Temporal Evolution Research Articles

Lava flow field development and lava tube formation during the 1858–1861 eruption of Vesuvius (Italy), unravelled by historical documentation, lidar data and 3D mapping

Somma-Vesuvius is well known for its powerful Plinian explosive eruptions, however during the last eruptive cycle (1631–1944), persistent activity took place on the stratovolcano as mild and violent Strombolian, and effusive eruptions, forming more than one hundred lava flow fields. An important mechanism of lava transport within lava flow fields is the formation and development of lava tubes. The presence of lava tubes in a flow field can greatly increase their distance of emplacement. Observations of lava tubes at Vesuvius have been documented in historical records and speleological reports but no modern scientific studies are available. This work focuses on lava tubes formed in the compound lava flow field of the long-lived 1858 eruption (from 27 May 1858 to 12 April 1861) that was fed by seven eruptive fissures. The temporal and spatial evolution of the 1858 lava flow field was reconstructed using historical documentation. The exposed lava flow field surface was analysed using a 1-m resolution lidar Digital Surface Model (DSM). Surveys to fully digitize the interior and the overlying surface of the largest lava tube found in the 1858 lava flow field were conducted using a terrestrial laser scanner, optical cameras, and an Unmanned Aerial Vehicle (UAV). The accurate 3D model obtained was used to precisely quantify the inner dimensions and to better constrain the morphologies of the lava tube. Observed internal features were described and used to gain information on the formation and activity of the lava tube. Our data allowed us to understand that the described lava tube formed as an inflated lava flow inside which lava flowed through during an extended period ultimately draining out completely at the end of the eruption. Understanding how lava flow fields develop and how lava tubes form on Vesuvius is crucial to re-evaluate the last effusive activity of the volcano and its impact on hazard assessment.

Optical and Radio Analysis of Systematically Classified Broad-lined Type Ic Supernovae from the Zwicky Transient Facility

Abstract We study a magnitude-limited sample of 36 broad-lined type Ic supernovae (SNe Ic-BL) from the Zwicky Transient Facility Bright Transient Survey (detected between 2018 March and 2021 August), which is the largest systematic study of SNe Ic-BL done in literature thus far. We present the light curves (LCs) for each of the SNe and analyze the shape of the LCs to derive empirical parameters, along with the explosion epochs for every event. The sample has an average absolute peak magnitude in the r band of M ¯ r , max = − 18.51 ± 0.15 mag. Using spectra obtained around peak light, we compute expansion velocities from the Fe ii 5169 Å line for each event with high enough signal-to-noise ratio spectra, and find an average value of v ph ¯ = 16 , 100 ± 1100 km s−1. We also compute bolometric LCs, study the blackbody temperature and radii evolution over time, and derive the explosion properties of the SNe. The explosion properties of the sample have average values of M ¯ Ni = 0.37 − 0.06 + 0.08 M ⊙ , M ¯ ej = 2.45 − 0.41 + 0.47 M ⊙ , and E ¯ K = ( 4.02 − 1.00 + 1.37 ) × 10 51 erg. Thirteen events have radio observations from the Very Large Array, with eight detections and five non-detections. We find that the populations that have radio detections and radio non-detections are indistinct from one another with respect to their optically inferred explosion properties, and there are no statistically significant correlations present between the events’ radio luminosities and optically inferred explosion properties. This provides evidence that the explosion properties derived from optical data alone cannot give inferences about the radio properties of SNe Ic-BL and likely their relativistic jet formation mechanisms.

Modifications of SPH towards three-dimensional simulations of an icy moon with internal ocean

There are some traces of the existence of internal ocean in some icy moons, such as the vapor plumes of Europa and Enceladus. This implies a region of liquid water beneath the surface ice shell. Since liquid water would be essential for the origin of life, it is important to understand the development of these internal oceans, particularly their temperature distribution and evolution. The balance between tidal heating and radiative cooling is believed to sustain liquid water beneath an icy moon’s surface. We aim to simulate the tidal heating of an internal ocean in an icy moon using 3-dimensional numerical fluid calculations with the Smoothed Particle Hydrodynamics (SPH) method. We incorporated viscosity and thermal conduction terms into the governing equations of SPH. However, we encountered two issues while calculating rigid body rotation using SPH with a viscous term: (1) conventional viscosity formulations generated unphysical forces that hindered rotation, and (2) there was artificial internal energy partitioning within the layered structure, which was due to the standard SPH formulations. To address the first issue, we modified the viscosity formulation. For the second, we adopted Density Independent SPH (DISPH) developed in previous studies to improve behavior at discontinuous surfaces. Additionally, we implemented radiative cooling using an algorithm to define fluid surfaces via the particle method. We also introduced an equation of state accounting for phase transitions. With these modifications, we have refined the SPH method to encompass all necessary physical processes for simulating the evolution of icy moons with internal oceans.

Simulation of thermal-mechanical coupling in Al alloy/steel inertia friction welding

Inertia friction welding (IFW) is a complicated thermal-mechanical coupling process. In-depth understanding of the thermal-mechanical process can improve the joint quality. In this study, a three-dimensional (3D) thermal-mechanical coupling model was successfully established to simulate the evolution of temperature, energy density, stress, strain, and flow behavior of the IFW process between 2219-O Al alloy and 304 stainless steel. The developed model was validated by comparing it with the measured temperature and deformation. Results show a transforming distribution of interface temperature from a M-shape to a squished V-shape during the IFW process. The characterization is attributed to the transformation of the accumulated friction energy density at the interface from a M-shape to an approximate V-shape due to the combined effect of varying sliding velocity and contact pressure. The Mises stress at the region near the interface is high and low at the steel and Al alloy sides, respectively. A significant portion of the kinetic energy stored in the flywheel is dissipated in the form of plastic deformation energy and cannot be neglected. The tracer particles show that most Al alloy at the initial interface is extruded to form flash, while a small amount of Al alloy is sticky and remains at the interface center. Increasing friction pressure and initial rotation speed result in higher interface temperature, axial shortening distance, and proportion of plastic work in total energy input.

Numerical simulation of the coupled thermal-hydro-mechanical fields for evaluating CO2 geo-sequestration storage in deep saline aquifers of Hainan Fushan Sag, China

Geo-sequestration storage of CO2 in saline aquifers is an important technological option to reduce carbon emissions in China in the future. The theory of geo-sequestration storage of CO2 in saline aquifers involves complex physical operations and chemical phenomena among reservoir stress, seepage, and temperature fields, and the numerical simulation models coupling thermal-hydro-mechanical (THM) fields are widely applied to analyze the evolution of multiphysical processes in CO2 reservoirs. Based on the geo-sequestration storage project of saline aquifer CO2 in Fushan Sag, Hainan Province, under the premise of considering the change of permeability and porosity after the mineralization of CO2 in the rock reservoirs, this study simulates the evolution of temperature, stress, and seepage fields during the sequestration process of saline aquifer Continuous CO2 injection wells clarifies the range of influence of the temperature field, seepage field, and the boundary of its stress field of saline aquifer Continuous CO2 injection wells in different sequestration periods. It reveals the influence mechanism of the fault structure on the reservoir CO2 sequestration pressure and capacity, and evaluates and estimates the CO2 injection storage capacity of Fu 1# well in Fushan Sag and on this basis. The THM numerical simulation model proposed in this study plays a crucial role in guiding theoretical evaluations of the CO2 geo-sequestration potential in saline aquifers and determining its technical parameters, thereby offering significant value for engineering applications.

Measurement of temporal evolution of antiferromagnetic domain change in nickel oxide driven by 2TO phonon mode excitation via pump-probe experiment

The temporal evolution of antiferromagnetic domain pattern changes under resonant excitation of the second-order transverse optical phonon mode in nickel oxide was investigated using mid-infrared free electron laser (MIR-FEL) pulses and visible nanosecond probe laser pulses at room temperature. We visualized the domain patterns through magneto-optical polarized microscopy in transmission and observed their temporal variation after the phonon excitation via MIR-FEL. We evaluated the differences throughout the timeline using the structural similarity index measure technique from domain images with and without MIR-FEL irradiation. We found that the MIR-FEL irradiation induces significant structural changes in the domain patterns. The maximum difference was observed at the timing of the MIR-FEL irradiation and exponentially recovered with the time constant of 1.4 ± 0.2 ms. This rather long-time constant could be owing to the spin relaxation, whilst further investigation is needed to confirm the underlying mechanism.

Unveiling and advancing grassland degradation research using a BERTopic modelling approach

Grassland degradation presents overwhelming challenges to biodiversity, ecosystem services, and the socio-economic sustainability of dependent communities. However, a comprehensive synthesis of global knowledge on the frontiers and key areas of grassland degradation research has <pg=>not been achieved due to the limitations of traditional scientometrics methods. The present synthesis of information employed BERTopic, an advanced natural language processing tool, to analyze the extensive ecological literature on grassland degradation. We compiled a dataset of 4,504 publications from the Web of Science core collection database and used it to evaluate the geographic distribution and temporal evolution of different grassland types and available knowledge on the subject. Our analysis identified key topics in the global grassland degradation research domain, including the effects of grassland degradation on ecosystem functions, grassland ecological restoration and biodiversity conservation, erosion processes and hydrological models in grasslands, and others. The BERTopic analysis significantly outperforms traditional methods in identifying complex and evolving topics in large datasets of literature. Compared to traditional scientometrics analysis, BERTopic provides a more comprehensive perspective on the research areas, revealing not only popular topics but also emerging research areas that traditional methods may overlook, although scientometrics offers more specificity and detail. Therefore, we argue for the simultaneous use of both approaches to achieve more systematic and comprehensive assessments of specific research areas. This study represents an emerging application of BERTopic algorithms in ecological research, particularly in the critical research focused on global grassland degradation. It also highlights the need for integrating advanced computational methods in ecological research in this era of data explosion. Tools like the BERTopic algorithm are essential for enhancing our understanding of complex environmental problems, and it marks an important stride towards more sophisticated, data-driven analysis in ecology.

On the Dynamism of Paintings Through the Distribution of Edge Directions

The digitization of artworks has recently offered new computational perspectives on the study of art history. While much of the focus has been on classifying styles or identifying objects, the analysis of more abstract concepts, such as the perception of motion or dynamism in still images, remains largely unexplored. Semioticians and artists have long explored the representation of dynamism in still images, but they often did so through theoretical frameworks or visual techniques, without a quantitative approach to measuring it. This paper proposes a method for computing and comparing the dynamism of paintings through edge detection. Our approach is based on the idea that the dynamism of a painting can be quantified by analyzing the edges in the image, whose distribution can be used to identify patterns and trends across artists and movements. We demonstrate the applicability of our method in three key areas: studying the temporal evolution of dynamism across different artistic styles, as well as within the works of a single artist (Wassily Kandinsky), visualizing and clustering a large database of abstract paintings through PixPlot, and retrieving similarly dynamic images. We show that the dynamism of a painting can be effectively quantified and visualized using edge detection techniques, providing new insights into the study of visual culture.

A complex network analysis of groundwater wells in and around the Doñana Natural Space, Spain

This paper proposes the use of Complex Network Analysis tools to study the spatial distribution and temporal evolution of groundwater abstraction wells within a region of interest. To that end a weighted undirected network connecting wells within a certain distance was built. The edge weights take into account the distance between the wells as well as their corresponding abstraction volumes. In addition to the conventional complex network metrics, two specific abstraction-based centrality indexes are proposed, based on the total impact of a groundwater abstraction well on its surrounding wells and on the total impact of surrounding wells on a specific location. The proposed approach has been applied to the area of the Doñana Natural Space, one of the most important wetlands in Europe. The topology of the network reveals a large number of small, disconnected components and a few large components. This is reflected also in the degree distribution, which follows a Power Law distribution. The network has a large average clustering coefficient, a small average path length and a low level of centralisation. Global and average local network efficiency are also high. The network is assortative in terms of degree-degree and strength-strength correlations. Eigenvector centrality of the nodes reveals the location of influential wells.

Numerical modelling of radionuclide uptake by bluefin tuna along its migration routes in the Mediterranean Sea after a nuclear accident

A numerical model which simulates the adsorption of radionuclides by migrating bluefin tuna in the Mediterranean Sea is described, in order to determine the level of contamination of these fish after a hypothetical nuclear accident and thus be able to assess the possible impact on human consumption. A 4–species foodweb model is incorporated into a Lagrangian model describing physical transport (advection, mixing, radioactive decay and interactions of radionuclides with sediments). Tuna is the last trophic level in the foodweb model and the equation providing the temporal evolution of radionuclide concentration in its flesh is solved along the fish trajectories, which were obtained through electronic tagging of fishes. The model was applied to the western Mediterranean, where several worst–case hypothetical accidents were simulated, both from a coastal nuclear power plant and from a vessel. Resulting 137Cs concentrations in migrating tuna were similar, or slightly higher, than reported background concentrations in these fishes and well below established safety levels. Maximum calculated concentrations in tuna flesh is in the order of 1 Bq/kg (wet weight). This is due to the rapid movement of the fishes, which spend only limited time over the most contaminated spots.

Evaporation and combustion of low asphaltene heavy oil droplets under conditions representative of practical applications

The evaporation and burning characteristics of isolated heavy fuel oil (HFO) droplets are experimentally studied at conditions representative of practical applications, producing novel, detailed results on the effect of oxygen concentration. Also, and for the first time for HFO, the temporal evolution of the flame stand-off ratio (based on CH* emission) and the cenosphere burning size are presented along with the droplet size histories. The effect of ambient oxygen concentration is first analyzed qualitatively, identifying different sub-stages. Later, a quantitative analysis is presented in terms of ignition delay time, micro-explosion regimes, shell swelling ratio, cenosphere size and several time metrics. The results show that, as ambient oxygen changes, the liquid and solid stages show marked differences not only in terms of time scales but also in the transitions of burning regimes. Cenospheres generated in oxygen-free conditions are found to be ∼1.4 times larger than those in droplet combustion. Although similar in size, the solid-to-liquid consumption time ratio in 5% O2 is found to be significantly longer, about twice than for the 10% O2 ambient. The characterization of these differences is thought to be relevant not only for oil flames but also for the promising future of heavy oil gasification applications.

Error and Traffic State Analysis of Floating Car Data

Abstract. Floating car technology is widely applied in traffic information collection and has become a common method for real-time and dynamic monitoring of urban traffic conditions. Using floating car data from Shanghai as the research object, this paper analyzes the positioning error of floating cars and finds that most errors are within 15 meters. The error is significantly affected by vehicle speed but less by time, and the error decreases as the road grade increases. Speed and travel time ratio are used as parameters to assess the spatiotemporal traffic state, exploring the spatial distribution and temporal evolution of speed, as well as the spatiotemporal distribution of traffic congestion. In the spatial distribution of speed, clustering analysis is used to deduce the temporal distribution pattern of speed on the same road segment. Based on this, corresponding traffic management measures can be applied to different road segments and at different times.

A JWST Medium-resolution MIRI Spectrum and Models of the Type Ia Supernova 2021aefx at +415 days

We present a JWST MIRI medium-resolution spectrometer spectrum (5–27 μm) of the Type Ia supernova (SN Ia) SN 2021aefx at +415 days past B-band maximum. The spectrum, which was obtained during the iron-dominated nebular phase, has been analyzed in combination with previous JWST observations of SN 2021aefx to provide the first JWST time series analysis of an SN Ia. We find that the temporal evolution of the [Co iii] 11.888 μm feature directly traces the decay of 56Co. The spectra, line profiles, and their evolution are analyzed with off-center delayed-detonation models. Best fits were obtained with white dwarf (WD) central densities of ρ c = 0.9−1.1 × 109 g cm−3, a WD mass of M WD = 1.33–1.35 M ⊙, a WD magnetic field of ≈106 G, and an off-center deflagration-to-detonation transition at ≈0.5 M ⊙ seen opposite to the line of sight of the observer (−30°). The inner electron capture core is dominated by energy deposition from γ-rays, whereas a broader region is dominated by positron deposition, placing SN 2021aefx at +415 days in the transitional phase of the evolution to the positron-dominated regime. The formerly “flat-tilted” profile at 9 μm now has a significant contribution from [Ni iv], [Fe ii], and [Fe iii] and less from [Ar iii], which alters the shape of the feature as positrons mostly excite the low-velocity Ar. Overall, the strength of the stable Ni features in the spectrum is dominated by positron transport rather than the Ni mass. Based on multidimensional models, our analysis is consistent with a single-spot, close-to-central ignition with an indication of a preexisting turbulent velocity field and excludes a multiple-spot, off-center ignition.

Sediment transport on rippled beds

We conduct an Euler-Lagrange, direct numerical simulation of a turbulent channel flow at a shear Reynolds number of Reτ=180 over an erodible particle bed. The particle bed consists of approximately 1.3 × 106 monodisperse particles, resulting in a bed thickness of around 12–13 particles. The particle density and size are chosen to achieve a ratio of 4 for the Shields stress to the critical Shields stress necessary for incipient motion such that particle transport occurs primarily as bedload. The simulation is run long enough for ripples to form. We track the temporal evolution of the particle flux and excess Shields stress for the entire bed as well as for the four regions of a ripple, namely, the crest, trough, lee side, and stoss side. We find that the particle flux and excess Shields stress closely match the Wong and Parker correlation when the particle bed is featureless at early time but diverge from the correlation when ripples form. This deviation primarily arises from particle transport in the trough and lee side regions. Conversely, particle transport in the crest and stoss side regions remains largely consistent with the Wong and Parker correlation. A root mean square-based correction for the bed is proposed to be used in conjunction with the Wong and Parker correlation. Additionally, ripples attain a self-similar profile in the shape and near-bed shear stress when they are sufficiently distant from their upstream neighbor. Any departure from self-similarity occurs when the upstream neighbor gets within close proximity.

Morphosedimentary evolution of the Belgica Mound Drift: Controls on contourite depositional system development in association with cold-water coral mounds

Small-scale contourite drift is an important component of continental margins that can record information about complex oceanographic processes. The Belgica Mound Drift is one example of a small-scale contourite drift. It is formed under the influence of cold-water coral (CWC) mounds and represents one of the most distal contouritic expressions influenced by the Mediterranean Outflow Water (MOW) in the NE Atlantic Ocean. Three distinct evolutionary stages have been identified from new high-resolution pseudo-3D reflection seismic data, each associated with a significant change in paleoceanography, affecting both bottom-current intensity and sediment input. The pre-drift stage (Pliocene–Early Pleistocene) corresponds to the regional RD1 erosive event, which was caused by the reintroduction of the MOW in the Porcupine Seabight, creating a distinct paleotopography that will influence all ensuing sedimentary processes. The second stage (Early Pleistocene–Middle Pleistocene) is the contourite drift inception in two distinct centres of growth, strongly steered by topographic obstacles such as the CWC mounds. During the third and final stage (Middle Pleistocene–present day), the contourite drift is developed under a more stable but less dynamic environment, characterised by more continuous and mounded aggradational stratification. The final stage of the contourite drift is related to the Middle Pleistocene Transition, with a spatially variable reduction in the MOW-related bottom currents and sediment input. The spatial and temporal evolution of this drift shows that its present-day morphology is controlled by the location of initial growth. Evolving moat morphology indicates that the intensity of the bottom currents generally increases during the drift evolution.This research presents a crucial paradigm for advancing our knowledge of elucidating the complexities of smaller-sized contourite systems in diverse oceanic environments.

Understanding heavy precipitation events in southern Israel through atmospheric electric field observations

Characterizing the interaction between meteorological variables such as humidity, wind speed, cloud cover, and precipitation with the atmospheric electric field is vital for improving the nowcast of extreme weather events such as heavy precipitation. With this aim, we provide minute-scale electric field observations in southern Israel. These were taken during low-pressure weather systems in winter, often termed ‘Cyprus Lows.’ We focus only on precipitating (‘wet’) events, where rain was measured at the surface during and after the cold front's passage. The mean |PG| values for ‘wet’ Cyprus Lows are higher (Hundreds to thousands V m−1) compared with the mean fair-weather values (~100–200 V m−1, and exhibit a sharp and rapid increase of the |PG| of up to tens of V m−1 min−1 during the arrival of the cold front and hundreds of V m−1 min−1 during precipitation. Then, we analyzed selected case studies in detail. The response of the |PG| to thunderstorm clouds, i.e., Cumulonimbus, is an increase to values of thousands of V m−1. The temporal evolution of the |PG| allowed us to identify the type of cloud and its life cycle stage. We suggest that using state-of-the-art 1 Hz measurements of the |PG| and deducing cloud patterns at strategic locations, such as in arid regions like southern Israel, may improve the nowcasting capabilities of localized heavy precipitation events.

Bayesian hierarchical spatiotemporal modelling of tuberculosis – Human immunodeficiency virus co-infection in Ethiopia

Understanding the epidemiological patterns of tuberculosis-human immunodeficiency virus (TB-HIV) co-infection over space and time is crucial because it assists in identifying areas with high risks that need special control strategies. This article aimed to determine districts in Ethiopia that are most vulnerable to TB-HIV co-infection by examining the spatiotemporal patterns of the co-infection across four years, from 2015 to 2018. The study’s data came from Ethiopia’s Federal Ministry of Health. The data was analysed by applying the Bayesian hierarchical spatiotemporal modelling. We considered four models with different space–time interaction structures via the Integrated Nested Laplace Approximation (INLA) in the R-INLA package. In addition, we have applied the Deviance Information Criterion to select the most suitable model. The mean raw annual TB-HIV relative risk (RR) continuously decreased from 2015 to 2018, and the raw RRs of co-infection varied over districts and years. The spatiotemporal model, which allows for space–time interaction with independent spatial random effect and dependent temporal random effect, was the preferred model for describing the variations in TB-HIV co-infection across different districts over time. The prior variance for the spatial structured random effect had a smaller precision mode than the spatial unstructured random effect. This difference reveals that the former accounted for more spatial autocorrelation than the latter, indicating an information-borrowing effect amongst districts. Furthermore, the findings exhibit that the relative risk of TB-HIV co-infection had significant spatiotemporal variation and clustering. Through this research, further information was obtained regarding the temporal evolution of the geographical spread of TB and HIV co-infection at the district level in the country. It also made it possible to determine districts that should receive priority for control actions because of their high risk of co-infection.

Effect of Source-Sink Misalignment and Sink Cavity Aspect Ratio on the Thermal Performance of Gallium Heat Sinks

Herein, we numerically investigated methods to improve heat dissipation from the base of a phase-change material (PCM) heat sink exposed to high heat flux (15 W/cm2). The sink cavity exhibited a rectangular cross-section with two opposite vertical walls of high thermal conductivity. These walls serve as heat-dissipating fins. We used gallium, a metallic PCM with high thermal conductivity and a low melting point to improve heat dissipation by leveraging buoyancy-driven circulations within the molten gallium. We adopted a method to position the sink base on top of the heat source surface in such a way to augment temperature gradients within the liquid to induce imbalances in gallium density and consequent internal circulations. In this technique, one vertical wall is subjected to forced-convective heat exchange with the surroundings, while the other remains adiabatic. Subsequently, for the case which showed optimum performance, we relaxed the adiabatic thermal boundary condition applied at one of the vertical walls and replaced it with convective heat transfer condition, and assessed the sink performance based on that. Further, we analyzed the impact of various aspect ratios of the sink cavity on the induced internal circulations and base cooling. Sink cavities with aspect ratios of 1.33, 0.96, and 0.64 corresponding to sink heights of 20, 17, and 14 mm, respectively, were examined under a consistent heat flux applied across a 15-mm base length. We also investigated the precise positioning of the sink base relative to the heat source surface and explored its influence on temperature gradients and gallium density imbalances crucial for induced internal circulations. Results indicate that a sink cavity aspect ratio of 0.96 yields the optimal heat dissipation from the base. Further, aligning the left bottom edge of the sink base with the left edge of the heat source surface maximizes heat dissipation to the ambient surroundings, thereby prolonging latent-heat dumping into the sink before the gallium completely melts. Any misalignment between the sink base and the surface of the heat source may have a profound impact on the temporal evolution of induced internal circulations within the molten gallium, essentially affecting heat dissipation from the sink base.

Element-specific ultrafast lattice dynamics in FePt nanoparticles

Light–matter interaction at the nanoscale in magnetic alloys and heterostructures is a topic of intense research in view of potential applications in high-density magnetic recording. While the element-specific dynamics of electron spins is directly accessible to resonant x-ray pulses with femtosecond time structure, the possible element-specific atomic motion remains largely unexplored. We use ultrafast electron diffraction (UED) to probe the temporal evolution of lattice Bragg peaks of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. The diffraction interference between Fe and Pt sublattices enables us to demonstrate that the Fe mean square vibration amplitudes are significantly larger that those of Pt as expected from their different atomic mass. Both are found to increase as energy is transferred from the laser-excited electrons to the lattice. Contrary to this intuitive behavior, we observe a laser-induced lattice expansion that is larger for Pt than for Fe atoms during the first picosecond after laser excitation. This effect points to the strain-wave driven lattice expansion with the longitudinal acoustic Pt motion dominating that of Fe.

Spatiotemporal structure of flow field by a dielectric-barrier-discharge plasma actuator using phase-resolved particle image velocimetry

A typical dielectric-barrier-discharge plasma actuator (DBD-PA) operating in continuous mode generates a self-similar starting vortex followed by a quasi-steady wall jet. In the burst mode, it generates periodic vortices, leading to a spatially wider mean flow field. The characteristics of periodic vortices can be controlled by adjusting the duty cycle α and burst frequency fb of the actuation signal. In the present study, the flow field generated by the actuator in burst mode is studied for 50%≥α≤90%, and 10 Hz ≥fb≤90 Hz. The Reynolds number of mean flow based on the maximum induced velocity and jet half-width ranges from 171 to 524. High-speed laser-sheet visualization and time-resolved particle image velocimetry are used for flow field measurements. The flow field generated during burst mode operation shows the strong influence of the burst frequency. Periodic vortices with comparable size and convection speed of starting vortex are generated at low fb. Dipoles and small-scale vortices are formed at moderate fb similar to the behavior of a transitional wall jet. Kelvin–Helmholtz instability is observed at higher burst frequency. An ornamental vortex consisting of several small periodic vortices is formed at high values of fb during the starting period. Based on the temporal evolution of momentum, the flow field generated by the DBD-PA can be classified into (i) starting flow regime, (ii) transitional flow regime, and (iii) periodic flow regime. The self-similarity of the mean flow field generated by the DBD-PA is verified by comparing the mean velocity profile at multiple downstream locations with the self-similar solutions of laminar and turbulent wall jets. The entrainment coefficient of the mean flow field is similar to that of an axisymmetric turbulent wall jet. The bi-orthogonal analysis shows that both coherent mode energy content and the entropy are minimum (about 5% only) for the continuous mode actuation. For the burst mode actuation, both the coherent mode energy content and the entropy are higher in magnitude compared to that of continuous mode and decrease with increasing burst frequency. The vortices in the periodic flow regime are locked in with fb. The distance between subsequent vortices, λx and flow frequency, f follow a power-law relationship, λx=cf−n, where c is a constant and n can be approximated as 2/3.