Velocity Reversal Research Articles

Effects of horizontal magnetic fields on flow morphologies and global transports in liquid metal thermal convection

We report an experimental study of Rayleigh–Bénard convection of liquid metal GaInSn in a cuboid cell with an aspect ratio of 0.5 under the effect of a horizontal magnetic field. The Rayleigh number spans a range of $3.8\times 10^5 \leqslant Ra \leqslant 1.1\times 10^7$ , while the magnetic field strength reaches up to 0.5 T, corresponding to a maximum Hartmann number to 2041. By combining temperature and velocity measurements, we identify several flow morphologies, including a novel cellular pattern characterized by four stacked vortices that periodically squeeze and induce velocity reversals. Based on the identified flow morphologies, we partition the entire ( $Ra, Ha$ ) parameter space into five distinct flow regimes and systematically investigate the flow characteristics within each regime. The temperature gradient and oscillation frequency exhibit scaling relationships with the combined parameters $Ra$ and $Ha$ . Notably, we observe a coupling between flow regime and global transport efficiencies, particularly in a regime dominated by the double-roll structure, which experiences a maximum 36 % decrease in heat transfer efficiency compared with the single-roll structure. The dependencies of heat and momentum transport on $Ra$ and $Ha$ follow scaling laws as $Nu \sim (Ha^{-2/3}RaPr^{-1})^{3/5}$ and $Re \sim (Ha^{-1}RaPr^{-1})^{4/3}$ , respectively.

Response of chemically active dimer motor in phase-separating binary fluid mixture: Motilityregulation and self-aggregation.

The design of synthetic chemically powered nanomotors often considers the fuel and product to be miscible. The propulsion properties of such motors can be altered if the binary fluid consisting of fuel and product is phase separating. The dynamical properties of a dimer motor in a phase-separating binary mixture are discussed. Depending on the strength of phase separation and the activity of the dimer, the single-motor propulsion velocity either decreases or reverses its direction. The velocity reversal is shown to be related to the generated fluid flow around the motor. The collective dynamics of the motors in such phase-separating fluid results in the formation of self-assembled structures.

Insight into intracontinental deformation and crustal reworking of ancient continents implied by crustal structure imaging of the Yangtze Craton

The crustal imprints from multistage tectonic activities in cratons offer valuable insights into continental evolution. Utilizing seismic data from two densely deployed, nearly perpendicular linear arrays, and a newly developed stepwise joint inversion of depth-domain receiver function and surface wave dispersion, we constructed a detailed crustal layering model for the Yangtze Craton. Our analysis revealed elongated double velocity reversal zones as salient features of the crust, which likely record ancient crustal reworking and juvenile crustal growth associated with Neoproterozoic rift-related magmatic processes. The interlayering of low- and high-velocity structures may contribute to the enduring stability of the Yangtze Craton. Additionally, superimposed layers separated by east-dipping interfaces and abrupt changes in crustal thickness in the boundary belts surrounding the Yangtze Craton document the crust's structural responses to intracontinental deformation during continent assembly.

Investigation of computational model for the natural circulation at dual channel facility

The current work investigates a computational model to study the thermal and hydraulic air behavior during the natural circulation at air ingression and accidents. This is done with the RHYS coupling ASYST VER 4 package. The test facility considered for the present study is a dual vertical channel facility comprised of two parallel channels connected to the upper and lower plenum. The flow fields in the heated and cooled channels were comprehensively characterized by analyzing axial temperature and velocity distributions using varied uniform iso-flux (100–1400 W/m2) and different outer surface temperatures (278, 288, 298, and 308 K). Temperature and velocity reversal recorded after maximal spots due to natural convection. The temperature rise from 278 to 308 K gave an average of 25.51 and 25.19° increase in air and inner wall temperatures, respectively, while air velocity increases at high cooling intensity (278 K) within the heated channel, in the cooled channel, low cooling intensity (308 K) resulted in higher velocity. The convective heat transfer is represented in terms of heat transfer coefficients, which are used to compute the Nusselt number. Additionally, the ASYST model was validated with data from literature sources, indicating strong agreement.

Numerical Study of Heat Transfer and Fluid Flow Characteristics of a Hydrogen Pulsating Heat Pipe with Medium Filling Ratio

Benefiting from its high thermal conductivity, simple structure, and light weight, the pulsating heat pipe (PHP) can meet the requirements for high efficiency, flexibility, and low cost in industrial heat transfer applications such as aerospace detector cooling and vehicle thermal management. Compared to a PHP working at room temperature, the mechanism of a PHP with hydrogen as the working fluid differs significantly due to the unique thermal properties of hydrogen. In this paper, a two-dimensional model of a hydrogen PHP with a filling ratio of 51% was established to study the flow characteristics and thermal performance. The volume of fluid (VOF) method was used to capture the phase distribution and interface dynamics, and the Lee model was employed to account for phase change. To validate the model, a comparison was conducted between the simulation results and experimental data obtained in our laboratory. The simulation results show that the pressure and temperature errors were within 25% and 5%, respectively. Throughout a pressure oscillation cycle, the occurrence of uniform flow velocity, acceleration, and flow reversal can be attributed to the changes in the vapor–liquid phase distribution resulting from the effect of condensation and evaporation. In addition, when the fluid velocity was greater than 0.6 m/s, dynamic contact angle hysteresis was observed in the condenser. The results contribute to a deeper understanding of the flow and heat transfer mechanism of the hydrogen PHPs, which have not been yet achieved through visualization experiments.

Biased motility-induced phase separation: from chemotaxis to traffic jams

We propose a one-dimensional model of active particles interpolating between quorum sensing models used in the study of motility-induced phase separation (MIPS) and models of congestion of traffic flow on a single-lane highway. Particles have a target velocity with a density-dependent magnitude and a direction that flips with a finite rate that is biased toward moving right. Two key parameters are the bias and the speed relaxation time. MIPS is known to occur in such models at zero bias and zero relaxation time (overdamped dynamics), while a fully biased motion with no velocity reversal models traffic flow on a highway. Using both numerical simulations and continuum equations derived from the microscopic dynamics, we show that a single phase-separated state extends from the usual MIPS to congested traffic flow in the phase diagram defined by the bias and the speed relaxation time. However, in the fully biased case, inertia is essential to observe phase separation, making MIPS and congested traffic flow seemingly different phenomena if not simultaneously considering inertia and tumbling. We characterize the velocity of the dense phase, which is static for usual MIPS and moves backward in traffic congestion. We also find that in presence of bias, the phase diagram becomes richer, with an additional transition between phase separation and a microphase separation that is seen above a threshold bias or relaxation rate.

Complex dayside particle precipitation observed during the passage of a solar wind rotational discontinuity

The dayside particle precipitation during the passage of a solar wind rotational discontinuity (RD) event on 10 April 2015 is examined and reviewed. The RD leads to complex structures at the magnetopause, boundary layer, mantle, and cusp even though the geomagnetic activity level remains low. Particle precipitation data from DMSP F17 reveal the formation of an unusual boundary layer where the low energy (cold) ions exhibit energy-latitude dispersion that is usually associated with mantle while the high energy (hot) ions look like typical magnetospheric ions. DMSP F17 and F19 observe a double cusp that is a signature of magnetic reconnection occurring at both high- and low-latitudes due to the dominant IMF By. A global MHD simulation of the event supports the existence of the simultaneous reconnections at high- and low-latitude magnetopause that are consistent with the anti-parallel and component merging models, respectively. Finally, Cluster C2, located at high-latitude and high-altitude in the southern hemisphere, observes velocity fluctuations and reversals with peak-to-peak amplitudes >800 km·s–1 as it crosses the magnetopause. Guided by the MHD simulation, the Cluster observation can be interpreted as the spacecraft crossing reconnection outflows while moving from one side of the X-line to the other.

Umbilical Cord Wraps around a Newborn's Legs like Ankle Shackles.

A 36-year-old woman, gravida 3, para 1 (previous cesarean section), with one medical abortion, and no history of systemic diseases presented an unremarkable medical history during prenatal visits. The final prenatal ultrasound at 38 weeks of gestation showed a vertex presentation, a weight of 2600 g, a normal amniotic fluid level, and the placenta located on the posterior wall of the uterus. Fetal cardiotocography conducted before delivery reported a reactive heart rate without decelerations. The Doppler wave analysis of the fetal umbilical artery was normal (the ratio of peak-systolic flow velocity to the end-diastolic flow velocity was 2.5) without the absence or reversal of end-diastolic velocity. The total score of the fetal biophysical profile by ultrasound was 8. The night before the scheduled cesarean section, she experienced heightened anxiety and was unable to sleep, noting "crazy" fetal movements throughout the night. During the cesarean section, it was discovered that the umbilical cord was wrapped around the newborn's legs, resembling ankle shackles. The fetal weight was 2740 g, and Apgar scores were 9 at the first minute and 10 at the fifth minute. The motility of the neonatal legs was normal without cyanosis or neurological adverse outcomes.

Collective variable model for the dynamics of liquid crystal skyrmions

Liquid crystal skyrmions are topologically protected spatially-localized distortions of the director field which exhibit particle-like properties including translational motion in oscillating electric fields. Here, we develop a collective variable model of the skyrmion dynamics, extending the approach of Long and Selinger proposed earlier for one dimensional systems. The model relates the skyrmion motion to a complex dynamics of the width of the twist wall around the skyrmion core. The width evolves in a non-reciprocal way, quantifying squirming deformations of the high twist region within on and off states of the field. We analyze in details the average skyrmion velocity as a function of the frequency and strength of the field as well as its duty cycle. The model predictions agrees qualitatively with experiments and results of numerical minimization of the Frank-Oseen model. Our results provide insights into the conditions necessary to observe velocity reversal as a function of the field parameters.

Particle-based model of liquid crystal skyrmion dynamics.

Motivated by recent experimental results that reveal rich collective dynamics of thousands-to-millions of active liquid crystal skyrmions, we have developed a coarse-grained, particle-based model of the dynamics of skyrmions in a dilute regime. The basic physical mechanism of skyrmion motion is related to squirming undulations of domains with high director twist within the skyrmion cores when the electric field is turned on and off. The motion is not related to mass flow and is caused only by the reorientation dynamics of the director field. Based on the results of the "fine-grained" Frank-Oseen continuum model, we have mapped these squirming director distortions onto an effective force that acts asymmetrically upon switching the electrical field on or off. The resulting model correctly reproduces the skyrmion dynamics, including velocity reversal as a function of the frequency of a pulse width modulated driving voltage. We have also obtained approximate analytical expressions for the phenomenological model parameters encoding their dependence upon the cholesteric pitch and the strength of the electric field. This has been achieved by fitting coarse-grained skyrmion trajectories to those determined in the framework of the Frank-Oseen model.

A minimalist approach for improved time imaging of a noisy vibroseis data: A case study from Jaisalmer Basin, Rajasthan

AbstractProcessing land seismic data, especially vibroseis data, is often challenging due to complicated near‐surface situations and source‐generated noise. The case study deals with noisy vibroseis data acquired in the Jaisalmer Basin. The near‐surface estimation in this area is difficult due to a possible velocity reversal manifested in the shingled patterns of the first breaks. The near‐surface workflow incorporates a model‐adaptive first break‐picking approach, essentially integrating two problems of first‐break‐picking and model estimation into a single problem. The signal‐conditioning workflow is based on cascaded scaling and single‐channel‐based noise reduction to prevent the removal of weak signals. Horizon‐based migration velocity analysis was used to focus reflectors on the constant velocity‐migrated stacks. This was particularly useful in areas with dubious velocity trends based on semblance panels. The velocity volume has structural consistency, which provides a better time‐migrated image. The workflow also incorporates a targeted post‐stack processing sequence to enhance continuity, sharpen discontinuities and improve the resolution, as notable by comparing the legacy‐processing results of the same dataset.

Use of QM/MM Surface Hopping Simulations to Understand Thermally Activated Rare-Event Nonadiabatic Transitions in the Condensed Phase.

We implement a rare-event sampling scheme for quantifying the rate of thermally activated nonadiabatic transitions in the condensed phase. Our Quantum mechanics/molecular mechanics (QM/MM) methodology uses the recently developed Interface for NonAdiabatic QM/MM in Solvent (INAQS) package to interface an elementary electronic structure package and a popular open-source molecular dynamics software (GROMACS) to simulate an electron transfer event between two stationary ions in a solution of acetonitrile solvent molecules. Nonadiabatic effects are implemented through a surface hopping scheme, and our simulations allow further quantitative insight into the participation ratio of a solvent and the effect of ion separation distance as far as facilitating electron transfer. We also demonstrate that the standard gas-phase approaches for treating frustrated hops and velocity reversal must be refined when working in the condensed phase with many degrees of freedom. The code and methodology developed here can be easily expanded upon and modified to incorporate other systems and should provide a great deal of new insight into a wide variety of condensed phase nonadiabatic phenomena.

The Reversibility Paradox: Role of the Velocity Reversal Step

The reversibility, or Loschmidt paradox, is a thought experiment in which microscopic reversibility is exploited to generate an apparently spontaneous trajectory in which entropy decreases, thus violating the Second Law of Thermodynamics (SLT). To achieve this, the system is allowed to evolve from a low-entropy to a high-entropy configuration, at which point all velocities of its molecules are reversed, thus producing the desired effect. In this paper we focus on the velocity reversal step. Implementing this step requires the measurement of velocities and positions, which must be stored in a memory. Even in the case of a reversible measurement, the erasure of the stored information, necessary to reset the measuring device to its original state, has an entropy cost that offsets the entropy decrease during the reverse evolution of the system. This cost is sufficient to explain why the described procedure does not violate the SLT.

High-precision and high-efficiency first-arrival slope tomography via eikonal solvers and the adjoint-state method

Abstract First-arrival slope tomography (FAST) introduces first-arrival slopes, corresponding to the horizontal components of the slowness vectors at the receiver and source positions to supplement first-arrival traveltime for better guiding ray propagation in the media until the best match is achieved with the observed data. FAST can recover the velocity model with higher resolution and precision than first-arrival traveltime tomography (FATT) but is computationally intensive. In this context, we propose an improved approach, referred to as high-precision and high-efficiency first-arrival slope tomography (HFAST). HFAST redefines one of the slopes using the reciprocity principle and simultaneously employs the first-arrival traveltime and slopes to ensure high-quality model building. On the other hand, HFAST extracts calculated data and derives the gradient of the misfit function from the solutions of relatively limited forward and inverse problems, resulting in a low computational cost. The cost of HFAST is proportional to the minimum between the receivers and sources, whereas the cost of FAST is scaled to the sum of the receivers and sources. Numerical experiments involving the checkerboard and SEAM II Foothill models demonstrate that HFAST can achieve a higher inversion precision than FATT, especially in the recovery of small-scale anomalies and the presence of velocity reversal. Moreover, HFAST is more computationally efficient than FAST and suitable for managing large data sets. Therefore, HFAST can be regarded as a valuable supplement to current first-arrival-based model building methods and has the potential to be applied in static corrections, prestack depth migration and waveform inversion in the future.

Neoichnology of vertebrate traces along the western barrier coast of Ukraine: preservation potential and subsurface visualization

A diverse quite of vertebrate traces covers beach, aeolian, and bay-side (deflation flats) surfaces along the NW Black Sea coast of Ukraine. These include avian, ungulate, and canid footprints, as well as mammal burrows (length >5 cm; depth ~2 cm). The preservation of biogenic structures is enhanced by rapid burial (low-energy sedimentation or event deposition), algal mat formation, and salt encrustation. Continuous high-frequency (800 MHz) ground-penetrating radar (GPR) imaging aided in visualizing subsurface sections of an active burrow complex within a beach-dune ridge. Images near an active fox burrow captured distinct subsurface anomalies (point-source hyperbolic diffractions) in the upper aeolian section above the water table. Unfilled tunnel sections are easily distinguished from buried roots and other targets based on signal velocity and polarity reversals relative to air-to-sediment response at the ground surface. The diffraction geometry (angle) is related to signal velocity, providing valuable information about relative saturation of the overlying substrate. Decimeter-scale deformation of shallow reflections may be attributed to tracking surfaces, with similar examples found immediately below modern surfaces affected by anthropogenic trampling. It is likely that muddy lagoonal tracking surfaces may be preserved under layers of sand (overwash or aeolian deposition) and, following saltwater expulsion, may be recognized in geophysical images as clear deformed paleo-surfaces. Heavy-mineral concentrations (e.g. magnetite-rich sand) are common for beach and dune horizons that have undergone reworking and such anomalies often accentuate physical and biogenic deformation structures. Due to moderate-to-high fraction of ferri- and paramagnetic minerals, these anomalies are also well-expressed in GPR images due to its electromagnetic signal response. A conceptual framework of trace preservation potential (taphonomy) and geophysical recognition (GPR) suitability is proposed for this coastal region, with implications to paleo-environmental reconstruction.

Study of magnetohydrodynamics-based-mixed convection & entropy generation within the rectangular enclosure with two obstacles for Cu-SiO2 multiwalled carbon nanotubes ternary hybrid nanofluids

This work employs the multirelaxation time lattice Boltzmann method (MRT-LBM) to investigate the heat transfer and entropy generation characteristics due to mixed convection in a rectangular enclosure with a discrete heater subjected to an external magnetic field. The enclosure contains Cu-SiO2 multiwalled Carbon nanotubes ternary nanofluid. In particular, the effect of Hartmann number (0 ≤ Ha ≤ 90), Richardson number (0≤ Ri ≤ 10), the volume fraction of nanofluids (0 ≤ ϕ ≤1), and the location of obstacles over the various aspects of thermohydrodynamics and entropy generation characteristics have been elaborated. Three cases are categorized based on the arrangement of the obstacles in the enclosure. Case 1 and Case 3 represent the alienated arrangement of the obstacles where the first obstacle is located below and above the second, respectively, while Case 2 represents the in-tandem arrangement of the obstacles. The results illustrate the reversal of flow velocity at Ha = 90 for all Ri. It is observed that case 2 is best suited for Nusselt number enhancement. It is seen that when Ri = 0.1 and 1 Case 2 and at Ri = 10, Case 1 augments heat transfer for Ha = 0.

Multispacecraft Observations of the Simultaneous Occurrence of Magnetic Reconnection at High and Low Latitudes During the Passage of a Solar Wind Rotational Discontinuity Embedded in the April 9‐11, 2015 ICME

AbstractNumerous questions remain about how solar wind directional discontinuities, for example, rotational discontinuities (RDs), affect energy and transport processes at the magnetopause. The impact on the dayside magnetosphere of a solar wind RD is studied. The study shows that the RD leads to complex structures at the magnetopause, boundary layer, mantle, and cusp even though the geomagnetic activity level remains low. At low altitudes, the Defense Meteorological Satellite Program spacecraft observe a double cusp that is a signature of magnetic reconnection occurring at both high and low latitudes due to the dominant IMF By, while Cluster C2, located at high‐latitude and high‐altitude in the southern hemisphere, observes velocity fluctuations and reversals with peak‐to‐peak amplitudes >800 km s−1 as it crosses the magnetopause. A global MHD simulation of the event shows that the C2 observations are consistent with the spacecraft crossing reconnection outflows while moving from one side of the X‐line to the other.

Heat Flow in a Periodically Forced, Thermostatted Chain

We investigate the properties of a harmonic chain in contact with a thermal bath at one end and subjected, at its other end, to a periodic force. The particles also undergo a random velocity reversal action, which results in a finite heat conductivity of the system. We prove the approach of the system to a time periodic state and compute the heat current, equal to the time averaged work done on the system, in that state. This work approaches a finite positive value as the length of the chain increases. Rescaling space, the strength and/or the period of the force leads to a macroscopic temperature profile corresponding to the stationary solution of a continuum heat equation with Dirichlet-Neumann boundary conditions.

Full-Scale Static Loading and Free-Vibration Tests of a Real Bridge with Friction Pendulum Bearing System (FPS) and Design Parameter Estimation

ABSTRACT Although the friction pendulum bearing system (FPS) has been widely used in the construction of buildings, bridges, and other structures to enhance their seismic performance, the FPS was adopted in bridge construction in Japan for the first time in 2020 on the Tokai-Hokuriku expressway. To validate the design hypotheses of the real bridge with FPS, a series of static and dynamic tests were performed, and the parameters of the FPSs were estimated. The girder of the bridge supported by four FPSs was pushed to various specified displacements considered in a maximum considered earthquake event in a quasi-static manner by hydraulic jacks equipped with large-caliber valves to enable quick pressure release. The load of the jacks was then suddenly released so that the bridge entered free vibration. The bearings in the site static tests are found to undergo a noncontinuous sliding motion (stick – slip) because a small loading rate of jacks is applied. This stick – slip phenomenon is confirmed in both the site and laboratory tests in cases where the sliding velocity either approached 0 or departed from 0, i.e. at every velocity reversal. The friction coefficient is estimated from the force and displacement data at the sliding points, and good agreement is observed between the site static tests and the laboratory tests. The friction coefficient model for dynamic analysis, accounting for the stick – slip effect and several dependencies, is calibrated by these test results. The free-vibration test results, including measured displacements and accelerations, show agreement with the simulation results based on a simplified model of the bridge with the FPSs. Finally, the unscented Kalman filter and its adaptive variant are applied to estimate the design parameters of the FPSs.

Eliminating the Wavefunction from Quantum Dynamics: The Bi-Hamilton–Jacobi Theory, Trajectories and Time Reversal

We observe that Schrödinger’s equation may be written as two real coupled Hamilton–Jacobi (HJ)-like equations, each involving a quantum potential. Developing our established programme of representing the quantum state through exact free-standing deterministic trajectory models, it is shown how quantum evolution may be treated as the autonomous propagation of two coupled congruences. The wavefunction at a point is derived from two action functions, each generated by a single trajectory. The model shows that conservation as expressed through a continuity equation is not a necessary component of a trajectory theory of state. Probability is determined by the difference in the action functions, not by the congruence densities. The theory also illustrates how time-reversal symmetry may be implemented through the collective behaviour of elements that individually disobey the conventional transformation (mathrm{T}) of displacement (scalar) and velocity (reversal). We prove that an integral curve of the linear superposition of two vectors can be derived algebraically from the integral curves of one of the constituent vectors labelled by integral curves associated with the other constituent. A corollary establishes relations between displacement functions in diverse trajectory models, including where the functions obey different symmetry transformations. This is illustrated by showing that a (mathrm{T}-obeying) de Broglie-Bohm trajectory is a sequence of points on the (non-mathrm{T}) HJ trajectories, and vice versa.