Longitudinal Structures Research Articles

Particle transport and turbulence modification in unstably stratified mixed convection within a horizontal channel

Particle transport and turbulence modification in an unstably stratified, particle-laden turbulent flow within a horizontal channel is studied via direct numerical simulations combined with Lagrangian point‒particle tracking techniques. Two-way coupling in momentum and energy is considered in dilute gas‒solid flows under mixed convection (synergistic effects of shear and buoyancy). For comparison, simulations of neutral turbulence are also conducted with equivalent parameters. The study reveals that large-scale longitudinal vortical structures induce significant spatial heterogeneity in the spanwise distribution of inertial particles. This heterogeneity is characterized by an increase in the particle concentration on the side of the cold plume and a corresponding decrease on the side of the hot plume. In the context of unstably stratified turbulence, particles reduce the fluid streamwise velocity and promote its profile toward symmetry. This phenomenon contrasts with that observed in neutral flows, where particles induce velocity asymmetry by dragging the upper flow and accelerating the lower flow. A quantitative analysis of the heat flux indicates that particles absorb heat as they settle, thereby reducing the average temperature of the flow. Buoyancy effects slow the settling and reinjection of particles, which in turn diminishes thermal energy absorption. Particles with higher inertia preferentially settle on the cooler plume side, minimizing their participation in heat exchange due to prolonged durations of repeated wall collisions.

Global Ionospheric Response During Extreme Geomagnetic Storm in May 2024

The main idea of the present study is to investigate in detail the time evolution of the spatial inhomogeneities connected with the ionospheric response to the geomagnetic storm registered in the period of 10–11 May 2024. The obtained ionospheric anomalies represented by the relative deviations of the global Total Electron Content (TEC) data have been utilized in the analysis. The used global TEC data have been converted to a coordinate system with a modip latitude and geographical longitude. In addition to the maps illustrating the global spatial distribution of the geomagnetically forced ionospheric anomalies, a presentation of the observed longitudinal structures by sinusoidal approximation has also been used. The resulting positive and negative responses have been studied depending on the magnetic latitude, local times and the behavior of the geomagnetic activity parameters during the considered event. The interpretation takes into account the known mechanisms for the effect of the geomagnetic storm on the electron density. A special attention is focused on the differences in the two hemispheres at high and mid latitudes, where a simultaneous direct impact of the particle precipitation and the change in the temperature regime of the neutral atmosphere has been assumed. The low-latitude response as a result of the Equatorial Ionization Anomaly (EIA) associated with Disturbed Dynamo Electric Fields (DDEFs) and its relationship with local time has also been considered.

Steady secondary flow in a turbulent boundary layer past a slender axisymmetric body

The turbulent boundary layer in a viscous incompressible fluid developing longitudinally past the surface of a thin cone or cylinder at a finite distance from the laminar–turbulent transition zone is studied. The characteristic Reynolds number, determined from the external flow velocity and the length of the body, is assumed to be large, and the thickness of the boundary layer is small and comparable to the radius of the body. The asymptotic method of multiple scales is used to find solutions to the Navier–Stokes equations. Instead of the traditional decomposition of the solution into time-averaged values and their fluctuations, the velocities and pressure are expressed as an asymptotic series consisting of steady and perturbed terms. As a result, the viscous steady flow (‘secondary’) that arises in the boundary layer as a mandatory component of fast turbulent fluctuations was described. Analytical and numerical solutions for the radial steady velocity are presented, describing the self-induced suction of fluid from the external flow into the boundary layer. Further analytical solutions are obtained for the longitudinal and circumferential velocities, which differ markedly from the laminar regime. The solutions found are somewhat similar to the degenerate (one-dimensional) case of self-sustaining longitudinal thin structures in turbulent shear flows. A qualitative comparison with direct numerical simulations is presented.

Quantitative revealing the solute segregation behavior at melt pool boundary in additively manufactured stainless steel using a novel processing method for precise positioning by HAADF-STEM

Laser-powder bed fusion (LPBF) enables the fabrication of complex metallic components by manipulating various laser scan strategies to control microstructure and texture. Multiple thermal cycling and rapid solidification lead to non-equilibrium, non-uniform microstructure, and micro-segregation at the melt pool boundary (MPB), whose accurate location is still invisible by transmission electron microscopy (TEM), and quantitative concentration remains imprecise. In this study, we proposed a novel method to make it clear by controlling the crystallographic texture of 316 L stainless steel through unique LPBF processing parameters to obtain a single-crystal-like microstructure of the cellular structures along the laser scanning direction. The accurate location of the track-track MPB is distinguishable by means of the transverse and longitudinal cellular dislocation structures on both sides. The edge-on state of the track-track MPB makes the quantitative concentration analysis precisely using high-angle annular dark-field scanning TEM with energy-dispersive X-ray spectroscopy, which is in good agreement with the Scheil-Gulliver solidification simulations.

Longitude Structure of Wavenumber 4 of the Ionosphere After Midnight Based on the OI135.6 nm Night Airglow Using FY‐3D Ionospheric Photometer

AbstractIn this study, based on the OI135.6 nm night airglow data of the FY‐3D Ionospheric Photometer (IPM) during the 2018–2021 geomagnetically quiet period, the global wavenumber 4 longitudinal structure of the equatorial ionization anomaly (EIA) at 2:00 local time was discovered, and the component of the wavenumber 4 was extracted from these structures. Compared with the OI135.6 nm night airglow data of the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) F18 during 2011–2014, there were significant differences in the variation pattern of the relative amplitude of the two versus solar activity and the seasonal variation in the proportion of the component of the wavenumber 4. Based on the neutral wind speed observation results of the Michelson Interferometer for Global High‐Resolution Thermospheric Imaging on board the Ionospheric Connection Explorer (ICON) from 2020 to 2021, the longitudinal structures of the 4 ionospheric waves after midnight are related to the cross‐equatorial meridional wind. We believe that the wavenumber 4 longitudinal structures after midnight originate from the semidiurnal eastward‐propagating with zonal wavenumber 2 (SE2) nonmigrating tide in the cross‐equatorial neutral wind rather than the diurnal eastward‐propagating with zonal wavenumber 3 (DE3) nonmigrating tide in from the zonal wind, which modulates the daytime wavenumber 4 longitudinal structures.

Heat transfer enhancement mechanics of different micro-disturbing structures for transcritical hydrocarbon fuel using 3-D secondary flow intensity analysis

Heat transfer enhancement based upon micro-disturbing structures becomes increasingly important for thermal protection of air-breathing propulsion systems. To explore the influence mechanism of different structures on heat transfer in regenerative cooling channels of scramjet engines, flow structures and heat transfer of supercritical-pressure n-Decane in mini-channels with dimple and micro-rib arrays are numerically investigated. Results indicate effective heat transfer enhancement inside the channels depends more on efficient transverse turbulence disturbance rather than longitudinal or perpendicular turbulence disturbance. Severe thermal stratification generates strong shear layers in channels, where transverse secondary flows bring much intenser flow mixing than longitudinal secondary flows, especially in transcritical temperature region where the best heat transfer enhancement is obtained. With same characteristic dimensions, longitudinal micro-disturbing structures like dimples have lower flow resistance, but transverse micro-disturbing structures like micro-ribs achieve stronger heat transfer enhancement. Besides, the performance of micro-ribs is much more sensitive to the dimensional change. Therefore, micro-ribs have larger working excursion for enhancing performance factor than dimples. With optimal structure parameters: hr = e = 0.3 mm, pr = 3 mm, hd = 0.225 mm, dd = 1.5 mm, overall performance factor in micro-ribbed channel is 1.14 times of that in dimpled channel. Those conclusions provide effective support for optimization design for the regenerative cooling structures.

Numerical study of PCM melting performance in a rectangular container with various longitudinal fin structures

This paper uses the enthalpy-porosity technique to conduct a three-dimensional numerical investigation on the effect of adding longitudinal fins in energy storage units to enhance the melting process, with a specific focus on natural convection. By reducing the extension length of the upper part of the fins and increasing the extension length at the bottom, the work aims to enhance heat transfer while reducing obstruction to convection. The effects of rectangular, trapezoidal, and triangular longitudinal fins with thicknesses of 0.2, 0.25, and 0.3 mm on the melting characteristics were compared while maintaining a constant filling volume of RT42 phase change material (PCM) within the rectangular container. The results indicate that the enhanced heat transfer capacity of fins with different structures increases as the fin length increases. Triangular fins exhibit the most significant heat transfer enhancement effect, with the least hindrance to convection. When the fin thickness is 0.2 mm, the triangular fin decreases the melting completion time by 15.7 %, increases the heat storage rate by 18.7 % compared to the rectangular fin, and exhibits optimal natural convection intensity and temperature distribution in the energy storage unit. Additionally, the maximum flow velocity and average Nusselt number can be increased by 21.2 % and 18.5 %, respectively. It was also found that triangular fins are more suitable for high-temperature working conditions.

On the influence of coastal structures on shoreline variability and beach interconnection

Approximately 24 % of sandy beaches worldwide are experiencing erosional processes often driven by natural forces that are intensified by human activities. Erosion becomes a significant issue, particularly when it affects areas of socioeconomic and ecological importance. Thus, this study aimed to utilize coastline data from a city in southern Brazil, extracted from satellite images using free computational tools, with three main objectives: (1) identify spatial and temporal trends and cycles; (2) assess the impact of coastal structures; and (3) identify beach interconnection indicating bypassing processes in sandy beaches. For the first two objectives, shoreline data from 2015 to 2023, as well as statistical data, were extracted using the CASSIE tool. These data were filtered to minimize tidal effects and remove outliers. Subsequently, they were visualized using a Hovmöller diagram to provide a systemic overview. Through this diagram, it was possible to identify sediment transport toward the north and a sub-annual cycle of seasonality, with erosion occurring during the southern hemisphere's spring months, a period characterized by high-energy events. The relationship between erosional/accretional stretches and anthropogenic structures highlighted the negative impact of longitudinal structures, which exacerbated erosional processes, and the limited effectiveness of transverse structures in retaining necessary sediment. Moreover, the detrimental effect of building a jetty on its adjacent beaches was identified due to the alteration of the refraction control point and consequently, the equilibrium state of the beaches, which are unable to adapt due to coastal urbanization. For the third objective, shoreline data detected by the CoastSat tool from 1984 to 2023 were used. These data were subjected to Pearson correlation analysis. Perpendicular barriers to the shoreline that exhibited low correlation indicate limits of circulation cells that prevent the continuous bypassing process. Conversely, high correlation values suggest that the barrier allows for the occurrence of this process.

Anatomical Variations to the Vertebral Artery and Posterior Inferior Cerebellar Artery are Associated with the Partial Persistence of Primitive Lateral Basirovertebral Anastomosis

The angioarchitecture of the hindbrain is homologous to that of the spinal cord, and its vascular system can be analyzed at the longitudinal and axial structures. During embryonic development, there are two main longitudinal arteries: the longitudinal neural artery and the primitive lateral basilovertebral anastomosis. Commonly observed variations are formed by the fenestration and duplication of either the vertebrobasilar artery, or cerebellar artery, which can be observed when the primitive lateral basilovertebral anastomosis partially persists. Understanding the pattern and development of blood supply to the hindbrain provides useful information of various anomalies in the vertebrobasilar junction and cerebellar arteries.

Pseudo-static Winkler springs for longitudinal underground structures subjected to shear waves

ABSTRACT Shear waves in underground longitudinal structures impose deformations that need to be accounted for in structural design. Simplified approaches for such structures typically consider the Winkler foundation model, assuming Euler-Bernoulli beam theory, which neglects shearing-induced distortions, especially significant when shear waves are a dominant component of the seismic motion. In order to overcome these limitations, Timoshenko beam models have been proposed in the literature. These approaches however depend on an appropriate determination of the ground springs. Existing analytical formulations often assume plane-strain conditions, inadequate for representing low frequencies and thus are not directly applicable for pseudo-static interaction analyses. The present paper develops analytical solutions to determine transverse and rotation Winkler springs for structures subjected shear waves. The proposed springs overcome the drawbacks of plane-strain models and can be construed as a generalisation of them. The springs are obtained as a function of the seismic wavelength and the ground-structure stiffness contrast. Results obtained are validated against solutions from the literature and numerical results from a full 3D finite-element model. A non-dimensional parametric study is also presented, that allow an expedited evaluation of ground springs for practical applications.

Climatology of the Nonmigrating Tides Based on Long-Term SABER/TIMED Measurements and Their Impact on the Longitudinal Structures Observed in the Ionosphere

This paper presents climatological features of the longitudinal structures WN4, WN3, and WN2 and their drivers observed in the lower thermospheric temperatures and in the ionospheric TEC. For this purpose, two long-term data sets are utilized: the satellite SABER/TIMED temperature measurements, and the global TEC maps generated with the NASA JPL for the interval of 2002–2022. As the main drivers of the longitudinal structures are mainly nonmigrating tides, this study first investigates the climatology of those nonmigrating tides, which are the main contributors of the considered longitudinal structures; these are nonmigrating diurnal DE3, DE2, and DW2, and semidiurnal SW4 and SE2 tides. The climatology of WN4, WN3, and WN2 structures in the lower thermosphere reveals that WN4 is the strongest one with a magnitude of ~20 K observed at 10° S in August, followed by WN2 with ~13.9 K at 10° S in February, and the weakest is WN3 with ~12.4 K observed over the equator in July. In the ionosphere, WN3 is the strongest structure with a magnitude of 5.9 TECU located at −30° modip latitude in October, followed by WN2 with 5.4 TECU at 30 modip in March, and the last is WN4 with 3.7 TECU at −30 modip in August. Both the climatology of the WSA and the features of its drivers are investigated as well.

An acoustic impedance design method for tubular structures with broadband sound insulations and efficient air ventilation

In this research, we proposed an acoustic impedance-based design method to increase the soundproofing bandwidths and maintain efficient air ventilation for tubular structures consisting of hollow air-flowing channels. This research has great potential to provide design guidelines based on an acoustic impedance model and even/odd mode analysis for hollow-tube ventilated devices containing resonance and antiresonances. We introduced a thin ventilated structure with a subwavelength thickness of 0.107 λ where λ corresponds to the lowest frequency within the filtering bandwidth, a 19.4 % cross-section open for air passage, and a 10 dB fractional bandwidth of 110.2 % (i.e., 90 % sound energy is blocked), which is broader than that of a conventional Fano-like structure of 44.7 %. To experimentally verify the proposed approach, two prototypes, including lateral-coupled and longitudinal-coupled devices, were designed, fabricated with 3D printing, and experimentally characterized via a commercial-available impedance tube system. The simulations matched the measurements and demonstrated the 10 dB fractional bandwidth of 65 % and 91.5 % and ventilation efficiencies of 21.4 % and 33.4 %, respectively. The main contribution of this work is that the proposed approach can be adopted for lateral or longitudinal coupled ventilated structures with broadband sound insulations and lower the effects of resonance-induced sound transmissions. Besides, the proposed acoustic-impedance model can significantly save the required tremendous computational resources and time compared to FEM simulations. Moreover, the proposed lateral and longitudinal ventilated structures are expected to benefit broadband sound filtering and noise reduction for thin panel or tubular silencers

A New Pathway for Tornadogenesis Exposed by Numerical Simulations of Supercells in Turbulent Environments

Abstract A simulation of a supercell storm produced for a prior study on tornado predictability is reanalyzed for the purpose of examining the fine-scale details of tornadogenesis. It is found that the formation of a tornado-like vortex in the simulation differs from how such vortices have been understood to form in previous numerical simulations. The main difference between the present simulation and past ones is the inclusion of a turbulent boundary layer in the storm’s environment in the present case, whereas prior simulations have used a laminar boundary layer. The turbulent environment contains significant near-surface vertical vorticity (ζ > 0.03 s−1 at z = 7.5 m), organized in the form of longitudinal streaks aligned with the southerly ground-relative winds. The ζ streaks are associated with corrugations in the vertical plane in the predominantly horizontal, westward-pointing environmental vortex lines; the vortex-line corrugations are produced by the vertical drafts associated with coherent turbulent structures aligned with the aforementioned southerly ground-relative winds (longitudinal coherent structures in the surface layer such as these are well known to the boundary layer and turbulence communities). The ζ streaks serve as focal points for tornadogenesis, and may actually facilitate tornadogenesis, given how near-surface ζ in the environment can rapidly amplify when subjected to the strong, persistent convergence beneath a supercell updraft. Significance Statement In high-resolution computer simulations of supercell storms that include a more realistic, turbulent environment, the means by which tornado-like vortices form differs from the mechanism identified in prior simulations using a less realistic, laminar environment. One possibility is that prior simulations develop intense vortices for the wrong reasons. Another possibility could be that tornadoes form in a wide range of ways in the real atmosphere, even within supercell storms that appear to be similar, and increasingly realistic computer simulations are finally now capturing that diversity.

Excitation of nonlinear longitudinal structures in spin‐1/2 quantum plasma in the presence of ion beam

AbstractThe formation of longitudinal, nonlinear structures in an interacting ion beam, spin‐1/2 quantum, electron‐ion plasma is reported by employing a separate spin evolution quantum hydrodynamic model (SSE‐QHD). The reductive perturbation method is followed to develop the KdV equations. In the presence of an ion beam and keeping the finite electron inertial effects, ion acoustic (IA) soliton and spin electron acoustic (SEA) soliton are found. The critical values of beam speed and spin polarization, required for the existence of solitary structures, are highlighted. It is prescribed that the present model supports rarefactive and compressive SEA soliton and compressive IA soliton. It is shown that the ion beam is essential for the existence of a compressive SEA solitary structure. We have also calculated and compared the energy densities of both solitons. Furthermore, the dependence of the nonlinear structures on the beam parameters and spin polarization are also examined. Our findings would be useful to understand the new spin‐dependent nonlinear structures as well as the features of nonlinear IA waves in ion beam–plasma interactions.

Climatology of Dayside E‐Region Zonal Neutral Wind Shears From ICON‐MIGHTI Observations

AbstractLarge vertical shears in the E‐region neutral zonal winds can lead to ion convergences and contribute to plasma irregularities, but climatological studies of vertical shears of horizontal winds in a global scale are lacking due to the limitations of data coverage. The Ionospheric Connection Explorer (ICON) Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) has provided neutral wind observations with an unprecedented spatial coverage. In this study, the climatology of dayside E‐region neutral wind shears has been examined using 2‐years’ data (2020–2021). Specifically, the study focuses on large wind shears with a magnitude larger than 20 m/s/km, since large wind shears are more likely to cause significant perturbation in the ionosphere‐thermosphere (I‐T) system. The results show that the probability of occurrence of large shears is strongly dependent on the altitude, with the vertical profile varying with shear direction, latitude, season, and local time. In general, below 110 km altitude, large negative shears of the eastward wind are most likely to happen during summer at 8–10 LT in 25°N–40°N latitudes, showing a high probability across nearly all longitudes. Meanwhile, large positive shears tend to occur in 10°S–10°N latitudes, with peak probabilities exhibiting roughly consistent longitudinal structures across 8–10 LT in all seasons. The discrepancies between positive and negative large shear distributions underlie different global tidal influences. The large‐shear occurrence probabilities above 110 km are generally small, except in latitudes above 25°N during the winter for positive shears.

Microbunching of thermionic cathode rf gun beams in the Advanced Photon Source s-band linac

We report on measurements of beams from thermionic cathode (TC) rf guns in the Advanced Photon Source S-Band Linac. These measurements include the macropulse out of both new and existing TC guns as well as the observation of microbunching within the micropulses of these beams. A gun chopper limits the macropulse FWHM duration to the 10-ns range. Our objectives were to analyse the new TC gun and investigate microbunching within a TC-rf-gun-generated beam. Our diagnostics elucidated longitudinal beam structures from the ns to the fs time scales. Coherent transition radiation (CTR) interferometers responding to far-infrared wavelengths were employed after each compression stage to provide the autocorrelations of the sub-ps micropulse durations. The first compression stage is an alpha magnet and the second a chicane. A CCD camera was used to image the beam via optical transition radiation from an Al screen at the end of the linac and also employed to measure coherent optical transition radiation (COTR) in the visible range. The COTR diagnostic observations, implying microbunching on a fs time scale, are presented and compared with a longitudinal space-charge impedance model.

Modelling the development of biological structures displaying longitudinal geometries in vitro: culturing pluripotent stem cells on plasma-treated, growth factor-coupled polycaprolactone fibres

Many biological structures such as nerves, blood and lymphatic vessels, and muscle fibres exhibit longitudinal geometries with distinct cell types extending along both the length and width of internal linear axes. Modelling these three-dimensional structures in vitro is challenging: the best-defined stem-cell differentiation systems are monolayer cultures or organoids using pluripotent stem cells. Pluripotent stem cells can differentiate into functionally mature cells depending on the signals received, holding great promise for regenerative medicine. However, the integration of in vitro differentiated cell types into diseased tissue remains a challenge. Engineered scaffolds can bridge this gap if the appropriate signalling systems are incorporated into the scaffold. Here, we have taken a biomimicry approach to generate longitudinal structures in vitro. In this approach, mouse embryonic stem cells are directed to differentiate to specific cell types on the surface of polycaprolactone (PCL) fibres treated by plasma-immersion ion implantation and to which with lineage-specifying molecules have been covalently immobilised. We demonstrate the simplicity and utility of our method for efficiently generating high yields of the following cell types from these pluripotent stem cells: neurons, vascular endothelial cells, osteoclasts, adipocytes, and cells of the erythroid, myeloid, and lymphoid lineages. Strategically arranged plasma-treated scaffolds with differentiated cell types could ultimately serve as a means for the repair or treatment of diseased or damaged tissue.

<i>In Situ</i> ALD-TEM System to Study Atomic Nucleation Phenomena - Enabled by Ultrathin Large Aspect Ratio Free-Standing Shell Structures

Today, Atomic Layer Deposition (ALD) sets the limits for achieving nanometer precision of thin, yet almost dense films. However, the initial growth process, determining possible film thinness, is poorly understood. A better understanding can be obtained with the help of in-situ characterization during film growth with high spatial and chemical resolution. Transmission Electron microscopy (TEM) would be a suitable and widely available technique to accomplish this objective. However, standard instruments have differing vacuum requirements than those necessary for ALD. During ALD, TEM detectors could be damaged as they are being exposed to corrosive volatile chemical compounds. Here we present a dedicated TEM holder design, where ALD deposition occurs inside a microchip containing a large area cavity surrounded by thin film Al2O3 membranes. These membranes act as windows for TEM characterization while decoupling the ALD process from the TEM environment. The microchip consists of longitudinal large overhang shell structures, themselves made by ALD and etched in an HF vapor etch processes. The set-up, which includes controlled heating, was tailored to ALD requirements, and passed a vacuum-pressure test. Post-mortem inspection of film growth on a silicon sample chip demonstrates the successful formation of an ALD Al2O3 film with 40 nm thickness inside the cavity. These results demonstrate the potential of the system to enable a range of experiments on growth phenomena that may lead to even thinner films and better control of interfaces than previously possible.

Evaluation of Local Scour along the Base of Longitudinal Training Walls

This study proposes a new empirical model for estimating local scour along the base of longitudinal training walls for granular riverbeds. The model’s performance was rigorously assessed through experiments conducted in an open-channel flume, encompassing variations in granulometric characteristics, slope, and flow rates. The investigation involved a comparative analysis of six commonly employed equations for scour estimation. The results consistently demonstrated a tendency of the selected equations to overestimate scour depth within the longitudinal structures. In contrast, the new proposed equation considers factors such as the well-graded granular bedding represented by the Coefficient of uniformity (Cu) and the embedment of the longitudinal wall. This allows for a more robust identification of the scour behavior of longitudinal walls. This research enhances our comprehension of local scour in riverbeds. It provides engineers and researchers with a valuable tool for more accurate predictions, thereby contributing to the improved design and maintenance of river environment structures.

Customization and Information Encoding of Longitudinally Polarized Light Fields

The tightly focused structured light field displays a non‐negligible component of the electric field in the direction of propagation, i.e., the longitudinal light field. The longitudinal light field results in various novel optical phenomena, such as transverse spin angular momentum and 3D topological light fields. However, it remains a challenge to generate customized longitudinal light fields. Here, based on inverse design theory, the longitudinal light field can be customized on demand under the tightly focusing condition, which allows to generate the longitudinal light field with controllable structured distribution for propagating waves or manipulate the complex structures for all the three components of the electric field. In experiment, an experimental system is built integrating the generation, customization, and detection of structured longitudinal light field, and the customization of the longitudinal light fields with various patterns is experimentally realized. The customized longitudinal light field structures may find applications such as information encoding, long‐distance light manipulation, and novel light–matter interactions. As an example, information encoding is realized by the structured longitudinal light fields.