Self-oscillation Research Articles

Oscillatory processes in a nonlinear system of an individual traction electric drive

When the tires interact with the road in contact, self-oscillations occur. The appearance of self-oscillations in the interaction zone can serve as a useful diagnostic sign of the development of the process of loss of adhesion in the work, cases of self-oscillations for an individual electric drive are investigated. In traction and driven modes, a "soft" mode of occurrence of self-oscillation of wheel rotation is possible. When braking, there is a "soft" mode of excitation of self-oscillations, and when the wheels are locked, a "hard" mode of self-oscillations of the translational movement of the wheel center. Keywords: self-oscillation; frictional interaction of the wheel with the base; "soft mode" of self-oscillation; "hard mode" of self-oscillation; Bendixon criterion; friction force; individual electric wheel drive

Light-Fueled Synchronization of Two Coupled Liquid Crystal Elastomer Self-Oscillators.

The synchronization and group behaviors of self-excited coupled oscillators are common in nature and deserve to be explored, for self-excited motions have the advantages of actively collecting energy from the environment, being autonomous, making equipment portable, and so on. Based on light-powered self-excited oscillators composed of liquid crystal elastomer (LCE) bars, the synchronization of two self-excited coupled oscillators is theoretically studied. Numerical calculations show that self-excited oscillations of the system have two synchronization modes, in-phase mode and anti-phase mode, which are mainly determined by their interaction. The time histories of various quantities are calculated to elucidate the mechanism of self-excited oscillation and synchronization. For strong interactions, the system always develops into in-phase synchronization mode, while for weak interaction, the system will evolve into anti-phase synchronization mode. Furthermore, the effects of initial conditions, contraction coefficient, light intensity, and damping coefficient on the two synchronization modes of the self-excited oscillation are investigated extensively. The initial condition generally does not affect the synchronization mode and its amplitude. The amplitude of self-oscillation always increases with increasing contraction coefficient, gravitational acceleration, and light intensity, while it decreases with the increasing damping coefficient. This work will deepen people's understanding of the synchronization behaviors of self-excited coupled oscillators, and the theoretical framework could be extended to scenarios involving large-scale synchronization of the systems with numerous interacting oscillators.

Self-Sustained Oscillation of Electrothermally Responsive Liquid Crystal Elastomer Film in Steady-State Circuits.

Self-excited oscillations have the advantages of absorbing energy from a stable environment and Self-control; therefore, Self-excited motion patterns have broader applications in micro devices, autonomous robots, sensors and energy-generating devices. In this paper, a Self-sustained curling liquid crystal elastomer (LCE) film-mass system is proposed on the basis of electrothermally responsive materials, which can realize Self-oscillation under a steady-state current. Based on the contact model and dynamic LCE model, a nonlinear dynamics model of LCE film in steady-state circuits is developed and numerical calculations are carried out using the Runge-Kutta method. Through numerical calculations, it is demonstrated that LCE film-mass systems have two motion patterns in steady-state circuits: namely, a Self-oscillation pattern and a stationary pattern. Self-sustained curling of LCE film originates from the fact that the energy absorbed by the system exceeds the energy dissipated due to the damping effect. In addition, the critical conditions for triggering Self-oscillation and the effects of several key dimensionless system parameters on the amplitude and period of Self-oscillation are investigated in detail. Calculation results show that the height of electrolyte solution, gravitational acceleration, elastic modulus of LCE film, limit temperature, curvature coefficient, thermal shrinkage coefficient and damping factor all have a modulating effect on the amplitude and period of Self-oscillation. This research may deepen the understanding of Self-excited oscillation, with promising applications in energy harvesting, power generation, monitoring, soft robotics, medical devices, and micro and nano devices.

Research of features of oscillating process’ behavior in the nonlinear system of individual traction drive of an electrobus

BACKGROUND: When a vehicle is in motion, self-oscillations which properties are dependent on slip rate in a contact patch may occur in the area of tire interaction with ground surface. Oscillations frequency will vary in dependence with value of wheel slip relative to ground surface. Soft self-oscillations are excited by variable set of initial conditions at full slip in traction and driven wheel rolling modes as well as in mixed braking mode with partial slip. Hard mode of self-oscillations occurs at full wheel slip in braking mode. These processes have a negative impact on the processes in electric drive and mechanical drivetrain reducing their efficiency and may cause damage of components. Oscillations in the system are excited by interaction forces of an elastic tire with ground surface featuring vertical oscillations due to elastic behavior of its interaction with road unevenness. AIMS: Research of features of oscillating process behavior in the nonlinear system of individual traction drive of an electrobus. METHODS: Simulation of self-oscillation excitation processes in the area of contact interaction of a wheel and road was carried out in the MATLAB/Simulink software package. RESULTS: The article features the results of simulation and experimental studies of self-oscillation excitation processes of the KAMAZ 6282 electrobus moving on asphalt-concrete surface. It was found that vertical wheel displacement when moving through unevenness lead to oscillating behavior of vertical reaction forces in contact patches and, as a consequence, to oscillating behavior of longitudinal reaction forces, torque and rotation velocity of the shaft of the traction electric motor of the individual drive. It was defined that tire oscillation frequency is 67 Hz that coincides with electric motor shaft rotation oscillation frequency and this value is the same for both experiment and simulation. CONCLUSIONS: Practical value of the study lies in ability of using the study results at development of self-oscillation processes exclusion algorithms as a part of vehicle control system.

Self-excited oscillations in variable density counter-current round jets

The paper investigates the global instability phenomenon in variable-density counter-current round jets. The analysis is performed using a large eddy simulation and the computations are carried out employing a high-order numerical code. The configuration of two co-axial nozzles, where suction applied in an annular nozzle is a driving force for the counterflow, is investigated. In order to simulate conditions conducive to the emergence of global instability a wide range of velocity and density ratios is considered. Depending on these parameters two different types of modes are observed, i.e., Mode I — shear layer mode, and Mode II — jet column mode. The former is recognized by a high level of perturbations in the shear layer which vanish away from the shear layer reaching a low level at the jet centerline. The latter is characterized by strong disturbances throughout the entire jet region. The results obtained are in remarkable agreement with the theoretical basis reported in the literature. They reveal that the critical velocity ratio for which global instability emerges decreases along with the density ratio. Additionally, the transition from Mode II to Mode I is found for sufficiently strong suction. In general, the lower the density ratio, the weaker suction is required to induce Mode I. Consistently to the literature, Mode II is characterized by a lower frequency of oscillations compared to Mode I for which an increase in density ratio and/or velocity ratio shifts the peaks of the velocity spectra toward higher frequencies.

Synergistic degradation and degradation pathways of methylene blue by plasma process combined with cavitation impinging stream reactor based on hydrodynamic cavitation

The plasma process is an advanced oxidation technique that boasts high efficiency and low cost. A combination of the plasma process with a cavitation impinging stream (CIS) reactor has been developed to investigate the degradation characteristics of methylene blue dye wastewater. The results indicate that under a pressure of 1.40 MPa and an impinging distance of 40 mm, the CIS reactor achieved a degradation rate of 52.83%, which was twice as much as self-excited oscillation cavitation alone. Furthermore, it was found that the optimum solution concentration for this reaction was 5.70 mg/L. Combining a plasma apparatus with a CIS reactor resulted in a degradation rate of 94.06% within 60 min, with a synergy index of 3.13. Intermediate products generated during the HC+plasma treatment were analyzed using liquid chromatography-mass spectrometry. Based on experimental and theoretical analyses, two possible pathways for methylene blue degradation were proposed. This study presents novel insights into the degradation pathway of methylene blue. The CIS+Plasma technology exhibits superior efficiency, higher cavitation yield, and lower operational costs, rendering it a viable option for large-scale wastewater treatment applications.

Analysis of the generation mechanism and evolution of the wheel high-order polygonal wear of subway trains

The present paper aims to study the generation cause of initial high-order polygonization of subway wheels as well as its progression pattern. In analyzing the wear characteristics of out-of-round (OOR) wheels of metro trains obtained via field measurements, the distribution of wheel high-order OOR wear was found to be closely related to the small-radius (i.e., a radius no greater than 400 m) curve track. The friction-induced self-excited oscillation caused by the saturated wheel-rail creep force was used as the frequency-fixing mechanism of the wheel OOR wear, and the wheel polygonization induced via this oscillation was evaluated by finite element simulation. Furthermore, the influence of rail corrugation on the development of wheel OOR wear was analyzed, and it is found that, when the vehicle negotiates a small-radius curve track, an unstable oscillation of 87 Hz induced by the saturated creep force can lead to wheel 12th ∼ 13th-order OOR wear—mainly distributed along the inner wheel tread far from the wheel back. During vehicle operation, the vibrations caused by the wheel 12th ∼ 13th-order polygonization can excite the eigenmodes of the wheelset, leading to an increased wear intensity. In addition, wheel 12th-order polygonal wear interacts with the rail corrugation of the same wear wavelength but different phases, which can cause wheel 10th ∼ 11th- and 13th ∼ 17th-order polygonization.

The investigation of lateral/heading characteristics and the influence of jet control at high attack angle

When the aircraft does high maneuvers at a high attack angle, the rapid transition of flight state induces a series of complex flow phenomena such as flow separation and vortex breakdown, leading to a series of uncommanded motions such as self-excited roll oscillation, which brings great danger to the flight of the aircraft. Therefore, it is important to investigate the mechanism of this phenomenon and suppress it. In this paper, the numerical method is based on Lattice Boltzmann Method (LBM) and Large Eddy Simulation (LES) for the SACCON standard flying-wing layout aircraft model. The accuracy of the numerical method is validated by comparing the simulation results of aerodynamic loads at different attack angles with the experimental results. Then the phenomena of the vortex breakdown and shedding at different roll angles and sideslip angles at high attack angles are analyzed, and the relationship between the jet flow coefficient and the control moment is obtained by adding the jet flow to the fuselage. Finally, the rolling motion of the aircraft is simulated in the dynamic simulation. The simulation results show that the model generates self-excited roll oscillations around a nonzero mean angle at high attack angles. The dynamic numerical simulation based on the LBM method can be an effective complementary tool for the study of the dynamic characteristics of the aircraft with a high attack angle.

Optimal cross-sectional area ratio between porous material and resonance tube for the onset of self-excited oscillations in standing-wave thermoacoustic engines

Optimal cross-sectional area ratio between porous material and resonance tube for the onset of self-excited oscillations in standing-wave thermoacoustic engines

Analysis of shock train leading shock structure under oscillatory backpressure

The leading shock in a steady shock train takes one of two forms: oblique or normal. However, the phenomenon and mechanism of the normal-to-oblique transition of the leading shock in a forced shock train have not been widely reported. In this study, leading normal and oblique shocks are experimentally observed at the same position and similar velocities in a forced oscillation shock train with an incoming Mach number of 1.83. The normal-to-oblique transition is found to be closely related to the self-excited oscillation of the shock train. In addition, we find that the direct use of free-interaction theory cannot accurately predict the leading shock angle in a moving shock train. Thus, free-interaction theory requires appropriate correction for this scenario.

Nonlinear effects on the self-excited chatter oscillations in motorcycle dynamics, including tyre relaxation

Nonlinear effects on the self-excited chatter oscillations in motorcycle dynamics, including tyre relaxation

Study on Optimization Design of Airfoil Transonic Buffet with Reinforcement Learning Method

Transonic buffet is a phenomenon of large self-excited shock oscillations caused by shock wave-boundary layer interaction, which is one of the common flow instability problems in aeronautical engineering. This phenomenon involves unsteady flow, which makes optimal design more difficult. In this paper, aerodynamic shape optimization design is combined with reinforcement learning to address the problem of transonic buffet. Using the deep deterministic policy gradient (DDPG) algorithm, a reinforcement learning-based design framework for airfoil shape optimization was constructed to achieve effective suppression of transonic buffet. The aerodynamic characteristics of the airfoil were calculated by the computational fluid dynamics (CFD) method. After optimization, the buffet onset angles of attack of the airfoils NACA0012 and RAE2822 were improved by 2° and 1.2° respectively, and the lift-drag ratios improved by 83.5% and 30% respectively. Summarizing and verifying the optimization results, three general conclusions can be drawn to improve the buffet performance: (1) narrowing of the leading edge of the airfoil; (2) situating the maximum thickness position at approximately 0.4 times the chord length; (3) increasing the thickness of the trailing edge within a certain range. This paper established a reinforcement learning-based unsteady optimal design method that enables the optimization of unsteady problems, including buffet.

Flame dynamics of an injection element operated with LOX/H2, LOX/CNG and LOX/LNG in a sub- and supercritical rocket combustor with large optical access

Hot fire tests were performed using a single-injector research combustor featuring a large optical access window ([Formula: see text] mm) for flame visualisation. Three test campaigns were conducted with the propellant combination of liquid oxygen and hydrogen, liquid oxygen and compressed-natural-gas, as well as liquid oxygen and liquefied-natural-gas at conditions relevant for main- and upper-stage rocket engines. The large optical access enabled synchronised flame imaging using ultraviolet and blue radiation wavelengths covering a large portion of the combustion chamber for various sets of sub- and supercritical operating conditions. Combined with temperature, pressure and unsteady pressure measurements, this data provides a high-quality basis for the validation of numerical modelling. Flame width, length and opening angle as features describing the flame topology were extracted from the imaging. The suitability of flame imaging using ultraviolet and blue radiation wavelengths as qualitative markers of heat release was evaluated. Two-dimensional distributions of the Rayleigh Index were calculated for intervals with and without high-amplitude, self-excited oscillations of the longitudinal acoustic resonance modes. The calculated Rayleigh Index values properly reflect the thermoacoustic state of the chamber, indicating that both types of imaging may be used for qualitative study of thermoacoustic coupling of high-pressure cryogenic flames.

Delay-induced self-oscillation excitation in the Fitzhugh–Nagumo model: Regular and chaotic dynamics

The stochastic FitzHugh–Nagumo model with time delayed-feedback is often studied in excitable regime to demonstrate the time-delayed control of coherence resonance. Here, we show that the impact of time-delayed feedback in the FitzHugh–Nagumo neuron is not limited by control of noise-induced oscillation regularity (coherence), but also results in excitation of the regular and chaotic self-oscillatory dynamics in the deterministic model. We demonstrate this numerically by means of simulations, linear stability analysis, the study of Lyapunov exponents and basins of attraction for both positive and negative delayed-feedback strengths. It has been established that one can implement a route to chaos in the explored model, where the intrinsic peculiarities of the Feigenbaum scenario are exhibited. For large time delay, we complement the study of temporal evolution by the interpretation of the dynamics as patterns in virtual space.

Experimental and CFD analysis of static and dynamic rotor stabilities in three-annular seals

At the sealing pressures of 1.25, 2.5, 5, 7.5 or 10 bar and at a rotation speed of zero to 8000 rpm, using experiment and CFD simulation, the static and dynamic stabilities of two three-annular seal basic designs were investigated. Those were the design with identical radial clearances at each annulus (ThASIC) and the one with 2 times larger radial clearance at the second annulus (ThASDC) in the presence of two axial dimensions for the connecting cavities of 1 and 3 mm. A possible mode of self-excited radial oscillations of a non-rotating shaft in a three-annular seal has been available from the experiments, and the explanation of this phenomenon occurrence has been given. The influences of the axial sizes of the connecting cavities and the clearance of the second annulus on the velocity distributions along the length and circumference of the seal have been studied. The analysis of the response amplitudes for a possible imbalance and wall motion orbit depending on the frequency of rotation with direct synchronous whirl of the shaft has shown that with an increase in the sealing pressure, the critical rotation speed and amplitude increases in the ThASDC design and the critical rotation speed decreases in the ThASIC design. The smallest whirl amplitude and a more centered position are typical for the ThASDC design with the enlarged connecting cavity of l2,4= 3 mm. The analysis of each annulus contribution to the magnitudes of hydrodynamic forces has been carried out, and depending on the sealing pressure and the seal design, the dynamic stiffness and damping coefficients have been got. The effect of the friction factor fsθ on the circumferential flow for different three-annular seal designs has been obtained. Its impact on the dynamic stability of the rotor (ThAS-RR design) and stator (ThAS-SR and ThAS-SRS designs) ribs, which were located in the connecting cavities of the three-annular seal, has been studied. The dynamic coefficients of stiffness, damping, and leakage for five different designs (ThASIC, ThASDC, ThAS-RR, ThAS-SR, and ThAS-SRS) of the three-annular seals have been compared. Due to the flow hydrodynamics analysis, the formation mechanisms for the decentering and destabilizing forces in the three-annular seals have been explained.

Enhancing ammonia combustion with minimum hydrogen blended in presence of self-excited intermittent pulsating oscillations

In this work, we propose and test a partial premixed fuel injection design of NH3−H2−O2 with double ring-shaped inlets to enhance ammonia combustion in an open-ended combustor by generating and sustaining pulsating combustion oscillations. Emphasis is being placed on determining the minimum amount of hydrogen being blended with ammonia in the presence of such self-excited pulsating oscillations. With the numerical model validated by comparing with experimental and theoretical data, we identify and systemically investigate three key thermodynamic parameters. They are shown to strongly affect the thermal, combustion, and emission performances. These parameters include the following: (1) total fuel mass flow rate ṁf; (2) mass fraction of hydrogen ω̇H2; and (3) the temperature TH of a heat exchanger implemented downstream of the combustor. It is interesting to observe that intermittent pulsating oscillations are sustained by such ammonia–hydrogen combustion. Furthermore, comparison is conducted between the present results and those with the classical single ring-shaped fuel inlet under the same flow and operating conditions. It is found that the exothermic heat of the proposed double-ring inlets is increased by 98.7% on average. The frequency of such intermittent oscillations is shown to increase with the decreased NH3 proportion. When pure hydrogen is supplied and passing through the outer ring inlet, the combustion limit can be greatly expanded, even if the inlet mass fraction of hydrogen is very small. The minimum hydrogen blended with ammonia is shown to be 0.1% to achieve a sustainable combustion and large-amplitude oscillations. The NO emission is found to be decreased, and H2O is shown to increase. The present study open ups an approach to enhance ammonia combustion by improving its flammability limit with the minimal hydrogen blended.

Investigation on Self-Excited Oscillation for Nonuniform Periodic Sheet Beam Traveling-Wave Tubes

The utilization of phase velocity tapering or jumping techniques in the design of traveling wave tubes (TWTs) has become a widely accepted practice for enhancing the efficiency of beam–wave interaction. However, these approaches are not without their challenges, as they can lead to a series of self-excited oscillation (SEO) issues that restrict further improvement in power. In this article, a nonlinear numerical method is explored for fast prediction of SEO in nonuniform periodic sheet beam TWTs (NP-SBTWTs). The oscillation thresholds at the upper and lower band edges are analyzed and discussed under different voltage and current conditions to determine the stable operating conditions for NP-SBTWT. In addition, the concept of “sideband absolute self-excited state” is proposed to be avoided in dispersion design, and the impact of actual reflection characteristics on stability is quantified by defining the eigenvalues of the feedback gain of the interaction structure based on the real reflection parameter. The results demonstrate that this approach distinguishes itself from conventional time-domain particle-in-cell (PIC) simulation techniques by its capacity to integrate authentic reflection parameters into the computation process to achieve a fast prediction of SEO in NP-SBTWT.

An experimental study on the erosion of sandstone by self-excited oscillation cavitation waterjet in submerged environment

Self-excited oscillation cavitation waterjet (SOCW) has good prospects for application in deep sea mineral extraction of its cavitation characteristics. This study conducted the erosion experiments of three types of sandstone, red sandstone, white sandstone, and black sandstone by SOCW under a submerged environment. The erosion characteristics of three types of sandstones under the impact of SOCW at various erosion times, standoff distance, and pressure were compared. It was found that, a) Red sandstone with larger porosity is more likely to produce a large damage area and has the best rock breaking effect. Taking the standard working condition of this experiment as an example (t = 50s, Pj = 16 MPa, S/De = 4), the erosion depth of red sandstone can reach 1.9 times that of white sandstone and 5.5 times that of black sandstone; the erosion area can reach 19.6 times that of white sandstone and 6.2 times that of black sandstone; the erosion quality can reach 5.2 times that of white sandstone and 7.4 times that of black sandstone. b) With the increase of erosion time and erosion pressure, the erosion index of sandstone shows an increasing trend. However, in this study, there is an optimal standoff distance S/De = 4 to maximize the erosion depth of the sandstone. Again, there is an optimal standoff distance to maximize the erosion hole area, S/De = 10 for the red sandstone and white sandstone, and S/De = 6 for the black sandstone. c) The special phenomenon of the pressure difference formed after the collapse of the SOCW bubble in the submerged environment, which acts on the moisture content fine pore space inside the sandstone, the fine pore zone, was observed by scanning electron microscope (SEM). The microscopic characterization of the water wedge effect was observed by computed tomography (CT), and the trend of crack development is shown by the binarization processed images of the sandstone internal damage section. This result can be a theoretical basis to guide the working parameters of nozzles in deep sea mining, especially for the selection of standoff distance.

Sunlight-powered self-excited oscillators for sustainable autonomous soft robotics

As the field of soft robotics advances, full autonomy becomes highly sought after, especially if robot motion can be powered by environmental energy. This would present a self-sustained approach in terms of both energy supply and motion control. Now, autonomous movement can be realized by leveraging out-of-equilibrium oscillatory motion of stimuli-responsive polymers under a constant light source. It would be more advantageous if environmental energy could be scavenged to power robots. However, generating oscillation becomes challenging under the limited power density of available environmental energy sources. Here, we developed fully autonomous soft robots with self-sustainability based on self-excited oscillation. Aided by modeling, we have successfully reduced the required input power density to around one-Sun level through a liquid crystal elastomer (LCE)-based bilayer structure. The autonomous motion of the low-intensity LCE/elastomer bilayer oscillator "LiLBot" under low energy supply was achieved by high photothermal conversion, low modulus, and high material responsiveness simultaneously. The LiLBot features tunable peak-to-peak amplitudes from 4 to 72 degrees and frequencies from 0.3 to 11 hertz. The oscillation approach offers a strategy for designing autonomous, untethered, and sustainable small-scale soft robots, such as a sailboat, walker, roller, and synchronized flapping wings.

Research on Abrasive Water Jet Polishing of Silicon Carbide Based on Fluid Self-Excited Oscillation Pulse Characteristics

A self-excited oscillating pulsed abrasive water jet polishing method is proposed to solve the problems of low removal efficiency in traditional abrasive water jet polishing and the influence of an external flow field on the material surface removal rate. The self-excited oscillating chamber of the nozzle was used to generate pulsed water jets to reduce the impact of the jet stagnation zone on material surface removal and increase the jet speed to improve processing efficiency. ANSYS Fluent was employed to simulate the processing flow field characteristics for different lengths of oscillation cavities. The simulation results indicate that the velocity of the jet shaft reached a maximum of 178.26 m/s when the length of the oscillation cavity was 4 mm. The erosion rate of the material is linear with the processing angle. A nozzle with a length of 4 mm of the self-excited oscillating cavity was fabricated for SiC surface polishing experiments. The results were compared with those of ordinary abrasive water jet polishing. The experimental results showed that the self-excited oscillation pulse fluid enhanced the erosion ability of the abrasive water jet on the SiC surface and significantly improved the material-removal depth of the abrasive water jet polishing SiC. The maximum surface erosion depth can be increased by 26 μm.