impact of ions on the cathode surface and the strong electric field formed near the cathode surface.

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.

Measurement and analysis of restitution coefficient between maize seed and soil based on high-speed photography

The restitution coefficient is an important elementary physical parameter related to the research and development of agricultural machinery.The kinematic model of maize seed in the falling and impacting processes was developed to measure the restitution coefficient between maize seed and soil.A test bench for measuring the restitution coefficient was designed and built referred to the theory of mirror reflection.The velocities for impacting maize seed were measured and analyzed in a three-dimensional space via high-speed photography,and then restitution coefficients of in different impact conditions were obtained.On this basis,this study took flat dent seed and round seed as samples.Single factor tests were conducted to analyze the influences of these factors on the restitution coefficient.The impact angle,falling height,soil compaction,soil moisture,maize moisture content and different parts of seed were selected as test factors.The corresponding regression equations were obtained by analysis.The results showed that,as the impact angle was bigger than 25°,the restitution coefficient increased with the increase of impact angle.The restitution coefficient had a linear decreasing trend with the increase of falling height.As the soil compaction strength was 200-350 kPa,the restitution coefficient increased with the increase of soil compaction.As the soil compaction strength was larger than 350 kPa,the changing trend of the restitution coefficient was relatively stable.As the soil moisture content was 13.5%-18%,the restitution coefficient decreased with the increase of soil moisture.As the soil moisture content was 18%,the restitution coefficient was the minimum.As the maize moisture content was 11%-16%,the restitution coefficient decreased with the increase of maize moisture content.The rotational motion always occurred in falling process of flat dent seed and round seed.The probabilities of crown part and lateral part of maize seed impacting with soil were the highest,and the restitution coefficient between crown part and soil was higher than that of other parts in the same condition.

High-speed photographic observation of collapse of two cavitation bubbles

Cavitation as a hydrodynamic phenomenon exists widely in water conservancy, shipbuilding, chemical and many other industries.Previous cavitation bubble dynamic studies mainly focused on single cavitation bubbles and their interaction with the wall. This paper studies the interaction between two cavitation bubbles under conditions with or without a wall. The results show that if the inception of two cavitation bubbles is not synchronized, the cavitation bubble of early inception collapse backwards the cavitation bubble of later inception; if the inception of two cavitation bubbles is synchronized, the two bubbles collapse towards each other; if a wall exists nearby, no matter whether the line connecting the centers of the two cavitation bubbles is vertical or parallel to the wall, the two cavitation bubbles collapse towards each other and then gradually merge, and the merged collapse body quickly moves to the wall. It is suggested that, as the number of cavitation bubbles increases, the cavitation erosion effect is not simply increased proportionally. Instead, mutual inhibitory effect may be demonstrated.

Determination of the shedding frequency of cavitation cloud in a submerged cavitation jet based on high-speed photography images

To accurately determine the shedding frequency of the cavitation cloud in a submerged cavitation jet,the spectral analysis and the proper orthogonal decomposition(POD)for high-speed photography images are performed.The spectrums of 6 different kinds of image signals(the area-averaged gray level,the line-averaged gray level,the point gray level,the cavitation length,width,and area)are calculated and compared.The line-averaged gray level is found to be optimal in determining the shedding frequency but an accurate frequency can only be obtained in the stable-frequency zone where the cavitation cloud sheds.In repeated experiments,the plateau-shape distribution of the main frequency is established with a deviation of 10.8%.A revised Reynolds number Re'is defined and the shedding frequency can be correlated to Re'by a power law when the cavitation number is less than 0.02.This relationship is validated by the experimental data in literature.The first mode of the POD characterizes the ensemble-average of the cavitation cloud while the second mode is the major part of the cavitation cloud transient components.The modes 2-5 are organized in pairs,which confirms the periodic feature of the cavitation cloud in the submerged cavitation jet.Near the nozzle exit,the modes 2-5 are symmetrically distributed in the jet shear layer.The shedding frequency of the cloud cavitation can also be precisely determined by performing the spectral analysis of the weighting coefficients of the mode 2.This paper shows that the two parameters,namely,the line-averaged gray level and the weighting coefficients of the mode 2,can be confidently used to calculate the shedding frequency of the cavitation cloud in a submerged cavitation jet.

Experimental determination of restitution coefficient of garlic bulb based on high-speed photography

Restitution coefficient(RC)of garlic bulb is an important mechanical property that is required to establish the kinematics model of bulb collision and research the damage mechanism of bulb collision.In this study,kinetic equations of bulb collision were established based on Hertz's contact theory.The kinematics characteristics,elastoplastic deformation and contact damage during bulb collision were analyzed by using high-speed photography.The effects of bulb mass,moisture content,collision material,material thickness and release height on the RC were investigated by mixed orthogonal experiments and single-factor experiments.The results showed that the movement of bulb in the compression stage was translation,and the movement in the rebound stage was translation and rotation.During collision,the larger the rotational angular velocity of the bulb was,the smaller the measured RC would be.The contact damage of bulb included internal damage of the tissue,epidermis stretch and tear.The significance of effects of factors on RC decreased with the following sequence:collision material,release height,material thickness,bulb mass,and moisture content.Collision material,release height,material thickness,and bulb mass were significant factors.The RC between the bulb and Q235,nylon,and rubber decreased sequentially.The RC decreased with the increase of release height and bulb mass.The RC increased with the increase of material thickness of Q235,while it decreased with the increase of material thickness of rubber or Nylon.The determination coefficients of the regression equations between the significant factors and the RC were all greater than 0.96.The results will be helpful for damage mechanism analysis and design of garlic production equipment.

Experimental investigation on de-icing by an array of impact rod-type plasma synthetic jets

Since flight accidents due to aircraft icing occur from time to time,this paper proposes an array of impact rod-type plasma synthetic jet de-icing methods for aircraft icing problems.The impact rod-type plasma synthetic jet actuator(PSJA)is based on the traditional PSJA with an additional impact rod structure for better de-icing in the flight environment.In this work,we first optimize the ice-breaking performance of a single-impact rod-type PSJA,and then conduct an array of impact rod-type plasma synthetic jet ice-breaking experiments to investigate the relationship between crack expansion and discharge energy,ice thickness and group spacing.The results show that the impact force and impulse of a single-impact rod-type PSJA are proportional to the discharge energy,and there exists a threshold energy Qmin for a single actuator to break the ice,which is proportional to the ice thickness.Only when the discharge energy reaches above Qmin can the ice layer produce cracks,and at the same time,the maximum radial crack length produced during the ice-breaking process is proportional to the discharge energy.When the ice is broken by an array of impact rod PSJAs,the discharge energy and group spacing together determine whether the crack can be extended to the middle region of the actuator.When the group spacing is certain,increasing the energy can increase the intersection of cracks in the middle region,and the ice-fragmentation degree is increased and the ice-breaking effect is better.At the same time,the energy estimation method of ice breaking by an array of impact rod-type PSJAs is proposed according to the law when a single actuator is breaking ice.

Research on cutting characteristics of fiber bundle with high-speed photography

Cutting is an essential and complicated process in many fields.Efficient and low-consumption cutting operations are of great significance for environmental protection and energy conservation.The development of high performance cutting parts relies on a deep understanding of the cutting process and cutting mechanism.In this research,a new type of cutting test bench with high-speed photography was developed,and the cutting tests were conducted on the jute fiber bundle from quasi-static cutting at 10 mm/s to dynamic cutting in the speed range of 0.6-2.4 m/s.The cutting process was captured by a high-speed camera.Analysis shows that compression exists before quasi-static cutting,and the compression force curve with respect to the compression ratio follows an exponential function.The cutting speed has a significant effect on cutting energy.The cutting energy consumption is not a monotonous function of cutting speed owing to the combined effect of elastic deformation and friction of fibers.The cutting energy increases with increasing cutting speed in the range of 0.6-1.2 m/s due to the increase of the friction within fibers and the friction between the blade and fibers.The cutting energy decreases with increasing cutting speed in the range of 1.2-1.8 m/s,and tends to be a fixed value when the cutting speed exceeds 1.8 m/s due to the stabilized elastic deformation and friction coefficient.From the perspective of energy saving,it is meaningless to increase the blade speed excessively when cutting fiber bundles.

水下爆炸的高速摄影测试技术研究

将高速摄影技术应用于室内水下爆炸实验研究,对爆炸容器、模型安装方法及光学测量系统进行了实验设计。通过水下爆炸实验,利用高速摄影仪观测了自由场及结构近场下气泡脉动的整个过程,获得了气泡脉动周期和最大半径,将实验数据与理论经验公式进行比较后发现,结果基本吻合。最后,对高速摄影测试技术提出改进措施。

Formation, microstructure and mechanical properties of double-sided laser beam welded Ti-6Al-4V T-joint

The T-joints of Ti?6Al?4V alloy were manufactured by double-sided synchronized laser beam welding with the homologous filler wire. The formation, microstructure and mechanical properties of welded joints as well as the correlations of each other were investigated. The results indicate that the quality of weld seams is good without defects such as discontinuity, beading, visible cracks or porosity, which is linked to the steady molten pool behavior and droplet transition. The morphologies of the heat affected zone (HAZ) located on the skin and stringer are disparate. The microstructure of the HAZ and fusion zone (FZ) is mainly comprised of acicular martensiticα′ phases. The microhardness of the HAZ and FZ is higher than that of the base metal (BM) and reaches a maximum value at the HAZ near FZ on the stringer. The tensile specimens along the skin and stringer fractured at the BM with ductile fracture surfaces.

水下爆炸气泡射流现象的试验研究

水下爆炸气泡塌陷产生的射流载荷会对舰船结构造成严重地毁伤,而射流现象又十分复杂.为研究水下爆炸气泡射流的产生条件以及载荷形式,文章设计了一系列的试验,通过高速摄影观测气泡运动过程,同时利用压力传感器对气泡周围的压力场进行记录.将试验结果与经典的Blake判断准则进行了比较分析,提出了气泡产生冲向靶板的射流条件.同时通过对试验中测量到的压力-时间曲线进行分析,可以看出射流的产生对气泡载荷的压力、冲量、能量都产生了很大影响,从能量的角度提出射流现象将使气泡能量集中,并对其正前方结构可能产生更大的毁伤.在分析试验结果的基础上分析了射流产生的机理,得到了不同引力和浮力方向情况下的射流形成条件和射流形成的载荷形式,为舰船结构抗爆研究提供参考.

实验室尺度水下爆炸气泡实验方法

为研究水下爆炸气泡的动态特性,建立了实验室尺度水下爆炸气泡实验方法,该方法采用高速摄影技术,在2 m×2 m×2 m的水箱中进行,分别从定性和定量的角度分析了水箱边界影响产生的的误差。研究了小当量PETN的水下爆炸气泡现象,高速摄影的速度控制在4000～5000s-1,精度控制在1%以内。实验结果表明:随着药量的增加,水箱边界的影响逐渐增加。在进行实验室尺度水下爆炸气泡实验时,为尽量减小边界对气泡运动产生的影响,需要控制使用的药量。

高雷诺数竖壁降膜流动特性的数值研究

采用流体体积法(VOF)对于高雷诺数竖壁降膜流场进行了数值模拟,分析了降水膜速度分布、温度分布、波速及平均水膜厚度等特性参数的变化规律。计算结果表明,随着流动向下游发展,平均水膜厚度先是迅速减小,然后减小速度趋缓,最后趋于一恒定值;当初始水膜厚度一定时,随着初始雷诺数的增大,平均水膜厚度逐渐增大,水膜波动的振幅逐渐减小,但水膜稳定性变差。采用VOF方法的模拟结果与高速摄影实验数据符合良好,说明VOF方法准确描述了绝大部分液膜流动特性,在总体上对于高雷诺数竖壁降膜流场进行了准确的预测。

UNDERWATER ACOUSTICS AND CAVITATING FLOW OF WATER ENTRY

The fluid mechanics of water entry is studied through investigating the underwater acoustics and the supercavitation.Underwater acoustic signals in water entry are extensively measured at about 30 different positions by using a PVDF needle hydrophone.From the measurements we obtain (1)the primary shock wave caused by the impact of the blunt body on free surface;(2)the vapor pressure inside the cavity;(3)the secondary shock wave caused by pulling away of the cavity from free surface;and so on.The supercavitation induced by the blunt body is observed by using a digital high-speed video camera as well as the single shot photography.The periodic and 3 dimensional motion of the supercavitation is revealed.The experiment is carried out at room temperature.

Failure characteristics of high stress rock induced by impact disturbance under confining pressure unloading

The failure characteristics under coupled static and dynamic loading were investigated by the improved split Hopkinson pressure bar （SHPB） with axial pre-pressure and confining pressure. The results show that the stress—strain curve of the rock under static-dynamic coupled loading is a typical class I curve when the dynamic load is comparatively high; With the decrease of the dynamic load, the stress—strain curve transforms to a typical class II curve. The dynamic failure process was recorded by high-speed photography. Analyses of fracture surface morphology show that the failure modes of specimens are tensile failure or combined shear failure when the impact load energy is low, but the failure modes of specimens become tensile failure when the impact load energy is high. The results of fractal dimension show that the elastic potential energy release leads to increase in the degree of crushing of samples when the energy of impact load is low under coupled static and dynamic loads with high stress.

Numerical and Experimental Study on Nugget Formation Process in Resistance Spot Welding of Aluminum Alloy

The weld nugget formation in the resistance spot welding(RSW) of aluminum alloy was investigated in the present study. The nugget formation process was directly observed by using a digital high-speed camera. Numerical simulation was also employed to investigate the nugget formation process. The results showed that for the RSW of two aluminum alloy sheets, a nugget was first formed in the workpiece/workpiece(W/W) interface and grew along the radial direction and axial direction of the sheets, and then it became a large elliptical nugget. For the RSW of three aluminum alloy sheets, two small nuggets were firstly formed in two W/W interfaces and grew along the axial direction and radial direction; finally they fused into one nugget. Besides, there existed a critical welding time, after which the nugget size remained nearly unchanged. This indicates that a long welding time is unnecessary for the RSW of aluminum alloy. In addition, the calculated nugget radius was compared with the experimental results, which showed that the simulation results agreed well with the experimental results.

Experimental investigations of mechanical and reaction responses for drop-weight impacted energetic particles

Low-velocity drop-weight impact experiments on individual and multiple Cyclotetramethylene tetranitramine (HMX) energetic particles were performed using a modified drop-weight machine equipped with high-speed photography components. Multiple particles experienced more severe burning reactions than an individual particle. Comparisons between impacted salt and HMX particle show that jetting in HMX is mainly due to the motion of fragmented particles driven by gaseous reaction products. Velocity of jetting, flame propagation, and area expansion were measured via image processing, making it possible to quantify the chemical reaction or mechanical deformation violence at different stages.

Detonation Shock Dynamics Relations for RDX-TNT Based Cast Explosives

RDX and TNT based explosives are very useful in performing research and development works especially when complex shaping of an assembly is required due to their molding capability and the range of ratios in which they can be mixed. In this paper, for these compositions, detonation shock dynamics （DSD） relations between normal detonation velocity D. and detonation wave curvature κ are determined for RDX： TNT explosives in weight ratios 40：60, 50：50 and 60：40. Experiments are performed with 50 mm diameter rate sticks of approximately 200 mm length using high speed rotating mirror camera and time measurement probes. The results show that first order DSD relation is indeed representative of these explosives. The slope of this relation increases by increasing TNT contents in these explosives. Going from 40% to 60% TNT, the data show an increase of more than 100% in negative slope of Dn-κ relation.