Small Angles Research Articles

Erosion Investigation of Dimple Wall using Erosion-Coupled Dynamic Mesh.

The erosion of the dimple walls is investigated experimentally and numerically. A mathematical simulation framework was proposed to describe quantitatively the morphological evolution of the dimple wall quantitatively. As the wall shape continues to evolve, the wall shear stress, mesh deformation, and erosion rate would decrease and gradually tend to be constant. Two distinct regions have been identified along the dimple's windward wall surface: the wall's central area and the lateral area. In the central region, the wall profile flare occurs mainly in the early stage. In the lateral region, profile flare occurs mainly in the later stages of erosion. The microhardness of the wall surface shows a positive correlation with the erosion rate. The liquid-solid two-phase impinges on the wall at a smaller angle, and the wall material removal process is mainly based on the microcutting and slip mechanism. The results provide theoretical implications for the design of dimple-shaped, wide-channel welded plate heat exchangers.

Bethe-Heitler lepton pair production in the deuteron breakup reaction* *Supported by the National Natural Science Foundation of China (12075003, 12175117, 12335006)

We study the lepton pair production via the Bethe-Heitler mechanism in the deuteron breakup reaction. The complete seven-fold differential cross section is calculated with final state interactions taken into account. The deuteron bound state is described by a relativistic covariant deuteron-nucleon vertex. The numerical results indicate that the differential cross section is highly dependent on the lepton's azimuthal angle in regions of small polar angles and exhibits sharp peaks in the distribution over the invariant mass of the generated lepton pair or the two nucleons in the final state. We demonstrate that such a nearly singular feature originates from the collinearity between the produced lepton or antilepton and the incident photon, and it is physically regularized by the lepton mass in our calculation. The final state interaction between the knocked-out nucleon and recoil nucleon redistributes the differential cross section over the missing momentum, with a significant enhancement at a large missing momentum and a suppression in the intermediate region. With a further decomposition of the final state interaction contribution, It is found that the on-shell term dominates the near quasi-elastic region, while the off-shell term dominates the other end. Additionally, we examine the contribution from the interference between the proton amplitude and neutron amplitude, which, as expected, is found negligible even if the proton-neutron rescattering is included. The results of this study can serve as inputs for the analysis and background estimation of multiple exclusive measurements at Jefferson Lab and future electron-ion colliders.

Failure mechanism of rock specimens with an elliptical pre-hole under different stress conditions

This paper conducted a load test on rock specimens containing an elliptical pre-hole using a combination of numerical simulation and theoretical solutions of linear elasticity. The aim is to investigate the impact of elliptical geometric parameters and stress conditions on the rock failure process while considering a consistent hole area. Additionally, laboratory experiments incorporating digital image correlation (DIC) techniques were used for verification. This result indicates that the impact of elliptical geometric parameters is primarily observed during the early stage of loading, while it has minimal influence on the deterioration effect and failure mode of rock specimens during the later stage of loading. Regarding the inclination angle, as its value increases, the pure tension region near the hole diminishes, and the failure point on the hole's edge shifts from the coordinate axis direction to the inclination direction before returning to its original direction. Regarding the axis ratio, as the value decreases, the pure tension region diminishes at small angles, while the opposite occurs at large angles. Confining pressure affects the whole loading process. The greater the confining pressure, the smaller the tensile stress in the rock specimen, the more the tensile failure mode is suppressed, and the larger the rock specimen strength is.

Giant enhancement of second harmonic generation from monolayer 2D materials placed on photonic moiré superlattice

Abstract We numerically investigate second harmonic generation (SHG) from a monolayer of 2D-material placed on photonic moiré superlattice fabricated by dielectric materials. The greatly enhanced local field at the resonance modes of moiré superlattice can dramatically boost the SHG response in 2D materials. Considering a typical 2D-material MoS2 monolayer placed on a photonic moiré superlattice of a twist angle 9.43°, the maximum SHG conversion efficiency reaches up to 10−1 at a relatively low intensity of fundamental light 1 kW/cm2, which is around 14 orders of magnitude larger than that from the monolayer placed on a flat dielectric slab without moiré superlattices. The SHG conversion efficiency from the monolayer can be further enhanced with the decrease of the twist angles of moiré superlattice due to the even more confinement of local field. The flat bands in the moiré superlattices formed by the small twist angles can particularly ensure the efficiency even under wide-angle illuminations. The results indicate that photonic moiré superlattice which can tightly confine light is a promising platform for efficient nonlinear optics.

On Design of Through-Hole Structure Inspired by Vascular Bundle of Bamboo for Multiple Compression Load Cases

In this paper, the multiangle’s crashworthiness of the bionic tubes inspired by the cylindrical fibers of bamboo vascular bundles as well as the through-hole structure are studied. A finite element model validated by the theoretical analysis and drop hammer impact experiments is used to assess the optimal core distribution ratio of the bionic tube at 0°, 10°, 20°, and 30° impacts. Finally the effects of the number of circular core and wall thickness on crashworthiness are investigated. The results show that the bionic tube has better crashworthiness at small angle impacts compared to the square tubes, and l14r6 is obtained as the optimum bionic tube: the SEA at small angles improves by 12.72% (0°), 14.91% (10°), and 18.12% (20°), respectively; however, the SEA at 30° impact decreases by 2.53%. As the number of circular core increases, the crashworthiness becomes more significant, but the deformation mode becomes worse. The SEA of the tube with 32 cores increases by an average of 1.85 times (0°), 1.89 times (10°), 1.34 times (20°), and 1.41 times (30°) for each impact condition, respectively, compared to the tube with single core.

Indentation and Detachment in Adhesive Contacts between Soft Elastomer and Rigid Indenter at Simultaneous Motion in Normal and Tangential Direction: Experiments and Simulations.

In reported experiments, a steel indenter was pressed into a soft elastomer layer under varying inclination angles and subsequently was detached under various inclination angles too. The processes of indentation and detachment were recorded with a video camera, and the time dependences of the normal and tangential components of the contact force and the contact area, as well as the average contact pressure and average tangential stresses, were measured as functions of the inclination angle. Based on experimental results, a simple theoretical model of the indentation process is proposed, in which tangential and normal contacts are considered independently. Both experimental and theoretical results show that at small indentation angles (when the direction of motion is close to tangential), a mode with elastomer slippage relative to the indenter is observed, which leads to complex dynamic processes-the rearrangement of the contact boundary and the propagation of elastic waves (similar to Schallamach waves). If the angle is close to the normal angle, there is no slipping in the contact plane during the entire indentation (detachment) phase.

High frequency attenuation of elastic waves transmitted at an angle through a randomly-fluctuating horizontally-layered slab

This paper is concerned with the modeling of elastic waves traveling at small incidence angles through a randomly-fluctuating horizontally-layered slab, in regimes where the wavelength is small compared to the thickness of the slab. The wave propagation problem is reset in a frame following the coherent front, which propagates in a homogenized medium. This homogenized medium is anisotropic because of the layering, and the equations obtained account explicitly for the coupling of quasi-P and quasi-S waves. The resulting model is governed by a set of coupled stochastic ordinary differential equations that can be approximated numerically very efficiently, and yields in particular estimates of the transmission and reflections coefficients of the slab. The latter compare favorably to the coefficients obtained in a full scale numerical simulation of the (micro-scale) wave equation, for a fraction of the cost.

Melting Process of Frozen Sessile Droplets on Superhydrophobic Surfaces.

Superhydrophobic surfaces can exhibit icephobicity in many ways due to their large contact angles and small rolling angles. The melting process of frozen droplets on superhydrophobic surfaces is still unclear, hindering the understanding of surface icephobicity. In this experimental study of the melting process of frozen sessile droplets on superhydrophobic surfaces, we find two types of melting morphologies with opposite vortex directions on a single-scale nanostructured (SN) superhydrophobic substrate and a hierarchical-scale micronanostructured (HMN) superhydrophobic substrate. Melting pattern visualizations and flow field measurements showed Marangoni convection and natural convection occurring in the melting sessile droplets. For the HMN superhydrophobic substrate, the internal flow was found to be dominated by Marangoni convection due to the temperature gradient along the surface of the droplet. For the SN superhydrophobic substrate, Marangoni convection was inhibited by the superhydrophobic particles at the surface of the droplet, which were shed from the fragile superhydrophobic substrate during the freezing-melting process, as confirmed by surface characterizations of the substrate and flow measurements of a water pool. These results will help researchers better understand the melting process of frozen droplets and in designing novel icephobic surfaces for numerous applications.

Moiré Superlattice Structure of Pleated Trilayer Graphene Imaged by 4D Scanning Transmission Electron Microscopy.

Moiré superlattices in graphene arise from rotational twists in stacked 2D layers, leading to specific band structures, charge density and interlayer electron and excitonic interactions. The periodicities in bilayer graphene moiré lattices are given by a simple moiré basis vector that describes periodic oscillations in atomic density. The addition of a third layer to form trilayer graphene generates a moiré lattice comprised of multiple harmonics that do not occur in bilayer systems, leading to nontrivial crystal symmetries. Here, we use atomic resolution 4D-scanning transmission electron microscopy to study atomic structure in bilayer and trilayer graphene moiré superlattices and use 4D-STEM to map the electric fields to show subtle variations in the long-range moiré patterns. We show that monolayer graphene folded into an S-bend graphene pleat produces trilayer moiré superlattices with both small (<2°) and larger twist angles (7-30°). Annular in-plane electric field concentrations are detected in high angle bilayers due to overlapping rotated graphene hexagons in each layer. The presence of a third low angle twisted layer in S-bend trilayer graphene, introduces a long-range modulation of the atomic structure so that no real space unit cell is detected. By directly imaging trilayer moiré harmonics that span from picoscale to nanoscale using 4D-STEM, we gain insights into the complex spatial distributions of atomic density and electric fields in trilayer twisted layered materials.

Cavitation study of H-Darrieus hydrokinetic turbines via numerical simulations

This paper presents a parametric study of H-Darrieus turbines in cavitating conditions. The objective of this work is to assess the impact of cavitation on turbine performances for a wide variety of configurations and under various operating conditions. A better insight on the most “cavitation-friendly” turbine is provided by determining the optimal design parameters in terms of solidity, deployment depth, number of blades and blade pitch angle. Such design parameters are studied here and help to identify and analyse the inception and effects of cavitation. Two-dimensional URANS numerical simulations are carried out within Star-CCM+ at a Reynolds number of 107 based on the turbine diameter. The cavitating conditions simulated in this paper correspond to near-surface deployments at free-stream velocities ranging from 2 to 3 m/s. In a first phase, the solidity is varied from 0.1 to 0.7 while the tip-speed ratio is varied accordingly for optimal power coefficient. It is shown that a higher solidity turbine operating at a lower tip-speed ratio is beneficial in cavitating conditions due to the low effective velocity on the blades. The chord-to-radius ratio is varied in the second phase of this study. Turbines with 1, 3, 6 and 9 blades for a constant solidity of 0.6 are simulated and the results show that a larger chord-to-radius ratio is detrimental in cavitating conditions. Thus, the high curvature effects are to be avoided by using a larger number of blades for a given turbine solidity. The third and final phase of this study simulates turbines where the blade pitch angle was varied from −6° to +2° for a medium turbine solidity of 0.3. The small negative blade pitch angles of −2° and −4° show promising results where cavitation is greatly reduced, showing an improvement on power coefficient of 45 % for a blade pitch angle of −4° compared to the no blade pitch angle case at a cavitation number of 35. Overall, this paper concludes on the fact that the blade’s effective velocity and angle of attack are the key parameters controlling cavitation and that a combination of the studied design parameters should be considered to completely avoid cavitation inception or limit its detrimental effect on performance.

Ways to increase the stability of the movement of cars at small and large angles of rotation of the controlled wheels

The controllability and stability of the car are the most important operational properties and components of the active safety of its movement. Currently, the issues of creating active security systems are actively engaged all over the world. The purpose of the work is to analyze ways to increase the stability of the movement of a two—axle wheeled vehicle at small and large angles of rotation of the controlled wheels. A new method is proposed for analyzing ways to increase the stability of a two-axle wheeled vehicle, characterized in that it allows, based on the application of the second Lyapunov method, to identify methods to increase the stability of movement at small and large angles of rotation of controlled wheels. The analysis of the stability of the movement of two-axle wheeled vehicles at large rotation angles of controlled wheels with various traction and steering drive schemes is carried out. It is proved that for a two-axle wheeled vehicle with all driving and steerable wheels at small angles, the corrective steering angle on the rear axle should be directed towards reducing the angle of rotation of the front wheels of the rear axle modulo and, in addition, the rotation of the rear steerable wheels should begin later than the rotation of the front, i.e. with a delay. Keywords: сontrollability and stability of wheeled vehicles; large rotation angles of the driven wheels; redistribution of torques

Convolutional neural networks for tracking coalescing waveforms in supersonic jet noise

High-speed schlieren images of the field surrounding a supersonic jet provide a rich database for testing convolutional neural network (CNN) classification methods due to the presence of many waveform structures of interest. One such structure is waveform coalescence, where waves intersect at small angles, which can lead to increased steepening and nonlinear distortion in the jet near field [Willis et al., AIAA Journal (2023)]. Although waves exhibiting coalescence behavior have been identified in narrow field-of-view (FOV) schlieren images, new methods are needed for large FOV images to improve computational efficiency. This presentation will explore methods to track and classify waves observed in large FOV schlieren images as coalescing or noncoalescing using CNNs. Using transfer learning, pretrained networks can be retrained for this problem, with training data obtained from narrow FOV schlieren or from two-dimensional simulated waveforms using the Khokhlov–Zabolotskaya–Kuznetzov (KZK) equation. The impact of model hyperparameters, choice of pretrained network, image scaling, and other factors on the results of waveform classification by the CNN will be explored. [WAW is supported by the ARL:UT Chester M. McKinney Graduate Fellowship in Acoustics.]

Analysis of Root Angulation of Maxillary Central Incisor Using Cone Beam Computed Tomography (CBCT)

Background: Variance in anatomical morphology is influenced by the axial inclination of the tooth. When looking at the axial tilt of the crown, it's common to assume that it follows the same axis as the root. This study aims to use Cone Beam Computed Tomography (CBCT) to determine the root angulation correlation in maxillary central incisors. Methods: This cross-sectional observational research was performed at Dow University of Health Sciences (DUHS). The CBCT scans of patients who matched the inclusion criteria were done by skilled radiography technicians and primary investigators on ROTOGRAPH EVO 3D. For statistical analysis, one-way ANOVA was utilized to examine the Root Angulation (RA) with different root positions. p-value &lt;0.05 was considered significant. Results: This study examined n=152 CBCT images. Mean age was 27.2 + 5.9 years, with 32(42.1%) men and 44(44.1%) females. Buccal subtype I was most prevalent (59, 38.8%) in maxillary central incisors, while buccal subtype III was least common (5, 3.3%). The root angulations varied significantly between root location classifications (p=0.007). These were intermediate root location (14.9 2.6 degrees) and buccal subtype III (14.28 2.25 degrees). The palatal root type had the least angle (3.73 1.5 degrees). Conclusion: The buccal root position was shown to be the most common root location. Buccal subtype I was by far the most common. Buccal subtype III and middle root location had the maximum root angle. The palatal root position had the smallest angle.

Narrow Width Farley‐Buneman Spectra Above 100 km Altitude

AbstractFor spectra associated with full turbulence the observed mean phase velocity of unstable Farley‐Buneman waves has been found not to exceed the ion‐acoustic speed, cs. This has been attributed to various nonlinear processes. However, weakly turbulent modes are also excited on the edge of the “instability cone.” These modes have to be actual eigenmodes predicted by linear instability theory near‐threshold conditions. Unlike the modes that are associated with strong turbulence, these weakly turbulent modes are affected by the ion drift. This can make the Doppler shift of narrow spectra reach as high as the E × B drift velocity in the upper portion of the unstable layer at small aspect angles. Slow narrow spectra are also predicted nearer the E direction. We have produced a model of the Doppler shift of narrow‐width spectra under various electric field conditions above 100 km altitude. While the fluid dispersion relation is used to clarity the physics, we have also found the eigenmodes from an accepted kinetic dispersion relation. The calculations include a new model of the ion‐acoustic speed based on an empirical model of the electron temperature and of ion frictional heating under strong electric field conditions. The model provides an explanation for various VHF observations of the Doppler shift of narrow spectra that have been called “Type III spectra” and “Type IV spectra” in the existing literature.

Simulation of Spatial Polarization Characteristics of a Triorthogonal Antenna Element for the Tasks of HF Band Bearing

Introduction. Achieving improved accuracy and sensitivity of the direction finding of radio signals represents a relevant research direction in the field of modern HF radio monitoring. HF radio waves propagate through ionospheric layers, which distorts the polarization parameters of the passing electromagnetic wave (EMW). One possible approach to improve the accuracy and sensitivity of the direction finding of HF radio signals consists in the use of an antenna element capable of accepting both components of the electromagnetic field in the antenna array.Aim. A comparative analysis of the proposed triorthogonal antenna element with existing solutions for the tasks of HF band bearing.Materials and methods. Antenna elements and their spatial polarization characteristics were simulated in the MATLAB environment using the Phased Array toolbox.Results. The spatial polarization characteristics of the triorthogonal antenna under study were constructed and compared with an asymmetric vertical vibrator and a biorthogonal antenna. The comparison showed that at small elevation angles, the triorthogonal antenna ensures an energy gain of up to 4.5 dB compared to a biorthogonal antenna and an asymmetric vertical vibrator. At elevation angles of 30…60° and over 60°, the increase in the quality of a signal received by the triorthogonal antenna element reaches 3 dB and 2 dB, respectively.Conclusion. According to the obtained spatial polarization characteristics, the triorthogonal antenna under study can be part of a large-base antenna array of the HF band. The use of this antenna will increase the accuracy and sensitivity of direction finding by means of matching the antenna element with the EMW polarization

A comparative analysis of intraglottal geometry and velocity flow fields: Excised human, canine, and synthetic vocal fold models

Synthetic vocal fold models imitate the characteristics of human vocal folds and offer advantages over tissue models, such as accessibility and prolonged lifespan, all while producing comparable vibration frequencies to those in humans. Nonetheless, due to their simplified design, they may not fully capture the intricate behavior observed in tissue models like the canine larynx model. Canine larynges, like human’s, exhibit a vertical mucosal wave, which yields a phase delay between the inferior and superior edges of the vocal folds free margin. Consequently, a divergent glottis forms during closing, producing intraglottal flow separation vortices (FSVs) that increase voice loudness and intelligibility. To bridge the knowledge gap between silicone and tissue vocal fold models, we compared the intraglottal geometry and velocity flow fields during phonation of these three models. At low subglottal pressures, the synthetic model displayed small divergence angles and no flow separation, where both tissues models exhibited FSVs. At higher pressures, FSVs observed in tissue models were correlated with increased glottal flow waveform skewing and higher MFDR values. These findings highlight why the excised canine larynx model should be the preferred choice over synthetic vocal fold models for studying the aerodynamics and fluid-structure interaction during phonation.

Preliminary Results of Marine Gravity Recovery by Tiangong-2 Interferometric Imaging Radar Altimeter

This paper presents for the first time the results of marine gravity recovery using the ocean observation data acquired by Tiangong-2 interferometric imaging radar altimeter (TG2 InIRA) which demonstrate not only the balanced accuracies of the north and east components of deflection of the vertical (DOV) as envisaged, but also the improved spatial resolutions of DOV compared with that by conventional altimeters (CAs). Moreover, much higher measurement efficiency owing to the wide-swath capability and the great potential in accuracy improvement of marine gravity field are also demonstrated. TG2 InIRA adopts the interferometry with short baseline and takes small incidence angles, by which wide-swath sea surface height (SSH) can be measured with high accuracy. Gravity recovery experiments in the Western Pacific area are conducted to demonstrate the performance, advantages and capability of TG2 InIRA. SSH data processing algorithms and DOV calculation have been designed by taking the wide-swath feature into account, based on which, the gravity anomalies are then calculated using the inverse Vening Meinesz formula. The derived gravity anomalies are compared with both the published gravity models and the shipborne gravity measurements. The results show that the accuracy of TG2 InIRA is equivalent to, or even a little better than, that of CAs. The fused gravity result using equal TG2 InIRA data and CAs data performs better than those using TG2 InIRA data alone or CAs data alone. Due to the signal bandwidth of TG2 InIRA is only 40 MHz which is much smaller than that of CAs, much higher accuracy can be hopefully achieved for future missions if larger signal bandwidth is used.

Scattering of low-energy Ne+ ions from the stepped surface of InGaP(001)&lt;110&gt; at the small angles of incidence

Scattering of Ne+ ions at small angles of incidence from a stepped InGaP(001) &lt;110&gt; surface by the E0=5 keV was simulated using computer simulation. The trajectories of dechanneled ions from defect surface, as well as their energies at the scattering and scattering angle, are studied. It is shown that before dechanneling, the frequency and amplitude of the trajectory of ions, which move the surface channel formed by the stepped atom, increase. The energy distributions of these ions are obtained and the part of the spectrum corresponding to these ions is determined. It has been established that the energetic dechanneled ions formed low intensity peaks on the low-energy part of the spectrum.

Comparison of different inversion strategies for electrical impedance tomography (EIT) measurements

SUMMARY Electrical impedance tomography (EIT) is a promising method to image the frequency-dependent complex electrical conductivity distribution of the subsurface in the mHz to kHz frequency range. In contrast to the well-developed electrical resistivity tomography (ERT) method, the inversion approach for EIT data is less established. Different inversion strategies have been proposed, but the implications of the differences between these methods have not been investigated yet. In this study, we aim to compare four different inversion strategies for EIT measurements. The first strategy (CVI) formulates the inverse problem in the complex number domain and is mathematically the most elegant method. The second strategy (RVI) is the established real-valued inversion method, which decouples the inversion of the real and imaginary parts and completely ignores the complex nature. The third strategy (ALT) is very similar to the RVI strategy in case of small phase angles, but it considers the complex coupling in the forward operator and alternately updates the real and imaginary parts of the model in the case of large phase angles. The fourth and final strategy (CVI+) was newly formulated in this study. It fully considers the complex nature of EIT measurements but separates the treatment of the real and imaginary part in terms of the data weighting and regularization. The different inversion strategies were tested with two synthetic models. The first model has a small phase contrast and the second model has a large phase contrast. In the case of a small phase contrast, the CVI strategy was able to resolve the distribution of electrical conductivity amplitude, but the inversion result for the phase angle was less reliable. The other three strategies presented similar results and the models were well resolved within the expected data misfit. In the case of a model with large phase contrast, only the newly formulated CVI + strategy was able to produce reliable results. It was found that the extremely large phase angle can have a significant influence on the modelled amplitude of data. The cross-sensitivity (i.e. the imaginary part of the sensitivity) that describes the influence on the real part of data due to a change in the imaginary part of model, or that on the imaginary part of data due to a change in the real part of model, provided unique information during the inversion. It was concluded that the CVI + strategy is theoretically the most comprehensive and correct approach for EIT inversion, but that in the case of small phase angles the RVI strategy has the practical advantage that no complex calculations are required, which substantially reduces the required computational effort.

Small-angle strabismus detection by telemedicine in an experimental model

To evaluate the ability of observers to accurately detect strabismus using an alternate cover testing approach via telemedicine and to assess the effect of various factors related to video conditions on the accuracy rate. Videos were made by the authors in which different angles of strabismus, up to 12Δ, were induced by prism and recorded using alternate cover testing. The videos were made under a variety of conditions that incorporated various head postures, lighting, fixation target position and motion, and viewing angle. The videos were shown to observers of varying levels of expertise. The accuracy of detecting the deviation was assessed and analyzed for statistical significance. The overall rate of detection was significantly affected by the angle of deviation, with 12Δ being correctly detected with 94% accuracy, 8Δ with 72% accuracy, and 4Δ with <50% accuracy. Esotropia and hypertropia were more accurately detected than exotropia for all angles tested. The level of training of the observer did not correlate with detection accuracy. However, accuracy was negatively affected by backlighting and movement of the fixation target during cover testing. Our data demonstrate that detection of even relatively small angles of strabismus can be accomplished with a high level of accuracy using telemedicine and an alternate cover testing strategy. For optimal results, it is important to consider conditions related to the fixation target and lighting.