Asymmetric Boundary Research Articles

Collective Effect of Fluid's Coriolis Force and Nanoscale's Parameter on Instability Pattern and Vibration Characteristic of Fluid-Conveying Carbon Nanotubes

In the present work, the effects of nanoscale parameter and Coriolis force together are investigated on vibrating eigenvalues of fluid-conveying carbon nanotube (CNT). A nonlocal Timoshenko beam and a plug flow model are implemented to derive fluid–structure interaction (FSI) governing equations of motion. These equations solved by Galerkin to obtain instability pattern, critical fluid velocities (CFVs), frequency and damping at different nanoscale parameter, boundary conditions, and aspect ratios. The results demonstrate existence of multiple types of instabilities and bifurcations, which are deviated from classic FSI buckling and flutters' instabilities, and caused by damping from coalition of nanoscale effect and fluid's Coriolis force, this phenomena are more noticeable in the CNTs with asymmetrical boundary conditions and smaller size.

Compact dual-band truncated patch antenna with fractal defected ground structure for wireless applications

In this paper, a compact, dual-band patch antenna is proposed over Minkowski fractal defected ground structure (DGS) for bandwidth enhancement of global positioning system (GPS) applications. The proposed design combines the truncated dual L-shaped slits cut on diagonal corners of radiating patch and fractal defect on the metallic ground plane. This concept shifts the frequencies to lower bands with improvement in antenna radiation properties. By deploying symmetrical and asymmetrical boundaries to the structure for the fractal DGS on metallic ground plane, improvement in bandwidth and gain are obtained. Compact antenna size is achieved for dual-band GPS frequencies of L1 (1.575 GHz) and L2 (1.227 GHz). The measured results for antenna prototype are (1.2–1.245 GHz): L2 band and (1.51–1.59 GHz): L1 band for 10 dB return loss bandwidth with better pattern radiation. Gain value with and without DGS is observed for compact antenna overall volume of 0.32λ0 × 0.32λ0 × 0.024λ0.

Effect of asymmetric boundary conditions on flexible pipes crushing

During installation, the tensioners of the pipe laying support vessels may impose asymmetric stress states on the inner carcass and the pressure armor of flexible pipes. Moreover, the possibility of admitting residual deformations in the cross section of flexible pipes after their installation is fundamental to allow the use of these structures in ultra-deep waters. However, these deformations are often not explicitly considered in predicting the collapse strength of these structures since the common practice is to consider symmetric stress states and collapse models with elliptical cross sections. This article thus presents a finite element approach that addresses the effect of asymmetric boundary conditions on the ovality of flexible pipes due to crushing loads. Their impact on the residual ovality in these structures after passing through the tensioners and the effect of these conditions on the wet collapse strength are also evaluated. By relying on the results from several numerical simulations, design strategies to address the asymmetric boundary conditions are discussed and a new methodology to measure ovality is proposed.

Atomistic Simulations of Dislocation Pileup: Grain Boundaries Interaction

Using molecular dynamics (MD) simulations, we studied the dislocation pileup–grain boundary (GB) interactions. Two Σ11 asymmetrical tilt grain boundaries in Al are studied to explore the influence of orientation relationship and interface structure on dislocation activities at grain boundaries. To mimic the reality of a dislocation pileup in a coarse-grained polycrystalline, we optimized the dislocation population in MD simulations and developed a predict-correct method to create a dislocation pileup in MD simulations. MD simulations explored several kinetic processes of dislocations–GB reactions: grain boundary sliding, grain boundary migration, slip transmission, dislocation reflection, reconstruction of grain boundary, and the correlation of these kinetic processes with the available slip systems across the GB and atomic structures of the GB.

Near Field Development of Planar Wakes Under the Effect of Asymmetric Initial Conditions

This paper reports an experimental investigation on the response of a planar wake past a flat plate to various upstream flow conditions. A tripping wire was placed on the upper side of the flat plate downstream the leading edge which resulted in asymmetric boundary layers. The near wake asymmetry was compared to their symmetrical counterpart at two different Reynolds numbers. The near wake dynamics were investigated using hotwire anemometry and flow visualizations. Self-similarity of the asymmetrical wakes was established. Asymmetry seemed to have the largest effect on the convection velocity of the large structures in the asymmetric laminar-turbulent wake.

Effect of Asymmetric Boundary Conditions on Couette–Poiseuille Flow of Power-Law Fluid

The effect of viscous dissipation on forced convective heat transfer for a Couette–Poiseuille flow of a power-law fluid subjected to asymmetric thermal boundary conditions is investigated here. The Couette–Poiseuille flow studied is limited to one with a maximum velocity, where the location that corresponds to the maximum velocity has to be solved numerically. A new analytical expression is obtained for the Nusselt number, in terms of the heat flux ratio applied to the top and bottom parallel plates and in terms of the Brinkman number as defined for the power-law fluid. Present results, when reduced to a few special cases, are in excellent agreement with published results. An asymptotic Brinkman number is observed in the Nusselt number versus Brinkman number plot, which shows a sign change in for the range of power-law indices tested.

Sources of Local Time Asymmetries in Magnetodiscs

The rapidly rotating magnetospheres at Jupiter and Saturn contain a near-equatorial thin current sheet over most local times known as the magnetodisc, resembling a wrapped-up magnetotail. The Pioneer, Voyager, Ulysses, Galileo, Cassini and New Horizons spacecraft at Jupiter and Saturn have provided extensive datasets from which to observationally identify local time asymmetries in these magnetodiscs. Imaging in the infrared and ultraviolet from ground- and space-based instruments have also revealed the presence of local time asymmetries in the aurora which therefore must map to local time asymmetries in the magnetosphere. Asymmetries are found in (i) the configuration of the magnetic field and magnetospheric currents, where a thicker disc is found in the noon and dusk sectors; (ii) plasma flows where the plasma flow has local time-dependent radial components; (iii) a thicker plasma sheet in the dusk sector. Many of these features are also reproduced in global MHD simulations. Several models have been developed to interpret these various observations and typically fall into two groups: ones which invoke coupling with the solar wind (via reconnection or viscous processes) and ones which invoke internal rotational processes operating inside an asymmetrical external boundary. In this paper we review these observational in situ findings, review the models which seek to explain them, and highlight open questions and directions for future work.

Boundary conditions, semigroups, quantum jumps, and the quantum arrow of time

Experiments on quantum systems are usually divided into preparation of states and the registration of observables. Using the traditional mathematical methods (the Hilbert space and Schwartz space of distribution theory), it is not possible to distinguish mathematically between observables and states. The Hilbert space as well as Schwartz space boundary conditions for the dynamical equations lead by mathematic theorems (Stone-von Neumann) to unitary group with —∞ < t < ∞. But in the experimental set-up, one clearly distinguishes between the preparation of a state and the registration of an observable in that state. Furthermore, a state must be prepared first before an observable can be measured in this state (causality). This suggests time asymmetric boundary conditions for the dynamical equations of quantum theory. Such boundary conditions have been provided by Hardy space in the Lax-Phillips theory for electromagnetic and acoustic scattering phenomena. The Paley-Wiener theorem for Hardy space then leads to semi-group and time asymmetry in quantum physics. It introduces a finite “beginning of time” t0 for a time asymmetric quantum theory, which have been observed as an ensemble of finite times t(i)0, the onset times of dark periods in the quantum jump experiments on a single ion.

Thermal AND Gate Using a Monolayer Graphene Nanoribbon.

The first ever implementation of a thermal AND gate, which performs logic calculations with phonons, is presented using two identical thermal diodes composed of asymmetric graphene nanoribbons (GNRs). Employing molecular dynamics simulations, the characteristics of this AND gate are investigated and compared with those for an electrical AND gate. The thermal gate mechanism originates through thermal rectification due to asymmetric phonon boundary scattering in the two diodes, which is only effective at the nanoscale and at the temperatures much below the room temperature. Due to the high phonon velocity in graphene, the gate has a fast switching time of ≈100 ps.

Thermally driven grain boundary migration and melting in Cu.

With molecular dynamics simulations, we systematically investigate melting of a set of Σ3〈110〉70.53° tilt grain boundaries (GB) in Cu bicrystals, including coherent twin boundaries (CTBs), 12 asymmetric tilt grain boundaries (ATGBs), and symmetric incoherent twin boundaries (SITBs), in the order of increasing length weight of SITB or GB energy. ATGBs decompose into CTBs and SITBs, which migrate and coalesce as a result of internal stress relaxation. GBs can be superheated or premelted, and GB melting temperature decreases exponentially with increasing SITB weight, owing to the systematics in GB microstructure. GB melting nucleates at disordered CTB-SITB junctions, and grows along SITBs and then into grain interiors, with the solid-liquid interfaces preferentially aligned with {111}.

Statistics and dynamics of the boundary layer reattachments during the drag crisis transitions of a circular cylinder

Time series of pressure measured on the periphery of the central section of a circular cylinder of aspect ratio 22 are used to investigate the dynamics of the local reattachments during the drag crisis transitions. A succession of multi-stable dynamics are identified and characterized through conditional statistical analysis as the Reynolds number is increased. The first transition marking an abrupt weakening of the periodic pressure fluctuations associated to the global shedding dynamics is accompanied by the appearance of symmetric and bistable perturbations. Afterwards, two scenarii of asymmetric and symmetric boundary layer reattachments are found. The asymmetric scenario leads to two transitions. During the first transition, the flow explores randomly three stable states to eventually stabilize on the state corresponding to the permanent reattachment on one side of the cylinder. The second transition is bistable and leads to the permanent reattachment on both sides. For the second symmetric scenario, the boundary layer reattachments occur simultaneously on both side of the cylinder. In that case, the flow explores randomly two stable states to eventually stabilize on the state of full reattachment. Pressure distributions of all of these states are characterized as well as their corresponding probabilities during the drag crisis transitions of the critical regime.

Heat transfer in a porous saturated wavy channel with asymmetric convective boundary conditions

The viscous flow in a wavy channel with convective boundary conditions is investigated. The channel is filled with a porous viscous fluid. Two cases of equal and different external convection coefficients on the walls are taken into account. Effect of viscous dissipation is also considered. The governing equations are derived employing long wavelength and low Reynolds number approximations. Exact closed form solutions are obtained for the simplified equations. Important physical features for peristaltic flow caused by the wavy wave are pumping, trapping and heat transfer rate at the channel walls. These are discussed one by one in depth and detail through graphical illustrations. Special attention has been given to the effects of convective boundary conditions. The results show that for Bi1≠Bi2, there exists a critical value of Brinkman number Brc at which the temperatures of both the walls become equal. And, for Bi1>Bi2 and Br>Brc, the temperature of the cold wall exceeds the temperature of hot wall.

The shear response of copper bicrystals with Σ11 symmetric and asymmetric tilt grain boundaries by molecular dynamics simulation.

Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11<1 1 0> tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination along the boundary plane. A non-planar structure with dissociated intrinsic stacking faults was prevalent in Σ11 asymmetric GBs of Cu. This type of structure can significantly increase the ductility of bicrystal models under shear deformation. A grain boundary can be a source of dislocation and migrate itself at different stress levels. The intrinsic free volume involved in the grain boundary area was correlated with dislocation nucleation and GB sliding, while the dislocation nucleation mechanism can be different for a grain boundary due to its different equilibrium structures.

扭曲向列相薄盒中线缺陷的研究

Within the Landau-de Gennes theory,the order reconstruction of s =±1/2 twist disclinations in a twisted nematic cell is investigated,using the two-dimensional relaxation iterative method.The biaxial structure of the defect core as the cell gap decreasing is explored.At a critical value of dc*≈ 9ξ(here ξ is the characteristic length for order-parameter changes),the exchange solution is stable,while the defect core solution becomes metastable,where the system starts to stretch the defect structure and the biaxiality starts to propagate inside of the cell.Comparing to the case with no initial disclination,the value at which the exchange solution becomes stable increases relatively.At a critical separation of dc≈ 7ξ,the system undergoes a biaxial transition,and the defect core merges into a biaxial wall with large biaxiality.The force reaches a maximum at d≈ 9ξ,and a local minimum at d≈7ξ.For weak anchoring boundary conditions,because of the weakened frustration,the asymmetric boundary conditions repel the defect to the weak anchoring boundary as the anchoring strength coefficient decreasing.

Stability properties of a heat equation with state-dependent parameters and asymmetric boundary conditions

In this work the stability properties of a partial differential equation (PDE) with statedependent parameters and asymmetric boundary conditions are investigated. The PDE describes the temperature distribution inside foodstuff, but can also hold for other applications and phenomena. We show that the PDE converges to a stationary solution given by (fixed) boundary conditions which explicitly diverge from each other. Numerical simulations illustrate the results.

Casimir-Like Macroscopic Propulsion and Environmental-Energy Conversion

According to Quantum Electrodynamics (QED), a hidden source of great momentum and energy, called the EM Quantum Vacuum, fills the Universe. A Quantum light-sail will use refraction to induce asymmetric boundary conditions in the isotropic EM radiation pressure of the Quantum Vacuum. In terms of Newton’s First Law, these asymmetric radiation-pressures act as outside-forces that push harder on one side of a light-sail than on its opposite side. This radiation-pressure will provide the motive force for a new class of macroscopic prime-movers, a kind of massless propulsion; a new environmental-energy conversion device.

Reactive impedance surface-based broadband circularly polarized Koch fractal boundary microstrip antenna

A circularly polarized (CP) broadband antenna is proposed for wireless applications in the range of 2–3 GHz frequency. It consists of asymmetrical Koch fractal boundary patch over a reactive impedance surface (RIS) substrate. The simulations of single-layer Koch fractal antenna, dual layer with square and fractal RIS elements are carried out in a systematic way for broadband CP radiation and corresponding results are presented. For better CP characteristics, properties of fractal curves and dimensions of RIS elements are optimized. The antenna with fractal RIS iteration order one (iteration1) is experimentally studied. The 10-dB return loss bandwidth is 50.35%, whereas 3-dB axial ratio bandwidth is 7.45%, which indicate that by applying fractals concept to RIS technique, with a single probe feed, broadband CP radiation can be obtained.

Effects of nanoparticle migration and asymmetric heating on mixed convection of TiO2–H2O nanofluid inside a vertical microchannel

The effects of nanoparticle migration on mixed convection of titania/water nanofluid inside a vertical microchannel have been investigated numerically via Runge–Kutta–Fehlberg method. A modified two-component heterogeneous model is employed for the nanofluid in the hypothesis that the Brownian motion and the thermophoresis are the only responsible mechanisms for nanoparticle migration. Because of small dimensional structures of microchannels, a linear slip condition is considered at the boundaries, which appropriately represents the non-equilibrium region near the interface. To impose different temperature gradients, the heat flux ratio of the right to the left wall (ε) is investigated in three different situations, namely the adiabatic right wall (ε=0), unequal heat fluxes at the walls (ε<1) and equal heat fluxes (ε=1). It is revealed that the asymmetric thermal boundary condition affects the direction of nanoparticle migration and distorts the symmetry of the velocity and temperature profiles. In the rich nanoparticle concentration region, the viscosity and the local conductivity increase, which lead to a stronger conduction and a weaker convection rate. Also, it is found that splitting the total amount of heat flux on the walls unevenly, is the most efficient way to enhance the heat transfer rate in the vertical microchannels.

Transition from smoldering to flaming fire in short cotton samples with asymmetrical boundary conditions

The transition from smoldering to flaming fire in cotton has been investigated for two different boundary conditions: (a) open boundaries and (b) a lightweight concrete block covering one side of the sample. In the experiments, cotton was exposed from below to an electrically heated hotplate. The cotton sample was 0.15m×0.15m×0.15m, with density between 20 and 100kg/m3.The boundary condition affected the transition from smoldering to flaming. No transition to flaming was observed with open boundaries. For cases with a lightweight concrete block, transition to flaming was observed in samples with densities 60, 80 and 100kg/m³, but not for 20 and 40kg/m³. The transition from smoldering to flaming occurred when smoldering and secondary char oxidation coexisted. This coexisting occurred in 1 of 5 experiments for each of the densities 60, 80 and 100kg/m³. A necessary condition for this coexistence to occur seems to be different smoldering velocities in different parts of the sample. Based on the experiments, the following scenario for the transition from smoldering to flaming fire is suggested: The smoldering front moves app. 0.5mm/min faster through the parts of the sample far away from the lightweight concrete block, than through the parts closer to it. Due to differences in smoldering velocity, a porous char (500–600°C) forms in parts of the sample far from the block while closer to the block the cotton will be hot but not combusted (200–300°C). Oxygen is easily transported into the char, and secondary char oxidation occurs. The secondary char oxidation ignites combustible gasses generated by the smoldering process in other parts of the sample resulting in a flaming fire.The experiments indicate that smoldering and secondary char oxidation must coexist in a sample for the transition from smoldering to flaming to occur. The block seems to be crucial to obtain different smoldering velocities in the sample leading to smoldering and char oxidation occurring simultaneously.

Structural behavior of sandwich panels with asymmetrical boundary conditions

Asymmetrically supported and loaded sandwich panels with a polyurethane soft core and isotropic or orthotropic steel faces were analyzed in laboratory tests and numerical simulations. The influence of the steel face microprofilation on the mechanical and kinematical responses of the sandwich panel is considered in the paper. The real experiments were conducted for two supporting systems. For each supporting system isotropic and orthotropic sandwich faces were taken into account. The numerical simulations, in which 2D composite shell elements were used, correspond to the real experiments. The laboratory tests showed that introducing the orthotropic face layer significantly increases the load capacity of the sandwich panel. In the case of asymmetrically supported systems effective flexural rigidity increases too. The paper demonstrates that a relatively simple FE model can be successfully used to assess the global behavior of sandwich panels in complex boundary conditions. Satisfactory consistency of the numerical and real results in the linear range of structural behavior was obtained. Further improvement of the model is possible by introducing a definition of the failure criteria.