Periodic Forcing Research Articles

Vibration-induced streaming flow near a sharp edge: Flow structure and instabilities in a large span of forcing amplitude.

The steady streaming generated near solid walls by the periodic forcing of a viscous fluid is known to be strongly enhanced near sharp structures, owing to centrifugal effects that lead to the generation of an intense jet from the sharp tip. This flow has been shown to provide efficient active mixing in microchannels, due to strong transverse velocity. The forcing is often prescribed by acoustic transducers, but it can also be generated from low-frequency time-periodic flow ensured by mechanical vibrations. In this paper, we study the flow structure generated by low-frequency forcing (typically 10Hz) around a sharp tip. Using direct numerical simulations, we extract both the time-periodic and steady responses within a large span of amplitude of vibrations. When the amplitude is smaller than the tip radius of curvature, we recover the flow structure observed at higher frequencies (>1kHz) in previous studies, namely, an intense symmetric central jet and a quadratic dependence for the characteristic streaming velocity with the oscillating velocity v_{s}∼v_{a}^{2}. At higher amplitudes, such a scaling no longer holds and the streaming flow pattern loses its left-right symmetry. We then analyze the mechanisms of the instability from the careful examination of the instationary flow fields, and we propose possible mechanisms for such a flow transition involving the coupling between the streaming jet and instationary vorticity.

Estimating Skill Mismatch in the Indian Labour Market: A Regional Dimension

The article provides an overview of the very pertinent skill mismatch issue in the Indian labour market. It specifically looks into the state-wise overeducation rates, a type of vertical skill mismatch where the education level of the worker exceeds the job requirements. The article tries to provide a regional dimension to the estimates of overeducation and addresses the problem by linking it to the incidence of vocational training among employees. Given that vocational training plays an important role in imparting industry-relevant skills, the article recommends its expansion as a necessary factor for curbing overeducation in states lagging in the industrial sector. Using the recent Periodic Labour Force Survey (PLFS) data, the article unveils trends in recent years where, though the propensity to take vocational training has increased, it is not sufficient to meet the industry’s demands. Also, the stunted proportion of individuals with government-sponsored vocational training signals either inferior quality or low access. The later fact is reinstated when we look at the bias towards higher income households in undertaking such training. Apart from the need to address the supply side of skilled labour, by making training programmes more inclusive, there is also a need to address the demand side to ensure proper alignment between the skills derived from training programmes and those required by the industry. Besides, given the huge informal labour market, there is an urgent need for proper certification and recognition of skills derived from training programmes to ensure their productive employment. The article shows a severe imbalance between the demand and supply sides and calls for active participation of different stakeholders, including the private sector, to address the problem. In this regard, the article shows instances of successful implementation of vocational training facilities in other countries, which can form a basis for rolling out well-defined and coherent programmes.

Heat Flow in a Periodically Forced, Thermostatted Chain

We investigate the properties of a harmonic chain in contact with a thermal bath at one end and subjected, at its other end, to a periodic force. The particles also undergo a random velocity reversal action, which results in a finite heat conductivity of the system. We prove the approach of the system to a time periodic state and compute the heat current, equal to the time averaged work done on the system, in that state. This work approaches a finite positive value as the length of the chain increases. Rescaling space, the strength and/or the period of the force leads to a macroscopic temperature profile corresponding to the stationary solution of a continuum heat equation with Dirichlet-Neumann boundary conditions.

Creation and Destruction of Jobs in Urban Labour Market: Role of Gender, Caste and Religion in India.

This study analyses labour market transitions from job to jobless or jobless to job by using Periodic Labour Force Survey (PLFS)of India in urban labour market by applying first-order Markov process in constructing transition matrices and panel logit regression model using personal level information from the PLFS in India for 2019-20. While education is assumed to be the most important factor influencing the process of transitions in the labour market, the circumstance variables like gender, caste and religion are expected to have significant effects on it in a country like India. The empirical results show a significant difference in labour market transitions between men and women. The movement of workers between job status was very low and the major part of the transition occurred in the form of job losses for each type of workers both among men and women and also among all working age group and young age group.

A probe into the fatigue crack growth in mechanical systems with hyperchaotic/chaotic dynamics

This study presents experimental and analytical fatigue frameworks for the dynamic fatigue life prediction of nonlinear mechanical systems under hyperchaotic oscillations. Fatigue in aluminium notched beams under hyperchaotic/chaotic vibrations is studied using a time-based analytical model. The fatigue lives obtained by the analytical model provided satisfactory correlations with those acquired by the dynamic fatigue experiments conducted on cracked aluminium beams. The model is then used to study the fatigue under Duffing chaotic, Duffing hyperchaotic, and periodic dynamic forcing. Results indicate that the classic linear cumulative damage rule delivers inaccurate high cycle fatigue (HCF) predictions under chaotic and hyperchaotic loading. Proper prediction of the HCF in metallic mechanical components experiencing chaotic and hyperchaotic dynamics requires time-based fracture-mechanics analytical models, such as that proposed in the current study. Under similar elastic energy inputs, hyperchaotic loading brings higher fatigue damage than chaotic and periodic loading. The fatigue life sensitivity to initial conditions is high under hyperchaotic, moderate under chaotic and next-zero under periodic loading.

Numerical study of the Poynting vector effect on periodically forced neutrally buoyant spherical particles at low Reynolds numbers — dynamics and rheology

In this paper, the effect of the Poynting vector force field on periodically forced neutrally buoyant spherical particles at low Reynolds numbers is studied numerically. The mathematical formulation of six nonlinear coupled integro–differential equations was obtained by modifying the Lovalenti and Brady formalism for the hydrodynamic force acting on a spherical particle. The governing equations were solved using an adaptive step size Runge–Kutta method by integrating the integral using the trapezoidal rule. The forces acting on the particle-provided phase space trajectories are similar to Jeffery’s orbits. The paper also discusses the possible outcomes of the rheology due to the Poynting vector on the periodically forced neutrally buoyant sparsely spaced non-interacting spherical particles in a fluid at low Reynolds numbers. The computed rheological parameters in this study are first normal stress difference, second normal stress difference, intrinsic pressure and relative viscosity. The study shows that the first normal stress difference is nearly zero and the second normal stress difference is nonzero indicating the dominance of the Poynting vector field over the magnetic field on the bulk stress of the fluid. The intrinsic pressure and the relative viscosity changed due to the stress caused by the electromagnetic force and periodic force on the spherical particles.

Suppression of quasiperiodic thermoacoustic oscillations via genetic programming

We use genetic programming (GP) to discover data-driven control laws for the suppression of quasiperiodic oscillations in a prototypical thermoacoustic system. We rank the control laws based on a predefined cost function that accounts for the pressure amplitude and the actuation effort. We then breed subsequent generations of control laws via a tournament process. We find that GP closed-loop control is more effective than GP open-loop control and conventional periodic forcing, producing a similarly high degree of amplitude suppression but with the lowest actuation effort. We also find that GP closed-loop control can identify unforeseen actuation mechanisms, providing new insight into the physical coupling between the heat release rate and pressure fields.

Research on a rotary piezoelectric wind energy harvester with bilateral excitation.

This paper describes a rotary piezoelectric wind energy harvester with bilateral excitation (B-RPWEH) that improves power generation performance. The power generating unit in the current piezoelectric cantilever wind energy harvester was primarily subjected to a periodic force in a single direction. The B-RPWEH adopted a reasonable bilateral magnet arrangement, thus avoiding the drawbacks of limited piezoelectric cantilever beam deformation and unstable power generation due to unidirectional excitation force. The factors affecting the power generation were theoretically analyzed, and the natural frequency and excitation force of the piezoelectric cantilever have been simulated and analyzed. A comprehensive experimental research method was used to investigate the output performance. The B-RPWEH reaches a maximum output voltage of 20.48Vpp when the piezoelectric sheet is fixed at an angle of 30°, the Savonius turbine number is 3, and the magnet diameter is 8mm. By adjusting the fixed angle of the piezoelectric sheet, the number of Savonius wind turbine blades, and the magnet diameter, the maximum voltage is increased by 52.38%, 4.49%, and 245.95%, respectively. The output power is 24.5 mW with an external resistor of 8 kΩ, and the normalized power density is 153.14 × 10-3 mW/mm3, capable of powering light-emitting diodes (LEDs). This structure can drive wireless networks or low-power electronics.

Effect of COVID-19 Pandemic on Employment and Earning in Urban India during the First Three Months of Pandemic Period: An Analysis with Unit-Level Data of Periodic Labour Force Survey.

Urbanisation has accelerated the pace of development throughout the world. Big cities provide employment and livelihood for workers because of which workers have always migrated from rural areas to cities. However, in India, most of the migrant workers are absorbed in the low-paid and low-skilled jobs in the widespread informal sector. With the outbreak of COVID-19, lockdown was declared suddenly without any notice in India during the last week of March 2019 and most of the urban informal sector workers suddenly lost their jobs, and since they had no protection, they werepushed into poverty. Detailed analysis of such losses is of utmost importance so that perfectly appropriate remedial measures can be taken by the government. Periodic Labour Force Survey (PLFS)report of 2019-20 has analysed the situation of labour market in India for four quarters from July 2019 to June 2020. Therefore, the last quarter of the data will give us the valuable information about the urban labour market during the first three months of the COVID-19 pandemic period. This study analyses the possible reasons behind decline in monthly earnings and labour market participation of urban people in India during the period of outbreak of COVID-19 pandemic, i.e. during the period from April 2020 to June 2020, using the data of fourth quarter from each of the PLFSs of 2017-18, 2018-19 and 2019-20 since they have identical seasonal conditions. We have used cross-tabulation method to find out employment and unemployment rates of people inurban areas according to gender and type of employment for the period, from July to June, for the years 2018, 2019 and 2020. We have also tried to find the reasons behind the decline in income of workers during the first three months of the pandemic period, i.e. during the fourth quarter of 2019-20, compared to the fourth quarter of 2017-18 and that of 2018-19 using the Mincerian wage equation. Our empirical results have shown that urban workers in India have lost jobs and suffered from significant decline in income during the first three months of the COVID-19 pandemic period in almost all types of employment.

The mechanism of wheel polygonal wear based on rotor dynamics

A rotor dynamic model of wheel-rail coupling system was established taking wheel eccentricity and initial polygon into consideration in order to investigate the mechanism of wheel polygonal wear. Considering the complex wheel-rail contact relationship, the equivalent yaw angle was proposed and the wheel-rail relative speed model was established. The law of wheel-rail self-excited vibration and its effect on polygonal wear were studied. The possibility of variable speed operation was investigated together with the sensitivity of the system parameters. The results show that the periodic longitudinal and lateral creep forces are essential for the wheel polygonal wear, which have the characteristics of supercritical Hopf bifurcation and ‘fixed wavelength’ and ‘fixed frequency’. While in some circumstances, longitudinal and lateral creep forces jointly cause polygonal wear, in other cases, longitudinal creep forces have almost no effect. By adjusting the longitudinal and lateral stiffness, primary suspension stiffness, and rail damping parameters, as well as by selecting an appropriate speed for variable speed operation, the wheel polygonal wear may be effectively suppressed. The findings offer a fresh viewpoint for the study of mechanisms and the prevention of wheel polygonal wear.

Improvement of energy storage density and energy harvesting performance of amphoteric Pr ion-modified lead-free Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) ceramics

Mechanical energy harvesting and energy storage through lead-free piezoelectric materials is an inevitable source of eco-friendly sustainable powering of electronic devices. Herein, we have synthesized amphoteric rare-earth element praseodymium (Pr) modified Ba0.85Ca0.15Ti0.9Zr0.1O3(BCZT) ceramics, with a cost-effective solid-state-reaction based two-step sintering method for the controlled grain growth. Their crystalline structures and surface morphology were investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The dielectric and ferroelectric properties of the ceramic capacitors were investigated and correlated with structural parameters. In the vicinity of the monotonic phase transition region, with the coexistence of orthorhombic and tetragonal symmetry, the Pr ion addition in BCZT improves the tetragonal phase, which widens its energy harvesting and storage arena. The appreciable energy harvesting ability was found for an optimized energy harvester with a composition of 0.04 wt% Pr added BCZT ceramic, with an open-circuit voltage of about 5.1 V (corresponding power density of 1.213 mW/cm3) from a simple finger tapping and bring about output voltage stability with a maximum output voltage of about 9 V over 10,000 cycles when periodic force is applied by a machine tapping. Furthermore, this optimized Pr added BCZT ceramic capacitor is capable of storing a substantial recoverable energy density of 81.9 mJ/cm3with a considerable energy storage efficiency of 76.4%. These upshots offer a head start in implementing these ceramic capacitors for effective energy harvesting and energy storage applications for powering futuristic miniature electronics.

A Simplified Method for Evaluating the Diaphragm Flexibility for Frame-Shear Wall Structure under Earthquake Load

The rigid floor assumption is commonly used in structural design, but it is not applicable to buildings with a large plane aspect ratio. This study designed nine frame-shear wall structures with the story of 3, 6, and 12, with a plane aspect ratio of 2, 3.33, and 4. Based on the design results, the finite element models were set up by ETABS. Both the rigid diaphragm and the flexible diaphragm cases were considered in each model. The effect of elastic diaphragm deformation on structural seismic performance was investigated, including fundamental period, top displacement, inter-story drift, and base shear force. The results indicate that the diaphragm deformation on 3-story structures is more significant than that on 6-story and 12-story structures. The diaphragm in-plane deformation increases with the aspect ratio. On the basis of the analysis results, a simplified formula to calculate the internal force amplification factor and a quantitative assessment method for evaluating the diaphragm flexibility were proposed, which can provide a reference for engineering design.

Dynamical behaviour of nonlinear structures in superthermal plasmas associated with external periodic force

Dynamical behaviour of nonlinear structures in superthermal plasmas associated with external periodic force

Improved subspace modal identification of industrial robots

AbstractIndustrial robots have become key components for manufacturing automations due to their larger workspaces and flexibility. However, low stiffness and high compliance of industrial robots may inevitably lead to vibration by self‐excitation or periodic force dependent on workspace configuration. Therefore, the knowledge of the robot's modal properties should be accurately required to enhance the operation accuracy of industrial robots. To improve the identification accuracy of experimental modal parameters of field industrial robots, an improved subspace identification method is proposed to perform nonlinear iterative optimization for updating the state parameters of industrial robots. Experimental response measurement of a six‐degrees‐of‐freedom industrial robot is carried out to obtain modal parameters under various poses. The identification results of the improved subspace modal method are preferable to that of the traditional method. Moreover, the reconstructed three‐dimension working frequency space is presented to exactly characterize experimental modal frequencies throughout its workspace. The proposed method effectively improves the identification accuracy of modal parameters when compared with the traditional algorithms and the influence of robots' pose change on modal parameters is also investigated by experimental modal measurements.

Dynamic stability of sandwich panels subjected to periodic axial loads

This paper presents an analysis for the dynamic stability of sandwich beams/wide plates subjected to periodic axial loads. The formulation of the problem is done by use of the Extended High-order Sandwich Panel Theory (EHSAPT). The equations of motion are derived from Hamilton’s principle and are expressed in terms of seven generalized displacements. A sandwich panel with simply supported edges is studied as an example, and the equations of motion for a given harmonic n are further derived. Two time-variations for the axial forces, namely, harmonic axial forces and step-wise periodic axial forces, are considered in this work. By considering the features of these two periodic load profiles, Floquet theory and Bolotin’s method are adopted to perform the dynamic stability analysis. Sandwich panels with different face-to-core thickness ratios are studied. Stability maps for varying frequency and amplitude of the forces are presented. Numerical examples show that when a sandwich panel is subjected to periodic loads, it is possible that it can experience dynamic instability even when the dynamic loads are much lower than the static critical loads.

Stochastic resonance of a multi-stable system and its application in bearing fault diagnosis

This paper investigates the stochastic resonance and mean-first passage time of a quad-stable potential in the presence of Gaussian white noise and periodic forcing. The analytical expressions of mean-first passage time and spectral amplification are obtained, respectively. It is found that even small noise intensity can lead to noise-assisted hopping between two adjacent potential wells for the case of small damping coefficient. For large noise intensity, the escape process of Brownian particles is accelerated in an underdamped nonlinear system. Moreover, the curve of spectral amplification displays a typical resonant peak at an optimal noise intensity, suggesting the onset of stochastic resonance. Meanwhile, with the decrease of periodic signal frequency, the peak value of spectral amplification is enhanced. Especially, an optimal quad-stable potential structure exists to maximize the stochastic resonance effect. The proposed multi-stable stochastic resonance method is applied to the fault diagnosis of inner and outer race bearing, and the quantum particle swarm optimization algorithm is used to optimize the system parameters and damping coefficient. The good agreement between fault frequency and theoretical value validates efficiency of the proposed method. Compared with the overdamped tri-stable stochastic resonance method, the performance of fault diagnosis is enhanced substantially by the proposed method.

Compound-Combination Synchronization for Fractional Hyperchaotic Models with Different Orders

In this paper, we introduce a new type of synchronization for the fractional order (FO) hyperchaotic models with different orders called compound-combination synchronization (CCS). Using the tracking control method, a theorem to calculate the analytical controllers which achieve our proposed synchronization is described and proved. We introduce, also, the FO hyperchaotic complex Lü, Chen, and Lorenz models with complex periodic forcing. The symmetry property is found in the FO hyperchaotic complex Lü, Chen, and Lorenz models. These hyperchaotic models are found in many areas of applied sciences, such as physics and secure communication. These FO hyperchaotic models are used as an example for our proposed synchronization. The numerical simulations show a good agreement with the analytical results. The complexity and existence of additional variables mean that it is safer and interesting to transmit and receive signals in communication theory. The proposed scheme of synchronization is considered a generalization of many types in the literature and other examples can be found in similar studies.

Random process with a turbulent power spectrum

Random processes with large fluctuations are simulated by a system of nonlinear stochastic equations describing coupled nonequilibrium phase transitions. It is shown that under the action of white noise a critical state can arise, which is characterized by a turbulent spectrum and a scale-invariant probability density function. Under the action of a periodic force on a random process with a turbulent spectrum, a resonant response of scale-invariant functions arises. The lumped system is generalized to the spatially distributed case. To study the sustainability and stationarity of a random process, the principle of maximum entropy is used.

Analysis of Finite Solution Spaces of Second-Order ODE with Dirac Delta Periodic Forcing

Second-order Ordinary Differential Equations (ODEs) with discontinuous forcing have numerous applications in engineering and computational sciences. The analysis of the solution spaces of non-homogeneous ODEs is difficult due to the complexities in multidimensional systems, with multiple discontinuous variables present in forcing functions. Numerical solutions are often prone to failures in the presence of discontinuities. Algebraic decompositions are employed for analysis in such cases, assuming that regularities exist, operators are present in Banach (solution) spaces, and there is finite measurability. This paper proposes a generalized, finite-dimensional algebraic analysis of the solution spaces of second-order ODEs equipped with periodic Dirac delta forcing. The proposed algebraic analysis establishes the conditions for the convergence of responses within the solution spaces without requiring relative smoothness of the forcing functions. The Lipschitz regularizations and Lebesgue measurability are not considered as preconditions maintaining generality. The analysis shows that smooth and locally finite responses can be admitted in an exponentially stable solution space. The numerical analysis of the solution spaces is computed based on combinatorial changes in coefficients. It exhibits a set of locally uniform responses in the solution spaces. In contrast, the global response profiles show localized as well as oriented instabilities at specific neighborhoods in the solution spaces. Furthermore, the bands of the expansions–contractions of the stable response profiles are observable within the solution spaces depending upon the values of the coefficients and time intervals. The application aspects and distinguishing properties of the proposed approaches are outlined in brief.

Harnessing Demographic Dividend Before it is Lost Forever in India.

Based on the secondary data taken from Population Census, and the Employment-Unemployment Surveys and Periodic Labour Force Survey of the National Sample Survey, it is found that Indian economy is passing through a critical phase of economic development in which it is likely to lose its demographic advantage. Because, in India while about 4.5 million people wereleaving agriculture every year prior to the Covid-19 pandemic years, the non-farm sectors job was not growing adequately to accommodate the persons leaving agriculture, and the newly educated non-farm job seekers. As a result there was an upsurge in educated youth unemployment (18% and about 24 million) rate, and hence the discouraged youth labour force. On the other hand, an increase in the share (from 8.0 to 10.2%) and growth (3.0-5.1%) of elderly population put a question on the process of harnessing demographic dividend in India. Based on these findings it is argued that an integrated approach of development is necessary to boost the labour force participation of youth and overall population to boost the growth of per capita national state domestic product (NSDP) in Indian states. This could be achieved through the promotion of micro and small enterprises along with infrastructure development along with a systematic emigration and remittances policy.