Asymmetric Boundary Conditions Research Articles

Terahertz Laser Combs in Graphene Field-Effect Transistors

Electrically injected terahertz (THz) radiation sources are extremely appealing given their versatility and miniaturization potential, opening the venue for integrated-circuit THz technology. In this work, we show that coherent THz frequency combs in the range $0.5~\mathrm{THz}<\omega/2\pi<10~\mathrm{THz}$ can be generated making use of graphene plasmonics. Our setup consists of a graphene field-effect transistor with asymmetric boundary conditions, with the radiation originating from a plasmonic instability that can be controlled by direct current injection. We put forward a combined analytical and numerical analysis of the graphene plasma hydrodynamics, showing that the instability can be experimentally controlled by the applied gate voltage and the injected current. Our calculations indicate that the emitted THz comb exhibits appreciable temporal coherence ($g^{(1)}(\tau)>0.6$) and radiant emittance ($10^{7}\,\mathrm{Wm^{-2}}$). This makes our scheme an appealing candidate for a graphene-base THz laser source. Moreover, a mechanism for the instability amplification is advanced for the case of substrates with varying electric permitivitty, which allows to overcome eventual limitations associated with the experimental implementation.

Analytical Approximate Solution for Large Post-Buckling Behavior of a Fixed-Pinned Beam Subjected to Terminal Force with Shear Force Effect

Based on Euler–Bernoulli beam theory, this paper investigates large post-buckling deformation of a slender elastic beam with fixed-pinned end. Owing to the asymmetric boundary conditions, it is difficult to establish analytic solution. Based on the Maclaurin series expansion and orthogonal Chebyshev polynomials, the governing differential equation with both sinusoidal nonlinearity and cosinusoidal nonlinearity can be reduced to a polynomial equation, and the geometry condition with sinusoidal nonlinearity can also be simplified to be a cubic polynomial integral equation. The admissible lateral displacement function to satisfy the fixed-pinned boundary conditions is derived in an elegant way. The analytical approximations are obtained with the harmonic balance method. Two approximate formulae for axial load and lateral load are established for small as well as large angle of rotation at the pinned end. These approximate solutions show excellent agreement with those of the shooting method for a large range of the rotation angle at the pinned end. Moreover, due to brevity of expressions, the present analytical approximate solutions are convenient to investigate effects of various parameters on the large post-buckling response of fixed-pinned beams.

Thermo-mechanical analysis of porous sandwich S-FGM plate for different boundary conditions using Galerkin Vlasov's method: A semi-analytical approach

abstract In the present paper, for the first time, an attempt has been made to obtain the solution for bending and stress under the thermal environment using Galerkin Vlasov's method. A non-polynomial based higher-order shear deformation theory with inverse tangent hyperbolic shape function is used to define the displacement field. The formulation is performed for the author's recently developed sandwich plate using a new modified sigmoid function-based functionally graded material (S-FGM) plate of different symmetric and non-symmetric configurations. Using a one-dimensional steady-state heat conduction equation, a new temperature distribution through the thickness based on modified sigmoid law is proposed. Three different types of porosity are considered viz. even, uneven but symmetric and uneven but non-symmetric. A new uneven non-symmetric porosity model is used in which micro-voids are varied in accordance with material property variation in the thickness direction in order to capture the accurate distribution of voids on the plate. The principle of virtual work is employed to derive equilibrium equations. An exact solution is obtained using the assumed solution with shape functions satisfying the edge boundary conditions. From the present study on static analysis, it is deduced that more refined and accurate results for plate in thermal environment, it is necessary to include the thermal effect on the stiffness of the plate in addition to the initial deflection of the plate. The effect of boundary conditions on stress and deflection distribution along the surface of the porous plate is studied, and it is observed that the distribution is prominently affected by the type of symmetric or asymmetric boundary conditions. The considerable increase in deflection and stress can be seen for even porosity distribution (P-1) in comparison to uneven symmetric (P-2) or uneven non-symmetric (P-3) porosity distribution. In addition, the maximum transverse shear stresses are offset more from the center of the sandwich plate with an increase in temperature differences with a maximum offset at ΔT = 300 K and minimum offset at ΔT = 0 K. Different examples are considered to check the accuracy and validation of the present formulation. The calculated outcomes and interpretations can be useful as a validation study for the imminent investigation of sandwich S-FGM plates having porosities in the thermal environment.

Vortex shedding patterns past a rectangular cylinder near a free surface

The effects of asymmetric boundary conditions on the flow past rectangular cylinders are relevant for many engineering applications. Pertinently, the influence of wall proximity on the flow around cylinders has been widely investigated, and the flow regimes depending on the distance to the wall have been identified. However, a comprehensive study of the flow around rectangular cylinders near a free surface is lacking. The classifications of the flow have not been tackled and the connection between the flow behaviors and the force variations with the depth has not been demonstrated. In this study, multiphase simulations over a broad range of depth-to-length ratios (0.3–4.5) and width-to-length ratios (0.7–5.0) were conducted. Flow results show that the anti-symmetric vortex shedding becomes asymmetric and then suppressed as the depth decreases. Three patterns of asymmetric flow depending on the width-to-length ratio were observed: “asymmetrically separated”, “one-sidedly reattached” and “asymmetrically reattached”. The force variations with the depth for rectangular cylinders with 0.7 <w/l < 1.5, 1.5 ≤ w/l < 2.8 and 2.8 <w/l ≤ 5.0 show different trends due to the occurrence of different asymmetric flows. The boundaries between different flow patterns were defined based on the force variations.

Brillouin-Mandelstam spectroscopy of stress-modulated spatially confined spin waves in Ni thin films on piezoelectric substrates

We report the results of micro-Brillouin-Mandelstam light scattering spectroscopy of thermal magnons in the two-phase synthetic multiferroic structure consisting of a piezoelectric [Pb(Mg1/3Nb2/3)O3](1−x)–[PbTiO3]x (PMN-PT) substrate and a Ni thin film. The experimental data reveal the first two modes of the perpendicular standing spin waves (PSSW) spatially confined across the Ni thin film. A theoretical analysis of the frequency dependence of the PSSW peaks on the external magnetic field reveals the asymmetric boundary condition, i.e. pinning, for variable magnetization at different surfaces of the Ni thin film. The strain field induced by applying DC voltage to PMN-PT substrate leads to a downshift of PSSW mode frequency owing to the magneto-elastic effect in Ni, and corresponding changes in the spin-wave resonance conditions. The observed non-monotonic dependence of the PSSW frequency on DC voltage is related to an abrupt change of the pinning parameter at certain values of the voltage. The obtained results are important for understanding the thermal magnon spectrum in ferromagnetic films and the development of the low-power spin-wave devices and quantum sensors.

Seismic analysis of a masonry cross vault through shaking table tests: The case study of the Dey Mosque in Algiers

This paper presents the results of a monodirectional shaking table test on a full-scale unreinforced masonry cross vault characterized by asymmetric boundary conditions. The specimen represents a vault of the mosque of Dey in Algiers (Algeria), reproducing in detail the mechanical characteristics of masonry, and the constructive details including the presence of some peculiar wooden logs placed within the vault\'s configurations were studied by using an incremental dynamic analysis up to the collapse of the vault without the steel bars. The use of an innovative high-resolution 3D optical system allowed measure displacement data of the cross vault during the shake table tests. The experimental results were analysed in terms of evolution of damage mechanisms, and in-plane and out-of-plane deformations. Moreover, the dynamic properties of the structure were investigated by means of an experimental modal analysis.

ASYMMETRIC PERIODIC BOUNDARY CONDITIONS FOR MOLECULAR DYNAMICS AND COARSE-GRAINED SIMULATIONS OF NUCLEIC ACIDS

Periodic boundary conditions are commonly applied in molecular dynamics simulations in the microcanonical (NVE), canonical (NVT) and isothermal-isobaric (NpT) ensembles. In their simplest application, a biological system of interest is placed in the middle of a solvation box, which is chosen ‘sufficiently large’ to minimize any numerical artefacts associated with the periodic boundary conditions. This practical approach brings limitations to the size of biological systems that can be simulated. Here, we study simulations of effectively infinitelylong nucleic acids, which are solvated in the directions perpendicular to the polymer chain, while periodic boundary conditions are also applied along the polymer chain. We study the effects of these asymmetric periodic boundary conditions (APBC) on the simulated results, including the mechanical properties of biopolymers and the properties of the surrounding solvent. To get some further insights into the advantages of using the APBC, a coarse-grained worm-like chain model is first studied, illustrating how the persistence length can be extracted from local properties of the polymer chain, which are less affected by the APBC than some global averages. This is followed by all-atom molecular dynamics simulations of DNA in ionic solutions, where we use the APBC to investigate sequence-dependent properties of DNA molecules and properties of the surrounding solvent.

Experimental Investigations on the Effect of Axial Homogenous Magnetic Fields on Propagating Vortex Flow in the Taylor-Couette System.

Experimental investigations of propagating vortex flow states (pV states) in a short Taylor–Couette system with asymmetric boundary conditions are presented. The flow state was established in a ferrofluid showing no magneto-viscous effect and was exposed to axial magnetic fields. It was found that the magnetic field led to a change in the spatial and temporal behavior of the pV state, indicating complex interactions between the flow field and magnetic field. A stepwise applied axial magnetic field destabilized the pV state, leading to an intermittent flow state. Gradually increasing the axial magnetic fields changed the temporal behavior of the regime. Up to magnetic field strengths of 20 kA/m, the orbital frequency, as a measure for the temporal periodicity, was increased with field strength.

Investigation of the Dyakonov-Shur instability for THz plasma waves in quantum gated cylindrical FET

Plasma waves may become unstable in the channel of field effect transistors (FETs) with asymmetric boundary conditions on source and drain, which is known as Dyakonov-Shur instability. In this letter, we extend the Dyakonov-Shur instability to the quantum gated cylindrical FET and obtain the dispersion equations describing unstable terahertz (THz) plasma waves using quantum cylindrical hydrodynamic equations. Research results show that the length of the channel and the wave vector of the circumferential direction inhibit the instability increment; however, the quantum effect, the radius of the channel, and the electron temperature enhance the instability increment. The oscillation frequency increases with the increase in the quantum effect, the wave vector of the circumferential direction, and the electron temperature, but the oscillation frequency increases with the decrease in the length and radius of the channel. Compared with the one-dimensional rectangular FET, THz plasma waves in the two-dimensional cylindrical FET have a higher oscillation frequency.

Tunable bistability of a clamped elastic beam

Bistable structures featuring two stable shapes are commonplace in natural and engineered systems. They can quickly snap from one stable state to the other in response to certain external stimuli, and so have found a range of applications in microswitches, actuators, energy harvesters, and mechanical metamaterials. Buckled beams represent a sub-class of bistable structures extensively studied, yet the relation between relevant boundary conditions and their bifurcation behaviors remains incompletely understood. Here we examine the bifurcation behavior of a buckled, elastic beam with both ends clamped in the absence of any external body force. Through a systematic investigation combining experiments, theory, and computational analysis, we identify that the beam can go through saddle–node bifurcation or pitchfork bifurcation (subcritical or supercritical) under asymmetric or symmetric boundary conditions, respectively. We construct the stability diagram in terms of the independent control parameters that can provide a precise prediction of the critical point upon bifurcation. Our results can not only provide powerful guidance on the design of fast switches, mechanical metamaterials, energy harvesters, or micro-electro-mechanical systems (MEMS) that exploit the bistability of elastic beams with clamped ends, but also have implications on a variety of biophysical phenomena and biomimetic structures.

A Novel Method for Numerical Analysis of 3D Nonlinear Thermo-Mechanical Bending of Annular and Circular Plates with Asymmetric Boundary Conditions Using SAPM

This study is the first report of numerical solution of nonlinear bending analysis for annular and circular plates based on 3D elasticity theory with asymmetric boundary conditions using semi-analytical polynomial method (SAPM). Orthotropic annular and circular plates are subjected to transverse loading and 3D bending analysis in the presence of symmetric and asymmetric boundary conditions is studied. For asymmetry cases, the plate boundaries are divided to two or three parts and various boundary conditions such as clamped, simply-supported and free edges are defined for each part. The asymmetry in one and two directions is studied. The influence of elastic foundations, mechanical and thermo-mechanical loadings are examined. Regarding this fact that no study has been done in the case of asymmetric boundary conditions, the obtained results are compared with FEM results by ABAQUS. The results show good agreement with the literatures and FEM results, which it shows that the presented method can use to analyze the 3D bending of plates under asymmetric conditions. Also, it is observed that 3D elasticity estimates some higher deflections than other theories. But, the obtained results by 3D elasticity theory and those obtained by FEM analysis in the case of asymmetric conditions are so close.

40.3: Inversion Charge for Memory Display under Passively Addressed Driving using Photo‐aligned Ferroelectric Liquid Crystal

A ferroelectric liquid crystal (FLC) panel driven by different driving signals has been test. The FLC layer thickness is 3um. Asymmetric boundary conditions, when only one of the ITO surfaces of the display matrix is covered with the photo‐aligning layer while another is not, have been used for providing high contrast, fast response and steady multiplex operation. The electro‐optical performance driven by different impulse signal is presented to show the minimum charge needed for the inversion of bi‐stable state.

Experimental study on the performance evaluation of active chilled beams in cooling operation under varied boundary conditions

This study investigates the thermal comfort and indoor air quality performance of Active Chilled Beams (ACB) in cooling operation mode in an open office environment with asymmetrical loads. Many studies on thermal comfort using ACB in cooling mode have been conducted; most of the studies confirm that thermal comfort is satisfactory because the temperature gradient and the airspeeds are acceptable at the occupant level. However, these studies do not specifically address the local discomfort in cooling mode. Furthermore, these studies do not consider performance under different ACB configurations or varied boundary conditions such as those found in real offices. This paper reports the results of an experimental study that addresses the above issues. A laboratory experiment was designed to simulate a multi-occupant open office with an ACB subjected to asymmetric boundary conditions. The results demonstrate that discomfort draft risk at the ankle level is higher when the ACB is oriented parallel to the window. Furthermore, the results suggest that shape (the type of ACB) and throw of ACB affect air distribution. The results emphasize the importance of properly selecting, orienting, and designing ACB, not just to offset the room loads, but to match the proportions and boundary conditions of the office.

Viscous Dissipation Effects in A Microchannel Caused by Oscillation of One Surface

The viscous dissipation effects in a microchannels caused by an oscillatory lower surface is investigated numerically. An asymmetric thermal boundary condition, particularly at upper plate insulated and lower plate with constant surface temperature is solved and analyzed in details graphically. Results reveal that effect of temperature field is strongly dependent on Brinkman number, while the thermal diffusion rate on the heat induced relies on the Prandtl number. The angular frequency has influence on the temperature field gradient.

Viscous Dissipation Effects in A Microchannel Caused by Oscillation of One Surface

The viscous dissipation effects in a microchannels caused by an oscillatory lower surface is investigated numerically. An asymmetric thermal boundary condition, particularly at upper plate insulated and lower plate with constant surface temperature is solved and analyzed in details graphically. Results reveal that effect of temperature field is strongly dependent on Brinkman number, while the thermal diffusion rate on the heat induced relies on the Prandtl number. The angular frequency has influence on the temperature field gradient.

13‐4: Passively Addressed Helix‐Free Ferroelectric Liquid Crystal for Fast Response Bi‐Stable Display

A helix free ferroelectric liquid crystal (FLC) panel using the passively addressed driving scheme has been realized. The FLC layer thickness is 4um. Asymmetric boundary conditions have been used for providing high contrast and steady multiplex operation. The electro‐optical performance of the panel shows fast switching time around 87us and good bi‐stability in the meantime. A 4um 64*64 passively addressed FLC display prototype has been achieved and successfully multiplexed.

Mutual synchronization of two lean-premixed gas turbine combustors: Phase locking and amplitude death

In a can-annular gas turbine combustion system, low-frequency self-excited combustion instabilities can arise from coupled interactions between adjacent can combustors, rather than from individual combustors. These interactions—in either a push–push or a push–pull mode – are caused by the presence of a cross-talk area upstream of the first stage turbine stator vanes. The mechanisms governing the selection and growth of the interaction modes are not well understood, particularly for systems with multiple oscillators (combustors). Here we describe experimental observations of mutual synchronization between two coupled-combustors, each containing a lean-premixed swirl-stabilized turbulent flame, subjected to symmetric and asymmetric inlet boundary conditions. Our results reveal that when two combustors oscillating at different natural frequencies are connected via a cross-talk area, they can become mutually synchronized with each other, exhibiting global oscillations at a common frequency. The presence of cross-talk communication can excite strong oscillations in the coupled dual-combustor system, even when each combustor is individually stable in isolation. By contrast, for certain asymmetric inlet boundary conditions, coupling the two combustors together stops them from oscillating completely, even though each combustor in isolation oscillates in a large-amplitude limit cycle. In nonlinear dynamics, such mode suppression is known as amplitude death and is a classical feature of coupled self-excited oscillators, with potentially wider applications for passive control of combustion instabilities. We analyze a large set of experimental data in non-dimensional domains, and demonstrate that the coupled mode is not defined by the phase difference between the heat release rate oscillations of the two combustors, but rather by the phase difference between their acoustic pressure fluctuations. Mode selection is found to be strongly correlated to the degree of asymmetry in the magnitude of the flames' heat release rate oscillations in adjacent combustors. As well as providing the first experimental evidence of amplitude death in a combustion system, this study reveals a previously unrecognized role of combustor-to-combustor interactions in provoking self-excited combustion instabilities in multiple-combustor gas turbine systems.

Thermal radiation effects on MHD flow with heat and mass transfer of micropolar fluid between two vertical walls

In the present study, the boundary layer free convective unsteady flow of an incompressible micropolar fluid under a uniform magnetic field is considered with thermal radiation in symmetric and asymmetric boundary conditions. The governing unsteady boundary layer momentum, angular momentum, energy and concentration equations with initial and boundary conditions are nondimensionalised and solved numerically. For validation of numerical solutions, analytical expression for velocity, micro-rotation, temperature and concentration profiles of steady state case have been obtained and compared through the tabular form. It is found that magnetic field , vortex viscosity and its steady state time has retarding effect on velocity and micro-rotation of micropolar fluid. For both cases (asymmetric and symmetric), thermal radiation parameter tends to improve velocity and micro-rotation profiles. Physically, due to increasing Nr, heat is generated in fluid flow, which leads to improvement in heat transfer as well as velocity throughout the fluid flow region.

Microstructural Modeling and Strengthening Mechanism of TiB/Ti-6Al-4V Discontinuously-Reinforced Titanium Matrix Composite.

A novel modeling method was proposed to provide an improved representation of the actual microstructure of TiB/Ti-6Al-4V discontinuously-reinforced titanium matrix composite (DRTMC). Based on the Thiessen polygon structure, the representative volume element (RVE) containing the complex microstructures of the DRTMC was first generated. Thereafter, by using multiple user-defined subroutines in the commercial finite element software ABAQUS, the application of asymmetric mesh periodic boundary conditions on the RVE was realized, and the equivalent elastic modulus of the DRTMC was determined according to the homogenization method. Through error analyses on the experimental and calculated results regarding the equivalent elastic parameters of the DRTMC, the rationality of generating the DRTMC finite element model by using the present method was validated. Finally, simulations based on four types of network-like models revealed that the present simplifications to the particle shape of the reinforcement phase had less of an influence on the overall composite strength. Moreover, the present study demonstrates that the DRTMC enhancement is mainly attributed to the matrix strengthening, rather than the load-transferring mechanism. The strengthening influences of the distribution forms of the reinforcement phases, including their distribution density and orientation, were studied further. It was found that both the higher distribution density and limited distribution orientation of the particles would increase the probability of overlapping and merging between particles, and; therefore, higher strength could be yielded when the volume fraction of the reinforcement phase reached a certain threshold. Owing to the versatility of the developed methods and programs, this work can provide a useful reference for the characterization of the mechanical properties of other composites types.

Exact results for sheared polar active suspensions with variable liquid crystalline order.

We consider a confined sheared active polar liquid crystal with a uniform orientation and study the effect of variations in the magnitude of polarization. Restricting our analysis to one-dimensional geometries, we demonstrate that with asymmetric boundary conditions, this system is characterized, macroscopically, by a linear shear stress vs. shear strain relationship that does not pass through the origin: At a zero strain rate, the fluid sustains a non-zero stress. Analytic solutions for the polarization, density, and velocity fields are derived for asymptotically large or small systems and are shown by comparison with precise numerical solutions to be good approximations for finite-size systems.