Active vibration control of a shaft bracket-plate coupled system

Active vibration control of a shaft bracket-plate coupled system is investigated. The vibration of the plate is controlled with electromagnetic vibration absorbers (EVAs), which are mounted around the feet of the shaft bracket to impede the transmission of vibration from the bracket apex to the plate. A dynamic model is established on the Timoshenko beam theory and the Kirchhoff thin plate theory to reveal the mechanism of vibration transmission. It is exhibited that all the induced forces and moments at the coupling points contribute much to the transverse responses of the plate. The feasibility of active control with the EVAs is evaluated numerically based on the controllability of the plate vibration. It is demonstrated that the two-point in-plane control is able to attenuate the plate vibration under the excitation of in-plane disturbance forces, while the multi-point control is effective in reducing the plate vibration regardless of the directions of disturbance forces. An experimental system is built to verify the performance of the two-point in-plane control. The results have shown that with the help of adaptive control, the two-point in-plane control is capable of suppressing the vibration of the foundation induced by the in-plane forces acting on the shaft bracket.

Investigation of the stability of transverse hydrogen injection combustion caused by plate vibration using the dynamic mode decomposition method

This article investigated the stability of transverse hydrogen injection combustion caused by the plate vibration. The finite-rate method is used to simulate the combustion. The unsteady flow field in the unstable phase of combustion is extracted. The unstable mode of the shock wave structure and the flame structure during the stage of combustion instability, the spatial and temporal characteristics of the dominant modes, as well as their stability are analyzed based on the dynamic mode decomposition (DMD) method. The results indicate that, according to the sequence of energy, the extracted first six orders modes of the shock wave structure and the flame structure have relatively low frequency with a negative growth rate and small numerical value, which presents a trend of weak convergence. The characteristics of the dominant structure of DMD modes show that the plate vibration has great effects on the reflected shock wave structure near the plate and on the upper wall surface, as well as the flame structure near the plate. According to the sequence of the mode energy and the growth rate, respectively, the extracted first six orders modes have relatively high frequency. Simultaneously, the structures of the modes extracted by the sequence of mode energy are more regular, while those extracted by the sequence of growth rate are more disorderly. The unstable shock wave structure is mainly manifested by the reflected shock wave in the vibration region and the shock wave structure reflected by the upper wall surface. The unstable flame structure is mainly concentrated near the vibration region and downstream areas.

Dynamic invariance in near field acoustic levitation

It is possible to levitate a mass by vibrating a flat disk located under the mass. This near-field acoustic levitation is particularly useful for eliminating friction between moving objects. This paper presents an experimental and theoretical study of the dynamic behavior of a levitating mass for different magnitudes of oscillation of the flat disk. The magnitude of the vibration of the mass appears to be independent of the amplitude of the vibration of the disk over a range of two orders of magnitude. This unusual behavior is due to the simultaneous changes of the stiffness of the air film and the natural frequency of the system as the plate vibration is changed. As the plate oscillation is reduced, the distance to resonance decreases, allowing an increase of the ratio of the output to input signals in such a way that the output remains constant. This result can be useful for improving the energy efficiency of the levitation.

Small plate vibration sound-absorbing device with a clearance and without surrounding restriction: Theoretical analysis and experiment

This article focuses on the theoretical analysis and experiment of a small plate vibration sound-absorbing device with a clearance and without surrounding restriction. Generally, a plate vibration sound-absorbing device absorbs sound in a frequency range lower than that of porous material, which is used as a sound-absorbing material, and its sound absorption coefficient is small. Furthermore, note that the plate is fixed at the periphery in a conventional plate vibration sound-absorbing device. Hence, when the plate is small, vibration barely occurs and effective sound-absorbing effect is not obtained. Given that the plate vibration sound-absorbing device has a restriction wherein a particular size of plate is required, it is primarily utilized for indoor finishing considering the architectural acoustics, and finding applications for small-sized machines has been challenging. Theoretical values in this work are calculated assuming that the clearance and back air layer for the small plate vibration sound-absorbing device constitute a Helmholtz resonator. Furthermore, experiments were conducted to perform comparisons against the theoretical value. The theoretical analysis was conducted by connecting the clearance and back air layer in parallel with the plate portion in the electrical equivalent circuit. By performing the experiments using various parameters, the sound absorption characteristics of the proposed plate vibration sound-absorbing device were determined. If the clearance is smaller than the boundary layer thickness, then the conventional resistive end correction cannot be applied.

Natural Frequency Measurements of the Fluid-elastic-coupled Shell Plate Vibration in a Large-sized Cylindrical Steel Tank by Microtremor Observations

Microtremor observations measured the natural frequencies of the fluid-elastic-coupled shell plate vibration (bulging) in a large flat-bottomed cylindrical steel tank with a 125,000-m3 capacity. Five peaks appear in the observed microtremor spectral ratios of the top or mid-height of the shell plate to the bottom on the tank foundation. Comparing the spectral ratios to the solutions obtained by FEM eigenvalue analysis assuming a fixed base suggests that the five peaks are the bulging modes of (m, n)=(1, 1 to 5), where m and n denote the vertical order and the circumferential wavenumber, respectively. The measured non-soil-coupled natural frequencies from the spectral ratio agree fairly well with those obtained from FEM analysis. The measured natural frequencies of the fundamental mode (m=n=1) also agree well with those projected by a simplified equation developed under the assumption of a fixed base, which is adopted in the seismic codes of the Japanese Fire Service Act. This equation should provide a reliable soil-coupled natural frequency of the fundamental mode for a tank situated on firm ground in which the storage-soil-coupled effects are presumed weak. Additionally, a simple method is presented to determine the non-soil-coupled natural frequency of the fundamental mode from the observed microtremor spectral ratios without referencing the FEM eigenvalue solutions. This simple method works very well for the tank examined.