Wall Speed Research Articles

Linear stability analysis of oblique Couette–Poiseuille flows

We perform a detailed numerical study of modal and non-modal stability in oblique Couette–Poiseuille profiles, which are among the simplest examples of three-dimensional boundary layers. Through a comparison with the Orr–Sommerfeld operator for the aligned case, we show how an effective wall speed succinctly characterizes modal stability. Large-scale parameter sweeps reveal that the misalignment between the pressure gradient and wall motion is, in general, destabilizing. For flows that are sufficiently oblique, the instability is found to depend exclusively on the direction of wall motion and not on its speed, a conclusion supported, in part, by the perturbation energy budget and the evolution of the critical layers. Closed forms for the critical parameters in this regime are derived using a simple analysis. From a non-modal perspective, pseudoresonance is examined through the resolvent and the $\epsilon$ -pseudospectra. An analysis of the unforced initial value problem shows that the maximum energy gain is highly dependent on both the magnitude and direction of the wall velocity. However, the strongest amplification is always achieved for configurations that are only weakly skewed. Finally, the optimal perturbations appear to develop via a lift-up effect enhanced by an Orr-like mechanism, the latter driven by cross-flow shear.

Effect of Sr substitution on the structural, dielectric and ferroelectric property of BaTiO3

We have performed a comprehensive study to explore the effect of Sr substitution on the structural and ferroelectric properties of BaTiO3(BTO) with compositions Ba1-xSrxTiO3for 0 ⩽x⩽ 1. The room temperature structural investigation inferred that the samples with compositionsx> 0.30 has cubic phase instead of tetragonal as for pristine BTO. The temperature dependent dielectric studies illustrate that all well-known three structural phase transitions of BTO are coming closer to each other and the cubic phase is shifted towards lower temperature with increasing Sr content. The frequency dependent dielectric measurements show that there exists the mesoscopic subdomain, whose relaxation time decreases with increasing Sr concentration. The Sr substitution enhanced the ferroelectric properties and maximum remnant polarization at room temperature is observed for 20% Sr substituted sample. The frequency dependent dielectric measurements illustrate the relaxation which could be due to the mesoscopic subdomain, and its relaxation time decreases with increasing Sr concentration. The frequency dependentP-Emeasurements at room temperature infer that for 30% Sr substituted sample, the ferroelectric domain switching is dominated by the rate of nucleation whereas in other compositions forx< 0.3, it is governed by the domain wall speed. The Raman measurements infer the rearrangement of the domain configuration with Sr substitution. Modification in the intensity of the E(TO4) and A(TO3) Raman modes with electric field is also observed for 30% Sr substituted sample and the origin of this is also discussed.

Strain-induced domain wall motion in ferrimagnetic nanowire

Abstract Controllable magnetic domain walls in ferrimagnets offer great promise as information carriers for future spintronic devices. It is crucial to find low-power-consuming and efficient methods of controlling domain walls. In this work, we propose to use the strain to drive the wall motion in ferrimagnets, and we theoretically reveal the important role of the net angular momentum. When the strain is applied, an anisotropy gradient is induced and effectively drives the wall motion. For a nonzero net angular momentum, the precession torques acting on the two sublattices cannot cancel out each other, which induces a rotation of the wall plane and speed down the wall propagation. Furthermore, the wall dynamics depends on several internal and external parameters, which allows one to manipulate the wall motion through tuning these parameters. Our work provides clear insight into the strain-induced domain wall motion in ferrimagnets, which is meaningful for future spintronic experiments and applications.

Shock waves at final stages of cavity collapse in non-homogeneous liquid with divergenceless flow

We show the emergence of shock waves at the final stages of the complete collapse of a spherical cavity in a liquid with a smoothly decreasing density. The well-known Rayleigh assumption of fluid homogeneity is altered, while maintaining that of divergenceless flow. The fundamental difference between both infinite liquids is that his has an infinite mass, while ours, a finite one. Given the ease of deformation of non-homogeneous media in relation to homogeneous ones, as observed in several materials, all Rayleigh results are modified, including the cavity wall speed and acceleration, total time of complete collapse, and distribution of pressure in the infinite liquid. Rather than the homogeneous Rayleigh fluid, our non-homogeneous liquid can support a finite local sound speed. As a result, we succeed to show the emergence of shock patterns at the final stages of the cavity collapse. The analytical formulation is compared with underwater implosion and explosion experiments and simulations. Possible applications as a benchmark test for hydrocodes are briefly discussed.

Control of a twisted domain wall motion supported by topology

We report the topology-mediated modulation of a twisted domain wall speed in a thick perpendicularly magnetized system. By exploiting the topological robustness of the direction of the Bloch wall component in the twisted domain wall, we show that the domain wall speed either increases or decreases depending on whether the transverse magnetic field is parallel or antiparallel to the Bloch wall component. The decrease in the speed is maintained until the antiparallel transverse reaches ∼0.3 T, indicating that the twisted domain wall can offer wide controllability supported by the topological robustness which involves an injection of a Bloch point. We also demonstrate that the transverse magnetic field suppresses the Walker breakdown, allowing high mobility domain wall motion for a wide range of perpendicular driving fields.

Examination of Couette Flow with a Pressure Gradient and Heat Conduction Using Molecular Dynamics Simulation

As computer capabilities improve, Molecular Dynamics simulations are becoming more important for solving various flow problems. In this study, Couette and Poiseuille flows at different wall temperatures were investigated using a hard-sphere Molecular Dynamics simulation approach. Although a low spacing ratio was used in the simulations, the results are valid for rarefied gas flows when proper scaling based on the Knudsen number was used because only binary collisions with a hard-sphere model were considered. The main focus of this study was the examination of the effects of various wall speeds, pressure gradients, and wall temperatures. A pressure gradient was generated by developing a modified selective periodicity condition in the flow direction. With the combined effect of the pressure gradient and the wall velocities, subsonic, transonic, and supersonic speeds in nanochannels were examined. With the combination of different parameters, 1260 simulation cases were conducted. The results showed that there are temperature and velocity slips that are dependent on not only the temperature and velocity values but also on the magnitudes of a pressure gradient. The pressure gradient also caused nonlinearities in temperature and velocity profiles.

Giant supermagnonic Bloch point velocities in cylindrical ferromagnetic nanowires.

Achieving high velocities of magnetic domain walls is a crucial factor for their use as information carriers in modern nanoelectronic applications. In nanomagnetism and spintronics, these velocities are often limited either by internal domain wall instabilities, known as the Walker breakdown phenomenon, or by spin wave emission, known as the magnonic regime. In the rigid domain wall model, the maximum magnon velocity acts as an effective "speed of light", providing a relativistic analogy for the domain wall speed limitation. Cylindrical magnetic nanowires are an example of systems without the Walker breakdown phenomenon. Here we demonstrate that the magnonic limit could be outstandingly surpassed in cylindrical nanowires with high magnetization, such as iron. Our numerical modeling shows the Bloch point domain wall velocities as high as 14 km s-1, well above the magnonic limit estimated in the interval 1.7-2.0 km s-1. The key ingredient is the three-dimensional conical shape of the domain wall, which elongates and breaks during the dynamics, expelling backwards pairs of Bloch points. This leads to domain wall acceleration, the effect, which resembles the "jet propulsion". This effect will be very important for three-dimensional networks based on cylindrical magnetic nanowires.

Extreme Domain Wall Speeds under Ultrafast Optical Excitation.

Time-resolved ultrafast EUV magnetic scattering was used to test a recent prediction of >10 km/s domain wall speeds by optically exciting a magnetic sample with a nanoscale labyrinthine domain pattern. Ultrafast distortion of the diffraction pattern was observed at markedly different timescales compared to the magnetization quenching. The diffraction pattern distortion shows a threshold dependence with laser fluence, not seen for magnetization quenching, consistent with a picture of domain wall motion with pinning sites. Supported by simulations, we show that a speed of ≈66 km/s for highly curved domain walls can explain the experimental data. While our data agree with the prediction of extreme, nonequilibrium wall speeds locally, it differs from the details of the theory, suggesting that additional mechanisms are required to fully understand these effects.

Approximation of the Couette-Poiseuille flow on an annular domain in compressible regime for a hyperloop pod

Fanno flow is one of the most studied problems in fluid mechanics, as it models the pressure losses on domains in which the characteristic length is considerably lower than the height. These types of flows are present in several applications in the industry, such as pipe flows in pneumatic and hydraulic circuits, oil pipelines or air conditioning and water distribution systems. One potential application can be found in hyperloop design, where flow at high speeds moves between the small gap between the capsule and the tube wall. In this case, the domain is annular-shaped instead of a single cylinder, and the interior wall moves at the capsule speed. Although this quasi-1D flow is strongly affected by the wall friction, it cannot be modelled using the standard Fanno expression. Thus, to model the pressure drop without extensive CFD techniques, a simplified 1D analytical expression is developed to estimate the friction losses. The model assumes a constant ratio between the shear stress on the interior and the exterior walls throughout the channel. Then, instead of the hydraulic diameter as in the standard Fanno expression, a new definition of an equivalent diameter is performed to consider the annular-shaped domain and the movement of the interior wall. The new equivalent diameter is always higher than the original used for Fanno flows (hydraulic one), even twice when the wall speed equals the mean speed. This effect leads to smaller pressure losses due to the movement of one of the walls and indicates that the wall movement is always beneficial. To validate the results obtained from the model, they are compared with an equivalent CFD simulation, leading to a maximum deviation of 5% on the Mach number and 7% on the total friction coefficient.

Квантование энергетических уровней в 1D квантовом колодце в случае мгновенных стационарных состояний при нерелятивистском движении стенки и микрочастицы

The paper considers the problem of finding energy levels in the 1D quantum well in case of its width alteration at the nonrelativistic rate. According to the reviewed literature, the exact solution is known only in the case of nonrelativistic motion of the 1D quantum well wall at the constant rate. It is shown that motion with the constant rate is physically unrealizable. Therefore, it is necessary to find at least small areas of the Schrodinger equation solution for a wider range of nonrelativistic alterations in the 1D quantum well width. Analysis results presented in the study show existence of such areas. The found areas correspond to the instantaneous stationary states satisfying the Bohr quantization condition. In this case, the Dirichlet condition is also satisfied on the moving wall. It means that in this case energy of the level with the n number also becomes a function of the k second quantum number, which takes into account dynamic alteration in the 1D quantum well width. Variants were found of the k second quantum number spectrum and of the quantum level spectrum in various cases of the wall continuous motion with zero initial speed and finite acceleration. Within the framework of the analysis used, formulas were obtained to change the difference between energies of the two arbitrary levels. An analysis was made for the boundaries of the wall speed and the 1D quantum well width in considering the nonrelativistic problem. The obtained results and their possible applications are under discussion, including analysis of the problems related to nanotechnology

Thermosolutal mixed convection in a partially heated and concentrated enclosure with various wall speed ratios

In this work, the effects of various wall speed ratios and different heating and concentrating area of the side walls on thermosolutal mixed convection are analyzed. Nine cases are considered depending on the location of heat and concentration sources. Both the horizontal walls are in motion in the same or opposite direction with different speed ratios. A portion of length B of the vertical walls is heated and concentrated uniformly while the remaining parts of these walls are in adiabatic condition. The horizontal walls are considered with cold temperature and low concentration. The governing equations are modeled to describe thermosolutal phenomena. These equations are solved by reconstructing a recently developed compact scheme. To strengthen the analysis performed in this study, we have validated our in-house code with the numerical and experimental data. Computed results are presented in terms of streamlines, isotherms, iso-concentrations, and average Nusselt and Sherwood numbers to evoke the thermosolutal phenomena for various physical parameters. The computed outcomes are scrutinized for different values of the well-defined parameters namely thermal Grashof number (), Prandtl number (), Richardson number (), buoyancy ratio (), wall speed ratios () and Lewis number (). This numerical study has been carried out for different combinations of partially active walls to reveal their effects on heat and mass transfer. It is observed that the thermal and solutal transfer performance becomes effective with maximum wall speed ratios and lower Richardson numbers.

Nonlinear Dynamic Characteristics of Rotating Thick Ring on the Elastic Foundation Based on Plane Stress Theory

Radial tires are widely used in transportation machinery, such as large aircraft, new energy vehicles, and engineering vehicles. The ring on the elastic foundation (REF) model is often used for dynamic analysis of radial tires, but the influence of elastic boundary on the dynamic characteristics of radial tires is ignored in this model. In this paper, based on the absolute nodal coordinate formulation (ANCF) and plane stress theory, a rotating thick ring on the elastic foundation (RTREF) model is established to explore the influence of elastic boundary position and other factors on dynamic characteristics. Because existing rectangular and triangular elements in ANCF can’t accurately describe radial/circumferential mesh of a thick ring, a novel ANCF-annular-sector element (AASE) is proposed for the RTREF model. By analyzing the influences of elastic boundary position, thick wall, and rotational speed on the vibration characteristics and nonlinear forced transient response of the RTREF model, it is demonstrated that even if the ring wall is thin, the influence of elastic boundary positions on the dynamic characteristics of the model can’t be ignored.

Bubble dynamics and speed of jets for needle-free injections produced by thermocavitation.

The number of injections administered has increased dramatically worldwide due to vaccination campaigns following the COVID-19 pandemic, creating a problem of disposing of syringes and needles. Accidental needle sticks occur among medical and cleaning staff, exposing them to highly contagious diseases, such as hepatitis and human immunodeficiency virus. In addition, needle phobia may prevent adequate treatment. To overcome these problems, we propose a needle-free injector based on thermocavitation. Experimentally study the dynamics of vapor bubbles produced by thermocavitation inside a fully buried 3D fused silica chamber and the resulting high-speed jets emerging through a small nozzle made at the top of it. The injected volume can range from to per shot. We also demonstrate that these jets have the ability to penetrate agar skin phantoms and ex-vivo porcine skin. Through the use of a high-speed camera, the dynamics of liquid jets ejected from a microfluidic device were studied. Thermocavitation bubbles are generated by a continuous wave laser (1064nm). The 3D chamber was fabricated by ultra-short pulse laser-assisted chemical etching. Penetration tests are conducted using agar gels (1%, 1.25%, 1.5%, 1.75%, and 2% concentrations) and porcine tissue as a model for human skin. High-speed camera video analysis showed that the average maximum bubble wall speed is about 10 to 25m/s for almost any combination of pump laser parameters; however, a clever design of the chamber and nozzle enables one to obtain jets with an average speed of . The expelled volume per shot (0.1 to ) can be controlled by the pump laser intensity. Our injector can deliver up to 20 shots before chamber refill. Penetration of jets into agar of different concentrations and ex-vivo porcine skin is demonstrated. The needle-free injectors based on thermocavitation may hold promise for commercial development, due to their cost and compactness.

Effects of cutting parameters on cutting force and cutting energy for crab‐apple

AbstractStudying the effects of cutting parameters on cutting force and energy consumption in the core removal and slicing process of crab‐apple can provide the theoretical basis for the design and optimization of crab‐apple core removal and slicing equipment. In this study, the single‐factor tests were carried out on a special fixture and microcomputer‐controlled electronic universal testing machine to study the effect of slicing speed, blade oblique angle, and blade‐edge shape in the slicing process and core removal tool wall thickness and speed in the core removal process on the cutting force and energy consumption. The test results showed that the peak force and energy consumption can be reduced by adopting the concave type blade compared to the convex type blade. Slicing speed and blade oblique angle have significant effects on the cutting performance of crab‐apple in the slicing process. When blade oblique angle and cutting speed increase, the cutting force and energy consumption both show a decreasing trend. For the core removal process, reducing the core removal tool wall thickness and increasing the core removal speed can reduce the cutting force and energy consumption and improve the processing quality.

The Effect of Circuit Exercise on Wall Climbing Speed on Speed Climbing Technique in Mapala Athlets University of PGRI Palembang

The problem of this research is that athletes do not have maximum speed in climbing speed climbing techniques. The purpose of the study was to determine the effect of circuit training on the speed of wall climbing in speed climbing techniques in mapala athletes at PGRI Palembang University. The research method is an experimental method of one group pretest-posttest design technique. The research population is 10 people and the sample is the total population. Data collection uses tests, with speed climbing technique speed test instruments. Data analysis used paired sample t test SPSS 16. The results of the study stated that there was an effect of circuit training on speed climbing wall speed climbing techniques in mapala athletes at PGRI Palembang University because Tcount T table (6.810 1.83).

Droplet collapse during strongly supercooled transitions

We simulate the decay of isolated, spherically symmetric droplets in a cosmological phase transition. It has long been posited that such heated droplets of the metastable state could form, and they have recently been observed in 3D multi-bubble simulations. In those simulations, the droplets were associated with a reduction in the wall velocity and a decrease in the kinetic energy of the fluid, with a consequent suppression in the gravitational wave power spectrum. In the present work, we track the wall speed and kinetic energy production in isolated droplets and compare them to those found in multi-bubble collisions. The late-time wall velocities that we observe match those of the 3D simulations, though we find that the spherical simulations are a poor predictor of the kinetic energy production. This implies that spherically symmetric simulations could be used to refine baryogenesis predictions due to the formation of droplets, but not to estimate any accompanying suppression of the gravitational wave signal.

Current-induced zero-field domain wall depinning in cylindrical nanowires

Multi-segmented cylindrical nanowires have properties that make them attractive for high-density, high-speed logic and memory applications. Investigations of the current-induced domain wall motion in cylindrical nanowires have, so far, typically been conducted with a background magnetic field. However, if performed at zero external field, they would be much more viable for their use in prospective electronic devices. Here, we present an all-magneto electrical method to consistently pin domain walls in multi-segmented nanowires and induce their de-pinning using current pulses. The experiments were conducted with compositionally modulated three-segmented nickel/cobalt/nickel and two-segmented cobalt/nickel nanowires of 190 and 150 nm diameter, respectively, where the soft/hard magnetic texture has been fairly studied. We find that for the 3 segmented nanowire, the domain wall can be de-pinned independent of the polarity of the pulse, while for the 2 segmented nanowire the domain wall de-pins only for one polarity. Applying current pulses of 1 × 1012 A/m2, we use a pulse width of 22 ns to estimate a lower boundary for the domain wall speed of 634.54 m/s in cobalt. We study the resistive heating effect from the DC measurement current to find a temperature increase of no more than 2 °C after more than 20 h of tests.

Design Analysis of Heat Sink Using the Field Synergy Principle and Multitarget Response Surface Methodology

This study describes a novel heat sink design approach employs the field synergy concept and multitarget response surface methodology (RSM). The multiobjective response surface methodology can be used to determine the simulation equations that will maximize the heat transfer of fins at various fin heights, fin angles, and fin circumferences, when considering the impact of jet flow heat exchange. The goal of the response value was to maintain the minimum possible average field coangle and fin temperature. The results show that the ideal heat sink size would be the following: fins with a height of 50 mm, an angle of 60 degrees, and the number of fins equal to five. We examined the impact of wall speed on the heat transfer caused by the field synergy angle. Our findings suggest that with the synchromesh of the display field, heat-dissipation efficiency rises.

Flow of a shallow film over a moving surface

The von-Karman–Pohlhausen averaging technique is employed to build a reduced model for the flow of a shallow film from a sluice gate or impacting jet over a moving surface. The viscous drag exerted on the film by the moving wall acts to arrest flow counter to the direction of the wall's motion, and force an adjustment toward the wall speed. For a (normally) impacting jet, this results in a range of wall speeds for which a steady state is reached in which all the fluid is eventually recirculated to flow along the wall, with a distinctive “heel” forming upstream of the impact region. For wall speeds below this range, the flow counter to the wall cannot be arrested, and unsteady states result. For wall speeds above this range, a different steady state emerges in which fluid is immediately diverted through and downstream of the impact region, eliminating any heel. The steady, heeled flow states predicted by the reduced model are in qualitative agreement with numerical simulations of the full two-dimensional problem.

Thermal suppression of bubble nucleation at first-order phase transitions in the early Universe

One of the key observables in a gravitational wave power spectrum from a first order phase transition in the early Universe is the mean bubble spacing, which depends on the rate of nucleation of bubbles of the stable phase, as well as the bubble wall speed. When the bubbles expand as deflagrations, it is expected that the heating of the fluid in front of the phase boundary suppresses the nucleation rate. We quantify the effect, showing that it increases the mean bubble separation, and acts to enhance the gravitational wave signal by a factor of up to order 10. The effect is largest for small wall speeds and strong transitions.