Bistable Point Research Articles

A bistable point absorber wave energy convertor with a mechanical motion rectifier

The design of novel wave energy converters (WECs) with the capability of low frequency energy capture is of great significance for the development of low frequency ocean wave energy. In this study, a novel point absorber WEC combined with a bistable mechanism and a mechanical motion rectifier (MMR) is proposed to achieve the goal of efficiently capturing wave energy in the low frequency range. In the WEC, the introduction of a rack-pinion-lever-spring (RPLS) bistable mechanism can not only enhance the capture efficiency in a low frequency range but also protect the system from excessive vibration amplitude through the nonlinear stiffness. Besides, one MMR based power take off (PTO) system is proposed to convert the bidirectional vibration into unidirectional rotation of the generator and extract more power via the mechanism of two one-way pinions. The dynamic model in the time domain for the nonlinear point absorber WEC with the RPLS bistable mechanism and the MMR based PTO system is established in which the convolution term is replaced by the state space method. The characteristics of the nonlinear dynamics and wave energy capture performance for the novel WEC are analysed in regular wave condition. The numerical results demonstrate that the peak capture width ratio of the proposed WEC can reach almost 2 times that of the traditional linear system in a lower frequency range.

Improving metrology with quantum scrambling

Quantum scrambling describes the spreading of information into many degrees of freedom in quantum systems, such that the information is no longer accessible locally but becomes distributed throughout the system. This idea can explain how quantum systems become classical and acquire a finite temperature, or how in black holes the information about the matter falling in is seemingly erased. We probe the exponential scrambling of a multiparticle system near a bistable point in phase space and utilize it for entanglement-enhanced metrology. A time-reversal protocol is used to observe a simultaneous exponential growth of both the metrological gain and the out-of-time-order correlator, thereby experimentally verifying the relation between quantum metrology and quantum information scrambling. Our results show that rapid scrambling dynamics capable of exponentially fast entanglement generation are useful for practical metrology, resulting in a 6.8(4)-decibel gain beyond the standard quantum limit.

Experimental assessment of the performance of a bi-stable point wave energy absorber under harmonic incident waves

Intentional introduction of a bi-stable nonlinear restoring element to the design of point wave energy absorbers (PWAs) has been recently investigated as a means to broaden the steady-state response bandwidth of the PWA and shift it towards lower frequencies. The underlying hypothesis, which is supported by many previous theoretical investigations, is that this will help (i) reduce the sensitivity of the PWA’s response to variations in the frequency and amplitude of the incident waves, and (ii) permit the PWA to effectively harness energy from the low-frequency high-energy content of the incident wave spectrum. In this study, we investigate this hypothesis experimentally by designing, fabricating, and assessing the response behavior of a prototype bi-stable PWA (B-PWA) excited by harmonic incident waves in an experimental wave flume. We create bifurcation maps in the wave frequency and amplitude parameters’ space and use them to define an effective frequency bandwidth outside which the B-PWA generates negligible power levels. We compare the effective bandwidth of three different B-PWAs (different bi-stable restoring elements) to the traditional linear design and demonstrate that bi-stability does not seem to improve the effective bandwidth over the linear design; at least not for the three designs considered. Moreover, the difficulty of defining the effective bandwidth of the B-PWA and its sensitive dependence on the wave amplitude adds more layers of complexity, which make the process of optimizing its performance a cumbersome task. Nonetheless, a key advantage remains in the ability of the bi-stable restoring force to push the effective bandwidth of the B-PWA towards lower frequencies, where most of the wave energy is trapped.

Response behavior of bi-stable point wave energy absorbers under harmonic wave excitations

To expand the narrow response bandwidth of linear point wave energy absorbers (PWAs), a few research studies have recently proposed incorporating a bi-stable restoring force in the design of the absorber. Such studies have relied on numerical simulations to demonstrate the improved bandwidth of the bi-stable absorbers. In this work, we aim to understand how the shape of the bi-stable restoring force influences the effective bandwidth of the absorber. To this end, we use perturbation methods to obtain an approximate analytical solution of the nonlinear differential equations governing the complex motion of the absorber under harmonic wave excitations. The approximate solution is validated against a numerical solution obtained via direct integration of the equations of motion. Using a local stability analysis of the equations governing the slow modulation of the amplitude and phase of the response, the loci of the different bifurcation points are determined as function of the wave frequency and amplitude. Those bifurcation points are then used to define an effective bandwidth of the absorber. The influence of the shape of the restoring force on the effective bandwidth is also characterized by generating design maps that can be used to predict the kind of response behavior (small amplitude periodic, large amplitude periodic, or aperiodic) for any given combination of wave amplitude and frequency. Such maps are critical toward designing efficient bi-stable PWAs for known wave conditions.

Parametric study on power capture performance of an adaptive bistable point absorber wave energy converter in irregular waves

The power capture performance of an adaptive bistable point absorber wave energy converter (WEC) in irregular waves is investigated in this paper. Based on the linear potential flow theory and Cummins equations, the equation of motion for the WEC is established in time domain. Then a parametric study is performed on the power capture performance of the WEC by considering the influences of different wave and system parameters such as the spring combination parameter, the stiffness of auxiliary springs and main springs, the original length value of the main spring, damping coefficient of the PTO systems and the significant wave height. The results show that in an irregular wave, each parameter will have a certain impact on the performance of the device. Appropriate device parameters need to be selected according to actual environmental parameters to ensure that the adaptive bistable WEC has better power capture performance than its linear and conventional bistable counterparts.

Evaluation of long-term power capture performance of a bistable point absorber wave energy converter in South China Sea

Nowadays, extensive research has been conducted to generate electricity from ocean waves, but there are not too many studies on the capture of wave energy in a specific ocean area. As the largest sea area in China's offshore waters, the South China Sea contains a large amount of wave energy and is regarded as a natural wave energy power plant. A comparative study was conducted to investigate the effects of wave energy converter (WEC) parameters on the long-term power capture performance of a bistable point absorber WEC installed in the South China Sea in this paper. Combined with the joint probability distribution of significant wave height Hs and peak wave period Tp in South China Sea, a mathematical model was developed for the evaluation of long-term power performance of WECs. The governing equations of the point absorber WEC were established using the hybrid frequency-time domain method. The dynamic response and captured power can be obtained after numerically solving the equations of motion with the 4th order Runge-Kutta Method. The results show that the bistable mechanism has great advantages in the long-term captured power of WEC. In general, a circular truncated cone-shaped buoy WEC with a larger buoy size and smaller nonlinear parameter tend to result in better long-term power capture performance. The insights given in this study provide a motivation and foundation for developing a complete set of performance optimization methods of point absorber WECs installed in South China Sea.

High-performance and robust bistable point absorber wave energy converter

A novel magnetic bistable device (MBD) is proposed to improve the efficiency of energy harvesting of a point absorber wave energy converter (WEC) by decreasing the potential barrier of the bistable energy absorber. The novel MBD is mainly composed of two pairs of coaxial permanent magnet rings with different sizes. For the permanent magnet rings, one pair has the same magnetic field direction to provide negative stiffness and the other magnet rings have nonlinear positive stiffness property with a reverse magnetic field direction. The nonlinear dynamic model is established by adopting the state space model approach during solving the convolution integral of radiation force. The capture width ratio of the WEC with the novel MBD is studied with the change of design parameters in regular wave. Then the effect of structural parameters for the novel MBD on the wave energy capture ratio is discussed. The results indicate that using the novel MBD can significantly improve the performance of point absorber WEC especially under low wave excitations in comparison with that of the WEC with traditional MBD.

Performance Analysis of an Adaptive Bistable Point Absorber Wave Energy Converter Under White Noise Wave Excitation

Performance analysis is conducted for an adaptive bistable point absorber wave energy converter under white noise wave excitation. The adaptive bistable wave power capture mechanism is composed of two oblique springs responsible for bistable characteristics, and two auxiliary springs realizing the ‘adaptive’ feature. White noise wave excitation is adopted to explore the effects of multi-frequency input in absence of different energy distribution characteristics. Time domain simulations are performed based on Cummins equations derived from linear potential flow theory. Results indicate that the adaptive bistable mechanism still bears a better power performance than its linear counterpart which may be well explained by its inherent ‘phase control’ feature in white noise wave excitation. A parametric study is also carried out to investigate how the power capture performance of the adaptive bistable point absorber wave energy converter is affected by main and auxiliary spring stiffness, random phase of wave excitation and power-take-off damping.

Exponential volume dependence of entropy-current fluctuations at first-order phase transitions in chemical reaction networks

In chemical reaction networks, bistability can only occur far from equilibrium. It is associated with a first-order phase transition where the control parameter is the thermodynamic force. At the bistable point, the entropy production is known to be discontinuous with respect to the thermodynamic force. We show that the fluctuations of the entropy production have an exponential volume-dependence when the system is bistable. At the phase transition, the exponential prefactor is the height of the effective potential barrier between the two fixed-points. Our results obtained for Schlögl's model can be extended to any chemical network.

Bi-stability of SUDR+K model of epidemics and test kits applied to COVID-19

Motivated by the many diverse responses of different countries to the COVID-19 emergency, here we develop a toy model of the dependence of the epidemics spreading on the availability of tests for disease. Our model, that we call SUDR+K, grounds on the usual SIR model, with the difference of splitting the total fraction of infected individuals in two components: patients that are still undetected and patients that have been already detected through tests. Moreover, we assume that available tests increase at a constant rate from the beginning of epidemics but are consumed to detect infected individuals. Strikingly, we find a bi-stable behavior between a phase with a giant fraction of infected and a phase with a very small fraction. We show that the separation between these two regimes is governed by a match between the rate of testing and a rate of infection spread at given time. We also show that the existence of two phases does not depend on the mathematical choice of the form of the term describing the rate at which undetected individuals are tested and detected. Presented research implies that a vigorous early testing activity, before the epidemics enters its giant phase, can potentially keep epidemics under control, and that even a very small change of the testing rate around the bi-stable point can determine a fluctuation of the size of the whole epidemics of various orders of magnitude. For the real application of realistic model to ongoing epidemics, we would gladly collaborate with field epidemiologists in order to develop quantitative models of testing process.

Mechanism and sensitivity for broadband energy harvesting of an adaptive bistable point absorber wave energy converter

The adaptive bistable point absorber wave energy converter (WEC) concept proposed by Zhang et al. (Applied Energy 2018(228), pp. 450–467) is further investigated by exploring the underlying mechanism and sensitivity related to the broadband energy harvesting of this device. The mechanism can be explained in two different ways: (i) the broadband power capture is due to the fact that the adaptive bistable mechanism introduces both negative (at low wave frequencies) and positive (at high wave frequencies) stiffness to the device; and (ii) an inherent ‘phase control’ feature is observed especially at low wave frequency region. The variation of initial conditions of floater motion only has an observable influence at intermediate wave frequencies corresponding to chaotic response of the floater. The broad frequency bandwidth can be realized through different combinations of main and auxiliary springs' stiffness. The effects of wave amplitude on broadband energy harvesting can be categorized according to an optimum amplitude (if existing) into a gradual reduction for a larger wave amplitude and a quick drop for a smaller amplitude.

Bistable behaviour and thermo-acoustic instability triggering in a gas turbine model combustor

In a liquid-fueled two-stage gas turbine model combustor, a bistable point where two different flame states can exist is studied in non-reacting and reacting conditions with the help of fast spray-PIV and chemiluminescence imaging. The first flame, named sV, is stabilized thanks to an internal reaction zone that modifies the air flow compared to non-reacting conditions. The second state, associated with a lifted flame, exhibits a strong thermo-acoustic instability linked to the quarter-wave mode of the chamber and piloted by a varying liquid-fuel supply to the flame. Using well-controlled air flow rate modulations, the switch from the quiet sV state to the lifted one is studied in a time-resolved fashion. The analysis shows that the sensitivity of the internal reaction zone of the sV state to perturbations is responsible for the change in flame shape which then triggers the onset of the thermo-acoustic instability.

Optimization of bistable viscoelastic systems

We consider the flow of a viscoelastic fluid in a symmetric cross geometry. For small driving pressures the flow is symmetric, but beyond a certain critical pressure the symmetric flow becomes unstable; two stable asymmetric solutions appear, and forcing of the unstable symmetric flow beyond the critical pressure gives rise to increased hydraulic resistance. We have combined a state-of-the-art implementation for viscoelastic flow modeling with topology optimization in a high level finite element package (COMSOL). We use this framework on the cross geometry with the aim to reduce the critical driving pressure corresponding to the point of bistability, such that the effect is enhanced. The point of bistability is, however, not explicitly contained in the solution, so we opt for a heuristic approach based on the dissipation ratio between the asymmetric and unstable symmetric flow solutions. We find a design that significantly reduces the driving pressure required for bistability, and furthermore is in agreement with the approach followed by experimental researchers. Furthermore, by comparing the two asymmetric solutions, we succesfully apply the same approach to a problem with two fluids meeting in the cross.

Static and dynamical aspects of the metastable states of first order transition systems

We numerically study the metastable states of the 2d Potts model. Both of equilibrium and relaxation properties are investigated focusing on the finite size effect. The former is investigated by finding the free energy extremal point by the Wang-Landau sampling and the latter is done by observing the Metropolis dynamics after sudden heating. It is explicitly shown that with increasing system size the equilibrium spinodal temperature approaches the bistable temperature in a power-law and the size-dependence of the nucleation dynamics agrees with it. In addition, we perform finite size scaling of the free energy landscape at the bistable point.

Natural ageing effects of Schottky-gate GaAs FETs in current-voltage characteristics and in 1/f noise spectrum

We study the current-voltage (I–V ) characteristics and 1/f (flicker) noise spectra of experimental samples (prototypes) of submicron GaAs field-effect transistors (FETs) with Schottky planar gate in the linear regime. The model of I–V characteristics, in which the FET channel is described by the Shockley model and the crowding of current lines in passive regions is taken into account, is proposed. Analysis of the measured set of I–V characteristics allowed us to define geometric and electric parameters of the devices more precisely. The natural ageing effects were revealed by analyzing the measurements performed at a 3-year interval. We make the conjecture that excess defects are present in the structure and they are probably represented by dipoles (donor-acceptor pairs). Concentration of excess defects is estimated and its significant decrease with time is detected. For analysis of 1/f noise, we used the hypothesis of the existence of bistable point defects, assuming that the latter result in resistivity fluctuations of the conducting regions. It is shown that defects in the channel give the main contribution to 1/f noise and their concentration decreases with time. Increased content of defects near the buffer layer was detected in some devices. The experimental data obtained allow us to suppose that the observed 1/f noise is caused by the excess defects revealed in the analysis of I–V characteristics of FETs with Schottky planar gate.

Nonlinear amplifier and frequency shifter using a tunable periodic drive

We consider the superposition of a weak and a strong force acting on an overdamped particle moving on either an asymmetric-periodic or a double-well potential. The velocity of the particle has only harmonics of the strong force, when the particle either oscillates near a minimum or runs away from it. Near a threshold drive (bistable point) separating these two dynamical regimes, the weak force drastically changes the velocity spectrum, greatly amplifying the mixing harmonics. This effect can be used either to amplify or to shift the frequency of a weak signal and can be observed in a wide variety of systems, including domain walls in a ferromagnet, SQUIDs, and tiny particles in a ratchet potential.

Experimentally realizable devices for domain wall motion control

Magnetic domain walls (MDWs) can move when driven by an applied magnetic field. This motion is important for numerous devices, including magnetic recording read/write heads, transformers and magnetic sensors. A magnetic film, with a sawtooth profile, localizes MDWs in discrete positions at the narrowest parts of the film. We propose a controllable way to move these domain walls between these discrete locations by applying magnetic field pulses. In our proposal, each applied magnetic pulse can produce an increment or step-motion for an MDW. This could be used as a shift register. A similarly patterned magnetic film attached to a large magnetic element at one end of the film operates as an XOR logic gate. The asymmetric sawtooth profile can be used as a ratchet resulting in either oscillating or running MDW motion, when driven by an ac magnetic field. Near a threshold drive (bistable point) separating these two dynamical regimes (oscillating and running MDW), a weak signal encoded in very weak oscillations of the external magnetic field drastically changes the velocity spectrum, greatly amplifying the mixing harmonics. This effect can be used either to amplify or shift the frequency of a weak signal.