Front Curvature Research Articles

Magnetic domain wall curvature induced by wire edge pinning

In this study, we report on the analysis of the magnetic domain wall (DW) curvature due to magnetic field induced motion in Ta/CoFeB/MgO and Pt/Co/Pt wires with perpendicular magnetic anisotropy. In wires of 20 μm and 25 μm, a large edge pinning potential produces the anchoring of the DW ends to the wire edges, which is evidenced as a significant curvature of the DW front as it propagates. As the driving magnetic field is increased, the curvature reduces as a result of the system moving away from the creep regime of DW motion, which implies a weaker dependence of the DW dynamics on the interaction between the DW and the wire edge defects. A simple model is derived to describe the dependence of the DW curvature on the driving magnetic field and allows us to extract the parameter σE, which accounts for the strength of the edge pinning potential. The model describes well the systems with both weak and strong bulk pinning potentials like Ta/CoFeB/MgO and Pt/Co/Pt, respectively. This provides a means to quantify the effect of edge pinning induced DW curvature on magnetic DW dynamics.

Icing of a droplet deposited onto a subcooled surface

ABSTRACT When a droplet impinges onto a subcooled surface, it starts freezing very rapidly if the surface is already covered with frost. In this study, the freezing process is investigated for water droplet and two subcooled substrates having different thermal properties: duralumin (high heat conductivity) and N-BK7 glass (low heat conductivity). Experiments performed on single droplets and modelling are carried out together. A new experimental technique based on laser-induced fluorescence is developed to reconstruct the evolution of the ice front within the freezing droplets with and without taking into account the volume expansion during the freezing process. Comparisons between the two substrates reveals how the freezing dynamics can have a significant effect on the shape of the freezing front. Early in the freezing process, the ice front displays a spherical shape for both substrates. However, the curvature of the ice front and the angle with the free liquid surface are dependent on the freezing rate and the substrate material. The formation of a pointy tip is observed only for the N-BK7 substrate, as the solidification takes a much longer time in this case. The contact angle at the solid/liquid/air tri-junction point allows explaining the formation of this pointy tip. A simplified 2D axisymmetric model, accounting for the enthalpy released at the phase change and the thermal diffusion in both the solid substrate and the freezing medium is compared to the experimental results. Although it is in general not able to mimic the exact shape of the ice front, it allows predicting the overall freezing rate with a good accuracy.

Research on fatigue crack propagation process of fracturing pumphead

Fracturing pumpheads are a key and crucial component in fracturing technology. Fatigue crack failure greatly decreases the pumphead working life and operational efficiency. This paper describes an analytical model for a five-cylinder fracturing pumphead using the submodel method with the re-meshing technique to analyze its fatigue crack propagation process. Changes in the crack face, a-KI curve (crack length a and stress intensity factor KI), and a-N curve (number of fatigue loading cycles N) during crack propagation are considered. The results show that the crack failure of the fracturing pumphead results primarily from mode-I cracks. During crack propagation, the curvature of the crack front gradually decreases due to the higher stress intensity factor KI at the pressured inner surfaces of the pumphead. The stress distribution on the pumphead greatly influences the change law for the a-KI curve. When the crack tip localizes in the high stress zone, KI increases with the crack length a; when the crack tip localizes in the low stress zone, KI decreases with the crack length a. The step structure of the pumphead also influences changes to the a-KI curve. Around the step structure, a wave peak appears in the a-KI curve, but the influence is localised and its effect on the a-N curve is not significant.

Phase Control of the Giant Resonant Goos-Hänchen Shift

It has been demonstrated that the lateral Goos-Hanchen shift of light beams reflected and transmitted through a layered dielectric structure can be effectively controlled by the focusing (defocusing) of the incident beam. The shift of the reflected and transmitted beams depends on the curvature of the phase front of the incident beam if the spectral line of transmitted radiation is sufficiently narrow. This dependence is due to the broadening of its spatial spectrum and to the difference of the reflection (transmission) coefficients of spatial harmonics forming the beam. As a result, Goos-Hanchen shifts change strongly compared to the case of the incident beam with a plane phase front and can even change sign.

The angular resolution of GRAPES-3 EAS array after improved timing and shower front curvature correction based on age and size

The angular resolution of an extensive air shower (EAS) array plays a critical role in determining its sensitivity for the detection of point γ-ray sources in the multi-TeV energy range. The GRAPES-3, an EAS array located at Ooty in India (11.4̂N, 76.7̂E, 2200 m altitude) is designed to study γ-rays in the TeV-PeV energy range. It comprises of a dense array of 400 plastic scintillators deployed over an area of 25000 m2 and a large area (560 m2) muon telescope. The development of a new statistical method has allowed the real time determination of propagation delay of each detector in the GRAPES-3 array. The shape of the shower front is known to be curved and here the details of a new method developed for accurate measurement of the shower front curvature is presented. These two developments have led to a sizable improvement in the angular resolution of the GRAPES-3 array. It is shown that the curvature depends on the size and the age of an EAS. By employing two different techniques, namely, the odd-even and the left-right methods, independent estimates of the angular resolution are obtained. The odd-even method estimates the best achievable resolution of the array. For obtaining the angular resolution, the left-right method is used after implementing the size and age dependent curvature correction. A comparison of the angular resolution as a function of EAS energy by these two methods shows them be virtually indistinguishable. The angular resolution of the GRAPES-3 array is 47′ for energies E>5 TeV and improves to 17′ at E>100 TeV, eventually approaching 10′ at E>500 TeV.

Density and Temperature Fluctuations behind a Shock Wave under the Influence of a Stratified Energy Source

For the conditions obtained in experiments on the interaction between a shock wave and the ionization-unstable plasma, the influence of a thermally stratified energy source on the front of a shock wave is numerically modeled on the basis of the full system of Navier–Stokes equations. It is shown that the front curvature detected in schlieren images is associated with a higher temperature of central layers in the source, while its disappearance is due to the multiple generation of Richtmyer–Meshkov instabilities in the gas density field. It is demonstrated that the redistribution of source energy into layers leads to the formation of local areas with a gas temperature increased by several times (in comparison with the values for a homogeneous source) behind a shock wave.

Objects interacting with solidification fronts: Thermal and solute effects

Objects, such as droplets, bubbles, or particles, interact with a moving solidification front in several material processing routes. The consequences of this confrontation, varying from instantaneous engulfment to complete rejection of suspended objects, regulates the solidified microstructure and thus, the functional properties of the material. In this study, we utilize in situ cryo-confocal microscopy to investigate the interplay between thermal conductivity mismatch (of the object and the melt) and the role of solute, which both impact the geometry of the ice-water solidification front in different manners. First, we experimentally validate the theoretical predictions of physical models on the distortion of thermal fields in the presence of foreign objects. Subsequently, we report on the dominating effect of solute on the front curvature with the dynamic evolution of premelted films. In the case of cellular front morphology, we depict that particles with a lower thermal conductivity than water induce a distortion of the encircling ice crystals, thereby modifying the final microstructure of the solid. Finally, our results demonstrate that to successfully predict and control the solidification microstructure, in the presence of objects, physical models incorporating both thermal and long-range solute effects are required.

On the Possible Relaxation of the ASTM E1921 and ASTM E1820 Standard Specifications with Respect to the Use of the Mini-CT Specimen

Specimen miniaturization is gaining a lot of interest in the fracture mechanics community thanks to recent methods accounting for specimen size correction. In particular, the miniature compact tension (mini-CT) specimen is gaining large international interest for irradiated surveillance materials used to monitor irradiation damage in reactor pressure vessels. Because of the limited available amount of some of these materials, eight mini-CT specimens can easily be machined from one broken Charpy specimen, avoiding the need of consuming surveillance materials. Specimen reorientation also becomes possible to investigate anisotropic effects. At this stage, ASTM E1921, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range(Superseded), the standard for brittle fracture, and ASTM E1820, Standard Test Method for Measurement of Fracture Toughness (Superseded), the standard for ductile fracture, are used for this geometry, although it does not fully satisfy the requirements of the standards. In particular, the specifications on the specimen dimensions relative to the specimen width are tight, given that the tolerances are a fraction of the specimen width. Given that the actual dimensions of the specimen are taken into account in the calculations, the analysis described in this article shows that the tight dimensional accuracy is not necessary. Moreover, given the limited thickness of the specimen, the curvature of the fatigue precrack tends to deviate from a quasi-straight front. As a result, a large number of mini-CT specimens do not fulfill the standard requirements with respect to crack front curvature. Previous tests demonstrated that actual cracks excessive curvature lead to similar results when compared with artificial fatigue quasi-straight cracks. The objective of this article is to experimentally demonstrate that some of the standard requirements can be relaxed without jeopardizing the quality of test data, in terms of master curve and crack resistance curve.

Small Incision Lenticule Extraction (SMILE) for Moderate and High Myopia: Seven-Year Outcomes of Refraction, Corneal Tomography, and Wavefront Aberrations.

Purpose To investigate the long-term outcomes of refraction, corneal tomography, and wavefront aberrations after small incision lenticule extraction (SMILE) for moderate and high myopia. Methods Prospective, nonconsecutive case series. A total of 26 patients (26 eyes) who underwent SMILE from May 2010 to March 2013 at the Fudan University Eye and ENT Hospital (Shanghai, China) were enrolled. The periods of follow-up were 1 month, 1 year, 5 years, and 7 years after surgery. The routine eye examinations included uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), manifest refraction, and corneal tomography. Results All surgeries were executed without any complications. At the final visit, an UDVA of 20/20 or better was achieved in 26 eyes (100%) and 11 eyes (42%) exhibited no change in CDVA. 9 eyes (35%) gained one line, 6 eyes (23%) gained two lines, and no eyes lost CDVA. 24 eyes (92%) and 26 eyes (100%) were within ±0.5 D and ±1.00 D of the target refraction, respectively. A mean refractive regression of −0.17 D was observed between 1 month and 7 years postoperatively. Mean corneal front curvature (MCFC) was significantly decreased between pre- and post-SMILE surgery (P < 0.0001). Higher-order aberrations (HOAs) and vertical coma were significantly increased after SMILE compared to those measured before surgery (all P < 0.001). There were no significant differences in trefoil and spherical aberration between pre- and post-SMILE surgery (all P > 0.05). Conclusion SMILE is an effective, safe, and stable procedure for moderate and high myopia, with relatively constant corneal stability and wavefront aberrations. This trial is registered with ChiCTR-ONRC-13003114.

Upstream actuation for bluff-body wake control driven by a genetically inspired optimization

The control of bluff-body wakes for reduced drag and enhanced stability has traditionally relied on the so-called direct-wake control approach. By the use of actuators or passive devices, one can manipulate the aerodynamic loads that act on the rear of the model. An alternative approach for the manipulation of the flow is to move the position of the actuator upstream, hence interacting with an easier-to-manipulate boundary layer. The present paper comprises a bluff-body flow study via large-eddy simulations to investigate the effectiveness of an upstream actuator (positioned at the leading edge) with regard to the manipulation of the wake dynamics and its aerodynamic loads. A rectangular cylinder with rounded leading edges, equipped with actuators positioned at the front curvatures, is simulated at $Re=40\,000$ . A genetic algorithm (GA) optimization is performed to find an effective actuation that minimizes drag. It is shown that the GA selects superharmonic frequencies of the natural vortex shedding. Hence, the induced disturbances, penetrating downstream in the wake, significantly reduce drag and lateral instability. A comparison with a side-recirculation-suppression approach is also presented, the latter case being worse in terms of reduced drag (only 8 % drag reduction achieved), despite the total suppression of the side recirculation bubble. In contrast, the GA optimized case contributes to a 20 % drag reduction with respect to the unactuated case. In addition, the large drag reduction is associated with a reduced shedding motion and an improved lateral stability.

Using Deep Learning in Ultra-High Energy Cosmic Ray Experiments

The extremely low flux of ultra-high energy cosmic rays (UHECR) makes their direct observation by orbital experiments practically impossible. For this reason all current and planned UHECR experiments detect cosmic rays indirectly by observing the extensive air showers (EAS) initiated by cosmic ray particles in the atmosphere. Various types of shower observables are analyzed in the modern UHECR experiments including a secondary radio signal and fluorescent light from the excited nitrogen molecules. Most of the data is collected by the network of surface stations which allows to measure the lateral EAS profile. The raw observables in this case are the time-resolved signals for the set of adjacent triggered stations. The Monte Carlo shower simulation is performed in order to recover the primary particle properties. In traditional techniques the MC simulation is used to fit some synthetic observables such as the shower rise time, the shower front curvature and the particle density normalized to a given distance from the core. In this talk we’ll consider an alternative approach based on the deep convolutional neural network trained on a large Monte-Carlo dataset, using the detector signal time series as an input. The above approach has proven its efficiency with the Monte-Carlo simulations of the Telescope Array Observatory surface detector. We’ll discuss in detail how the network architecture is optimized for this particular task.

Experimental limit on the validity of Markstein relation

A PIV-based system has been set-up for the simultaneous measurement of the local propagation velocity and the flame stretch of laminar jet premixed flames. The contributions of the flame front curvature and the strain rate of the fresh gas flow to the flame stretch are separately discussed. Laminar jet flames of methane-air lean mixtures have been studied with a secondary flame in a coflow configuration to avoid blow-off. In weakly curved flame tips (low inlet gas velocity), these measurements confirm the prediction of linear velocity-stretch dependence and allow the indirect determination of the Markstein length and planar flame propagation velocity, by extrapolating the experimental results to the zero-stretch condition. In moderately curved flame tips (large inlet gas velocity), experiments show a faster than linear increase of the flame velocity with the stretch. The transition between these two regimes occurs suddenly when a threshold value of the inlet gas velocity is exceeded.

Development of Growth Theory for Vapor–Liquid–Solid Nanowires: Wetting Scenario, Front Curvature, Growth Angle, Linear Tension, and Radial Instability

In this paper, we report that under wetting conditions (or modes) of nanowire (NW) growth, when a nonplanar crystallization front emerges under a catalyst droplet, a shift in the three-phase line (TPL) of the vapor–liquid–crystal interface occurs under thermodynamically stable conditions when the angle with respect to the droplet surface, termed the growth angle, is fixed. The growth angle of the NWs is determined not from a geometrical perspective but on the basis of the physical aspects of the processes occurring around the TPL, revealing a size dependence caused by the influence of linear tension of the three-phase contact of a vapor–liquid crystal. The observed radial periodic instability of the NWs is described according to the size dependence of the thermodynamic growth angle, which induces negative feedback in the system. Under the influence of linear tension and positive feedback, the tips or needles of NWs can be formed.

Enhancing Accuracy Determination of Sources of Radio Emission Coordinates by Linear Antenna Arrays

Two methods have been proposed for enhancing the accuracy in determining the coordinates of radio emission sources by using phase systems based on reducing the dynamic and random errors in terms of the curvature of electromagnetic wave phase front during processing the signals at inputs of linear antenna array. The reduction of dynamic errors stipulated by the limited number of terms of the Maclaurin expansion of the distance-to-source function is achieved by estimating each value of distance and bearing without use of iterative methods. The reduction of random errors occurring during the source motion is performed at the expense of preliminary estimation of values of phase difference of received signals in the process of bearing estimation and values of the difference of phase differences in determining the distance by using the least squares method in the sliding window.

Influence of submerged heating on vertical Bridgman crystal growth of silicon under travelling magnetic field

The effect of a submerged heater on the Bridgman crystal growth process under travelling magnetic field is studied. It is known that the magnetic field generates melt flow from crystal center to crucible wall which can suppress morphological instability development, but it also increases the curvature of the crystallization front. The advantage of the submerged heater method is the ability of better control over the temperature distribution in the vicinity of the solid-liquid interface, as well as the constancy of the growth zone geometry, in particular the height of the melt above the phase boundary, which helps reduce the axial heterogeneity of the resulting crystal. Within the steady state approach we perform numerical simulation of Bridgman growth process under simultaneous action of travelling magnetic field and a submerged heater. It is shown that the submerged heater can intensify the flow in the radial direction generated by travelling magnetic field near the growing crystal while the curvature of the crystallization interface remains almost unaffected. On the other hand, it is also possible to decrease significantly the flow close to the crystallization front.

Selection of the optimal heat treatment conditions using advanced phase transformation models

The paper focuses on the problem of optimal cooling cycle selection for steel sheets after hot rolling in order to obtain a given phase composition using complex numerical model. The proposed model driven by diffusion of carbon with the moving boundary (Stefan problem) was designed and implemented on the basis of Level Set equations coupled with diffusion equation. Application of implicit method for interface evolution allows to overcome the limitations associated with the complicated representation of material microstructure, but also in a natural way reflects the impact of front curvature and influence of chemical forces on the velocity of the interface. The article discusses the details of the model as well as the applied optimization technique. Three test scenarios are analyzed and the results are discussed.

Detonation wave propagation in micro-scale groove charges

The detonation wave propagation characteristics in micro-scale groove charges are very important for optimizing the structure of the Micro-Electro-Mechanics System explosive train and improving its detonation reliability. Focusing on the problem of detonation wave propagation of micro-scale groove charges under strong confinement, the effects of charge density, groove size and confinement are considered. A theoretical model of curved detonation wave propagation in a micro-scale groove charge under a strong confinement was established by means of equivalent mass correction. The mathematical expression for the detonation velocity was derived and a numerical calculation method of detonation velocity and shock front shape was given using MATLAB software. An experiment was designed to test the detonation velocities for micro-scale groove charges with a booster explosive. The results closely agreed with the calculations, validating the propagation model of curved detonation waves. The results show that the smaller the groove size, the bigger the detonation velocity loss and the curvature of shock front in the central axis. When the charge size was 0.6×0.6mm, the detonation velocity loss was 11.49%. The detonation velocity and maximum streamline deflection angle increase with increasing charge density and size. The increase of streamline deflection angle reduces the detonation velocity of the explosive. However, the streamline deflection angle changes by only a small amount in the micro-scale with an effect on the detonation velocity of less than 1%. The detonation velocity has a strong correlation with charge size and density. This paper contains theoretical guidance for the design and performance optimization of charge structures in the MEMS explosive train.

ADDTID: An Alternative Tool for Studying Earthquake/Tsunami Signatures in the Ionosphere. Case of the 2011 Tohoku Earthquake

In this work, we characterized the ionospheric disturbances generated during the Japan Tohoku earthquake of 11 March 2011, by means of the Atomic Decomposition Detector of Traveling Ionospheric Disturbances (ADDTID) algorithm. This algorithm automatically detects and characterizes Traveling Ionospheric Disturbances (TIDs) from Global Navigation Satellite System (GNSS) measurements. Applying the high-precision estimates of ADDTID, the propagation parameters would make it easier to distinguish TIDs from different origins, in particular the characteristics conforming the acoustic gravity waves driven by the earthquake/tsunami. This method does not assume that disturbances follow a circular pattern of propagation, and can estimate the location by the propagation pattern of tsunami wavefronts and related TIDs. In this work, we present in a single framework a description of phenomena observed by different researchers. By means of the ADDTID algorithm, we detect: (a) simultaneous TIDs of different characteristics, where the detection was robust against the curvature of the wave fronts of the perturbations and the accuracy of the estimated parameters. The results were double-checked by visual inspection from detrended Vertical Total Electron Content (VTEC) maps and keogram plots, and the parameters of the slow-speed TIDs were consistent with the tsunami waveform measurements; (b) different wavefronts between the west and east TIDs around the epicenter, consistent in time and space with the post-earthquake tsunami; (c) complete evolution of the circular TIDs driven by the tsunami during the GNSS observable area; (d) fast and short circular TIDs related to the acoustic waves of earthquake; (e) the pre-seismic activity consisting of a set of fast westward TIDs, and the comparison with neighboring days; (f) the location estimation of the tsunami wavefront along the coast and the possible use as early warning. Finally, we report disturbances that had not been previously published with a possible application to local and real-time detection of tsunamis.

Experimental study on the excitation of thermoacoustic instability of hydrogen-methane/air premixed flames under atmospheric and elevated pressure conditions

The current work examines the excitation of thermoacoustic instability of lean premixed hydrogen-methane/air low swirl flames under both atmospheric and elevated pressure conditions (up to 0.3 MPa). Under a given pressure condition, The tests were conducted at different bulk velocities (U), hydrogen proportions (ηH), and equivalence ratios (Ф). Results show that thermoacoustic instability can be excited by increasing one of these variables while keeping others the same. It was found that pressure elevation has a minor effect on the oscillation frequency. Moreover, it was demonstrated that the current instability is induced by large coherent structures. The effect of pressure elevation on the excitation of thermoacoustic instability is found to be Φ dependent. As indicators of the flame response to impinging vortices, the curvature and local flame surface area features were calculated with images captured with the planar laser-induced fluorescence of the OH radical (OH-PLIF) method. Results demonstrate a great similarity between the flame front evolution and the instability trend, implying that the effect of the chamber pressure on the instability trend can be indicated by the change in the flame front curvature and local flame surface area.

Theoretical and numerical investigations of rod growth of an Ni–Zr eutectic alloy

In this work, the directional solidification of NiZr–NiZr $$_2$$ eutectics under isothermal conditions is investigated through numerical and theoretical means. Multiple three-dimensional phase-field simulations, including a large-scale simulation, are performed to study the free pattern selection and the velocity–spacing relation of the evolving solidification microstructures. The computed velocities for different spacings of the stoichiometric NiZr rods in the as-well stoichiometric NiZr $$_2$$ matrix are compared with the predictions of the classical Jackson–Hunt analysis. Due to certain simplifying assumptions invoked in the original theory which are not entirely representative of the numerically realized microstructures, significant deviations are observed between the two. In view of this, an extended theory is formulated accounting for the global hexagonal arrangement of the evolving rods as well as the solidification front curvatures. Owing to that, a superior compliance is achieved between the analytical and simulated growth kinetics. The key elements of symmetry incorporation are of particular importance, especially in applications to ternary systems.