Non-symmetric Structure Research Articles

Symmetry-breaking for the control of combustion instabilities of two interacting swirl-stabilized flames

The concept of ‘symmetry breaking’ for the control of self-excited combustion dynamics is experimentally investigated in a lean-premixed, swirl-stabilized, two-nozzle, model gas turbine combustor. The present experimental investigation considers two fundamental asymmetries that are rarely explored in tandem – non-uniform fuel split and non-symmetric mean flow field. The equivalence ratio of each nozzle is varied between 0.57 and 0.73, including even and uneven fuel split conditions, and the structure of the mean flow field is altered by means of the swirl numbers of each nozzle, either S1=S2 or S1 ≠ S2, representing symmetric and non-symmetric mean flow structures, respectively. The bifurcation behavior of the system is then examined, with particular emphasis on the contributions of flame–flame and nozzle–flame interactions. A non-uniform fuel split is found to have a substantial effect on the system's stability, for both symmetric and non-symmetric mean flame structures, but the stability maps differ remarkably. In the symmetric mean flow, the instability occurs in one region near the even split line. In the non-symmetric case, on the other hand, the unstable region is divided into two regions with considerably lower amplitudes. There is a limit to the asymmetry-induced stability gain, since the instability occurs over a broader region in the stability map considered in the present investigation. The experimental data presented in this paper will help to resolve uncertainties associated with flame interactions in the description of self-excited instabilities.

Enhanced Matrix Power Function for Cryptographic Primitive Construction

A new enhanced matrix power function (MPF) is presented for the construction of cryptographic primitives. According to the definition in previously published papers, an MPF is an action of two matrices powering some base matrix on the left and right. The MPF inversion equations, corresponding to the MPF problem, are derived and have some structural similarity with classical multivariate quadratic (MQ) problem equations. Unlike the MQ problem, the MPF problem seems to be more complicated, since its equations are not defined over the field, but are represented as left–right action of two matrices defined over the infinite near-semiring on the matrix defined over the certain infinite, additive, noncommuting semigroup. The main results are the following: (1) the proposition of infinite, nonsymmetric, and noncommuting algebraic structures for the construction of the enhanced MPF, satisfying associativity conditions, which are necessary for cryptographic applications; (2) the proof that MPF inversion is polynomially equivalent to the solution of a certain kind of generalized multivariate quadratic (MQ) problem which can be reckoned as hard; (3) the estimation of the effectiveness of direct MPF value computation; and (4) the presentation of preliminary security analysis, the determination of the security parameter, and specification of its secure value. These results allow us to make a conjecture that enhanced MPF can be a candidate one-way function (OWF), since the effective (polynomial-time) inversion algorithm for it is not yet known. An example of the application of the proposed MPF for the Key Agreement Protocol (KAP) is presented. Since the direct MPF value is computed effectively, the proposed MPF is suitable for the realization of cryptographic protocols in devices with restricted computation resources.

Temperature-Selective Dual Radical Generation from Alkyl Diiodide: Applications to Synthesis of Asymmetric CABC Multi-Block Copolymers and Their Unique Assembly Structures.

Temperature-selective radical generation from a newly designed alkyl diiodide (I-R2 -R1 -I) was studied. R1 -I and I-R2 had different reactivities for generating alkyl radicals in the presence of a tetraoctylammonium iodide (ONI) catalyst. Taking advantage of the temperature selectivity, we used the alkyl diiodide as a dual initiator in ONI-catalyzed living radical polymerization to uniquely synthesize CABC non-symmetric multi-block copolymers. Because of their non-symmetric structure, CABC multi-block copolymers form unique assemblies, that is, Janus-type particles with hetero-segment coronas and flower-like particles with hetero-segment petals.

H-BN-layer-induced chiral decomposition in the electronic properties of multilayer graphene

We discuss the chiral decomposition of non-symmetric stacking structures. It is shown that the low-energy electronic structure of a Bernal stacked graphene multilayer deposited on h-BN consists of chiral pseudospin doublets. N-layer graphene stacks on the h-BN layer have N/2 effective bilayer graphene systems and one effective h-BN layer if N is even or effective graphene bilayers plus one graphene monolayer modified by an h-BN layer if N is odd. We present the decomposition procedure and derive the recurrence relations for the effective parameters characterizing the chiral subsystems. In this case, the effective parameters consist of interlayer couplings and on-site potentials in contrast to pure graphene multilayer systems where only interlayer couplings are modified. We apply this procedure to discuss the Klein tunnelling phenomena and quantitatively compare the results with pure graphene multilayer systems.

Heat Transfer in Air Flow Across a Single-Row Bundle of Tubes With Spiral Grooves

An experimental study has been made of the heat transfer and hydraulic resistance in air flow across a single-row bundle of tubes with spiral grooves on the exterior surface and of a bundle of smooth tubes of the same diameter. Cutting a spiral groove of width 3 mm and depth 2 mm with a pitch of 20 mm on a tube with an outside diameter of 22 mm increased the heat-transfer coefficient in the range of Reynolds numbers 6200–16,000 by 40–53% compared to the smooth-tube bundle. The hydraulic resistance of the tube bundle decreased by 22%. The Reynolds analogy factor increased, on the average, by 41%. Tubes with spiral grooves, in the investigated range of the velocities of flow, have shown a certain increase in the Strouhal number to values of 0.24–0.36. Computer simulation has been carried out of the hydrodynamics of the single-row bundle of smooth-wall tubes and the tubes with spiral grooves. It has been shown that the enhancement of heat transfer and the decrease in the hydraulic resistance alike are attributable to the appearance of nonsymmetric vortex structures induced by the flow across the spiral grooves.

Equivalent Circuits for Nonsymmetric Reciprocal Two Ports Based on Eigenstate Formulation

Equivalent circuit topologies for two-port reciprocal nonsymmetric structures are proposed as an extension of the well-known symmetric lattice network. An eigenstate formulation approach that allows the separation of the eigenmodes in the equivalent circuit, as in the symmetric lattice network, is adopted. Four topologies based on series or parallel immittances with the combined use of some transformers are proposed. Particular cases, such as symmetric networks and parallel and series single elements, are derived from the proposed topologies. Moreover, power dissipation in these topologies is also analyzed. A modification in the reference planes is further introduced to achieve power dissipation orthogonality in the two branches of the circuit, as happens in the symmetric lattice network. Additionally, real transformation ratios for the transformers and nonnegative real parts for the immittances are obtained. Finally, the equivalent circuit is extracted for a complementary strip slot with different degrees of asymmetry. This illustrative example confirms the usefulness of the proposed topologies, which can be regarded as an extension of the symmetric lattice network, since they preserve the eigenstate decomposition.

Photophysical properties of chiral covalent organic cages

Chiral organic compounds are the excellent second-order nonlinear optical (NLO) materials due to their intrinsic non-symmetric structures and combined with the merits of organic compounds. Here, the ground electron structures, excited-state electron transition, and second-order NLO property of novel chiral covalent organic cages (COCs) (Solomek et al., 2017) consisting of naphthalene-1,4:5,8-bis(dicarboximide) (NDI) units, have been fully investigated by DFT/TDDFT. The simulated electron absorption wavelengths are in good agreement with experimental ones, allowing us to assign their electron transition characters with high confidence. Based on the experimental structures, we designed eight compounds to probe the effect of different substitutions on photophysical properties. It is found that the substitution of NH2 and NO2 groups at opposite side of NDI and replacement of cyclohex-1,2-diyl with benzene are the most effective way to not only tune energy gaps and electron transition properties but also enhance the NLO response. For instance, the second-order NLO value of compound 2-h is about 80 times as large as the organic urea molecule. Our work is also important for fully understanding photophysical properties and extending potential applications of chiral COCs.

Elfin: An algorithm for the computational design of custom three-dimensional structures from modular repeat protein building blocks.

Computational protein design methods have enabled the design of novel protein structures, but they are often still limited to small proteins and symmetric systems. To expand the size of designable proteins while controlling the overall structure, we developed Elfin, a genetic algorithm for the design of novel proteins with custom shapes using structural building blocks derived from experimentally verified repeat proteins. By combining building blocks with compatible interfaces, it is possible to rapidly build non-symmetric large structures (>1000 amino acids) that match three-dimensional geometric descriptions provided by the user. A run time of about 20min on a laptop computer for a 3000 amino acid structure makes Elfin accessible to users with limited computational resources. Protein structures with controlled geometry will allow the systematic study of the effect of spatial arrangement of enzymes and signaling molecules, and provide new scaffolds for functional nanomaterials.

Alloy-Like Behavior of the Thermal Conductivity of Nonsymmetric Superlattices

ABSTRACTIn this work, we show a phenomenological alloy-like fit of the thermal conductivity of (A)d1:(B)d2 superlattices with d1 ≠ d2; that is, a nonsymmetric structure. The presented method is a generalization of the Norbury rule of the summation of thermal resistivities in alloy compounds. Namely, we show that this approach can be also extended to describe the thermal properties of a crystalline and ordered system composed of two or more elements and has a potentially much wider application range. Using this approximation, we estimate that the interface thermal resistance depends on the period and the ratio of materials that form the superlattice structure.

Anisotropic elastic modulus, high Poisson's ratio and negative thermal expansion of graphynes and graphdiynes

Graphyne (GY) and graphdiyne (GDY) are two-dimensional one-atom-thick carbon allotropes highly considered to substitute graphene in electronic applications because of the prediction of non-null band-gap. There are multiple configurations of GY structures not yet fully investigated in literature. In this work, by means of classical molecular dynamics simulations, the Young's modulus, Poisson's ratio and linear thermal expansion coefficient (TEC) of all originally proposed seven types of GYs and corresponding GDYs are calculated. The dependence of these properties with the density of the structure is investigated for the first time. Quadratic increasing of the TEC of GY and GDY structures with density was found. The elastic modulus of GYs and GDYs were shown to be more sensitive to their density than general porous materials. In particular, non-symmetric structures are much softer along the armchair direction than along zigzag direction, implying that the elasticity along armchair direction of GY and GDY structures are similar to that of porous gels materials. Values larger than unity were found for the Poisson's ratio of some non-symmetric GYs and GDYs. A simple honeycomb mechanical model is shown to capture the observed values of Poisson's ratio of GYs and GDYs.

Emergency Control of AR Drone Quadrotor UAV Suffering a Total Loss of One Rotor

In this paper, an emergency fault-tolerant controller is developed for quadrotor unmanned aerial vehicles (UAVs) suffering a total loss of one actuator (rotor/motor). The proposed controller is based on control allocation where the infected actuator is exempted from the control effect, and control efforts are redistributed among healthy actuators. As a result, the emergency controller transforms the infected quadrotor into a trirotor and uses the remaining actuators to ensure that the UAV continues following its path. The main advantage of the new controller is that it uses the same simple proportional-integral-derivative (PID) controller applied in fault-free cases by redistributing its controls, rather than designing complex and memory consumable controllers. Experimental results using the augmented reality (AR) Drone 2 platform show that by implementing the quadrotor to trirotor conversion maneuver, the nominal PID controller gains powerful fault-tolerant capabilities and the infected UAV is successful in maintaining its desired path. Performance degradation seen as oscillations did not cause the drone to crash despite the total loss of yaw control, the presence of activation delay, the nonsymmetric structure of the trirotor, and its odd number of rotors that produces imbalance of the rotor moments.

Image feature representation with orthogonal symmetric local weber graph structure

Image feature representation is a hot topic in the computer vision field. Inspired by Weber's law and local graph structure (LGS), we propose a novel image feature representation descriptor, called orthogonal symmetric local weber graph structure (OSLWGS). It contains two components: differential excitation pattern (DEP) and orthogonal symmetric LGS (OSLGS). In particular, DEP is extended by bringing difference of Gaussian (DoG), which can make OSLWGS robust to image noise. In addition, OSLGS can overcome some defects of LGS including non-symmetric and single horizontal structure problems. Furthermore, 2D OSLWGS histogram is generated by fusing DEP and OSLGS to improve the discriminative power and obtain more precise image description. And then, it is further encoded into 1D histogram and classified via sparse representation. Extensive experiments on FERET, CMUPIE, LFW, Yale B, simulated YALE partial occlusion, RawFooT and PhoTex databases validate the effectiveness of the proposed OSLWGS. Experimental results demonstrate that the proposed algorithm is an efficient and robust method compared with some state-of-the-art approaches.

Wideband analytical equivalent circuit for coupled asymmetrical nonaligned slit arrays.

Microstructured metallic devices have been extensively studied because of their interesting properties for controlling the transmission, reflection, and absorption of electromagnetic waves. A very simple implementation is an array of infinitely long parallel metal strips printed on a dielectric substrate. In the past few years, several analytical models have been reported based on the use of equivalent circuits with distributed and lumped components to account for the electrical performance of these structures. However, the proposed models are restricted to highly symmetrical configurations of the basic unit cell of the periodic structure. The purpose of this paper is to present the nontrivial extension of such circuit models to deal with nonsymmetrical structures. More specifically, a wideband equivalent-circuit model will be developed to describe the scattering properties of a pair of coupled different nonaligned slit gratings printed on a dielectric slab of arbitrary thickness. The relevant consequences of the lack of symmetry of the structures under study will be thoroughly discussed. The obtained equivalent network can be straightforwardly used to model stacked structures with an arbitrary number of nonsymmetrical striplike arrays.

Effect of the 6PBT stirrer eccentricity and off-bottom clearance on mixing of pseudoplastic fluid in a stirred tank

The aim of this work is to investigate the effect of the shaft eccentricity on the flow field and mixing characteristics in a stirred tank with the novel stirrer composed of perturbed six-bent-bladed turbine (6PBT). The difference between coaxial and eccentric agitations is studied using computational fluid dynamics (CFD) simulations combined with standard k−ε turbulent equations, that offer a complete image of the three-dimensional flow field. In order to determine the capability of CFD to forecast the mixing process, particle image velocimetry (PIV), which provide an accurate representation of the time-averaged velocity, was used to measure fluid velocity. The test liquid used was 1.25%(wt) xanthan gum solution, a pseudoplastic fluid with a yield stress. The comparison of the experimental and simulated mean flow fields has demonstrated that calculations based on Reynolds-averaged Navier-Stokes equations are suitable for obtaining accurate results. The effects of the shaft eccentricity and the stirrer off-bottom distance on the flow model, mixing time and mixing efficiency were extensively analyzed. It is observed that the microstructure of the flow field has a significant effect on the tracer mixing process. The eccentric agitation can lead to the flow model change and the non-symmetric flow structure, which would possess an obvious superiority of mixing behavior. Moreover, the mixing rate and mixing efficiency are dependent on the shaft eccentricity and the stirrer off-bottom distance, showing the corresponding increase of the eccentricity with the off-bottom distance. The efficient mixing process of pseudoplastic fluid stirred by 6PBT impeller is obtained with the considerably low mixing energy per unit volume when the stirrer off-bottom distance, C, is T/3 and the eccentricity, e, is 0.2. The research results provide valuable references for the improvement of pseudoplastic fluid agitation technology.

Trirotor mechatronic design and reduction of dynamic model inputs by aerodynamic forces identification

This paper outlines a mechatronic design and a remote measurement system of a trirotor unmanned aerial vehicle (UAV). Furthermore, with particular emphasis on the experimental aspect, this study presents also a real dynamic model with reduced inputs made by experimental aerodynamic forces/torques identification. The trirotor presents a real control challenge compared with the quadrotor system. Indeed, in contrast to the quadrotor, the trirotor presents a non-symmetric structure and it has an odd number of rotors which causes a yaw moment issue. The developed mechanical design and the actuators characteristics are then presented. Experimental tests show that we can have a reduced dynamic model. Indeed, with empirical manipulations we deduce some relations between drag torque and thrust forces. Such result allows us to reduce the trirotor developed model inputs. Finally, this study is a preliminary phase before the real-time control implementation.

Diffraction inspired unidirectional and bidirectional beam splitting in defect-containing photonic structures without interface corrugations

It is shown that strong diffractions and related dual-beam splitting can be obtained at transmission through the nonsymmetric structures that represent two slabs of photonic crystal (PhC) separated by a single coupled-cavity type defect layer, while there are no grating-like corrugations at the interfaces. The basic operation regimes include unidirectional and bidirectional splitting that occur due to the dominant contribution of the first positive and first negative diffraction orders to the transmission, which is typically connected with different manifestations of the asymmetric transmission phenomenon. Being the main component of the resulting transmission mechanism, diffractions appear owing to the effect exerted by the defect layer that works like an embedded diffractive element. Two mechanisms can co-exist in one structure, which differ, among others, in that whether dispersion allows coupling of zero order to a wave propagating in the regular, i.e., defect-free PhC segments or not. The possibility of strong diffractions and efficient splitting related to it strongly depend on the dispersion properties of the Floquet-Bloch modes of the PhC. Existence of one of the studied transmission scenarios is not affected by location of the defect layer.

Parallel preconditioners for monolithic solution of shear bands

Shear bands are one of the most fascinating instabilities in metals that occur under high strain rates. They describe narrow regions, on the order of micron scales, where plastic deformations and significant heating are localized which eventually leads to fracture nucleation and failure of the component.In this work shear bands are described by a set of four strongly coupled thermo-mechanical equations discretized by a mixed finite element formulation. A thermo-viscoplastic flow rule is used to model the inelastic constitutive law and finite thermal conductivity is prescribed which gives rise to an inherent physical length scale, governed by competition of shear heating and thermal diffusion. The residual equations are solved monolithically by a Newton type method at every time step and have been shown to yield mesh insensitive result. The Jacobian of the system is sparse and has a nonsymmetric block structure that varies with the different stages of shear bands formation.The aim of the current work is to develop robust parallel preconditioners to GMRES in order to solve the resulting Jacobian systems efficiently. The main idea is to design Schur complements tailored to the specific block structure of the system and that account for the varying stages of shear bands.We develop multipurpose preconditioners that apply to standard irreducible discretizations as well as our recent work on isogeometric discretizations of shear bands.The proposed preconditioners are tested on benchmark examples and compared to standard state of practice solvers such as GMRES/ILU and LU direct solvers. Nonlinear and linear iterations counts as well as CPU times and computational speedups are reported and it is shown that the proposed preconditioners are robust, efficient and outperform traditional state of the art solvers.

Interfacial stresses in sandwich structures subjected to temperature and mechanical loads

Through sequential coupling of the differential equations for interfacial shearing and peeling stresses, concise closed-form strength-of-material solutions for these stresses in symmetric and non-symmetric sandwich structures subjected to the combined loadings of temperature, stretching, and bending have been developed. The free-edge effects of the interfacial shearing stress, the maximum magnitude of the interfacial shear stress, and the location of its occurrence are accurately modelled using a single high-frequency hyperbolic function. The analytical solutions have been validated against finite element analysis (FEA) solutions using four sandwich structures: one symmetric, two mildly asymmetric, and another severely asymmetric; and for three load cases: differential thermal expansion, differential mechanical stretching, and mechanical bending. The maximum deviation between the analytical and the FEA solutions for the interfacial shearing and peeling stresses for all the structures and load cases, including that for the severely asymmetric sandwich structure, was less than 15%.

CONSIDERATIONS ON THE DIFFUSE SEISMICITY ASSUMPTION

Second-order effects were historically included by the effective length method (K concept). All the studies about that methodology have been developed in frame plane, with regular rectangular frames. The new way to include those effects is the use of second-order analysis, direct analysis method or alternative simplified options. This methodology was included in ANSI AISC360 in the 2005 version and in the 2010 version. As before, the studies already developed for DAM analysis are in plane. In this paper, the K concept is revisited by numerical analysis, and extended to the 3D space. Using models of symmetric and non-symmetric industrial steel structures in plane, 3D stability analyses were developed, and the results were compared with plane behavior. Several conclusions and recommendations were exposed, resulting from the analyzed models. Keywords: Second-order analyses, steel structures, irregular 3D frames.

Structural and Electronic Properties of Layered Arsenic and Antimony Arsenide

Layered materials exhibit intriguing electronic characteristics and the search for new types of two-dimensional (2D) structures is of importance for future device fabrication. Using state-of-art first principle calculations, we identify and characterize the structural and electronic properties of two 2D layered arsenic materials, namely, arsenic and its alloy AsSb. The stable 2D structural configuration of arsenic is confirmed to be the low-buckled two-dimensional hexagonal structure by phonon and binding energy calculations. The monolayer exhibits indirect semiconducting properties with gap around 1.5 eV (corrected to 2.2 eV by hybrid function), which can be modulated into a direct semiconductor within a small amount of tensile strain. These semiconducting properties are preserved when cutting into 1D nanoribbons, but the band gap is edge dependent. It is interesting to find that an indirect to direct gap transition can be achieved under strain modulation of the armchair ribbon. Essentially the same phenomena can be found in layered AsSb, except a weak Rashba induced band splitting is present in AsSb due to the nonsymmetric structure and spin orbit coupling. When an additional layer is added on the top, a semiconductor–metal transition will occur. The findings here broaden the family of 2D materials beyond graphene and transition metal dichalcogenides and provide useful information for experimental fabrication of new layered materials with possible application in optoelectronics.