Self-oscillatory Regimes Research Articles

Self-Oscillation of a Liquid Crystal Elastomer String-Mass System Under Constant Gradient Temperature

Abstract Recent experiments have found that a fiber-mass system can self-oscillate along the vertical direction under a non-uniform temperature field, which necessitates significant vertical space. To address the challenge in adapting to situations with limited vertical space, the current work introduces a self-oscillating string-mass system, comprising of a mass ball and a thermally responsive liquid crystal elastomer string exposed to a constant gradient temperature. By employing theoretical modeling and numerical simulation, we have identified two motion regimes of the system, namely, the static regime and the self-oscillation regime, and elucidated the mechanism of self-oscillation. Utilizing the analytical method, we derived the expressions for bifurcation point, amplitude, and frequency of the self-oscillation, and investigated the impact of system parameters on these aspects, which were verified by numerical solutions. Compared to a fiber-mass system, the string-mass system has superior stability to deal with small horizontal disturbances, can amplify its amplitude and frequency limited by small thermal deformation of material, and saves a significant amount of vertical space. Given these attributes, such self-oscillating string-mass system presents novel possibilities for designing energy harvesters, active machinery, and soft robots.

Multi-dimensional chaos initiated by short pulses in non-autonomous radio-physical generator

A non-autonomous model of the Anishchenko–Astakhov generator in the regime of periodic and chaotic self-oscillations is considered. A periodic sequence of short pulses is considered as an external force. It is shown that the synchronization picture is close in structure to the classical synchronization picture observed in a two-dimensional system, but the pulse action leads to the excitation of chaotic oscillations, including those characterized by a different spectrum of Lyapunov exponents. In particular, it is shown appearance of hyperchaos and chaos with additional close to zero Lyapunov exponent. Phenomenological scenarios for the development of multi-dimensional chaos related to destruction of two-frequency tori are described. Hyperchaos is formed via hierarchy of discrete Shilnikov attractors arise as a result of sequence of Neimark-Sacker bifurcations. Chaos with additional close to zero Lyapunov exponent occurs as impact of saddle tori appeared via sequence of torus-doubling bifurcations.

Analysis of the Flow Structure in a Supersonic Channel with Cavity

The results of numerical study of supersonic flow in a channel with cavity are given. The calculated oscillation spectra are analyzed using the fast Fourier transform. Two types of oscillatory modes can be distinguished in the resulting periodic self-oscillatory regime. The first type of the modes corresponds to acoustic vibrations caused by the passage of sound waves along the cavity and calculated using the modified Rossiter formula. The second type of the modes corresponds to the frequencies of flow-rate oscillations caused by mass transfer between the cavity and the external flow. It is shown that the flow structure is modified when fuel is supplied in front of the cavity. Active combustion occurs in the layer of mixing fuel and oxygen from air. The flow pattern demonstrates the onset of Kelvin–Helmholtz instability on the interface between the main flow and the reacted gas. It is shown that an increase in the supplied fuel pressure leads to a decrease in the oscillation frequency and an increase in the characteristic size of oscillations.

Coupling-induced bistability in self-oscillating regimes of two coupled identical Spin-Torque Nano-oscillators

We consider a system of two coupled identical Spin Torque Nano-Oscillators (STNOs) with cylindrical geometry. The STNOs have free layers in the vortex magnetization configuration and fixed layer with magnetization in the out-of-plane direction. The magnetization dynamics in the two STNOs are analyzed by using a Thiele-like equation governing the dynamics of the vortex core positions. The coupling is assumed to be linear and symmetric and this enables to characterize it by a single complex parameter. We consider numerical simulations of the model as a function of the coupling strength k and phase ϕk. Depending on their values, different motions of the vortex cores are observed, e.g. periodic, and quasi-periodic. Each type of dynamics can be classified according to the symmetry property shown by the trajectories. In this respect, the transition between different types of motion corresponds to a symmetry-breaking process and we describe it as a bifurcation process. Fixing the value of the coupling constant phase, we found that there is a range of coupling constant strength in which the periodic and quasi-periodic motions coexist and both are stable. This bistability is an interesting phenomenon in connection to research on neuromorphic circuits based on STNOs.

Liquid crystal elastomer self-oscillator with embedded light source.

Light sources that switch periodically over time have a wide range of application value in life and engineering, and generally require additional controller to periodically switch circuits to achieve periodic lighting. In this paper, a self-oscillating spring oscillator based on optically responsive liquid crystal elastomer (LCE) fiber is constructed, which consists of a embedded light source and a LCE fiber. The spring oscillator can oscillate autonomously to achieve periodic switching of the light source. On the basis of the well-established dynamic LCE model, a nonlinear dynamic model is proposed and its dynamic behavior is studied. Numerical calculations demonstrate that the spring oscillator presents two motion regimes, namely the self-oscillation regime and the static regime. The self-oscillation of spring oscillator is maintained by the energy competition between light energy and damping dissipation. Furthermore, the critical conditions for triggering self-oscillation are also investigated in detail, as well as the key system parameters that affect its frequency and amplitude. Different from the existing abundant self-oscillating systems, this self-oscillating structure with simple structure and convenient fabrication does not require complex controller to obtain periodic lighting, and it is expected to provide more diversified design ideas for soft robots and sensors.

Nonlinear dynamics of a spin-Hall nano-oscillator based on the canted antiferromagnet.

The theory of the nonlinear dynamics of a canted antiferromagnet-based (AFM) spin-Hall oscillator with weak ferromagnetism caused by the strong Dzyaloshinskii-Moriya interaction between magnetic sublattices is rigorously studied. The AFM oscillator's frequency tuning is carried out both due to a DC spin-polarized current flowing through the normal metal layer and an external permanent magnetic field. A feature of the operation of this oscillator is the presence of a hysteresis region between the subcritical (damping) and overcritical (self-oscillating) regimes. We show that it is necessary to reduce an effective easy-plane anisotropy field of the AFM or choose antiferromagnetic material with a smaller exchange field between magnetic sublattices to get a smaller hysteresis region. The action of an external permanent magnetic field on an antiferromagnet leads to the presence of two stable equilibrium states, and the self-oscillating regime is characterized by two-mode generation. We find the conditions for the absence of two-mode generation on the "current density-magnetic field strength" plane. We believe that our results can be helpful for the practical development of a sub-THz frequency tunable oscillator based on the AFM with weak ferromagnetism.

Вплив шорсткості поверхні камери енергетичної установки на низькочас-тотні автоколивання холодного робочого газу

Dynamic processes in the combustion chamber have a significant effect on the characteristics of the working processes of solid-propellant rocket engines (LPREs). Pressure jumps and a sharp increase in the local temperature of the combustion products in non-stationary engine operation modes can lead to overrating values of operating parameters and a failure of the LPRE combustion chamber structure. The dynamic processes in the LPRE combustion chamber develop in a complex interconnection of a large number of physical and chemical processes that occur in the gas-dynamic part of the working space of the engine chamber and often lead to self-oscillating modes of engine operation. This is evidenced by numerous data on LPRE fire tests. This paper presents the results of a numerical study of the effect of the LPRE chamber inner surface roughness on LPRE operating parameter low-frequency self-oscillations. The study was made using up-to-date computer simulation means and analysis. Low-frequency (up to 1,000 Hz) oscillations in an LPRE combustion chamber were studied for a power plant test chamber in cold operation with the use of two different approaches to numerical modeling of the dynamics of in-chamber processes: the development and study of a 3D model of the dynamic system of combustion chamber structure – combustion products using the finite element method and the development and study of an axisymmetric 2D model of engine chamber gas flow using the finite volume method. The study revealed a self-oscillatory flow regime caused by combustion product vorticity and acoustic feedback due to vortices colliding with the chamber components or the LPRE nozzle. It was shown that accounting for the wall roughness increased gas vorticity in the gas–solid dynamic interaction zone and the chamber gas oscillation amplitude (on the average, by a factor of 2.5 at a maximum wall roughness height of 56 ?m). The calculated gas flow pattern in the vorticity zones of the chamber and the low-frequency gas pressure oscillation parameters are in qualitative agreement with the experimental ones.

How fast can vanadium dioxide neuron-mimicking devices oscillate? Physical mechanisms limiting the frequency of vanadium dioxide oscillators

The frequency of vanadium dioxide (VO2) oscillators is a fundamental figure of merit for the realization of neuromorphic circuits called oscillatory neural networks (ONNs) since the high frequency of oscillators ensures low-power consuming, real-time computing ONNs. In this study, we perform electrothermal 3D technology computer-aided design (TCAD) simulations of a VO2 relaxation oscillator. We find that there exists an upper limit to its operating frequency, where such a limit is not predicted from a purely circuital model of the VO2 oscillator. We investigate the intrinsic physical mechanisms that give rise to this upper limit. Our TCAD simulations show that it arises a dependence on the frequency of the points of the curve current versus voltage across the VO2 device corresponding to the insulator-to-metal transition (IMT) and metal-to-insulator transition (MIT) during oscillation, below some threshold values of . This implies that the condition for the self-oscillatory regime may be satisfied by a given load-line in the low-frequency range but no longer at higher frequencies, with consequent suppression of oscillations. We note that this variation of the IMT/MIT points below some threshold values of is due to a combination of different factors: intermediate resistive states achievable by VO2 channel and the interplay between frequency and heat transfer rate. Although the upper limit on the frequency that we extract is linked to the specific VO2 device we simulate, our findings apply qualitatively to any VO2 oscillator. Overall, our study elucidates the link between electrical and thermal behavior in VO2 devices that sets a constraint on the upper values of the operating frequency of any VO2 oscillator.

Effect of Cavitator and Nozzle Parameters on the Efficiency job of the impulse Jet Generator

Studies of fluid jet flows in the presence of a ventilated cavity with a negative cavitation number, carried out at the Institute of Mechanics of Moscow State University, showed that under certain conditions, self-cavitational oscillations with a high intensity of pressure pulsations occur in such a hydraulic system. It was shown that it is possible in principle to use the self-oscillation regime to create periodic impulsed jets during outflow from a converging nozzle into the atmosphere. The influence of the parameters of the cavitator and the outlet nozzle on the intensity of the impact of the outflowing liquid on the screen located perpendicular to the outflow direction is studied. It is shown that an increase in the length of the nozzle can significantly increase the efficiency of the generator, and a smooth narrowing of the channel in front of the cavitator can increase the operating range of the generator in the direction of larger gas blows. It is shown that there is a scale effect - with increasing head pressure, the relative intensity of self-oscillations decreases, but tends to reach a horizontal asymptote.

Electromechanical resonances and field-emission-induced self-oscillations of single crystal diamond needles

Due to its outstanding mechanical characteristics, diamond is an ideal material for use in micro- and nano-electromechanical systems. In this paper, we report on the investigation of vibrational properties of singly clamped needlelike diamond microcrystallites with nanoscale tips. The single-crystal diamond needles were produced by selective oxidation of polycrystalline films grown using chemical vapor deposition. The study of resonant oscillations driven by the AC voltage indicated that the elastic modulus of such diamond needles is close to that of bulk single crystal diamond. A self-oscillation regime induced by the DC voltage during field emission from the apex of a diamond needle is also demonstrated. It is shown that this regime can be used for efficient DC–AC conversion in microdevices. The high structural quality of diamond needles, their remarkable mechanical properties, and the relative ease of their mass fabrication make them promising candidates for application in various electromechanical systems, field-emission devices, and scanning probe techniques.

Studies of the Fractional Selkov Dynamical System for Describing the Self-Oscillatory Regime of Microseisms

A non-linear fractional Selkov dynamic system for mathematical modeling of microseismic phenomena is proposed. This system is a generalization of the previously known Selkov system, which has self-oscillatory modes and is used in biology to describe glycolytic vibrations of the substrate and product. The Selkov fractional dynamical system takes into account the influence of heredity and is described using derivative fractional orders. The article investigates the Selkov fractional dynamic model using the Adams–Bashforth–Moulton numerical method, constructs oscillograms and phase trajectories, and studies the equilibrium points. Based on the spectra of the maximum Lyapunov exponents, it is shown that in the fractional dynamic model there can be relaxation and damped oscillations.

Synchronization of a Passive Oscillator and a Liquid Crystal Elastomer Self-Oscillator Powered by Steady Illumination

Self-oscillators have the advantages of actively harvesting energy from external steady environment, autonomy, and portability, and can be adopted as an engine to drive additional working equipment. The synchronous behavior of self-oscillators and passive oscillators may have an important impact on their functions. In this paper, we construct a self-oscillating system composed of a passive oscillator and an active liquid crystal elastomer self-oscillator powered by steady illumination, and theoretically investigate the synchronization of two coupled oscillators. There exist three synchronous regimes of the two coupled oscillators: static, in-phase, and anti-phase. The mechanisms of self-oscillations in in-phase and anti-phase synchronous regimes are elucidated in detail by calculating several key physical parameters. In addition, the effects of spring constant, initial velocity, contraction coefficient, light intensity, and damping coefficient on the self-oscillations of two coupled oscillators are further investigated, and the critical conditions for triggering self-oscillations are obtained. Numerical calculations show that the synchronous regime of self-oscillations is mainly determined by the spring constant, and the amplitudes of self-oscillations of two oscillators increase with increasing contraction coefficient, light intensity, and spring constant, while decrease with increasing damping coefficient. This study deepens the understanding of synchronization between coupled oscillators and may provide new design ideas for energy harvesters, soft robotics, signal detection, active motors, and self-sustained machinery.

Self-oscillation and self-rotation of an optically-responsive liquid crystal elastomer pendulum

Liquid crystal elastomers (LCEs) have been widely used in active machinery, optical machinery, robotics, biological materials, medical and other fields, with advantage of good optical sensing performance. In this work, dynamics of a LCE-based optically-responsive pendulum system is theoretically investigated by proposing a nonlinear dynamic model. The self-oscillation and self-rotation can be triggered by steady illumination, in which the damping dissipation is compensated by the energy input from environment illumination to maintain the periodic motion. With theoretical modeling and numerical calculation, three typical motion regimes of the LCE pendulum are found, namely, static regime, self-oscillation regime and self-rotation regime. For different physical parameters, the critical conditions to excite the three motion regimes are given quantitatively. Results demonstrate that the motion regime, amplitude, and period of the LCE pendulum can be controlled by adjusting the illumination region, light intensity, damping coefficient, and gravitational acceleration. The proposed optically-responsive LCE pendulum can provide potential applications in research on sensing environment, energy harvesting and motion actuation of active machines, etc.

Laboratory Modeling of the Processes of Sound Excitation by Self-Oscillating Regimes in Flows in Heat-Exchange-Systems’ Pipelines

Laboratory Modeling of the Processes of Sound Excitation by Self-Oscillating Regimes in Flows in Heat-Exchange-Systems’ Pipelines

CFD modeling of a self-oscillatory airflow regime in the test ventilated room with plane supply opening

The current contribution presents the results of numerical simulation of the airflow and heat transfer in a model room of rectangular shape ventilated with a single plane jet under the conditions of the experiment by Mataoui et al. (2001). The 2D and 3D unsteady Reynolds-averaged Navier-Stokes approaches were used. Numerical solutions were obtained with the CFD package ANSYS Fluent based on the finite volume method with the cell-centered variable arrangement. The position of the air supply nozzle corresponds to the self-oscillatory experimental conditions. The results of the computational data validation are reported. Effects of the low-frequency velocity oscillations on the heat transfer parameters are discussed.

Performance characteristics of bistable active ring resonators based on ferrite films

A bistability phenomenon in a ring resonator consisting of a delay line on surface spin waves and a microwave amplifier has been experimentally investigated. It is shown that an enhancement of the gain coefficient above a specified value provides an appearance of a hysteresis at the resonator transmission characteristic. A frequency range of the bistability broadens due to an increase of the gain coefficient as well as the external magnetic field. While the value of the gain is limited from above by a transition of the ring into a self-oscillating regime, an increase in the magnetic field from 1150 Oe up to 3150 Oe provides an expansion of the frequency range of the hysteresis from 77 kHz to 185 kHz.

Hysteresis in Supersonic Flow Past a Plane Cascade of Cylindrical Rods

—The results of numerical simulation of the interaction of supersonic flow with a permeable screen in form of an infinite plane cascade (lattice) of circular cylinders are given. The interaction regime in which the shocks ahead of the cylinders are localized on the scale of the cascade step is considered. The multi-block computational technique in which the viscous boundary layers are resolved by means of local grids using the Navier–Stokes equations, while the effects of inteferrence between the shock-wave structures in supersonic wake are described within the framework of Euler’s equations. The action of shock waves induced by the neighboring elements of lattice to the near-wake region behind the intermediate elements can ambiguously affect the aerodynamic lattice performance as well as generate time-dependent phenomena in the wake. The flow regimes are classified depending on continuous increase and decrease in the free-stream supersonic air flow in the Mach number range from 2.4 to 4.2 with reference to the lattice of the 80% permeability. The sources of the hysteresis behavior of the lattice aerodynamic drag with respect to the Mach number and the mechanisms of the onset of self-oscillating wake flow regimes are discussed.

Analysis of the Particles’ Movement Over the Flat and Profiled Rotating Discs surface in the Velocity Layer of the Viscous Gas

Experiments on the rotating in the air cones with vertex angle β = 120º and flat disc shown that on frequencies Ω ≥ 2.5 hertz exists a qualitative difference in movement for the particles with diameters d ≈ 1 mm and d ≈ 0.1 mm. The particles with d ≈ 0.1 mm move in the near-surface region, the particles with d ≈ 1 mm jump up to 3 cm. Comparison of the spherical and aspheric (ellipsoid with axles d, d and 4 /3 d) particles' kinematics moving shown the inevitability of the large particles jump occurrence. Large particles come to self-oscillation regime by reason of periodically appearance of the Magnus force. Small particles are localized in the velocity layer

Chaotic and Hyperchaotic Self-Oscillations of Lambda Diode Composed by Generalized Bipolar Transistors

This paper is focused on the investigation of self-oscillation regimes associated with very simple structure of lambda diode. This building block is constructed by using coupled generalized bipolar transistors. In the stage of mathematical modeling, each transistor is considered as two-port described by full admittance matrix with scalar polynomial forward trans-conductance and linear backward trans-conductance. Thorough numerical analysis including routines of dynamical flow quantification indicate the existence of self-excited dense strange attractors. Plots showing first two Lyapunov exponents as functions of adjustable parameters, signal entropy calculated from generated time sequence, sensitivity analysis, and other results are provided in this paper. By the construction of a flow-equivalent chaotic oscillator, robustness and long-time geometrical stability of the generated chaotic attractors is documented by the experimental measurement, namely by showing captured oscilloscope screenshots.

Experimental plasma maser as a broadband noise amplifier. I. Long pulse

Operated in the noise amplification regime, a plasma maser underwent a series of experiments. The pulse duration was 50 ns, the suppression of feedback was provided by microwave absorbers, transition from the noise amplification to a self-oscillation regime was demonstrated, and the maximum power of 10 MW with the efficiency of 3% was measured. For different plasma concentrations, the linear amplification rate was ranging from 0.1 to 0.2 cm−1. The frequency band of an emission pulse was 3–15 GHz, and the noise spectrum was stable during the pulse. The experiment provided the guidelines for further development, which are in the pulse duration reduction for abandoning the microwave absorber. The short-pulse plasma maser should be ∼35 cm long with the pulse duration < 4 ns. It will have more power and efficiency, a pulse spectrum width of 3 GHz, and tunable frequency band overlapping 3–14 GHz with the rise of plasma concentration.