Warhead fragments motion trajectories tracking and spatio-temporal distribution reconstruction method based on high-speed stereo photography

High speed photography technique is potentially the most effective way to measure the motion parameter of warhead fragment benefiting from its advantages of high accuracy,high resolution and high efficiency.However,it faces challenge in dense objects tracking and 3D trajectories reconstruction due to the characteristics of small size and dense distribution of fragment swarm.To address these challenges,this work presents a warhead fragments motion trajectories tracking and spatio-temporal distribution reconstruction method based on high-speed stereo photography.Firstly,background difference algorithm is utilized to extract the center and area of each fragment in the image sequence.Subsequently,a multi-object tracking(MOT)algorithm using Kalman filtering and Hungarian optimal assignment is developed to realize real-time and robust trajectories tracking of fragment swarm.To reconstruct 3D motion trajectories,a global stereo trajectories matching strategy is presented,which takes advantages of epipolar constraint and continuity constraint to correctly retrieve stereo correspondence followed by 3D trajectories refinement using polynomial fitting.Finally,the simulation and experimental results demonstrate that the proposed method can accurately track the motion trajectories and reconstruct the spatio-temporal distribution of 1.0×10^(3)fragments in a field of view(FOV)of 3.2 m×2.5 m,and the accuracy of the velocity estimation can achieve 98.6%.

Investigating Jet Appearance in Different Background Lights with High-Speed Frame Photography

Shaped charge jet formation process is studied under the conditions of different background lights by means of high speed frame photography. In order to shoot true jet appearance, the glass tube in which jet moves is vacuumized. The experiment results show that observing jet appearance with the double reflecting mirrors system is feasible as long as the vacuum of the glass tube can meet the requirement of experiment.

The technique of step acousto-optical deflection and its application in high speed interferometric photography

This paper presents the technique of step acousto-optical deflection and its application in high speed interferometric photography. When a light beam carrying the interferogram information passes through an acousto-optical deflector, it is deflected into 10 patterns (φ6 mm-φ8 mm in diameter) in time-sequence and recorded on a fixed film under the action of step pulsed ultrasonic waves. The exposure time of every pattern is adjustable within the range of 10^(-5)-3 ×10^(-6) s and the interval between two patterns is 10^(-5) s. The multi-frame interferograms of the transient combustion field of the first fire composition ignited by laser are recorded by using this method with combination of a laser interferometer, and the temperature distribution and its change with time is calculated quantitatively.

Droplet transition for plasma-MIG welding on aluminium alloys

The synchronous acquisition system of droplet image inspection and arc electric signals were established and the droplet transition characteristics of aluminum alloys were researched in the plasma-MIG welding process.Typical droplet transition modes include globular transfer mode,short circuiting transfer mode,metastable spray transfer mode and projected transfer mode.The result indicates that MIG droplet transfer frequency and droplet transfer modes are changed by introducing the plasma arc in the plasma-MIG welding process compared with the MIG welding on the aluminum alloys,which broadens the range of welding parameters when the stable welding process proceeds.The metastable spray transfer and projected transfer mode are proved to be the most optimal modes by comparing the stability of electronic signal,droplet transition,weld appearance and weld penetration.

Arc Characteristic and Metal Transfer of Pulse Current Horizontal Flux-Cored Arc Welding

In this work, we utilized high-speed video photography to investigate the arc characteristics, metal transfer behavior, and welding spatter of the pulse-current flux-cored arc welding (P-FCAW) process in the horizontal position. The results indicate the presence of a stable “flux pole” during both the pulse-on and pulse-off periods when the mean current ranged from 140 to 170 A. The existence of this “flux pole” was beneficial for droplet transfer in the “axial droplet transfer” mode. With respect to welding spatter, with an increase in the welding current, we observed three kinds of spatter—explosive spatter, rebounded droplet spatter, and scattering spatter. With an increase in the pulse current from 310.6 to 345.6 A, the deflection of the arc reduced from 30.4° to 16.6°, which positively influenced the arc rigidity, particularly in the horizontal position. © 2017, Tianjin University and Springer-Verlag Berlin Heidelberg.

Effects of a flexible net barrier on the dynamic behaviours and interception of debris flows in mountainous areas

Flexible net barriers are a new type of effective mitigation measure against debris flows in valleys and can affect the kinematic energy and mass of debris flows. Here, ten flume tests were performed to study the dynamic behaviours of debris flows with differences in volumes, concentrations(solid volume fraction), and travel distances after interception by a uniform flexible net barrier. A high-speed camera was used to monitor the whole test process, and their dynamic behaviours were recorded. A preliminary computational framework on energy conversion is proposed according to the deposition mechanisms and outflow of debris flow under the effects of the flexible net barrier. The experimental results show that the dynamic interaction process between a debris flow and the flexible net barrier can be divided into two stages:(a) the two-phase impact of the leading edge of the debris flow with the net and(b) collision and friction between the body of the debris flow and intercepted debris material. The approach velocity of a debris flow decreases sharply(a maximum of 63%) after the interception by the net barrier, and the mass ratio of the debris material being intercepted and the kinetic energy ratio of the debris material being absorbed by the net barrier are close due to the limited interception efficiency of the flexible net barrier, which is believed to be related to the flexibility. The energy ratio of outflow is relative small despite the large permeability of the flexible net barrier.

Simulation and Experimental Study of Dynamic Characteristics of a Pneumatic Actuator in Air Splicer

Splicing process parameters determined by dynamic characteristic of pneumatic actuator in air splicer have a significant influence on the performance of spliced yarn. Both gas thermodynamic and pneumatic actuator dynamic models, which were solved by the Runge-Kutta algorithm, were established to analyze the relationship among structural parameters of a pneumatic actuator and splicing process parameters such as splicing duration and gas consumption. Additionally,a visualization test bench to observe the dynamics of the pneumatic actuator and a mass flow measurement system to track splicing duration and gas consumption were designed. Comparisons between experimental data and simulation results show that the mathematical model accurately accounts for the dynamic characteristics of the pneumatic actuator,and consequently predicts splicing process parameters, which provides a theoretical foundation for the design optimization of air splicer.

Laser induced self-propagating high-temperature synthesis of TiNi alloy

TiNi alloy, especially porous TiNi, a good biocompatible material, can be made by laser induced self-propagating high-temperature synthesis (SHS). A 40-W CO2 laser was used to ignite the powders of Ti and Ni, and TiNi intermetallic compound was synthesized by SHS in a reaction kettle of stainless steel. High-speed photography, X-ray diffraction, and scanning electron microscopy were used to investigate and analyze the reaction process, phase composing, and microstructure of the product, respectively. The influence factors on the reaction process and the product were discussed. The results indicate that laser induced SHS is an efficient, energy-saving method; The phase ingredient of the product consists of TiNi, Ti2Ni, and Ni3Ti. With the increase of the preparing pressure of the sample, the reacting rate decreases; With the increase of the laser power and the preheating temperature, the reacting rate increases. Under the condition of 30℃/min, the synthesis reaction had been carried out consistently and completely.

Calculation and verification of formula for the range of sprinklers based on jet breakup length

Jet breakup length is an important parameter which reflects the length of sprinkler range.Based on the linear instability theory,the dispersion equation of cylindrical jet was established and the theoretical value of jet breakup length was calculated.The jet breakup length and initial amplitude of surface wave were measured by applying the high-speed photography technology.Meanwhile,the numerical simulation was conducted by combining Level Set-VOF method for describing the jet breakup length to verify the theoretical and experimental results.Within the jet velocity and working pressure range of discussion,the results of comparison showed that the theoretical analysis gave a reasonable explanation to the influence of jet velocity,nozzle diameter and nozzle cone angle on jet breakup length.Comparing the theoretical value of jet breakup length with the experimental and simulated values,the three results accorded one another.The experimental jet breakup lengths were the lowest and the simulation values were the largest,and the relative error was less than 10%,especially the theoretical value was closer to the average value.For choosing the theoretical calculation of jet breakup length,a semi-empirical and semi-theoretical formula of range for the rotating sprinkler was concluded by the curve fitting method and the fitting formula was verified.The results showed the high accuracy of the ranges determined by this formula and the average relative error was less than 2.5%.The new formula was in good agreement with the data of different types of sprinklers comparing with other empirical formulas,and the error was only 5%.Meanwhile,the possibility of using this formula widely to determine the ranges of same series of sprinkler was confirmed.

Experimental Investigation of the Dynamic Deformation of LY12 Aluminum Plate Heated by Strong Electrical Current

The moire interferometry method was used with holographic gratings for high temperature and high speed photography to measure the dynamic deformation of LY12 aluminum plate with a central hole under tensile loading condition and heated by a strong current. Transient fullfield fringe patterns were acquired during the heating process. Transient temperature of the specimen was simultaneously recorded by a temperature measurement system. The experimental results show that dynamic moire fringe patterns can be measured during high heating rate conditions using high speed photography.