Wall Velocity Research Articles

Bubble wall velocities with an extended fluid Ansatz

We compute the terminal bubble wall velocity during a cosmological phase transition by modelling non-equilibrium effects in the plasma with the so-called “extended fluid Ansatz”. A ϕ 6 operator is included in the Standard Model effective potential to mimic effects of new physics. Hydrodynamical heating of the plasma ahead of the bubble is taken into account. We find that the inclusion of higher order terms in the fluid Ansatz is typically relevant, and may even turn detonation solutions into deflagrations. Our results also corroborate recent findings in the literature that, for a Standard Model particle content in the plasma, only deflagration solutions are viable. However, we also show that this outcome may be altered in a theory with a different particle content.

Modal stability analysis of the density-stratified plane Couette–Poiseuille flow

Shear and density stratification strongly affect the flow mechanism of the different atmospheric and ocean flows. In this paper, we investigate the stability characteristics of plane Couette–Poiseuille (CP) flow with stable density stratification in the vertical direction. A modal stability analysis is carried out to examine the exponentially growing instability of stratified plane CP flow under different controlling parameters. The domain of the flow is periodic in streamwise and vertical directions. The stability problem is solved numerically using the spectral collocation method. The present analysis is carried out for Reynolds number Re=104 with different speeds of moving wall and different strengths of stratification. The results show that the mass diffusivity impact on the flow instability mechanism is almost negligible beyond the Schmidt number Sc≥20. The three-dimensional mode is generally the most unstable mode for density-stratified CP flow. However, the most unstable mode in unstratified CP flow is always two-dimensional. In contrast to unstratified CP flow, the density-stratified CP flow is unstable even moving wall velocity exceeds 70% of the center velocity of the plane Poiseuille flow component. It is also observed that the moving wall velocity and density stratification simultaneously affect the stability of the flow, which shows the importance of the shear and stratification in the flow.

Topologically protected metastable states in classical dynamics

We propose that domain walls formed in a classical Ginzburg–Landau model can exhibit topologically stable but thermodynamically metastable states. This proposal relies on Allen–Cahn’s assertion that the velocity of domain wall is proportional to the mean curvature at each point. From this assertion we speculate that domain wall behaves like a rubber band that can winds the background geometry in a nontrivial way and can exist permanently. We numerically verify our proposal in two and three spatial dimensions by using various boundary conditions. It is found that there are possibilities to form topologically stable domain walls in the final equilibrium states. However, these states have higher free energies, thus are thermodynamically metastable. These metastable states that are protected by topology could potentially serve as storage media in the computer and information technology industry.

Subclinical Echocardiographic Parameters and Biomarker Approach to Characterize the Pathophysiology of Carcinoid Syndrome

Introduction: Carcinoid heart disease is a rare cause of right-sided valvular dysfunction, an important complication in patients with carcinoid syndrome. Objectives: We aim to present our own experience regarding early myocardial dysfunction in patients with carcinoid syndrome. Methods: 64 patients (35-90 year old, 62,5% female, 37,5% male patients) with carcinoid syndrome were evaluated between October 2017 and June 2021. All patients underwent two-dimensional echocardiography with additional tissue Doppler and speckle-tracking techniques. For all patients there were measured urinary level of 5- hydroxyindolacetic acid, serotonin and NT-proBNP values. Results: The following echocardiographic features: global longitudinal strain of both ventricles, mean systolic velocity of the left ventricle, systolic velocity of the free wall of right ventricle, correlated with high values of specific biomarkers: serotonin, and urinary 5-hydroxyindolacetic acid, and also with the presence of hepatic metastases and poorly differentiated tumors. Discussion: In this study we identified several patients with early myocardial dysfunction due to carcinoid syndrome, representing a predictive model of poor prognosis, requiring further investigations. Conclusion: Echocardiographic assessment based on the measurement of systolic velocities, as well as the global longitudinal strain of both ventricles, together with specific biomarkers may contribute to better recognition of carcinoid heart disease in clinical practice, but this needs validation through further studies enrolling larger number of patients.

In-situ thermal annealing effects on domain wall velocity in Pt/Co/Pt trilayer thin films with perpendicular magnetic anisotropy

The Pt/Co/Pt trilayer system is widely studied for its perpendicular magnetic anisotropy (PMA). In this work we delve into the effects of thermal annealing from the perspective of magnetic domain wall dynamics. We find that the coercivity of the films remains constant while the domain wall velocity decreases drastically with thermal annealing (≤473 K). The decrease in the creep scaling constant with annealing temperature, confirm the enhanced pinning effect. Our results demonstrate that annealing temperature can significantly affect the magnetic micro-structure as well as magnetic domain dynamics in Pt/Co/Pt films with PMA.

Convection flow of nano-encapsulated phase change material in wavy chamber with double sliding walls, flame-shaped heating source, and magnetic force

This article aims to improve the convection flow within a two-dimensional chamber by utilizing nano-encapsulated phase change material (NEPCM). The chamber center contains a flame-shaped heating source, and the bottom wall is wavy. Forced convection occurs due to the motion of the chamber's side walls, while natural convection arises from the thermal gradient across the side walls. The Galerkin-finite element (GFEM) approach was employed to analyze the system's governing equations. The study investigates the impact of the following factors on heat transfer: wall velocity (Re = 10–1000), wall movement directions (both walls moving in a positive direction, both walls moving in a negative direction, and one wall negative and the other positive), intensity of the magnetic field (Ha = 0–100), and cavity porosity (Da = 10−5 – 10−2). The results demonstrate that increasing Re and Da improves the average Nusselt number (NuAvg). Conversely, raising the intensity of the magnetic field and moving the side walls in the opposite direction reduces NuAvg. At Re = 1000, increasing Da (10−2 instead of 10−5) leads to a 139% increase in NuAvg, and decreasing Ha and moving walls in the same directions increase NuAvg by 11.5% and 640%, respectively.

Domain Dynamics and Resistive Switching in Ferroelectric Al1–xScxN Thin Film Capacitors

AbstractIn this paper, using a combination of pulse testing measurements and piezoresponse force microscopy (PFM), an investigation of the polarization reversal behavior and the accompanying resistive switching in the Al0.72Sc0.28N thin film capacitors is reported. The obtained results reveal a transition from the nucleation‐limited switching (NLS) in the low field range toward the more uniform switching described by the Kolmogorov–Avrami–Ishibashi (KAI) model in the high field range. It is found that the Al0.72Sc0.28N capacitors exhibit an unusually steep change in the switching time– it decreases by five orders of magnitude with a moderate increase of the applied field. This feature is caused by a significantly higher activation field value (≈126 MV cm−1) in comparison with the conventional perovskite ferroelectrics. PFM visualization of the field‐induced domain dynamics has allowed the evaluation of the nucleation rate and domain wall velocity. Furthermore, capacitors in the polydomain state generated by partial switching of polarization exhibit a significant (up to two orders of magnitude) increase in the steady‐state conductance. This effect is likely caused by the injection of strongly inclined conducting 180° domain walls. Resistance tunability offers additional functionalities to the Al1‐xScxN devices where conductive domain walls are used as active elements.

In-plane dynamic impact mechanical properties of novel bi-directional hierarchical honeycomb

A novel bi-directional hierarchical honeycomb is proposed. A finite element numerical model was constructed using Abaqus/Explicit, the model's accuracy was verified, and a series of in-plane impact studies were carried out. Firstly, the mechanical properties of axial impact were analyzed for different bi-directional hierarchical honeycombs. The results show that the mechanical properties of NBHH-h1h22h3-III and NBHH-h1h2h3h4-IV are the best and almost identical. Subsequently, the mechanical properties of NBHH-XhxYhy-IIs with different layers were analyzed. Research results show that NBHH-h33h4-II has a 77.96 % increase in specific energy absorption (SEA) compared to NBHH-h13h2-II, and NBHH-h13h2-II has a 116.02 % increase in SEA compared to NBHH-3h1h2-II. A parametric study of NBHHs was conducted. The mechanical properties of NBHHs with different layer ratio coefficients were analyzed, and it was found that the mechanical properties of NBHHs with l2 = 12 and k = 0.2 are the best. Finally, the effects of wall thickness and velocity on NBHHs were analyzed. Increasing the wall thickness appropriately can effectively improve the impact resistance of the structure. Bi-directional hierarchical design provides a practical reference for enhancing the in-plane mechanical properties of honeycombs.

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.

Pulsatile pressure-driven non-Newtonian blood flow through a porous stenotic artery: A computational analysis

Blood flow through a narrowed artery, such as those caused by plaque buildup, can lead to various cardiovascular diseases. Therefore, studying this phenomenon is essential for developing better treatments and understanding the underlying mechanisms. In this study, a mathematical model is developed to simulate blood flow through stenotic arteries with porous walls. The familiar Navier-Stokes equations are executed to simulate flow, whereas the Carreau fluid model is employed to quantify blood rheology. This model illustrates both Newtonian and non-Newtonian characteristics depending on the shear rate. Other than that, the blood vessel is assumed to have a moderate level of stenosis with porous saturated walls. The solution of the governed partial differential equations is carried out using a finite difference scheme. Our research addresses the impacts of permeability and rheological parameters, stenosis size, Brinkman number, and Prandtl number on blood velocity, flow rate, resistance to flow, wall shear stress, and temperature distribution. The findings demonstrate that while flow resistance reduces, the velocity, shear stress, and flow rate increase with high permeability values. Conversely, as the stenosis amplitude increases, the magnitude of wall shear stresses, velocity, and flow rates decrease. In addition, blood rheology also has the potential to transfigure the aforementioned factors. We also found that the temperature distribution increases with the Brinkman number, stenosis size, permeability, and power law exponent. It drives down, however, by raising the values of Weissenberg number and Prandtl number. Collectively, our findings contribute to a better understanding of the complicated phenomena of blood flow in stenotic vessels with porous saturated walls. The findings of this study may be valuable in establishing improved therapies for cardiovascular disorders and formulating more precise mathematical models to forecast blood flow in vivo.

Exact analytical solutions for micropolar magnetohydrodynamic flow: Insights into velocity slip and heat transfer characteristics

In this study, we investigate the laminar steady flow of a micropolar fluid along a permeable flexible surface subject to a uniform magnetic field acting normal to the surface, with wall transpiration and velocity slip at the surface. This analysis yields multiple exact solutions for the stretching/shrinking sheet, offering insights into the complex dynamics of the fluid–surface interaction. In addition, to enhance the realism of our model, we incorporate a power-law temperature distribution, providing a comprehensive understanding of the temperature profile in the context of the micropolar fluid flow. The multiplicity and existence of solutions in special cases are presented for both stretching and shrinking sheets. In particular, a unique solution is found for the stretching sheet in the cases of both suction and injection while dual solutions exist for the shrinking sheet over the range μ>μc in suction, and no solution exist for the injection. For the skin friction, dual solutions appear in the region 0≤a≤ac, but single solution exists for a<0. Furthermore, dual solutions are found for the heat transfer gradient in the regions −∞≤a≤ac and μc≤μ≤∞. Moreover, this research contributes to the broader comprehension of fluid dynamics on flexible surfaces and their intricate thermal behaviors.

The violent collapse of vapor bubbles in cryogenic liquids

Vapor bubbles in cryogenic fluids may collapse violently under subcooled and pressurized conditions. Despite important implications for engineering applications such as cavitation erosion in liquid propellant rocket engines, these intense phenomena are still largely unexplored. In this paper, we systematically investigate the ambient conditions leading to the occurrence of violent collapses in liquid nitrogen and analyze their thermodynamic characteristics. Using Brenner’s time ratio χ, the regime of violent collapse is identified in the ambient pressure–temperature parameter space. Complete numerical simulations further refine the prediction and illustrate two classes of collapses. At 1 < χ < 10, the collapse is impacted by significant thermal effects and attains only moderate wall velocity. Only when χ > 10 does the collapse show more inertial features. A mechanism analysis pinpoints a critical time when the surrounding liquid enters supercritical state. The ultimate collapse intensity is shown to be closely associated with the dynamics at this moment. Our study provides a fresh perspective to the treatment of cavitation in cryogenic fluids. The findings can be instrumental in engineering design to mitigate adverse effects arising from intense cavitational activities.

Spatial discretization effects in spanwise forcing for turbulent drag reduction

Wall-based spanwise forcing has been experimentally used with success by Auteri et al. (Phys. Fluids, vol. 22, 2010, 115103) to obtain large reductions of turbulent skin-friction drag and considerable energy savings in a pipe flow. The spatial distribution of the azimuthal wall velocity used in the experiment was not continuous, but piecewise constant. The present study is a numerical replica of the experiment, based on a set of direct numerical simulations (DNS); its goal is the identification of the effects of spatially discrete forcing, as opposed to the idealized sinusoidal forcing considered in the majority of numerical studies. Regardless of the discretization, with DNS the maximum drag reduction is found to be larger: the flow easily reaches complete relaminarization, whereas the experiment was capped at 33 % drag reduction. However, the key result stems from the observation that, for the piecewise-constant forcing, the apparent irregularities of the experimental data appear in the simulation data too. They derive from the rich harmonic content of the discontinuous travelling wave, which alters the drag reduction of the sinusoidal forcing. A detailed understanding of the contribution of each harmonic reveals that, whenever for example technological limitations constrain one to work far from the optimal forcing parameters, a discrete forcing may perform very differently from the corresponding ideal sinusoid, and in principle can outperform it. However, care should be exercised in comparison, as discrete and continuous forcing have different energy requirements.

Criterion for ultra-fast bubble walls: the impact of hydrodynamic obstruction

The Bödeker-Moore thermal friction [1] is usually used to determine whether or not a bubble wall can run away. However, the friction on the wall is not necessarily a monotonous function of the wall velocity and could have a maximum before it reaches the Bödeker-Moore limit. In this paper, we compare the maximal hydrodynamic obstruction, a frictional force that exists in local thermal equilibrium, and the Bödeker-Moore thermal friction. We study the former in a fully analytical way, clarifying its physical origin and providing a simple expression for its corresponding critical phase transition strength above which the driving force cannot be balanced out by the maximal hydrodynamic obstruction. We find that for large parameter space, the maximal hydrodynamic obstruction is larger than the Bödeker-Moore thermal friction, indicating that the conventional criterion for the runaway behavior of the bubble wall may have to be modified. We also explain how to apply efficiently the modified criterion to particle physics models and discuss possible limitations of the analysis carried out in this paper.

Impact of wall velocity ratios on mixed convection heat transfer in porous cavities with nanoliquids

Impact of wall velocity ratios on mixed convection heat transfer in porous cavities with nanoliquids

Reynolds-number scaling of wall-pressure–velocity correlations in wall-bounded turbulence

Wall-pressure fluctuations are a practically robust input for real-time control systems aimed at modifying wall-bounded turbulence. The scaling behaviour of the wall-pressure–velocity coupling requires investigation to properly design a controller with such input data so that it can actuate upon the desired turbulent structures. A comprehensive database from direct numerical simulations (DNS) of turbulent channel flow is used for this purpose, spanning a Reynolds-number range $Re_\tau \approx 550\unicode{x2013}5200$ . Spectral analysis reveals that the streamwise velocity is most strongly coupled to the linear term of the wall pressure, at a Reynolds-number invariant distance-from-the-wall scaling of $\lambda _x/y \approx 14$ (and $\lambda _x/y \approx 8$ for the wall-normal velocity). When extending the analysis to both homogeneous directions in $x$ and $y$ , the peak coherence is centred at $\lambda _x/\lambda _z \approx 2$ and $\lambda _x/\lambda _z \approx 1$ for $p_w$ and $u$ , and $p_w$ and $v$ , respectively. A stronger coherence is retrieved when the quadratic term of the wall pressure is concerned, but there is only little evidence for a wall-attached-eddy type of scaling. An experimental dataset comprising simultaneous measurements of wall pressure and velocity complements the DNS-based findings at one value of $Re_\tau \approx 2$ k, with ample evidence that the DNS-inferred correlations can be replicated with experimental pressure data subject to significant levels of (acoustic) facility noise. It is furthermore shown that velocity-state estimations can be achieved with good accuracy by including both the linear and quadratic terms of the wall pressure. An accuracy of up to 72 % in the binary state of the streamwise velocity fluctuations in the logarithmic region is achieved; this corresponds to a correlation coefficient of $\approx$ 0.6. This thus demonstrates that wall-pressure sensing for velocity-state estimation – e.g. for use in real-time control of wall-bounded turbulence – has merit in terms of its realization at a range of Reynolds numbers.

Monitoring the Velocity of Domain Wall Motion in Magnetic Microwires.

An approach was proposed to control the displacement of domain walls in magnetic microwires, which are employed in magnetic sensors. The velocity of the domain wall can be altered by the interaction of two magnetic microwires of distinct types. Thorough investigations were conducted utilizing fluxmetric, Sixtus-Tonks, and magneto-optical techniques. The magneto-optical examinations revealed transformation in the surface structure of the domain wall and facilitated the determination of the mechanism of external influence on the movement of domain walls in magnetic microwires.

In silico parametric analysis of femoro-jugular venovenous ECMO and return cannula dynamics

Background: Increasingly, computational fluid dynamics (CFD) is helping explore the impact of variables like: cannula design/size/position/flow rate and patient physiology on venovenous (VV) extracorporeal membrane oxygenation (ECMO). Here we use a CFD model to determine what role cardiac output (CO) plays and to analyse return cannula dynamics. Methods: Using a patient-averaged model of the right atrium and venae cava, we virtually inserted a 19Fr return cannula and a 25Fr drainage cannula. Running large eddy simulations, we assessed cardiac output at: 3.5–6.5 L/min and ECMO flow rate at: 2–6 L/min. We analysed recirculation fraction (Rf), time-averaged wall shear stress (TAWSS), pressure, velocity, and turbulent kinetic energy (TKE) and extracorporeal flow fraction (EFF = ECMO flow rate/CO). Results: Increased ECMO flow rate and decreased CO (high EFF) led to increased Rf (R = 0.98, log fit). Negative pressures developed in the venae cavae at low CO and high ECMO flow (high CR). Mean return cannula TAWSS was >10 Pa for all ECMO flow rates, with majority of the flow exiting the tip (94.0–95.8 %). Conclusions: Our results underpin the strong impact of CO on VV ECMO. A simple metric like EFF, once supported by clinical data, might help predict Rf for a patient at a given ECMO flow rate. The return cannula imparts high shear stresses on the blood, largely a result of the internal diameter.

Magnetization spiral structure and high domain wall velocity induced by inertial effect

In ultrafast magnetism, i.e., the short time scales such as picosecond and femtosecond, we report that the spiral structure of magnetization can be formed by the inertial effect. Comparing with the case of fast magnetism in nanosecond time scale, we can obtain a higher velocity of domain wall in ultrafast magnetism due to the driving of inertial moment. There is a critical value τc of angular momentum relaxation time τ for the inertial breakdown in uniaxial ferromagnet. Under the action of the inertial effect the domain wall width shows the characteristics of increases firstly as τ<τc, and then decreasing as τ>τc. It directly leads to the positive or negative mobility for the domain wall velocity as the inertia is less than or exceeds the critical value, respectively. In the high frequency mode (i.e., high field mode), we obtain the limit domain wall velocity, i.e., V→1/ατ. The smaller the inertia or damping, the faster the domain wall moves. For the case of biaxial anisotropic ferromagnets, the domain wall width and velocity decreases with the increasing inertia term, respectively. In the presence of inertia effect, there is an another critical magnetic field hc. The domain wall width decreases with the increase of the magnetic field as h<hc, while increases as h>hc. However, the domain wall velocity shows a nearly linear increase with the magnetic field. These results indicate that the ultrafast magnetism has more advantages than fast magnetism. It also provides a good control technique for rapid information processing in the future.

Kinetics of N- to M-Polar Switching in Ferroelectric Al1-xScxN Capacitors.

Ferroelectric wurtzite-type aluminum scandium nitride (Al1-xScxN) presents unique properties that can enhance the performance of non-volatile memory technologies. The realization of the full potential of Al1-xScxN requires a comprehensive understanding of the mechanism of polarization reversal and domain structure dynamics involved in the ferroelectric switching process. In this work, transient current integration measurements performed by a pulse switching method are combined with domain imaging by piezoresponse force microscopy (PFM) to investigate the kinetics of domain nucleation and wall motion during polarization reversal in Al0.85Sc0.15N capacitors. In the studied electric field range (from 4.4 to 5.6MVcm-1), ferroelectric switching proceeds via domain nucleation and wall movement. The currently available phenomenological models are shown to not fully capture all the details of the complex dynamics of polarization reversal in Al0.85Sc0.15N. PFM reveals a non-linear increase of both domain nucleation rate and lateral wall velocity during the switching process, as well as the dependency of the domain pattern on the polarization reversal direction. A continuously faster N- to M-polar switching upon cycling is reported and ascribed to an increasing number of M-polar nucleation sites and density of domain walls.