Lock-in State Research Articles

Significant influence of fluid viscoelasticity on flow dynamics past an oscillating cylinder

This study focuses on numerically investigating the impact of fluid viscoelasticity on the flow dynamics around a transversely forced oscillating cylinder operating in the laminar vortex shedding regime at a fixed Reynolds number of $Re = 100$ . Specifically, we explore how fluid viscoelasticity affects the boundary between the lock-in and no lock-in regions and the corresponding wake characteristics compared with a simple Newtonian fluid. Our findings reveal that fluid viscoelasticity enables the synchronization of the vortex street with the cylinder motion at lower oscillation frequencies than those required for a Newtonian fluid. Consequently, the lock-in region boundary for a viscoelastic fluid differs from that of a Newtonian fluid and expands in the non-dimensional cylinder oscillation amplitude and frequency parameter space. In the primary synchronization region, the wake of a Newtonian fluid exhibits ‘2S’ (two single vortices) and ‘P+S’ (a pair of vortices and a single vortex) shedding modes. In contrast, a ‘2P’ (two pairs of vortices) vortex mode is observed for a viscoelastic fluid within the same region. To gain a deeper understanding of the differences in the coherent flow structures and their associated frequencies between the two fluids, we employ the data-driven reduced-order modelling technique, known as the dynamic mode decomposition (DMD) technique. Utilizing this technique, we successfully extract and visualize the two competing fundamental frequencies (cylinder oscillation and natural vortex shedding frequencies) and their associated flow structures in the case of the no lock-in state, whereas only the dominant cylinder oscillation frequency and associated flow structure in the case of the lock-in state. Furthermore, we propose that the presence of excess strain resulting from the stretching of polymer molecules in viscoelastic fluids leads to a distinct difference in the wake structure compared with Newtonian fluids. This observation aligns with the findings obtained from the $Q$ -criterion and vorticity transport analysis of the wake.

Investigations on lock-in vortex-induced vibration of an airfoil at a high angle of attack based on detached eddy simulation

Large-scale modern wind turbines at standstill are prone to vortex-induced vibration (VIV). In this study, coupled fluid–solid dynamics of the wind turbine airfoil at a 90° attack angle are performed using the detached eddy simulation. The fully developed vibration responses with different structural dampings are explored in detail. The frequency lock-in regime is determined, and the corresponding phase differences between the lift and displacement are presented with the Lissajous curve. The dominant surface pressure mode and wake flow exhibit significant three-dimensional flow characteristics in unlock-in conditions, while a strong spanwise correlation in lock-in conditions is detected. The pressure fluctuation on the suction side in the lock-in state is observed to be more significant than in the unlock-in state. The effect of the distributed airfoil surface pressure on VIV is evaluated by considering the contribution value and the cyclic aerodynamic work density. With the decrease in structural damping, the aerodynamic work near the leading edge gets enhanced and the negative work region is reduced, leading to a higher amplitude of VIV. The beat vibration and hysteresis behavior at the critical reduced velocity are also analyzed in both the time domain and frequency domain.

Multi-scenario prediction and path optimization of industrial carbon unlocking in China

China's industry is facing a carbon lock-in (CLI) dilemma. Based on the complexity and dynamics of CLI, exploring diversified unlocking paths is an important way to achieve emission reduction goals. This paper innovatively applies system dynamics (SD) to the study of CLI prediction and unlocking path, and systematically constructs an industrial CLI SD model, trying to break the positive feedback mechanism of CLI through multiple causality and feedback chains. This paper simulates and predicts the changing trend of the CLI level of China's industry and 8 major subsectors from 2020 to 2050. At the same time, the random forest algorithm is used to select the regulatory variables of policy analysis. Compared with the previous research methods, the accuracy and objectivity of the selection of regulatory variables are improved, so as to test the effectiveness of the unlocking path under each scenario. The results show that (1) the whole industry faces a serious CLI problem and is still in a deep lock-in state. From the perspective of subsectors, the light, electromechanical, and textile industries have achieved unlocking; the oil and extractive industries are in a moderately lock-in state; the chemical, steel, and power industries are in a deep lock-in state; and the CLI situation is serious. (2) The results of the random forest algorithm show that R&D investment, the proportion of fixed asset investments in the energy industry, car sales, energy structure, and carbon emission industry structure are the most significant factors affecting CLI in each subsystem. (3) Compared with the existing economic development model, the technological, institutional, social, and comprehensive unlocking scenarios can all enable industry to achieve carbon peak and carbon unlocking in advance. However, the effect of each scenario is as follows: comprehensive unlocking scenario > technological unlocking scenario > institutional unlocking scenario > social unlocking scenario. (4) The unlocking times of the oil, chemical, steel, and power industries are relatively late in each scenario. To realize carbon unlocking in industry, these four areas should be focused on in the future. This study helps policymakers formulate reasonable policies to accelerate industrial carbon unlocking and promote the implementation of the “dual carbon” strategy.

Numerical investigations of two vibrating cylinders in uniform flow using overset mesh

Abstract In this research, flow around two vibrating cylinders in a tandem arrangement is simulated at Reynolds number Re = 200 using the dynamic overset mesh technique in finite volume-based commercial software. This investigation aims to study the combined influences of the spacing between the two identical circular cylinders and their excitation frequency in the flow. The cylinders are excited by a transverse forced vibration in a uniform cross-flow by applying a simple harmonic motion. The gap distance between the vibrating cylinders is chosen to be L/D = 1.5 and 4, and the vibration amplitude is kept constant at A/D = 0.25. The study focuses on three frequency ratios of the cylinders’ excitation frequency to Strouhal shedding frequency of the single stationary cylinder f e/f s = 0.8, 1.0, and 1.2. Simulation results showed that the flow characteristics over the two vibrating circular cylinders differ from that of a single vibrating cylinder. Also, it is observed that the lock-in state (resonance) for the two vibrating cylinders and the vortex wake patterns are highly affected by the gap distance between cylinders and the excitation frequency.

An EMG-based wearable multifunctional Eye-control glass to control home appliances and communicate by voluntary blinks

The purpose of this work is to first apply the electromyogram (EMG) of corrugator supercilii muscles as the only signal to control home appliances and communicate with others by means of voluntary blinks, especially for the severely disabled. An EMG-based wearable eye-control glass is constructed as a multifunctional human–machine interface (HMI) device in this work. Twelve healthy subjects participated in the experiment to perform the home appliance eye-control task and the eye-communication task. This multifunctional eye-control glass is low-cost, easy-to-use, and wearable, and the experiment results indicate the high selectivity of voluntary blinks/natural blinks and left-eye blinks/right-eye blinks, excellent sensitivity, fast response time, high accuracy, low individual differences, and a prominent information transfer rate of the wearable device. This study provides a novel way for the construction of wearable eye-control devices to help healthy and disabled people conveniently control home appliances and communicate with the external world. It is gentler for users when applying this EMG-based wearable eye-control glass in various practical applications, like in the cinema or a meeting, than gesture and voice interactions. For disabled people without speaking ability, users can perform simple communication and express their needs with clinical staff with the help of this wearable device. Especially, this novel method greatly facilitates the daily lives of patients in the lock-in state through blink control. Moreover, the relationship between voluntary blinks and the movement of corrugator supercilii muscles is proven.

Vortex shedding analysis of flows past forced-oscillation cylinder with dynamic mode decomposition

Direct numerical simulations are performed for flow past circular cylinders by the lattice Boltzmann method coupled with immersed moving boundary method. By analyzing the flows past a single cylinder at a wide range of cross-flow or in-line oscillation amplitude (0.25≤A/D≤1.5) and frequency (0.5≤fe/f0≤1.5), the results find that the vortex shedding modes inside and outside “lock-in” interval are of significant difference. The vortex shedding mode in the “unlock-in” state is 2S, but C(2S) and P + S shedding modes can be found in the lock-in state. Dynamic mode decomposition is used to analyze characteristic flow features, which shows that mode 1 is the main factor reflecting the flow field structure and mode 2 represents the vortex shedding mode in this work. The vortex shedding modes of flows past a tandem and side-by-side cross-flow double oscillating cylinders are systemically investigated. For tandem double oscillation cylinders, the results of modal decomposition suggest that the shear layer of upstream oscillating cylinder is separated behind the downstream cylinder at a space rate of L/D≤2, but separated behind the upstream cylinder at L/D≥3. Mode 2 at L/D=4 differs from other vortex shedding modes due to the strong inhibition effect by the downstream cylinder on the vortex formation of upstream cylinder. For side-by-side double oscillation cylinders, the wake of two cylinders is a single vortex street at H/D=1, a bistable flow at H/D=2 or 3, a coupled vortex street at H/D=4, and close to a single cylinder at H/D>4. The results of modal decomposition are disordered at H/D=2 due to the interaction between two cylinders and effect of gap flow.

Organizational path transformation in response to disruptive environmental changes: The role of middle managers

Organizations need to shift from the existing path and form a new path (i.e., realizing path transformation) after disruptive environmental changes, and yet it is a difficult process hardly known in the literature. This study reports on a comparative case study to examine middle managers’ roles in organizational path transformation. We show that middle managers can facilitate organizational path transformation first by breaking the lock-in state via individual exploration and collective assessment, and then by triggering self-reinforcing effects to form a new strategic pattern via external resource acquisition and synergistic internal deployment. In contrast, middle managers who conduct individual exploration, individual assessment, and independent resource deployment can create obstacles for organizational path transformation. Organizational contexts are likely the underlining cause for the differences between these two kinds of behaviors. This research contributes to the literature by revealing how middle managers facilitate or impede path transformation, and by identifying organizational contexts that influence their different behaviors.

An experimental study regarding the effect of streamwise vorticity on trailing edge vortex induced vibrations of a hydrofoil

A set of experiments testing the effect on vortex induced vibrations from three different trailing edge designs on a generic hydrofoil has been carried out at the Waterpower Laboratory of NTNU. One of the designs, a Donaldson type semi-blunt trailing edge not a-typical for use in hydraulic machinery serves as a reference. The other two, a trailing edge with boundary layer vortex generators and a trailing edge with rounded serrations cut into the blade, both introduce streamwise vorticity in the wake of the hydrofoil, as shown by particle image velocimetry measurements. The trailing edge of the hydrofoil is made exchangeable, so that a direct comparison of the strain intensity levels measured with a structurally embedded strain gauge bridge located close to the trailing edge can be made. The results indicate that the vortex generators are effective at mitigating the lock-in state of resonance and significantly reduces the maximum strain intensity measured, compared to the more conventional reference design. Velocity measurements both at lock-off conditions as well as close to resonance reveal a de-correlation of the streamwise velocity fluctuations in the wake for this design, suggesting that the von Kármán type vortex shedding is weakened even at resonance conditions. A de-correlation of the streamwise velocity fluctuations in the wake of the serrated trailing edge is also observed at lock-off conditions, but the vortex shedding appear to become gradually more coherent, as well as strengthened, for this design as the foil vibrations increase closer to resonance.

Flow field characteristics of a two-dimensional moonpool under uniform flow conditions

ABSTRACT Moonpools are holes in the middle of a ship's hull and through its deck. In this study, the flow mechanisms in moonpools in calm water were investigated. Star-CCM+ software was used to calculate the flow within a two-dimensional (2D) moonpool under uniform flow conditions. A 2D moonpool numerical calculation model was constructed, and the convergence analysis results and experimental data were used to verify the accuracy of the calculated results. Flow field and free surface fluctuations are the main factors that cause the drag force of the moonpool. The drag force increases with the flow velocity, and the drag coefficient presents different trends at different stages. With an increase in the Froude number, Fr, the 2D moonpool enters the frequency lock-in state of the piston and sloshing natural frequency. Additionally, increasing the moonpool length results in a nonlinear change in the flow state.

Lock-in phenomenon in the wake flow past an oscillating circular nano-cylinder

In this paper, the lock-in/synchronization phenomenon in the wake flow around an oscillating nano-cylinder is studied with molecular dynamics simulation. The results show that the lock-in valley of the velocity fluctuation also occurs in nano-scale. Similar to those in normal scales (usually >1 mm), three regimes of lock-in, transition, and no-lock-in states are also obtained. Unlike in normal scales, the concurrence of the density and velocity fluctuation waves with different phase and the same frequency makes the fluctuation frequency of the lift force different from that of the velocity. The oscillation of the nano-cylinder can intensify the lift force fluctuation, especially in the lock-in state. The intensity of the lift force increases with the amplitude of the nano-cylinder vibration. The lock-in valley of the lift force usually covers a wider range of frequency than that of the velocity. Improving the inflow Reynolds number can reduce the coverage range of the lock-in valley in frequency. The lock-in valley does not vary apparently as the Jz number, which represents the interaction intensity between fluid molecules, increases until it approaches Jz ≈ 1.4. The rise of Kn number can promote the occurrence of the lock-in phenomenon.

Dynamics and wake structure of a near-wall flexible cylinder

Interactions between the structural dynamics and wake of a vibrating flexible cylinder close to a wall are investigated using three-dimensional direct numerical simulation at reduced velocity Ur = 5-25, gap-to-diameter ratio G/D = 0.8, aspect ratio L/D = 50, and Reynolds number Re = 500. The 1st to 4th cross-flow (CF) vibration modes and the 1st to 9th in-line (IL) vibration modes are excited, and four types of modal groups for the ratio of IL to CF modes are identified, including groups ‘n:n’, ‘2n-1:n’, ‘2n:n’, and ‘2n+1:n’, where n refers to the excited mode in the CF direction. With the increase of Ur, the vibration responses are first dominated by one specific mode and then co-dominated by multi-modes with comparable intensities. Within the same modal group, the maximum CF and IL amplitudes and mean drag/lift coefficients decrease with Ur under smaller Ur conditions, whereas they increase with Ur under larger Ur conditions. The cylinder oscillation exhibits a standing-wave behavior and a mixture of standing and traveling behavior at the lower and higher Ur cases, respectively. Compared to the isolated case, the reduction of the IL vibration frequency by half and the enhanced IL amplitude are caused by the vortex shedding suppression from the lower side of the cylinder and by the corresponding IL lock-in state along the span. The vortex sheddings at the antinode and node of the CF displacement are in ‘2S’ and weak ‘2S ’ modes, with the energy input and output from the wake, respectively; whereas, for the isolated case, they are in ‘2P’ and ‘2S’ modes, respectively.

Firms’ internationalization through clusters: A keywords bibliometric analysis of 152 top publications in the period 2009-2018

While countless studies on the role of clusters in regional economic developments and business performance have been done, some disadvantages and limitations also have been identified. Limitations such as small local markets, limited resources, isolation, and over-independence which lead companies to a lock-in state regarding knowledge and innovation can be solved by means of internationalization or foreign market expansion. Therefore, the internationalization of clusters still needs more attention. Furthermore, by conducting a bibliometric study based on the keywords from previous research, this investigation intends to identify the principal and most influential items, their relationship, and evolution, related to this topic. This work listed the top 30 journals based on SCImago Rank (SJR) in the category of Marketing, Management, and Internationalization, to select authoritative papers on the topic of internationalization of clusters. Based on the title, time of publication (2009 to 2018), and content, 152 articles were selected. Using SciMAT software, all 584 keywords used in these articles were first categorized and then examined in terms of the networks between them, their h-index, centrality, density, and their evolution over the course of ten years (divided into two five-year periods). The results show that in the first five years, keywords such as, Network, Location-distance, and Global, have been at the core of this research topic. While in the second five year period, these keywords have yielded their position to new subjects such as Social capital, Cluster-type, and Development. Moreover, this research correspondingly confirms that the keyword bibliometric analysis produces similar results to a quantitative content analysis. These results therefore open the door to updated research questions for further and better development of the topic.

How to escape the dilemma of charging infrastructure construction? A multi-sectorial stochastic evolutionary game model

Providing effective charging infrastructure is an essential step in popularizing electric vehicles, which is conductive to improving pollution induced by traditional fuel vehicles and thus improving the environmental sustainability. The most crucial factor in escaping the dilemma of charging infrastructure construction lies in the synergies of multi-sectorial. This paper develops a stochastic evolutionary game model grounded on the strategy adoption of charging infrastructure construction involving governments (public), companies (private), and consumers (civil). We collect the data of Shanghai for simulation to compare the influence of strategy adoption. Our findings suggest that ⅰ) unlike the public and civil sector, in the current situation, the private sector will adopt the defector strategy quickly and lock in the defection state as time goes on; ⅱ) the additional cost effect caused by public-civil co-construction scenario is significant positive to the public sector but almost no impact on the civil sector; ⅲ) civil's strategy adoption has directional differences under two cost sharing scenarios (private-civil and public-civil) of charging infrastructures construction. In the private-civil scenario, if the cost of charging infrastructures construction is transferred to the civil sector, not only will it not change the private-sectorial defection state, but it will also cause civil sector to shift from cooperation to defection. However, at the same sharing ratio of the public-civil scenario, the civil sector is still willing to cooperate. ⅳ) reducing incentives of electric vehicles or increasing costs of internal combustion engine vehicles is necessary, which will not change the lock-in cooperation state of civil sector, and it can improve the cooperation willingness of publicsector.

Transverse flow-induced vibrations of a sphere in the proximity of a free surface: A numerical study

In this paper, we present a numerical study on the transverse flow-induced vibration (FIV) of an elastically mounted sphere in the vicinity of a free surface at subcritical Reynolds numbers. We assess the interaction dynamics and the vibration characteristics of fully submerged and piercing spheres that are free to vibrate in the transverse direction. We employ the recently developed three-dimensional two-phase flow–structure interaction solver to investigate fully and partially submerged configurations of an elastically mounted sphere. To begin, we examine the vortex-induced vibration (VIV) phenomenon and the vortex-shedding modes of a fully-submerged sphere vibrating freely in all three spatial directions. We systematically verify and analyze the mode transitions and the motion trajectories in the three degrees-of-freedom (3-DOF) for the Reynolds number up to 30000. We next simulate the transversely vibrating (1-DOF) full-submerged sphere over a wide range of reduced velocities 3≤U∗≤20, whereby the reduced velocity is adjusted by changing the freestream Reynolds number. The VIV response amplitude and the topology of the wake structure are compared with the measurements for the mode I and mode II response branches. We further look into the effect of the free surface on the FIV response of a transversely vibrating sphere in the proximity of a free surface. The response dynamics of the sphere is studied for three representative values of normalized immersion ratio (h∗=h∕D, where h is the distance from the top of the sphere to undisturbed free-surface level and D is the sphere diameter), at h∗=1 (fully submerged sphere with no free-surface effect), h∗=0 (where the top of the sphere touches the free surface) and h∗=−0.25 (where the sphere pierces the free surface). At the lock-in range, we observe that the amplitude response of the sphere at h∗=0 is decreased significantly compared to the case at h∗=1. It is found that the vorticity flux is diffused due to the free-surface boundary and the free surface acts as a sink of energy that leads to a reduction in the transverse force and amplitude response. When the sphere pierces the free surface at h∗=−0.25, the amplitude response at the lock-in state is found to be greater than all the submerged cases studied with the maximum peak-to-peak amplitude of ∼2D. We find that the interaction of the piercing sphere with the air–water interface causes a relatively large surface deformation and has a significant impact on the synchronization of the vortex shedding and the vibration frequency. The streamwise vorticity contours and pressure distribution are employed to understand the VIV characteristics and wake dynamics. Increased streamwise vorticity gives rise to a relatively larger transverse force to the piercing sphere at h∗=−0.25, resulting in greater positive energy transfer per cycle to sustain the large-amplitude vibration. Lastly, we study the sensitivity of large-amplitude vibration on the mass ratio, m∗, and, Froude number, Fr, at the lock-in state.

Consistency of splitting frequency difference with longtudinal modes spacing variation in Zeeman dual-frequency laser

Phase anisotropy in laser resonant cavity will bring about an influence on laser frequency and polarization, such as laser frequency splitting, of which the frequency difference is determined by their introduced phase retardation. For a helium-neon laser with a small phase retardation in the cavity, the two split modes are very close to each other whose burned holes are overlapped. Then only one mode oscillates while the other is always in lock-in state due to strong mode competition, which forms hidden frequency split. Meanwhile the spacing between adjacent longitudinal modes deviates from original value and produces a certain variation equal to twice the hidden splitting frequency difference. As a result the longitudinal modes spacing variation is dominated by the phase retardation. On the other hand, by applying transverse magnetic field to a laser tube along the polarization direction, the neon atoms will undergo transverse Zeeman effect and be divided into two groups to provide the gain for polarized light beams parallel to the magnetic field and perpendicular to the magnetic field respectively. Then the laser mode competition is greatly weakened so that the two split modes can oscillate simultaneously to obtain the frequency difference. In order to make profound study of the consistency between longitudinal mode spacing variation and splitting mode frequency difference in the presence of transverse magnetic field, the samples of tilted quartz plate or half wave plate is placed into laser cavity to produce phase retardation. By the two mentioned methods, the splitting frequency difference varying with phase retardation of samples is deduced to make a comparison. Two measurements show that the average relative deviation is less than 1%, while the experimental results accord with theoretical analyses quite well. In this way splitting frequency difference of Zeeman dual-frequency laser can be determined accurately, and a new method to measure the phase retardation of half wave plate is provided.

Effects of technology lock-in on enterprise innovation performance

PurposeThis study explores the effects of different types of technology lock-in on enterprise innovation performance. On this basis, the authors aim to provide technological innovation suggestions for the long-term development of Chinese enterprises.Design/methodology/approachA total of 211 high-tech enterprises in China were taken as the research sample. Data were collected through questionnaire surveys and secondhand data. Linear and nonlinear regression models were used to test the hypotheses.FindingsDifferent types of technology lock-in had different effects on enterprise innovation performance. Initiative lock-in and passive lock-in showed an inverted U-shaped relationship with innovation performance. Economies of scale were positively correlated with innovation performance, while self-lock-in was negatively correlated with it.Practical implicationsThis research can help enterprises reconceptualize technology lock-in. It can also help enterprises in different stages of development carry out targeted technological innovation and management strategies.Originality/valueThis research enriches our understanding of technology lock-in. By breaking with previous conceptualizations of a simple linear relationship between technology lock-in and innovation performance, this study proposes and confirms different technology lock-in has different relationships with enterprise innovation performance. This study is important for enterprises to reassess their technology lock-in state and carry out targeted technological innovation strategies. This research enriches the theory of path dependence and its combination with other theories. And this study also reveals the current research on technology lock-in is insufficient.

Comparison of anomalous magnetic properties of non-collinear CoCr2O4 and NiCr2O4 nanoparticles

Temperature-dependent magnetic properties of cobalt chromite (CoCr2O4) and nickel chromite (NiCr2O4) nanoparticles (NPs) have been compared by using magnetic measurements. The average particle size of CoCr2O4 and NiCr2O4 NPs deduced from TEM images was found to be 48 and 46 nm, respectively. The NiCr2O4 NPs showed paramagnetic to ferrimagnetic transition (Tc) at 84 K which is relatively lower than the Tc = 98 K for CoCr2O4 NPs. The temperature dependent Ms value for CoCr2O4 NPs showed deviation from Bloch’s law at low temperatures which may be due to the existence of lock-in and strong conical spin spiral state. NiCr2O4 NPs showed a good Bloch’s law fit with high value of Bloch’s constant B = 0.00183 K−1 and low value of Bloch’s exponent b = 1.23 than their respective bulk value which indicates the presence of disordered surface spins and finite size effect in these NPs. Interestingly, a high value of Hc (5 K) was observed for NiCr2O4 NPs relative to CoCr2O4 NPs which is attributed to the large canted angle between the oxygen and metallic cations in NiCr2O4 NPs. Temperature dependent coercivity (Hc) for both the NPs were fitted with Kneller’s law. The Kneller’s law fit to Hc vs. T data revealed an exponent α = 0.46 for CoCr2O4 NPs which indicates the non-interacting particles with uni-axial symmetry, while it was 0.74 for NiCr2O4 NPs indicating the assembly of randomly oriented particles. In summary, CoCr2O4 and NiCr2O4 NPs showed a different low temperature magnetic response which is also relatively different than counter ferrite NPs.

Magnetic phase diagram and dielectric properties of Mn doped CoCr2O4 nanoparticles

Abstract Effects of Manganese (Mn) substitution on the magnetic and dielectric properties of spinel phase Co1−xMnxCr2O4 (0.0 ≤ x ≤ 1) nanoparticles (NPs) have been studied. X-ray diffraction and Fourier transform infrared spectroscopy analysis showed the normal spinel structure for all the samples. Average crystallite size showed increasing trend with Mn doping and lies in the range 32–47 nm. The undoped CoCr2O4 NPs displayed a typical phase transition from paramagnetic (PM) to ferrimagnetic (FiM) state at TC = 100 K along with conical spiral magnetic state at TS = 27 K and lock-in transition state at TL = 13 K. The TC, TS and TL values were shifted towards lower temperatures with increasing Mn content and finally an antiferromagnetism (AFM) is obtained for MnCr2O4 NPs at TN = 20 K. A magnetic phase diagram was developed which showed the presence of different phases in these nanoparticles and attributed it to change in magnetic interactions with increasing Mn content. Saturation magnetization (MS) showed an increasing trend with Mn content due to the large magnetic moment of Mn ions. Nanoparticles showed an overall increasing trend of the dielectric constant with Mn concentration which is due to possible hopping of Mn2+ to Mn3+ and as well as increasing average crystallite size with Mn doping. In summary, increase in Mn doping revealed reduction in TC, TS, and TL values, an increasing MS trend, an AFM phase introduction and improved dielectric properties for Co1−xMnxCr2O4 NPs.

Steady-state visual evoked potential (SSVEP)-based brain–computer interface (BCI) of Chinese speller for a patient with amyotrophic lateral sclerosis: A case report

This study applied a steady-state visual evoked potential (SSVEP) based brain–computer interface (BCI) to a patient in lock-in state with amyotrophic lateral sclerosis (ALS) and validated its feasibility for communication. The developed calibration-free and asynchronous spelling system provided a natural and efficient communication experience for the patient, achieving a maximum free-spelling accuracy above 90% and an information transfer rate of over 22.203 bits/min. A set of standard frequency scanning and task spelling data were also acquired to evaluate the patient’s SSVEP response and to facilitate further personalized BCI design. The results demonstrated that the proposed SSVEP-based BCI system was practical and efficient enough to provide daily life communication for ALS patients.

Lock-in phenomenon of vortex shedding in oscillatory flows: an analytical investigation pertaining to combustors

An analytical investigation is performed to understand the lock-in phenomenon, observed in vortex shedding combustors. Several aeroengine afterburners and ramjets use a bluff body to stabilize the flame. The bluff body sheds vortices. During the occurrence of high-amplitude combustion instability, the frequency of vortex shedding locks in to the frequency of the chamber acoustic field. This phenomenon is termed vortex-acoustic lock-in. In general, there is a two-way coupling between the vortex shedding process and the acoustic field, making analytical investigation difficult. Since the frequency of the latter remains largely unaltered, performing an investigation to study the response of vortex shedding to external excitation not only allows one to gain insights, but also make the problem analytically tractable. We begin with a lower-order model available in the literature to describe the vortex shedding process in non-reacting flows, arising from sharp corners in the presence of upstream velocity excitation. The continuous time domain model is transformed to a discrete map, which connects the time instances of two successive vortex shedding events. The frequency and amplitude of excitation are varied to study the instantaneous vortex shedding time period, as the response of the system. In the absence of forcing, the iterates of the map form a period-1 solution with the frequency equalling the natural vortex shedding frequency. On increasing the amplitude of excitation, quasi-periodic behaviour of the iterates is observed, followed by a period-1 lock-in solution, where vortex shedding occurs at the excitation frequency. On further increasing the amplitude, de-lock-in occurs. From the map, an analytical solution is extracted, which represents the lock-in state. The condition and thereby the region in the frequency–amplitude parameter space where a general$p:1$lock-in occurs is then identified. Several important analytical expressions, such as for (1) critical threshold frequency above which lock-in occurs, (2) boundary of lock-in region in the parameter space, that are of direct importance to the design of quieter combustors are obtained. The study also identifies the transition of higher-order$p:1$to$1:1$lock-in state, through a series of lock-in and de-lock-in steps, whose occurrence could be verified from future experiments.