Explore the vast range of titles available.

Microresonators have a variety of scientific and industrial applications. The measurement methods based on the natural frequency shift of a resonator have been studied for a wide range of applications, including the detection of the microscopic mass and measurements of viscosity and stiffness. A higher natural frequency of the resonator realizes an increase in the sensitivity and a higher-frequency response of the sensors. In the present study, by utilizing the resonance of a higher mode, we propose a method to produce the self-excited oscillation with a higher natural frequency without downsizing the resonator. We establish the feedback control signal for the self-excited oscillation using the band-pass filter so that the signal consists of only the frequency corresponding to the desired excitation mode. It results that careful position setting of the sensor for constructing a feedback signal, which is needed in the method based on the mode shape, is not necessary. By the theoretical analysis of the equations governing the dynamics of the resonator coupled with the band-pass filter, it is clarified that the self-excited oscillation is produced with the second mode. Furthermore, the validity of the proposed method is experimentally confirmed by an apparatus using a microcantilever.

Most of pneumatic soft robots rely on external rigid controllers and valves to achieve rhythmic movements. This article introduces a soft pneumatic pressure regulation system with self‐rhythmic characteristics and simple structure. In this system, the hose generates self‐excited oscillations due to jet force, which realizes the transformation of constant pressure to periodically varying pressure. This mechanism allows soft robots to perform rhythmic movements. A mathematical model is developed to describe the self‐excited oscillations of the jet hose. Numerical simulations are conducted to analyze the impact of various parameters on system oscillations. The system operates under pressures from 90 to 150 kPa. By adjusting the pressure, hose length, and jet hole diameter, the oscillation frequency of the pressure can be tuned between 5.9 and 11.1 Hz. The comparison between simulation results and experimental data verifies the correctness of the mathematical model. Finally, a soft robot capable of crawling based on anisotropic friction is designed and fabricated. Powered solely by the soft pneumatic pressure regulation system, the robot achieves self‐rhythmic crawling. By adjusting the air source pressure, hose length, and jet hole diameter, the robot's crawling speed can be effectively controlled, ranging from 2.5 to 6.8 mm s−1. This study presents a soft pneumatic pressure regulation system that uses self‐excited hose oscillations to convert constant pressure (90–150 kPa) into rhythmic output. Simulations and experiments demonstrate tunable oscillation frequencies (5.9–11.1 Hz) through adjustments in pressure, hose length, and jet hole diameter. The system can control soft robot crawling (speed from 2.5 to 6.8 mm s−1).

The Koopman operator provides a powerful way of analysing nonlinear flow dynamics using linear techniques. The operator defines how observables evolve in time along a nonlinear flow trajectory. In this paper, we perform a Koopman analysis of the first Hopf bifurcation of the flow past a circular cylinder. First, we decompose the flow into a sequence of Koopman modes, where each mode evolves in time with one single frequency/growth rate and amplitude/phase, corresponding to the complex eigenvalues and eigenfunctions of the Koopman operator, respectively. The analytical construction of these modes shows how the amplitudes and phases of nonlinear global modes oscillating with the vortex shedding frequency or its harmonics evolve as the flow develops and later sustains self-excited oscillations. Second, we compute the dynamic modes using the dynamic mode decomposition (DMD) algorithm, which fits a linear combination of exponential terms to a sequence of snapshots spaced equally in time. It is shown that under certain conditions the DMD algorithm approximates Koopman modes, and hence provides a viable method to decompose the flow into saturated and transient oscillatory modes. Finally, the relevance of the analysis to frequency selection, global modes and shift modes is discussed.

Self-excited motions have the advantages of directly harvesting energy from the environment, autonomy, and portability of the equipment, and consequently, the development of a wealth of new self-excited motion modes can greatly expand the application of active machines. In this paper, a rotator capable of self-excited movement is proposed, which consists of a liquid crystal elastomer (LCE) bar and a regular bar with an axle. Based on the dynamic LCE model, through theoretical modeling and numerical calculation, it is found that the LCE rotator has three motion modes, namely static mode, oscillation mode and rotation mode. The detailed dynamical process reveals the mechanism of self-excited oscillation and rotation. In this paper, the effects of parameters such as light intensity, damping coefficient, dimensionless gravitational acceleration, length ratio, illumination region and initial angular velocity on the self-excited oscillation and rotation are further studied systematically, and the corresponding limit cycles are given by various cases. The results show that the light intensity, damping coefficient and length ratio have important influence on the motion mode, while the initial angular velocity does not affect the motion mode. The influence of various parameters including light intensity and illumination region on the amplitude and frequency of self-excited oscillation is also studied. It is found that the amplitude mainly depends on light intensity and damping. This study can deepen people's understanding of non-equilibrium self-excited motions and provide promising applications in the fields of energy harvest, power generation, monitoring, soft robotics, medical devices and micro–nano-devices.

Based on the theory of chaotic modulation, the influence of the control parameters of impingement jets on the stability of the system has been studied. The range of the simulation control parameters is determined through trial calculation. The influence of the main control parameters of the nonlinear system on the stability of pressure and pressure gradient trajectories has been studied numerically. The fluctuation curves of the damping force, recovery force, and kinetic energy and potential energy have been further studied. The results show that the main control parameters affect the waveform of the self-excited oscillation and the energy conversion of the impingement jet system. When the control parameters take specific values, the nonlinear system bifurcates and a regular periodic solution can be obtained. The results suggest the values of the control parameters that form a clear synergistic effect, which facilitates the conversion utilization of self-excited oscillation energy, and in which a stable oscillation can be formed.

With a focus on the shock oscillation phenomenon of a supersonic inlet at a high Mach number, the influence of isolator overflow on shock oscillation is studied in this paper. The shock wave dynamic model with overflow was established by the theoretical method, and the integrated numerical model of internal flow and external flow in the inlet was established too. The theoretical analysis of rate of overflow and overflow position on the flow field is carried out, and the changes of flow field parameters are studied by numerical simulation under different overflow positions. The results showed that both increasing the rate of overflow and setting the overflow gap close to the shock front were beneficial to reducing the flow parameters’ oscillation. In the viscous flow field, the overflow gap restricted the forward development of the local separation region of the shock train system, thus constraining the shock wave movement process, which could significantly reduce the parameter oscillation. In model C with two groups of overflow gaps, pressure oscillations of sampling point PU8 and PL8 were reduced to 29.81% and 30.56% relative to without overflow, and the corresponding rate of overflow was within 3.6%, which indicated that the appropriate overflow gap setting could effectively suppress the self-excited oscillation in the inlet.

The high-frequency pulse flow, equivalent to the natural frequency of rocks, is generated by a self-excited oscillating cavity to achieve resonance rock-breaking. The flow field and oscillating mechanism of the self-excited oscillating cavity were simulated using the large eddy simulation method of Computational Fluid Dynamics (CFD). A field-scale testing apparatus was developed to investigate the impulse characteristics and verify the simulation results. The results show that the fluid at the outlet at the tool is deflected due to the pulse oscillation of the fluid. The size and shape of low-pressure vortices constantly change, leading to periodic changes in fluid impedance within the oscillating cavity. The impulse frequency reaches its highest point when the length–diameter ratio is 0.67. As the length–diameter ratio increases, the tool pressure loss also increases. Regarding the cavity thickness, the impulse frequency of the oscillating cavity initially decreases, then increases, and finally decreases again. Moreover, both the impulse frequency and pressure loss increase with an increase in displacement. The numerical simulation findings align with the experimental results, thus confirming the validity of the theoretical model. This research provides theoretical guidance for the practical application of resonance rock-breaking technology.

To enhance the erosion efficiency in traditional abrasive water jet processing, an abrasive water jet processing method based on self-excited fluid oscillation is proposed. Traditional abrasive water jet methods suffer from reduced jet kinetic energy due to the presence of a stagnation layer, which hinders efficient material removal. By integrating a self-oscillation chamber into the conventional abrasive water jet nozzle, the continuous jet is transformed into a pulsed jet, thereby increasing the jet velocity and enhancing the kinetic energy of the process. This modification aims to improve material removal efficiency. Using Ansys Fluent, we simulated the material removal efficiency on workpiece surfaces with varying lengths of self-oscillation chambers. The simulation results reveal that the optimal length of the self-oscillation chamber for maximum erosion is 4 mm. SiC materials were used to evaluate the impact of self-oscillation chamber length (L), jet pressure (P), abrasive flow rate (M), and abrasive grain size (D) on erosion. Experimental results show that the self-oscillation chamber increases erosion depth by 33 μm. The maximum erosion depths recorded were 167 μm when L = 4 mm, 223 μm when P = 16 MPa, 193 μm when M = 80 g/min, and 268 μm when D = 2000 μm. Overall, the self-excited oscillation effect enhances the erosion efficiency of the waterjet by 14%. This study further elucidates the factors influencing erosion behaviors in oscillating abrasive water jet processing.

Use of the microcantilever in an atomic force microscope (AFM) that has been enhanced with a diamond abrasive grain has emerged as a powerful nanolithography technique. Vibration-assisted mechanical nanolithography, particularly when using self-excited oscillations generated by the microcantilever, enhances machining efficiency, and the machining depth can be controlled conveniently by manipulating the microcantilever’s steady-state amplitude. In this research, the formation mechanism of the machined grooves is investigated. Two machining modes are proposed: one involves the impacts of the diamond abrasive grain equipped on the microcantilever’s tip when used as the formation tool, while the other relies on pressing and rubbing of the diamond abrasive grain. Furthermore, effective control of the machining depth in both machining modes via amplitude manipulation is proposed theoretically. Mechanical nanolithography experiments are performed using a redesigned microcantilever, and the machining mode is determined by observing the new machining tool’s vibration profile during the machining process. As a result, under reduced pressing loads, grooves are formed by the tool’s impacts, whereas when the pressing load exceeds a threshold, the sample surface is pressed and rubbed by the diamond abrasive grain. Furthermore, the effectiveness of machining depth control through amplitude manipulation is demonstrated experimentally.

To better polish the inner surface of the pipe fittings and hole parts, this paper proposed a novel polishing method that was based on the self-excited oscillating pulse effect of abrasives flow. Firstly, the basic principle of self-excited oscillating polishing flow was introduced, and then the simulation model and experimental platform of a self-excited oscillating abrasive flow polishing were established. Through fluid simulation and stainless steel inner surface polishing experiments, the relationships between polishing parameters (inlet velocity, inlet initial gauge pressure, abrasive particle concentration, and abrasive particle size), turbulence intensity, and wall shear forces were analyzed. The results of experiments showed that the factors affecting the variation of roughness are sorted from large to small into inlet velocity, inlet initial gauge pressure, abrasive particle concentration, and abrasive particle size. In addition, A polishing comparison experiment was carried out. At the inlet A, the inner surface roughness Ra can reach 46 nm after 14 h of polishing without an oscillation cavity, while in the case of a self-excited oscillation cavity, the inner surface roughness Ra can reach 46 nm after 12 h of polishing. Compared with the non-oscillating abrasive flow polishing, the polishing efficiency with the self-oscillating abrasive flow is 15% higher, and the over-polishing phenomenon can be suppressed.