High-speed Video Observations Research Articles

Spatter detection and tracking in high-speed video observations of laser powder bed fusion

Purpose In laser powder bed fusion (L-PBF) additive manufacturing, spatter particles are ejected from the melt pool and can be detrimental to material performance and powder recycling. Quantifying spatter generation with respect to processing conditions is a step toward mitigating spatter and better understanding the phenomenon. This paper reveals process insights of spatter phenomena by automatically annotating spatter particles in high-speed video observations using machine learning. Design/methodology/approach A high-speed camera was used to observe the L-PBF process while varying laser power, laser scan speed and scan strategy on a constant geometry on an EOSM290 using Ti-6Al-4V powder. Two separate convolutional neural networks were trained to segment and track spatter particles in captured high-speed videos for spatter characterization. Findings Spatter generation and ejection angle significantly differ between keyhole and conduction mode melting. High laser powers lead to large ejections at the beginning of scan lines. Slow and fast build rates produce more spatter than moderate build rates at constant energy density. Scan strategies with more scan vectors lead to more spatter. The presence of powder significantly increases the amount of spatter generated during the process. Originality/value With the ability to automatically annotate a large volume of high-speed video data sets with high accuracy, an experimental design of observed parameter changes reveals quantitively stark changes in spatter morphology that can aid process development to mitigate spatter occurrence and impacts on material performance.

High Speed Video Observations of Vocal Fold Kinematics While Playing a Clarinet.

In this pilot study, flexible high-speed video (HSV) technology with synchronized audio is used to visualize vocal fold behavior in a wind instrumentalist. Specifically, this study aims to contribute to describing the vocal fold behavior of a professional clarinet musician playing varying tones and melodies. Vocal folds of a healthy 26-year-old professional clarinet musician were recorded utilizing a HSV camera coupled to a flexible endoscope, which allowed the synchronous recording of audio with vocal fold movement at the onset, playing, and offset of playing. Two raters experienced with analyzing vocal folds described vocal fold motion of each sample. Samples were processed through a software model to determine the vocal fold movement relative to their position at rest. Digital kymograms (DKGs) were additionally obtained to visualize vocal fold micromovements throughout each sample. At the onset of playing, the raters observed the vocal folds moving to a paramedian position. Vocal fold adduction varied according to the task performed and ranged from 58% to 77% of the original resting glottis width. The calculated changes in glottis width and DKGs were consistent with the descriptions by the raters. This study demonstrates the utility of flexible HSV in observing vocal fold motion before, during, and after events other than sustained phonation. The incomplete adducted vocal fold postures observed while playing the clarinet not only differ from phonation but also differ depending on task. These various postures may contribute to voice fatigue in wind instrumentalists or to various disorders experienced during their careers.

Correlated Lightning Electric Field and High-Speed Video Observations of Recoil Leaders Recorded in Rzeszow, Poland

Correlated Lightning Electric Field and High-Speed Video Observations of Recoil Leaders Recorded in Rzeszow, Poland

Gas Bubble Evolution during ECM Process can be suppressed by Pulse Parameters: High Speed Videographic Observations during Early Stages of Bubble Evolution

The precision in electrochemical machining (ECM) is adversely affected by the generation of gas bubbles due to the inherent electrochemical reactions occurring at the cathode (tool) and anode (workpiece) surface. These gas bubbles are known to cause variations in electrolyte conductivity and lead to uneven dissolution and limit the achievable flatness and surface finish. The demands for precision ECM make it important to investigate the bubble evolution phenomenon. In the prior state of the art, this has been done by means of transparent electrodes and high-speed video observations. The current work involves high-speed video experiments with a specific intention to observe gas bubbles during the early stages of evolution and attempts to address two main research questions: (i) Is it possible to suppress gas bubble evolution in ECM using pulse parameters? (ii) Is it possible to do ECM without gas bubbles? The results reveal that shorter pulse-on times and longer pulse-off times reduce the growth of the concentration boundary layer, thereby suppressing bubble generation. Furthermore, by combining the aforementioned electrical pulse settings with an appropriate flow rate, there is no significant detection of gas bubbles in the experiments conducted here, as can be concluded from the fact that the interelectrode gap (IEG) remains transparent. The industrial applicability-oriented analysis focused on evaluating the final IEG and did not reveal any significant difference due to shorter flow lengths. It is further suggested that by employing high flow rates in combination with shorter pulse-on times, the pulse-off time can be further shortened to raise duty cycles so as to facilitate economic machining without the influence of gas bubbles.

Experimental characterization of the flowfield and cavitation physics of a tip-loaded hydrofoil

This paper reports an experimental study of tip vortex flowfield and cavitation inception of a tip-loaded hydrofoil. Vortex strength, wandering, and turbulence statistics are characterized using stereo particle image velocimetry (SPIV) in a water tunnel facility, at a chord Reynolds number of 1.3×106. Cavitation physics are characterized using high-speed videography and dual-hydrophone acoustic cavitation measurements. The loading of the rectangular planform hydrofoil has a maximum at 65% span, 56% greater than that at the root, i.e., the hydrofoil loading is representative of non-elliptically loaded open propellers. Acoustic cavitation inception is quantified and is observed to precede visual cavitation onset using unaided and high-speed imaging. Measurements reported here show that vorticity fluctuations are nearly of the same magnitude as the ensemble vorticity. Instantaneous measurements of vorticity at the trailing edge, 12-chord downstream, and one-chord downstream positions are reported. Their peak magnitudes are located adjacent to the ensemble vortex center and are between four and five times the ensemble mean. The fluctuating vorticity measurements, taken in conjunction with high-speed video observations, provide insight into the hydrodynamic conditions responsible for intermittent cavitation events. The reported measurements elucidate instantaneous and mean turbulence physics associated with vortex cavitation and can provide a validation basis for numerical simulations.

Understanding the effects of temporal waveform modulation of laser emission power in laser powder bed fusion: Part II - Experimental investigation

The laser powder bed fusion (LPBF) process has historically been operated with high-brilliance fibre laser sources with continuous wave (CW) emission. Nonetheless, temporal waveform modulation of the laser emission power at high-frequency levels can provide a means to enhance the deposition process by modifying the melt dynamics and solidification mechanisms. In order to disclose the effect of different waveform shapes and their parameters, an experimental study using an open LPBF system was conducted. This paper is the second part of an investigation on this topic, which aims to validate the analytical model proposed in the first part of the work. The LPBF system that was developed enabled the power emission profiles to be programmed during single-track depositions. Four different waveform shapes were tested (namely square wave, ramp up, ramp down and triangle wave) at different levels of waveform amplitude (ΔP= 200–400 W) and different frequencies (fw = 2–4–6–8 kHz) during the single-track deposition of stainless steel AISI316L. High-speed imaging acquisitions allowed the melt dynamics to be disclosed and the melt-oscillation frequency to be identified. Larger waveform amplitudes and waveforms with sudden variations of emission power generated melt ejections and process instabilities. Stable conditions could be identified when employing ramp up and triangle waveforms with ΔP = 200. Melt-surface oscillation frequency corresponded to the values imposed via the modulation of the laser emission power, thus validating the analytical model of Part I, which correlated the melt-surface temperature to the recoil pressure induced over the molten pool. Optical microscopy images and metallographic cross-sections confirmed the high-speed video observations. Three-dimensional reconstructions of the depositions via focus variation microscopy allowed the build rates and roughness of the single tracks to be determined. Build rates obtained in stable deposition conditions with waveform modulation are analogous to values obtainable with CW emission, and beneficial effects over the roughness were reported.

A new theoretical performance landscape for suction feeding reveals adaptive kinematics in a natural population of reef damselfish.

ABSTRACTUnderstanding how organismal traits determine performance and, ultimately, fitness is a fundamental goal of evolutionary eco-morphology. However, multiple traits can interact in non-linear and context-dependent ways to affect performance, hindering efforts to place natural populations with respect to performance peaks or valleys. Here, we used an established mechanistic model of suction-feeding performance (SIFF) derived from hydrodynamic principles to estimate a theoretical performance landscape for zooplankton prey capture. This performance space can be used to predict prey capture performance for any combination of six morphological and kinematic trait values. We then mapped in situ high-speed video observations of suction feeding in a natural population of a coral reef zooplanktivore, Chromis viridis, onto the performance space to estimate the population's location with respect to the topography of the performance landscape. Although the kinematics of the natural population closely matched regions of high performance in the landscape, the population was not located on a performance peak. Individuals were furthest from performance peaks on the peak gape, ram speed and mouth opening speed trait axes. Moreover, we found that the trait combinations in the observed population were associated with higher performance than expected by chance, suggesting that these combinations are under selection. Our results provide a framework for assessing whether natural populations occupy performance optima.

Microscopic high-speed video observation of oxygen bubble generation behavior and effects of anode electrode shape on OER performance in alkaline water electrolysis

An important difficulty associated with alkaline water electrolysis is the rise in anode overpotential attributable to bubble coverage of the electrode surface. For this study, a system with a high-speed video camera was developed, achieving in-situ observation of bubble generation on an electrode surface, monitoring an area of 1.02 mm 2 at 6000 frames per second. The relation between polarization curve (current density up to 3.0 A cm −2 ) and oxygen bubble generation behavior on nickel electrodes having cylindrical wires and rectangular wires of different sizes (100–300 μm) was clarified. The generated bubbles slide upward, contacting the electrode surface and detaching at the top edge. Observations indicate that small electrodes have short bubble residence time and thin bubble covering layer on the electrode. As a result, the small electrode diameter contributes to smaller overpotential at high current density. • Microscopic high-speed video observation system for bubble generation was developed. • Bubble motion was observed in a 1.02 mm 2 area at 6000 frames per second. • Small cylindrical electrodes showed high performance for oxygen evolution reaction. • Generated bubbles move up the electrode surface and detach at the electrode top. • Small electrodes provide short electrode residence time and small detached bubbles.

Influence of Surface Tension and Evaporation on Melt Dynamics of Aluminum Alloys for Laser Powder Bed Fusion

One of the most important factors during Laser Powder Bed Fusion is a robust and stable process to generate reproducibly dense and crack-free components with the same homogenous and fine-grained microstructure. Among other factors, surface tension influences the robustness of this process. Three aluminum alloys were investigated by means of 3D laser printing with respect to their ability to form lightweight and high-performance structures. The alloys include two Al-Cr based alloys (Zicromal®: Al-Cr-Zr-Mn, Scancromal®: Al-Cr-Sc-Zr) and an Al-Mg based alloy (Scalmalloy®: Al-Mg-Sc-Mn-Zr). The surface tensions of their liquid melts were measured by means of the oscillating droplet method in electromagnetic levitation. Scalmalloy® exhibited a surface tension that was by 3-8% smaller than the other two alloys, over a temperature range of 500 K above their liquidus temperature; the evaporation analysis showed that Scancromal® exhibits significantly less evaporation compared to the other two alloys by an order of magnitude. During the Laser Powder Bed Fusion process, Scalmalloy® presented the most unstable melt dynamics, and Scancromal® appeared to be more robust and stable with less weld spatters and negligible dust deposition, as evidence from the high-speed video observations. The results support the idea that surface tension and evaporation are potentially crucial factors influencing the melt dynamics and also demonstrate the capability of additive manufacturing that produces customized aluminum alloys for aerospace applications.

Advanced Optical Diagnostics for Characterization of Arc Plasmas

This article presents the selected approaches of optical diagnostics applied to thermal arcs. Advanced methods for the determination of temperatures and species density distributions are described and examples of recent developments are presented, such as the consideration of optical depth in optical emission measurements, the use of broadband light sources for measurements of spatially resolved densities, or the combination of spectral analysis with high-speed video observation for the purpose of surface temperature determination. Finally, the potential of the presented diagnostics to solve future challenges of thermal arc research is demonstrated and discussed.

Rotational speed measurements of small spherical particles driven by acoustic viscous torques utilizing an optical trap

Two orthogonal standing acoustic waves, generated by piezoelectric excitation, can form a two-dimensional pressure field in microfluidic devices. A phase difference of the excitation waves can be employed to rotate spherical µm-sized silica particles by a torque mediated through the viscous boundary δ around the particle. The measurement of the rotational rate is, so far, limited to high-speed cameras and their frame rate, and gets increasingly difficult when the sphere gets smaller. We report here a new method for measuring the rotational rate of µm sized spherical particles. We utilize an optical trap with high-speed position detection to overcome the frame rate limitation of wide field image recording. The power spectrum of an optically trapped, rotating particle reveals additional peaks corresponding to the rotational frequencies—compared to a non-rotating particle. We validate our method at low rotational rates against high-speed video observation. To demonstrate the potential of this method we addressed a recent controversy about the rotation of particles with a relatively large viscous boundary layer δ. We measured steady-state rotational rates up to 229 Hz (13.8 × 103 rpm) for a particle with a radius R ≈ δ. Recent numerical research suggests that in this regime the existing theoretical approach (valid for ) overpredicts the steady-state rotational rate by a factor of 10. With our new method we also confirm the numerical results experimentally.

The ping-pong ball water cannon

The course Phy Ex was created by Yves Couder in the Paris VII university to teach experimental physics through projects. In this article, we present this teaching method through a particular project that took place in the autumn semester 2019: the ping-pong ball water cannon. In this experiment, a glass containing water and a floating table tennis ball is dropped from some height to the ground. Following the impact, the ball is ejected vertically upwards at speeds that can be several times the impact speed. We report the student team's initial dimensional and order-of-magnitude analysis, and describe the successive experimental setups that showed (1) that free flight is essential for the phenomenon to occur, (2) that the order of magnitude of the ball ejection momentum is correctly predicted by a momentum balance based on integrating the pressure impulse during impact and (3) that making the ball surface more wettable, or stirring the liquid, drastically increases the momentum transfer. The proposed explanation, confirmed by direct high-speed video observations, is that the immersion depth of the ball increases during free fall due to capillary forces or vortex depression-in the absence of buoyancy-and that the enormous excess pressure on the bottom of the ball during impact drives the ball up towards its buoyancy equilibrium. The transfered momentum is sufficient to expel the ball at high velocity, very similar to the formation of liquid jets in collapsing cavities in liquids.

High-Speed Video Observation, Currents, and EM-Fields From Four Negative Upward Lightning to the Peissenberg Tower, Germany

In this article, we analyzed the optical images from high-speed video recordings and the time-correlated waveforms of the current, the electric field, and the magnetic field from four negative upward lightning which struck the Peissenberg Tower, Germany. Two of them were self-initiated ones and two of them were other-triggered ones. From the illumination in the video images, leader activity could clearly be detected during the occurrence of the first of the other-triggered lightning. In contrast, no illumination by leader activity could be found during the occurrence of the second other-triggered lightning and the time period of 2.5 s before. In addition, the current pulses superimposing the initial continuous current/continuing current were examined differing between those which developed in luminous or nonluminous branches. From the total of 26 current pulses which developed in nonluminous branches, 25 (96%) of them had a 10–90%-rise time less than 4 μs, and 23 (88%) of them had a 10–90%-rise time even less than 2 μs. Because the current waveforms resemble those of weak return stroke, we assume that the current pulses indeed represent the leader/return stroke mode of charge transfer to ground. The return strokes were extremely weak, with an arithmetic mean of 2.59 kA (GM: 2.08 kA) for the current peak. On the other hand, from the total of 74 current pulses which developed in already luminous branches, none of them had a 10–90%-rise time less than 4.4 μs. The arithmetic mean is 29.8 μs and the geometric mean is 20.5 μs. Due to the relatively long rise time, we assume that these current pulses represent the M-component mode of charge transfer to ground.

Evaluating capacitive wetness sensors for measuring deposition in electrostatically charged spraying operations

Standard methods for measuring pesticide deposition include water sensitive paper and fibre (paper or cord) collectors, which require time consuming manual steps to gather, process and interpret results. Measurement of pesticide deposition is important in evaluating the efficacy of a spray, which affects the total crop yield. A lack of knowledge about spray quality often results in the need to spray to excess, which can contaminate the soil and damage neighbouring ecosystems and the atmosphere. The suitability of PHYTOS 31 capacitive leaf wetness sensors (Meter Group, Pullman, WA, USA) for spray sensing applications was analysed. The best fit calibration function for droplets of ~ 1 μL volume was linear (R2 = 0.985). The best fit calibration function for drops > 10 μL was a weighted sum of a linear and power function (R2 = 0.984). Thus, the calibration function should be chosen according to the anticipated size of droplet. The PHYTOS 31 sensors and water sensitive papers were used in a field test, comparing electrostatically charged and uncharged sprays. Both measurement techniques show that charged spray is more effective in covering parts of the plants which do not have a direct line of sight to the nozzle. The techniques disagree on quantitative coverage measurement, with the water sensitive papers reporting areal coverage two to four times higher than the capacitive sensors. High-speed video observations show that this is due to contraction (de-wetting) of the drop on the capacitive sensors. The surface of the capacitive sensors is more hydrophobic than the papers. The hydrophobicity of the capacitive sensors appears to be a better match to plant leaves than that of the papers. Capacitive sensors are a suitable measurement tool for evaluating liquid coverage in the field, but further calibration is necessary to be confident in the quantitative data.

High-speed video observations on fork lightning events in Malaysia

Malaysia is one of the countries with the highest lightning strikes incidence in the world. Yet, Malaysians are still taking a proper lightning protection indifferent manner. This might be due to lack of knowledge on lightning characteristics in Malaysia. Therefore, this study presents an unusual lightning phenomenon with multiple grounding points that might cause a disastrous event, called fork lightning. Between November and December 2018, there were three different patterns of fork lightning mechanism successfully recorded by using high speed camera with 2800 frame per second. Temporal analysis of the lightning progress had been done on the samples based only on the camera records which then played back by using Wondershare Filmora video editor. These three samples of fork lightning images were acquired among 37 lightning events recorded in this study. The first sample of fork lightning fully established the forked branch 0.35 ms before the return stroke occurred. On the contrary, second fork lightning established its forked branch 0.35 ms after the return stroke. Both forked branches for third sample simultaneously fully established with the return stroke event. Obviously, these events had striking the ground in up to ten milliseconds of time duration. Thus, there is always a possibility that they might cause serious damage and should not be neglected in designing a lightning protection system.

Progress in the Study of Transient Luminous and Atmospheric Events: A Review

Transient luminous events (TLEs) such as sprites, blue jets (BJs) and elves have been studied intensively during the last three decades, and much is now known of their properties. This progress is caused by several factors including satellite optical observations, ground-based measurements of sprite-produced electromagnetic fields, the use of high-speed video observations and telescopic cameras with high resolution that enables one to trace the dynamics of sprite and BJ development. In this paper, we review various types of TLEs, including recently discovered dancing sprites, gnomes, ultraviolet (UV) atmospheric flashes and other effects. The sprite initiation, visible evolution, streamer structure, and their relationship with intra-cloud (IC) process are discussed. Considerable study has been given to ULF/ELF measurements which can provide us with important information on the delayed sprite generation and the role played by IC processes in the perturbation of the lower ionosphere above the sprite. A set of electrodynamic and transport kinetic equations describing the TLEs are complicated because the number densities, mobilities of electrons and ions, reaction constants and other parameters are strongly dependent on altitude. Because of this, the majority of theoretical study of TLEs and other large-scale optical phenomena at high altitude are based on numerical modeling of the basic kinetic, transport and electrodynamic equations describing TLEs evolution, whereas the analytical theory remains a formidable task to be accomplished. In this paper, we review a few analytical results, which have been recently derived from simple physical models of the TLEs phenomena. In the remainder of this paper, we focus our attention on the properties of UV flashes in the mesosphere, which have been observed onboard Russian microsatellites “Universitetsky-Tanyana” and “Vernov.” Such a kind of optical flash is referred to as a transient atmospheric event, which differs from the TLEs in optical energy, duration and other parameters.

Rotational Maneuvers of Copepod Nauplii at Low Reynolds Number

Copepods are agile microcrustaceans that are capable of maneuvering freely in water. However, the physical mechanisms driving their rotational motion are not entirely clear in small larvae (nauplii). Here we report high-speed video observations of copepod nauplii performing acrobatic feats with three pairs of appendages. Our results show rotations about three principal axes of the body: yaw, roll, and pitch. The yaw rotation turns the body to one side and results in a circular swimming path. The roll rotation consists of the body spiraling around a nearly linear path, similar to an aileron roll of an airplane. We interpret the yaw and roll rotations to be facilitated by appendage pronation or supination. The pitch rotation consists of flipping on the spot in a maneuver that resembles a backflip somersault. The pitch rotation involved tail bending and was not observed in the earliest stages of nauplii. The maneuvering strategies adopted by plankton may inspire the design of microscopic robots, equipped with suitable controls for reorienting autonomously in three dimensions.

Comparisons of optical characteristics of two upward lightning flashes triggered by a nearby positive cloud-to-ground lightning

The propagation characteristics of two upward negative lightning flashes, which were initiated from the tops of two tall grounded structures immediately after the return stroke (RS) initiation of one nearby positive cloud-to-ground (CG) lightning flash, have been investigated and compared based on high-speed video observations. Two upward flashes show significant difference in their propagation characteristics mainly due to the existence of lower cloud layer near the Canton tower. Upward flash initiated from the top of Canton tower produces several large luminous pulses, with the enhanced luminosity mostly concentrating near the lower cloud, and it propagates faster beyond the lower cloud, while the brightness of the other upward flash does not vary much and distributes almost uniformly along the discharge channel. The major reason for these differences might lie in the instantaneous negative charge neutralization inside the lower cloud with the positive charges along the discharge channel, when the positive leader propagates upward across the lower cloud layer. After about 90 ms the upward flash initiated from the tall building produces a dart-leader subsequent RS, but it extinguishes faster than the other one.

Study of tip vortex cavitation inception and vortex singing

Tip vortex cavitation (TVC) is an important cavitation phenomenon in marine propeller. The formation and evolution of tip vortex cavitation are hot topics consistently both in engineering application and mechanism research. In this paper some recent studies on tip vortex cavitation inception and the noise of tip vortex cavitation evolution are presented. The effects of both flow field and water qualities on tip vortex cavitation inception are considered by experiments and numerical simulations. The results show that besides the average minimum pressure in the vortex core the turbulence fluctuation and water qualities including air content and nuclei distribution have great influence on tip vortex cavitation inception. Based on the idea of first nucleus cavitating in tip vortex core new prediction formula for tip vortex cavitation inception is proposed. The synchronous technique of high speed video observation and noise measurement are adopted to study the development of tip vortex cavitation. S-type total noise characteristics are obtained when cavitation number from low to high. Vortex singing is found in the case where the tip vortex cavitation just before leaves the tip region. The excited mechanism of vortex singing is proposed by analyzing the wave propagation on the interface of vortex cavity.

Optical characteristics of branched downward positive leader associated with recoil leader activity

The propagation characteristics of one downward positive leader with mainly four forked branches in one natural positive lightning have been investigated based on high-speed video observations. One branch develops horizontally; one branch travels downward eventually reaching the ground, and two faint branches are reilluminated by recoil leader (RL) activity multiple times. Four branches share some interesting correlation during their development. At the early phase, the horizontal branch and descending branch are inactive or propagate slowly when RL activity is more active. After the appearance of RL activity each time, the horizontal branch propagates slightly faster. After that, the two luminous branches propagate more actively and faster, while RL process becomes diminished and suppressed. Both downward and horizontal branches speed up as the downward one is approaching the ground. The two-dimensional (2-D) propagation speed for the downward branch ranges from 4.25 × 104 m/s to 3.85 × 105 m/s, with an average value of 1.01 × 105 m/s, and it ranges from 2.69 × 104 m/s to 3.61 × 105 m/s with an average speed of about 1.22 × 105 m/s for the horizontal one.