Mode Switching Phenomenon Research Articles

Assessment of failure mechanism of a strip footing on horizontal ground considering flow rules

Numerical investigation of the failure mechanism developed beneath a strip footing resting on non-dilatant cohesionless soil has been investigated in a finite element framework. The observed failure mechanism is manifested in terms of incremental displacement and incremental deviatoric strain patterns. A series of simulation on foundation models were carried out to examine the influence of various factors on the failure mechanism, namely meshing pattern, mesh refinement schemes, soil shear strength properties and embedment of the footing. The numerical outcomes have been verified against sophisticated laboratory investigations available in the literature. The developed failure mechanism is found to be asymmetric and markedly one-directional, the nature being prevalently independent to the variation in the influencing factors. Failure mechanism is found to progressively evolve following a mode-switching phenomenon, exhibiting transition from elastic to plastic deformations at- or near-failure. The deviations of the numerically obtained evolving failure mechanism from the same as pre-assumed in the conventional bearing capacity theories are explored and reported herein.

Mode switching characteristics of PSR B0329+54 at 150 MHz

We present the results of 60 hours of observations of PSR B0329+54 with the LOFAR PL-612 station located in Baldy near Olsztyn, Poland and managed by University of Warmia and Mazury in Olsztyn (UWM). Observations were conducted in August/September 2016 and in May and August 2017 using the HBA antennas, at the central frequency of about 140 MHz, and they were conducted in form of six 10-hour semi-continuous observing sessions. The main goal of the analysis was the study of the mode switching phenomenon in this pulsar, and our results show that at this frequency the abnormal profile mode is present only for about 12.6% of time on average, which is lower than for the analysis of a very large set of 1.5 GHz observations performed at Urumqi observatory in 2011. Also worth mentioning is the fact, that the results shown in this paper also demonstrate the first scientific output concerning pulsar observations with the PL-612 station.

Identification of the formation of resonant tones in compressible cavity flows

Identification of the fluid dynamic mechanisms responsible for the formation of resonant tones in a cavity flow is challenging. Time-frequency non-linear analysis techniques were applied to the post-processing of pressure signals recorded on the floor of a rectangular cavity at a transonic Mach number. The results obtained, confirmed that the resonant peaks in the spectrum were produced by the interaction of a carrier frequency (and its harmonics) and a modulating frequency. High-order spectral analysis, based on the instantaneous wavelet bi-coherence method, was able to identify, at individual samples in the pressure–time signal, that the interaction between the fundamental frequency and the amplitude modulation frequency was responsible for the creation of the Rossier–Heller tones. The same technique was also able to correlate the mode switching phenomenon, as well as the deactivation of the resonant tones during the temporal evolution of the signal.

Effect of equivalence ratio on the modal dynamics of azimuthal combustion instabilities

The present paper investigates the effect of equivalence ratio on the modal dynamics of self-excited azimuthal combustion instabilities in a laboratory-scale annular combustor. It is shown that operating at different equivalence ratios not only affects the mode of oscillation (whether the instabilities are mixed, or predominantly spinning or standing modes), but also the way in which mode switching between these states occurs. Using pressure time-series data obtained at multiple locations around the annulus the phenomenon of mode switching is investigated through the spin ratio, the orientation of the nodal lines and the envelope of the pressure oscillations. These three parameters all suggest that mode switching events occur almost periodically, and over azimuthal convective time scales. Moreover, analysis of the spin ratio and orientation of the nodal lines show that these parameters are correlated, and that their mean rates of change or trajectories also have a preferred direction. Therefore, these quantities were found to oscillate backwards and forwards in-phase with each other, as opposed to the mode simply rotating in one direction, providing new insight into the nature of modal dynamics of self-excited azimuthal modes.

Mode Switching of High Index Contrast Photonic Crystal Surface Emitting Lasers

This paper investigates the phenomenon of mode switching for GaAs-based high index contrast photonic crystal surface emitting lasers (PCSELs) with deeply etched air holes. The room-temperature lasing characteristics and angular resolved photoluminescence spectra of PCSELs show that the lasing mode would change from the fundamental guided mode to a high-order guided mode as the filling factor (FF) increased, resulting from the diffraction of deeply etched semiconductor/air arrays with high index contrast. Higher order guided modes could coherently couple out of photonic crystal and would significantly influence the laser output of PCSELs with a larger air FF, leading to rich far-field characteristics.

Non-fragile state estimation for discrete Markovian jumping neural networks

In this paper, the non-fragile state estimation problem is investigated for a class of discrete-time neural networks subject to Markovian jumping parameters and time delays. In terms of a Markov chain, the mode switching phenomenon at different times is considered in both the parameters and the discrete delays of the neural networks. To account for the possible gain variations occurring in the implementation, the gain of the estimator is assumed to be perturbed by multiplicative norm-bounded uncertainties. We aim to design a non-fragile state estimator such that, in the presence of all admissible gain variations, the estimation error converges to zero exponentially. By adopting the Lyapunov–Krasovskii functional and the stochastic analysis theory, sufficient conditions are established to ensure the existence of the desired state estimator that guarantees the stability of the overall estimation error dynamics. The explicit expression of such estimators is parameterized by solving a convex optimization problem via the semi-definite programming method. A numerical simulation example is provided to verify the usefulness of the proposed methods.

Characterization of instabilities in a Rotating Detonation Combustor

Rotating Detonation Combustor (RDC) operation is investigated under different air and fuel flow rates, and varied geometries to reveal four fundamentally different instabilities. Select test points are chosen to study these instabilities using qualitative and quantitative tools. First, for RDC operation at lean limits, or with subsonic air injection, or with large fuel injection orifices, the detonation wave inside the combustor undergoes aperiodic chaotic propagation around the combustor annulus characterized by incoherent pressure-time traces. The high incoherence in recorded pressure, along with the considerable variation in subsequent pressure peaks suggests numerous failure and re-initiation of the detonation wave. Second, almost all operating points at the variety of conditions tested exhibit some degree of low frequency sinusoidal oscillations characterized by periodic waxing and waning of subsequent detonation peak pressures. It occurs between 200 Hz and 500 Hz for the operating maps studied. Third, the phenomenon of mode switching in the RDC is also defined as instability since the sudden change in the number of detonation waves existing in the combustor is temporally unpredictable, and often unpredictable for a given geometry. It is found that RDC operation is more stable when there are multiple detonation waves inside the chamber. With a back-pressurizing convergent nozzle, at certain operating points, the RDC exhibits axisymmetric pulsed operation like the Pulsed Detonation Combustor (PDC). There is significant evidence to suggest that these longitudinal pulsed detonations (LPD) are manifested due to shock-reflection from the RDC exit followed by a subsequent shock-initiation of the fresh reactants. The frequency of this instability is around 3.8 kHz for the test case investigated.

Synthesis and characterization of mononuclear copper(II) complexes of pyridine 2-carboxamide: Their application as catalyst in peroxidative oxidation and antimicrobial agents

Four water soluble copper(II) complexes, [Cu(HL)2(H2O)2]Cl2 (1), [Cu(HL)2(ClO4)2] (2), [Cu(HL)2(SCN)2] (3) and [CuL2]⋅8H2O (4), where HL is pyridine 2–carboxamide, have been synthesized and characterized by various spectroscopic techniques. Structures have been determined by single crystal X-ray crystallography. The pH induced inter-conversion of Cu(HL)2(H2O)2]Cl2 (1) and [CuL2]⋅8H2O (4) through co-ordination mode switching was investigated thoroughly with the help of absorption spectroscopy. Complexes 1–3 were found to be active catalysts for the oxidation of toluene, ethyl benzene and cyclohexane in the presence of hydrogen peroxide as the oxidant under mild conditions. Toluene was oxidized to benzyl alcohol and benzaldehyde, ethyl benzene was oxidized to 1-phenylethanol and acetophenone and cyclohexane was oxidized to yield cyclohexanol and cyclohexanone Antimicrobial activities have been investigated with these copper(II) complexes against gram + ve bacteria, gram −ve bacterial and fungal species. X-ray structural characterization, pH-induced co-ordination mode switching phenomenon, peroxidative oxidation catalysis and antimicrobial activity of water soluble, discrete mononuclear complexes of copper(II) using the simplest pyridine 2–carboxamide ligand (HL) and its deprotonated form have been investigated thoroughly.

State estimation with guaranteed performance for switching-type fuzzy neural networks in presence of sensor nonlinearities

This paper investigates the state estimation with guaranteed performance for a class of switching fuzzy neural networks. A switching-type fuzzy neural networks (STFNNs) model is proposed which captures external disturbances, sensor nonlinearities, and mode switching phenomenon of the fuzzy neural networks without the Markovian process assumption. For such a model, a state estimation problem is formulated to achieve the guaranteed performance: the estimation error system is exponentially stable with certain decay rate and a prescribed H∞ disturbance attenuation level. A novel sufficient condition for this problem is established using the Lyapunov functional method and the average dwell time approach, and the estimator parameters are explicitly given. A numerical example is presented to show the effectiveness of the developed results.

Ocean wave transmission and reflection between two connecting viscoelastic ice covers: An approximate solution

An approximate solution for wave transmission and reflection between open water and a viscoelastic ice cover was developed earlier, in which both the water and the ice cover were treated as a continuum, each governed by its own equation of motion. The interface conditions included matching velocity and stresses between the two continua. The analysis provided a first step towards modeling the wave-in-ice climate on a geophysical scale, where properties of the ice cover change with time and location. In this study, we derive the wave transmission and reflection from one viscoelastic material to another. Only two modes of the dispersion relation are considered and the horizontal boundary conditions are approximated by matching the mean values. The reflection and transmission coefficients are first determined for simplified cases to compare with earlier theories. All results show reasonable agreement when the same physical parameters are used. Behaviors of the transmission and reflection coefficients are then obtained for a range of viscoelastic covers. A mode switching phenomenon with increasing ice shear modulus is found. This phenomenon was pointed out in the study of wave propagation from open water to a viscoelastic cover. For two connecting viscoelastic covers, such mode switching is found to terminate with increasing viscosity. Together with an earlier investigation of wave dispersion in a viscoelastic ice cover, the present study provides a way to implement theoretical results in a numerical model for wave propagation through a heterogeneous ice cover. In discretizing a continuously changing ice cover over the geophysical scale, on top of the energy advection, energy transmission between computational cells due to the heterogeneity can be estimated using the present method, while the attenuation and wave speed within each cell are from the previously obtained dispersion relation. In addition, on floe scales, this study provides a way to determine wave scattering from an ice floe imbedded in grease or brash ice.

LONG-TERM MONITORING OF MODE SWITCHING FOR PSR B0329+54

The mode switching phenomenon of PSR B0329+54 is investigated based on the long-term monitoring from September 2003 to April 2009 made with the Urumqi 25m radio telescope at 1540 MHz. At that frequency, the change of relative intensity between the leading and trailing components is the predominant feature of mode switching. The intensity ratios between the leading and trailing components are measured for the individual profiles averaged over a few minutes. It is found that the ratios follow normal distributions, where the abnormal mode has a wider typical width than the normal mode, indicating that the abnormal mode is less stable than the normal mode. Our data show that 84.9% of the time for PSR B0329+54 was in the normal mode and 15.1% was in the abnormal mode. From the two passages of eight-day quasi-continuous observations in 2004, and supplemented by the daily data observed with 15 m telescope at 610 MHz at Jodrell Bank Observatory, the intrinsic distributions of mode timescales are constrained with the Bayesian inference method. It is found that the gamma distribution with the shape parameter slightly smaller than 1 is favored over the normal, lognormal and Pareto distributions. The optimal scale parameters of the gamma distribution is 31.5 minutes for the abnormal mode and 154 minutes for the normal mode. The shape parameters have very similar values, i.e. 0.75^{+0.22}_{-0.17} for the normal mode and 0.84^{+0.28}_{-0.22} for the abnormal mode, indicating the physical mechanisms in both modes may be the same. No long-term modulation of the relative intensity ratios was found for both the modes, suggesting that the mode switching was stable. The intrinsic timescale distributions, for the first time constrained for this pulsar, provide valuable information to understand the physics of mode switching.

Multiobjective Evolutionary Structural Optimization Using Combined Static/Dynamic Control Parameters

A thermal protection system (TPS) is responsible for protecting a spacecraft’s components from melting due to high reentry temperatures. In the design of TPS, both maximum thermal stress and minimum natural frequency must be considered due to the combined thermoacoustic environment inherent in high-speed vehicle applications. A multiobjective structural optimization method for the three-dimensional acreage TPS design is developed using an evolutionary structural optimization (ESO) algorithm. The static control parameter used to find the optimum in minimum thermal stress design is modified to address an irregular mode-switching phenomenon, as well as to improve the modal stiffness in dynamic analysis. Two objectives are optimized simultaneously, namely, the maximization of fundamental natural frequency and the minimization of maximum thermal stress. The proposed modified control parameter is demonstrated in the design of a metallic TPS using the method of weighted objectives. The results are then compared with the conventional ESO sensitivity approach. This work concludes by applying the methodology which makes use of both topology and shape optimization in the design of an acreage TPS.

Mode-switching and nonlinear effects in compressible flow over a cavity

Multiple distinct peaks of comparable strength in unsteady pressure autospectra often characterize compressible flow-induced cavity oscillations. It is unclear whether these different large-amplitude tones (i.e., Rossiter modes) coexist or are the result of a mode-switching phenomenon. The cause of additional peaks in the spectrum, particularly at low frequency, is also unknown. This article describes the analyses of unsteady pressure data in a cavity using time-frequency methods, namely the short-time Fourier transform (STFT) and the continuous Morlet wavelet transform, and higher-order spectral techniques. The STFT and wavelet analyses clearly show that the dominant mode switches between the primary Rossiter modes. This is verified by instantaneous schlieren images acquired simultaneously with the unsteady pressures. Furthermore, the Rossiter modes experience some degree of low-frequency amplitude modulation. An estimate of the modulation frequency, obtained from the wavelet analysis, matches the low-frequency peak seen in the autospectrum. Higher-order spectral methods were employed to investigate potential quadratic nonlinear interactions between the Rossiter modes and to determine if they are responsible for the low-frequency mode present in the autospectrum. In turn, this low-frequency mode could interact with the Rossiter modes to modulate their amplitude. Significant nonlinearities, in the form of sum and difference frequencies of the Rossiter modes, are present in the l/d=2 cavity at M∞=0.4, while nonlinear effects are much smaller in the l/d=4 at M∞=0.6. The bispectral analysis indicates that quadratic interactions between Rossiter modes in the near-field pressure are not responsible for the observed low-frequency peak in the pressure autospectrum. Furthermore, the low-frequency mode does not exhibit a strong nonlinear coupling with the Rossiter modes.

Numerical simulations of strain localization in inelastic solids using mesh-free methods

In this paper, a comprehensive account on using mesh-free methods to simulate strain localization in inelastic solids is presented. Using an explicit displacement-based formulation in mesh-free computations, high-resolution shear-band formations are obtained in both two-dimensional (2-D) and three-dimensional (3-D) simulations without recourse to any mixed formulation, discontinuous/incompatible element or special mesh design. The numerical solutions obtained here are insensitive to the orientation of the particle distributions if the local particle distribution is quasi-uniform, which, to a large extent, relieves the mesh alignment sensitivity that finite element methods suffer. Moreover, a simple h-adaptivity procedure is implemented in the explicit calculation, and by utilizing a mesh-free hierarchical partition of unity a spectral (wavelet) adaptivity procedure is developed to seek high-resolution shear-band formations. Moreover, the phenomenon of multiple shear band and mode switching are observed in numerical computations with a relatively coarse particle distribution in contrast to the costly fine-scale finite element simulations. Copyright © 2000 John Wiley & Sons, Ltd.

Self-driven mode switching of earthquake activity on a fault system

Theoretical results based on two different modeling approaches indicate that the seismic response of a fault system to steady tectonic loading can exhibit persisting fluctuations in the form of self-driven switching of the response back and forth between two distinct modes of activity. The first mode is associated with clusters of intense seismic activity including the largest possible earthquakes in the system and frequency-size event statistics compatible with the characteristic earthquake distribution. The second mode is characterized by relatively low moment release consisting only of small and intermediate size earthquakes and frequency-size event statistics following a truncated power law. The average duration of each activity mode scales with the time interval of a large earthquake cycle in the system. The results are compatible with various long geologic, paleoseismic, and historical records. The mode switching phenomenon may also exist in responses of other systems with many degrees of freedom and nonlinear dynamics.

On the mode development in the developing region of a plane jet

The development of the mode arrangement in the developing region of an acoustically excited plane jet is extensively studied using hot-wire measurements. Two single-wire probes are located within the two shear layers of the jet to detect the flow structure patterns simultaneously. The jet is excited at its fundamental frequency in either varicose mode (m=0) or sinuous mode (m=1) condition. Experimental results show that different excitation mode conditions lead to different spreading rates and velocity fluctuation distributions in the developing flow field. The frequency spectral distributions for the m=1 excitation exhibit double peaks of fa and fb, rather than a single subharmonic f0/2 for the m=0 excitation. These three components constitute a relationship of (fa+fb)/2=f0/2. The mode switching phenomenon is found to be prominent under the m=0 case, while less pronounced for the m=1 case. The mode development is mainly governed by the evolution of the primary instabilities in the jet shear layers.

Properties of individual and integrated pulses drawn into mode changing phenomenon in PSR 0943+10 at 40, 103 and 430 MHz

AbstractPulsar 0943+10 demonstrates very obviously that the mode switching phenomenon includes simultaneous and dramatic alterations of all principal properties of radioemission, i.e. intensity, polarization (both linear and circular) and drifting behaviour of individual pulses over a broad frequency range. On the basis of observations of PSR 0943+10 at the frequencies 40 and 430 MHz, a complete scenario of the transition process between two modes of radiation has been revealed. The ‘pre-switching transition process’ is found which manifests itself as attenuation of the pulse intensity of the current mode during hundreds of the pulse periods preceding a switch.

The mode-switching phenomenon in pulsars

The mode-switching phenomenon in pulsars