This research investigated unstable inner cylinder whirl in a double rotating cylinder system in which the axis of the inner cylinder is supported elastically in the radial direction and the axis of the outer cylinder is fixed. It was assumed that the two cylinders stay in parallel so that the gap between the cylinders is axially uniform. This paper presents a theory to analyze the complex eigenvalues of the coupled system of the gap flow and the motion of the inner cylinder. The gap flow model takes into account the inertial force and turbulence effect, and derives an equation for the unsteady pressure. The motion of the inner cylinder has two degrees of freedom in the plane normal to the axis, and is coupled with the unsteady pressure of the gap. This theory was compared with experiments carried out by changing the combination of rotational speeds of the inner and outer cylinders, including counter-rotation where two cylinders rotate in opposite directions. The theoretical and experimental results were generally consistent in terms of the whirl frequency, damping ratio, and the boundary of the unstable whirl at the combinations of rotational speeds. In particular, both the theory and experiments indicated that the instability of the whirl increases as the sum of the rotational speeds of the two cylinders rises.

An investigation is described of the hydroelastically coupled motion of a cylindrical tank partially filled with liquid in response to travelling seismic wave excitation. The continuous phase variation of the vertical excitation along the base causes flexible tank wall vibration modes with positive circumferential wave numbers. The stress analysis for this crucial case is conducted in this paper. An analytical explanation of the wave velocity dependences of the circumferential wave components of the stress cannot readily be given because the sinusoidal excitation term has a trigonometric function in its argument. To enable such an explanation, the travelling seismic wave is expressed in terms of the Bessel function. A stochastic response analysis is also conducted. Solving moment equations becomes difficult because the determination of their excitation terms requires a quadruple integral over the Duhamel time-integral interval for each present time. To solve this problem, the exponential term in the auto-correlation function of the excitation is expressed by a Fourier series.

Operating cranes is challenging because payloads can experience large and dangerous oscillations. One of the solutions to reduce the sway oscillation of the crane payload is to use a passive damper. This method is simple but has limited effectiveness because without adaptability and flexibility to prevent adverse movements and amplify the favorable ones. This paper proposes an on-off damping controller for the semi-active damper to improve the passive damper. The on-off damping control aims to amplify the radial motion of the damper to increase the energy dissipated, then reduce the sway vibration. The effectiveness of the proposed controller is verified by numerical simulations of a 2D crane.

This paper discusses a new identification method for a linear single-degree-of-freedom system that uses a Gaussian random response and is based on the maximum likelihood estimation (MLE) method. The likelihood function of the proposed method consists of the analytical solution of the Fokker–Planck equation. We have already published a paper on theoretical and numerical considerations. However, in that study, the experimental verification of the proposed identification method was not performed. Therefore, in this study, we conduct an experimental verification of the proposed identification method. First, the identification algorithm is formulated in a spring-mass-damper system subjected to white noise excitation by a moving foundation to correspond to the actual experimental setup. A preliminary experiment in terms of the excitation source is conducted using a vibration speaker. In addition, the experimental modal analysis is performed to confirm the validity of the vibratory system. The fundamental operation test of the identification method is performed using the actual experimental random response data, and a dependency survey of the number of samples is conducted. From the results, the convergence behaviors of the estimation value are observed with an increasing number of samples in the spring constant and the ratio between the diffusion coefficient and the damping constant. In addition, benchmark tests are conducted using the half–power method (HPM) based on spectral analysis and the auto-regressive method (ARM) based on time–series analysis. In the case of spring constant estimation, the behaviors of the estimation value that converge to the true value are observed in all identification methods. In the ratio between the diffusion coefficient and damping constant, the behavior of the estimation value that converges to the true value is observed only in the proposed identification method.

The vibration-induced flow (VIF), in which a mean flow is induced by the interaction between the system vibration and micro-structures, has been studied as a fluid/particle micro-manipulation method that does not require an external pump. While the use of VIF with a wide variety of vibrations is expected to realize sophisticated fluid manipulation, numerical tools to predict these unsteady flows remain difficult. In this study, we have performed a numerical simulation of VIF with different vibrations and micropillar cross-sections. A proposed numerical model, which directly solves the continuity and Navier-Stokes equations in the coordinate system moving with the vibrating micropillar, enables us to avoid the introduction of a moving boundary, and therefore has a significant advantage in numerical stability and accuracy. The immersed boundary technique allows us to embed arbitrary complex micro-structures in the Cartesian computational domain without requiring boundary-fitted meshes for each geometry. The dependencies of characteristics of flow on vibration parameters, such as vibration frequency, amplitude, direction, and the shape of micro-structures, were investigated and compared with the experimental results obtained by the particle image velocimetry (PIV) measurement. Excellent agreement between the numerical and experimental results validates that the present numerical approach can be a powerful tool to design functional VIF systems, such as mixing, particle/cell transport, trapping, and separation.

Micro-pressure wave emission from tunnel portals in high-speed railways causes environmental issues. Tunnel hoods installed in the tunnel portals reduce the micro-pressure wave emission by reducing the pressure gradient of the compression wavefront generated in the tunnels. This study investigated the influence of the hood cross-sectional area on reducing the pressure gradient of the compression wavefront under the conditions in the current Shinkansen specifications, assuming that train running speeds would increase in the future. Scale model experiments were conducted using axisymmetric cross-section models of the tunnel, hoods, and trains; the high-speed train model was launched into the tunnel and hood models. The effect of the hood on the reduction of the micro-pressure wave was estimated by measuring the pressure gradient of the compression wavefront generated in the tunnel model. The experimental results show that the effect of the hoods depends on their length and cross-sectional area, even if there are apertures on the sidewall of the hoods. Furthermore, when there are no side apertures on the hoods and the cross-section is longitudinally uniform, the optimum hood cross-sectional area at which the maximum pressure gradient of the compression wavefront is the lowest depends on the train speed and nose length. The findings of this study will provide useful insights into the structural design of tunnel hoods in high-speed railways.

Spacecraft are exposed to acoustic loads during flights to space by launch vehicles. The endurance of spacecraft structure is verified by a ground acoustic test specified in test standard documents of the aerospace community. These standard documents require the execution of ground acoustic tests in a reverberant chamber on the assumption that it can generate a diffuse sound field which is normally defined as “completely isotropic and equal probability of energy flow in all directions”. However, in the space development industry and related studies, the directions of incident sounds which cause non-diffuseness has not been identified by the experimental comparison with the theoretical values of an ideal diffuse sound field (e.g. reverberation times, homogeneity of sound pressures and spatial correlation functions). Therefore, the conventional standard documents do not clearly specify the quantitative requirements regarding the directional properties of sound fields inside reverberant chambers. To specify the quantitative requirement on the directional properties of sounds, this paper focused on calculating the degree of isotropy of a sound field by the direct measurement of directional properties of sound waves as angular spectra. In this paper, the method to obtain the angular spectrum based on the theory of the expansion of the plane wave into spherical harmonics by using a spherical microphone array was firstly combined with the existing formula of the isotropy indicator, which allowed us to determine the frequency range when we apply the combined method to sound fields. The method to determine the applicable frequency range was demonstrated and the process of obtaining the angular spectrum and isotropy indicator was verified by numerical simulation. Finally, the combined method was applied to the sound field in the reverberant chamber of JAXA 1600m³ acoustic test facility.