Self-oscillation Research Articles

Numerical investigation of ozone decomposition by self-excited oscillation cavitation jet

Abstract Extreme environmental changes caused by the cavitation bubble collapse, such as high pressure, high temperature and the microjet, will cause pyrolysis reaction at the gas and liquid interface inside the bubble. Self-excited pulsed cavitation jet has an instantaneous strong pulse pressure, which leads to local hot spots surrounding the cavitation bubbles. The generation of strong oxidizing free radicals promotes easy ozone conversion into oxygen. Numerical simulations were conducted for ozone decomposition by cavitation jet. Three groups of different collision angles were applied to compare and analyze the ozone degradation reaction. Results showed that the collision angle has a certain influence on the chemical reaction intensity, the degradation of ozone, and oxygen production. At the collision angle of 180°, the chemical reaction was the most violent, with ozone degradation and oxygen production at the highest level, followed by 120° and lowest at 90°.

Self-Sustained Collective Motion of Two Joint Liquid Crystal Elastomer Spring Oscillator Powered by Steady Illumination.

For complex micro-active machines or micro-robotics, it is crucial to clarify the coupling and collective motion of their multiple self-oscillators. In this article, we construct two joint liquid crystal elastomer (LCE) spring oscillators connected by a spring and theoretically investigate their collective motion based on a well-established dynamic LCE model. The numerical calculations show that the coupled system has three steady synchronization modes: in-phase mode, anti-phase mode, and non-phase-locked mode, and the in-phase mode is more easily achieved than the anti-phase mode and the non-phase-locked mode. Meanwhile, the self-excited oscillation mechanism is elucidated by the competition between network that is achieved by the driving force and the damping dissipation. Furthermore, the phase diagram of three steady synchronization modes under different coupling stiffness and different initial states is given. The effects of several key physical quantities on the amplitude and frequency of the three synchronization modes are studied in detail, and the equivalent systems of in-phase mode and anti-phase mode are proposed. The study of the coupled LCE spring oscillators will deepen people’s understanding of collective motion and has potential applications in the fields of micro-active machines and micro-robots with multiple coupled self-oscillators.

Light-fueled self-excited vibration of a liquid crystal elastomer spring oscillator

Self-excited oscillation has the advantages of harvesting energy directly from the environment, autonomy and device portability. Design of self-oscillating systems can greatly expand their applications in self-sustained active machines. In this paper, we propose a new spring oscillator composed of a liquid crystal elastomer (LCE) fiber capable of self-excited vibration under steady illumination. Based on the well-established dynamic LCE model first proposed by Finkelmann et al., the self-excited vibration of the LCE spring oscillator is theoretically investigated. It is found that the vibration of the spring oscillator has two motion regimes, namely the oscillation regime and the static regime. Under steady illumination, the vibration of the spring oscillator can be self-sustained due to the coupling between the light-driven deformation of the LCE fiber and its own motion, in which the energy input from the constant environment compensates for the damping dissipation of the system. Moreover, some of the properties of the oscillations are affected by the system parameters. The initial velocity does not affect the motion regime of the light-powered vibration, and its period and amplitude generally depend on the intrinsic properties of the system, which possesses great robustness. The results may have potential applications in energy harvesters, soft robotics, signal sensors, active motors, and self-sustained machines.

Standing waves in the plenum of an open jet wind tunnel: Resonance and self-excited oscillation

The low-frequency pressure pulsations in a large low-speed open jet wind tunnel were studied using a 1:20 scaled model wind tunnel. It was found that plane pressure standing waves existed in all three directions of the wind tunnel’s plenum. It was confirmed that the large pressure pulsations in the transverse direction of the plenum were caused by the resonance of the plane pressure standing waves in that direction with the edgetone feedback (vortex-sound feedback between the nozzle and the collector). The analysis of the standing waves in the vertical direction of the plenum suggested that they might come from self-excited oscillations. For the model wind tunnel with a 1:20 strictly scaled collector, the standing waves in the transverse direction of the plenum were much stronger than those in the other two directions. By increasing the width of the collector, the intensity of the standing waves in the transverse direction was significantly reduced to a level similar to that of the standing waves in the vertical direction. It was also found that the standing waves in the transverse direction were significantly enhanced after a floor was added to the test section.

The characteristics and mechanisms of self-excited oscillation pulsating flow on heat transfer deterioration of supercritical CO2 heated in vertical upward tube

Heat transfer deterioration (HTD) of supercritical CO2 heated in a tube influences the efficiency and safe operation of the system due to the occurrence of local high temperature. To suppress and delay the HTD, the characteristics and mechanisms of self-excited oscillation pulsating flow on HTD of supercritical CO2 are studied by experiment and simulation at pressure 8 MPa, mass fluxes from 350 to 800 kg/m2·s, heat fluxes from 30 to 200 kW/m2. The Helmholtz oscillator is introduced into the inlet of the vertical tube for generating a pulsating flow of supercritical CO2. The heat transfer performance is compared with that of without Helmholtz oscillator at the inlet of the tube. The results show that the self-excited oscillation pulsating flow improve the heat transfer performance significantly. The heat transfer parameters present oscillations with small amplitude along flow direction before pseudo-critical point (Tpc). The average heat transfer coefficient can be up to 3.4 times and the enhancement takes place mainly at the HTD region which is located at the entrance section of the heated tube. The effect of suppressing HTD is more significant at higher heat flux, and the peak of wall temperature can be reduced by 100 K at a heat flux of 200 kW/m2. Compared to the steady flow, the mechanism analysis of the self-excited oscillation pulsating flow based on radial distributions of velocity, turbulent kinetic energy (TKE), and density reveals that the velocity distribution of “M−shape” appears later and gentler. The production and diffusion of TKE are improved at log layer (30 < y+ < 0.2r). In addition, the period and the amplitude do not show monotonous trends on heat transfer performance. The effects of pulsating parameters on HTD are optimized that the heat transfer performance with the period of 0.016 s and the amplitude of 100 kg/m2·s is the best in calculated cases.

Amplitude control for sensorless self-excited oscillation of cantilever based on a piezoelectric device

Amplitude control for sensorless self-excited oscillation of cantilever based on a piezoelectric device

Experimental amplitude and frequency control of a self-excited microcantilever by linear and nonlinear feedback

It is well known that the micro scale deviations of mechanical properties of a sample can be detected by measurement methods that use microcantilever as resonators. Those methods use the natural frequency shift of a resonator, thus we need to recognize the frequency shift caused by the effects of a sample on a resonator with high sensitivity and accuracy. Experimental approaches based on self-excited oscillation enable the detection of these shifts even when the resonator is immersed in a high-viscosity environment. In the present study, we experimentally and theoretically investigate the nonlinear characteristics of a microcantilever resonator and their control by nonlinear feedback. We show that the steady-state response amplitude and the corresponding response frequency can be controlled by cubic nonlinear velocity feedback and cubic nonlinear displacement feedback, respectively. Furthermore, the amplitude and frequency of the steady-state self-excited oscillation can be controlled separately. These results will expand application of measurement methods that use self-excited resonators.

Global stability analysis of flexible channel flow with a hyperelastic wall

We consider the stability of flux-driven flow through a long planar rigid channel, where a segment of one wall is replaced by a pre-tensioned hyperelastic (neo-Hookean) solid of finite thickness and subject to a uniform external pressure. We construct the steady configuration of the nonlinear system using Newton's method with spectral collocation and high-order finite differences. In agreement with previous studies, which use an asymptotically thin wall, we show that the thick-walled system always has at least one stable steady configuration, while for large Reynolds numbers the system exhibits three co-existing steady states for a range of external pressures. Two of these steady configurations are stable to non-oscillatory perturbations, one where the flexible wall is inflated (the upper branch) and one where the flexible wall is collapsed (the lower branch), connected by an unstable intermediate branch. We test the stability of these steady configurations to oscillatory perturbations using both a global eigensolver (constructed based on an analytical domain mapping technique) and also fully nonlinear simulations. We find that both the lower and upper branches of steady solutions can become unstable to self-excited oscillations, where the oscillating wall profile has two extrema. In the absence of wall inertia, increasing wall thickness partially stabilises the onset of oscillations, but the effect remains weak until the wall thickness becomes comparable to the width of the undeformed channel. However, with finite wall inertia and a relatively thick wall, higher-frequency modes of oscillation dominate the primary global instability for large Reynolds numbers.

Nanoscale cutting using self-excited microcantilever

The application of self-excitation is proposed to improve the efficiency of the nanoscale cutting procedure based on use of a microcantilever in atomic force microscopy. The microcantilever shape is redesigned so that it can be used to produce vibration amplitudes with sufficient magnitudes to enable the excitation force applied by an actuator to be transferred efficiently to the tip of the microcantilever for the cutting process. A diamond abrasive that is set on the tip is also fabricated using a focused ion beam technique to improve the cutting effect. The natural frequency of the microcantilever is modulated based on the pressing load. Under conventional external excitation conditions, to maintain the microcantilever in its resonant state, it is necessary to vary the excitation frequency in accordance with the modulation. In this study, rather than using external excitation, the self-excitation cutting method is proposed to overcome this difficulty. The self-excited oscillation is produced by appropriate setting of the phase difference between the deflection signal of the microcantilever and the feedback signal for the actuator. In addition, it is demonstrated experimentally that the change in the phase difference enables us to control the amplitude of the self-excitation. As a result, control of the cutting depth is achieved via changes in the phase difference.

A polishing method using self-excited oscillation abrasive flow for the inner surface of workpiece

A polishing method using self-excited oscillation abrasive flow for the inner surface of workpiece

Photodriven Self-Excited Hydrogel Oscillators

Self-excitation of smart materials that respond to stimuli is important for building soft robots capable of self-sustainable untethered locomotion. However, in these machines the sophisticated interaction and energy flow between materials and stimuli are not well understood. The authors study a self-excited hydrogel cantilever oscillator irradiated by constant light, and find that the synergy between the photoinduced moment and oscillation ensures positive work input into the cantilever to overcome damping. Scaling analysis of the energy flow of the system reveals how the stable oscillation amplitude is governed by various geometric and material parameters.

Research on the Mechanism of Rijke Tube Self-excited Sound

Combustion equipment generates self-excited vibration called combustion vibration. The noise generated by this ｒ vibration is undesirable because it can damage structures and degrade their performance. Because this vibration phenomenon is complex, it is very difficult to model. Therefore, the purpose of this study is to attempt modeling of Rijke tubes, which are prone to combustion vibration from the perspective of heat transfer engineering, and to reproduce the sound pressure waveform of Rijke tubes using the cellular automaton (CA) method. Since previous studies have summarized the properties of a Rijke tubes and the rules of the CA method, we referred to them and incorporated them into this study. Previous studies have shown that the particle velocity in the heat source section including time delay causes self-excited oscillation. In this study, we derived an equation for the particle velocity with time delay by using heat transfer engineering based on the expansion and contraction of air. The sound pressure waveform of the Rijke tube could be quantitatively reproduced by numerically calculating the time of passage through the heat source and determining the number of time delay from time to time．

Low-frequency shock train oscillation control in a constant area duct

The self-excited shock train oscillation control using partial removal of boundary layer flow in a constant area duct is studied numerically using unsteady Reynolds Averaged Navier–Stokes simulation. The effect of varying the suction flow ratio on the shock train oscillatory characteristics is analyzed using steady and unsteady statistics, space–time contour, power spectra, and cross correlation analysis. For the present study, a constant area duct of height 0.032 mm, and the aspect ratio of 25, at freestream Mach number of 2.0 and back pressure ratio of 0.52 is considered. The removal of the boundary layer through an angled suction slot with three different suction flow ratios is performed. Numerical results indicate that the removal boundary layer restricts the bifurcation process of the shock train and appears to be a single curved normal shock at a higher suction flow ratio. Also, the transition of regular reflection to the Mach reflection type is noted. The suction flow from the top and bottom slot tends to initiate a lateral oscillation that forms a wavy mixing flow region. The power spectral density contour suggests that the increase in suction flow ratio will tend to increase the dominant frequency ranges from (0.034 to 0.094). The cross correlation indicates the presence of downstream pressure disturbance that moves toward the upstream direction. With suction flow, a disturbance that emerges from the suction slot moves in the opposite direction and dominates at a lower suction flow ratio and these disturbances disappear at a large suction flow ratio.

Study on the disturbance effect of pulsating flow and heat transfer in self-excited oscillation shear layer

The fluid movement motion has an important influence on the evolution of the pul?sating flow in the hot runner. Using the large eddy simulation numerical method, the instantaneous velocity, wall shear stress, boundary-layer thickness, and Nusselt number of hot runner section under different structural parameters at an inlet pressure of 5000 Pa were studied. The research results showed that the backflow vortex can be formed in the hot runner, and the fluid at the axis center of hot runner can form a pulsating flow under the squeezing action of the backflow vortex. The pulsating flow had a strong disturbance effect on the fluid around the axis center and accelerated the heat exchange between the fluid around the axis center and the wall. The disturbance effect of pulsating flow gradually strengthened with the flow of the main flow to the downstream. When d2/d1 was 1-1.8, the wall shear stress first increased and then decreased, and the wall heat transfer efficiency first increased and then decreased. The maximum wall shear stress was 36.4 Pa. When L/D was 0.45-0.65, the boundary-layer thickness first decreased and then increased, and the heat transfer efficiency first increased and then decreased. The minimum boundary-layer thickness was 0.392 mm and the maximum Nusselt number was 138. When d2/d1=1.4 and L/D=0.55, the maximum comprehensive evaluation factor reached 1.241, and the heat transfer efficiency was increased by 24.1%.

Numerical Simulation of a Two-Dimensional Helium Gas Jet with Side-Jets Formation

Side jets are generated when the jet density is smaller than that of the ambient gas. It has been suggested that the side jets are related to self-excited oscillations due to the local absolute instability that occurs when the density ratio is less than 0.7, and vorticity generation due to baroclinic torque caused by the cross product of density gradient and pressure gradient. In addition, it has been suggested that a side jet is also formed by a pair of streamwise vortices with different rotation directions. First of all, in order to confirm previous hypotheses, we have tried the formation of side jets in a helium gas jet by a numerical simulation using ANSYS Fluent. As a result, it is shown that a side jet is formed between a streamwise vortex pair.

Investigation on Jet Breaking Mechanism of Self-Excited Oscillation Pulse Nozzle

Investigation on Jet Breaking Mechanism of Self-Excited Oscillation Pulse Nozzle

Internal flow and bearing characteristics in tilting pad journal bearing with fluid structure intersection

Journal bearings have the advantage of being damped. However, at high rotational speeds, self-excited oscillations called oil whip occur, leading to damage of bearing and shafts. Tilting Pad Journal Bearings are used in high-speed rotating machinery as they are effective against oil whip. The Reynolds equation has been used as a typical calculation method to investigate bearing characteristics, but it is difficult to apply when the bearing is complex. In this paper, a prediction method of bearing characteristics of Tilting Pad Journal Bearing using CFD was constructed. To simulate the motion of the pad as it oscillates against the fluid force, Dynamic Fluid Body Intersection was studied. In order to compare with the calculation results, an active oscillating rotor using magnetic bearings was constructed. As a result, the calculation results were roughly consistent with the experimental results, and a prediction method by CFD was constructed.

Study on Modeling of Self-Excited Mechanism of Edge Tone.

One type of aerodynamic noise caused by airflow is edge tone, which is generated when airflows collide with edge-shaped objects and are separated at the edges. This edge tone is called self-excited sound, but the self-excitation mechanism has not yet been clarified, and quantitative modeling is not yet possible. Therefore, this study examines a mathematical model of edge tones and analyzes the self-excitation mechanism. The hypothesis of the self-excitation mechanism proposed in this study is as follows. "A dipole source above and below the edge induces a vertical particle velocity on the jet axis, which causes the jet to oscillate vertically. The jet vibrates perpendicularly. The oscillation of the jet causes the dipole sources above and below the edge to grow moment by moment, and the value of the particle velocity perpendicular to the axis also increases, resulting in self-excited oscillation of the jet." In the computational model, the dipole source is assumed to be a point source in the opposite phase, and the impulse response of the particle velocity to an impulse input of volume velocity is calculated using the Fourier series transform. As a result, a self-excited phenomenon was generated by assuming a dipole source with an assumed waste time at the edges.

The effect of the filling degree of the tumbling mill chamber on the bifurcation value of the froude number

The influence of the filling degree of the drum chamber on the bifurcation value of the Froude number in the case of self-excitation of self-oscillations of a two-fraction granular charge with a minimum swing is considered. Such a pulsating mode of the charge movement is used in the self-oscillating grinding process in a tumbling mill. The coarse fraction that simulated the milling bodies consisted of spherical particles of an incoherent granular material with a relative size in the chamber ψb=0.00733. The fine fraction, simulating the material to be ground, was cement with a relative particle size ψm≈0.092∙10-3. The value of the adopted analogue of the kinematic viscosity of the two-fraction granular loading approached the value of 10-3 m2/s. The main variable factor in experimental studies was the filling degree of the drum chamber with loading at rest κb=0.25, 0.35, and 0.45. An additional factor was the degree of filling the gaps between the spherical particles of the coarse fraction with particles of the fine fraction κb=0, 0.25, 0.5, and 1. The method of visual analysis through the transparent end wall of the chamber of transient processes of the loading behavior with a smooth change and fixation of the velocity was applied. The bifurcation minimum value of the rotation speed was recorded, at which the steady-state circulation mode of the load movement turns into a transient pulsation one with a minimum swing. A decrease in the bifurcation values of the Froude number Frb on the cylindrical surface of the chamber with an increase in κb has been established. An increase in the intensity of the decrease in Frb with an increase in κm was revealed. The recorded effect is due to an increase in the connected properties of a two-fraction loading during self-excitation of self-excited oscillations with an increase in κb and κm. The numerical values of the boundaries of the range of bifurcation values of the Froude number for a tumbling mill Frb=0.0484–1.17 have been determined. The obtained Frb range corresponds to the Reynolds value in the range Re=40–197. The maximum Frb value is obtained with coarse grinding. An increase in the likelihood of self-excitation of self-oscillations of the intra-chamber loading with a decrease in the fineness of grinding was revealed

Effect of Nozzle Outlet Shape on Cavitation Behavior of Submerged High-Pressure Jet

A submerged high-pressure water jet is usually accompanied by severe cavitation phenomenon. An organ pipe nozzle can greatly improve the cavitation performance of the jet, making use of the self-excited oscillation of the flow. In order to study the effect of organ pipe nozzles of different nozzle outlet shapes on cavitation behavior of submerged high-pressure jet, in this paper we build a high-pressure cavitation jet experiment system and carried out a high-speed photography experiment to study cavitation cloud characteristics of a high-pressure submerged jet. Two organ pipe nozzles with and without a whistle were compared. The dynamic characteristics of the cavitation cloud was extracted through the POD method, it was found that the result effectively reflect the dynamic characteristics of the cavitation jet. The reconstruction coefficients of mode-1 obtained by the POD can better reflect the periodic time-frequency characteristics of cavitation development. The effect of the nozzle outlet shape on the cavitation behavior of organ pipe nozzle was analyzed based on unsteady numerical simulation, and it was found that the jet generated by the nozzle with a divergent whistle had a larger vorticity in the shear layer near the outlet. Further, stronger small-scale vortex and much severe cavitation occurred from the nozzle with a divergent whistle.