Journal of Sound and Vibration最新文献

This paper characterizes the linear flow-acoustic impedance of circular sub-millimeter orifices exposed to a grazing and grazing-bias flow configuration using a multi-microphone three-port measurement framework. Measurements are performed for fully developed turbulent flows and the static aerodynamic pressure difference over the perforated plate is monitored and controlled. By counteracting any static pressure difference over the perforations, the effect of a purely grazing flow is studied and quantified using existing scaling laws. The increase in perforation resistance is observed to scale with Mach number, whereas a frequency dependent skin friction velocity correlation is seen to better fit the decreasing trend in equivalent length. However, for both impedance variables, the coefficients of the empirical macro-perforated models are revisited to match the measured data for sub-millimeter dimensions. In a grazing-bias flow configuration, the bias flow component is seen to dominate the perforation’s acoustic behavior for pressure differences larger than $100\text{Pa}$. Furthermore, a pressure difference as low as $25\text{Pa}$ is observed to already significantly increase the resistance and decrease the equivalent length compared to the purely grazing flow situation and results in a different frequency dependency with respect to the orientation of the bias flow component. A such, the pressure difference over the perforations also becomes an important parameter to monitor in grazing flow measurements, especially for perforated plates with sub-millimeter dimensions and low percentage of open area.

The current multi-shaker random control methods concentrate on the simulation of stationary vibrations. This work presents an innovative technique for multi-shaker non-stationary random vibration environmental control test. Firstly, an amplitude modulation method is introduced to generate multi-input non-stationary random signals with time-varying root mean square values, whose kurtoses and moving RMS values are specified by amplitude modulation functions. Then, the generated signals are applied as excitation signals for the non-stationary random vibration environmental control test. The response signals of a linear vibration system under non-stationary excitations are derived in the frequency domain. The derived formulas for kurtoses and moving RMS values are also applicable to non-stationary response signals. Meanwhile, the stationary power spectral densities of response signals are put forward. When the amplitude modulation functions are zero mean Gaussian distributions, the power spectral densities of the response signals can be directly calculated. Ultimately, the control of kurtoses, moving RMS values and stationary power spectral densities of response non-stationary random signals is realized. Simulation and experimental examples both adopt a two-input two-output beam vibration system and the results prove the correctness and practicability of the proposed method. The presented control procedure is concise and efficient in simulating multi-shaker non-stationary random vibration environments.

A vibration-based local damage identification method is proposed for truss-like structures with breathing cracks. Firstly, a bilinear model is established to represent the characteristics of breathing cracks, and the features of such cracks are detected through reciprocity and homogeneity tests on structural input-output signals. Since the crack is influenced by tensional and compressive forces on truss members, only one damage factor is required to quantify the stiffness reduction of each member. Subsequently, a dynamic equilibrium equation concerning the damage factor is constructed to identify the new load term. The integral equation method, which relies on the virtual work principle, is employed to obtain the displacement responses and loads under all tensile conditions. Finally, the mean value of cumulative damage coefficients for each truss member can be derived as their corresponding damage factor. Numerical examples involving a plane truss structure and a truss offshore platform are utilised to illustrate the effectiveness of this damage identification method. The results demonstrate exceptional precision and efficiency in accurately identifying various damages in truss structures under different excitations and signal-to-noise ratios. A actual space truss structure confirm that this damage identification method exhibits high-level precision in identifying single-member damages as well as multi-member damages with breathing cracks locally.

This paper presents an analytical study to predict the effect of noise in a thermoacoustic system. The starting point of our analysis is the acoustic analogy equation with two source terms: one represents the fluctuating heat release rate, and the other one represents the noise. The heat release rate is nonlinear and modelled by a generalised-law with amplitude-dependent time-lag and coupling coefficients. A Green’s function approach is used to convert the acoustic analogy equation (a PDE) into an integral equation. An essential element in this approach is the tailored Green’s function of the combustion chamber. We calculate this analytically for a 1-D combustion chamber with general end conditions and a non-uniform mean temperature. The integral equation is then used for predictions in the time-domain and frequency-domain. We focus on the following three phenomena: transient oscillations, noise-induced triggering, and hysteresis. Our predictions are in line with experimental observations reported in earlier studies: (1) Noise speeds up the time it takes a transient oscillation with growing amplitude to reach its limit cycle. (2) Noise can launch a linearly stable system into an unstable state. (3) Noise reduces the width of a hysteresis zone. Both white noise and pink noise are considered; pink noise is found to be more effective.

The paper investigates the flow-induced vibration and sound wave propagation of a rotationally oscillating circular cylinder resting on a nonlinear elastic support and subject to compressible viscous flow with Reynolds number of $Re=150$ and Mach number of $Ma=0.2$. An implicitly coupled fluid-structure interaction method based on an arbitrary Lagrangian-Eulerian framework is adopted to predict the dynamic responses of the cylinder. Direct numerical simulations of Navier-Stokes equations are performed to resolve the unsteady flow and sound waves. A nonlinear forced synchronization phenomenon, referred to as ‘lock-on’, occurring between nonlinear vortex-induced vibration and rotational excitation of the cylinder is examined. Two synchronization regions are identified, the primary lock-on and the tertiary lock-on. It is found that the nonlinear elastic mount significantly affects the synchronous patterns of the cylinder by amplifying higher-order harmonic components of the vibration response of the cylinder and altering the separating bubbles in the wake. Moreover, large rotational excitation of the cylinder delays the boundary layer separation, altering the shedding vortex patterns. Asynchronization between the rotational excitation and the vibration of the cylinder modulates the sound waves, while the synchronization produces dipole sound propagation modes containing high-order harmonic wave components.

Evaluating the hydroelastic responses of underwater cementitious structural elements is critical for ensuring the sustainability and durability of energy-saving marine infrastructures. Existing work on the hydroelastic analysis of porous structures has been mostly developed using the general elastic constitutive relation; however, it fails to capture the influence of saturation. To fill this knowledge gap, we for the first time propose a novel fluid-porous structure interactive model that incorporates the combined effects of hydrodynamic pressure and saturation-induced pore pressure. One more pioneering effort is to solve this nonlinear hydroelastic problem by introducing peridynamic differential operator (PDDO). It is worth noting that the introduction of PDDO removes the inherent drawback employing the local-theory based techniques, namely being prone to singularities arising from the presence of discontinuity. The accuracy and reliability of the proposed numerical framework are validated by comparing the results with the degraded model in the reported literature. Moreover, our results highlight that the angular frequencies are underestimated when ignoring the effect of saturation in foamed concrete beams. The presented method provides a profound understanding of the underwater structural dynamic monitoring that benefits the design of marine infrastructures.

Rotating machinery shafting is typically comprised of multiple components that are assembled to ensure reliable operation, proper alignment minimal vibration. However, conventional rotordynamics approximates the shafting as a single, continuous member, neglecting contact interfaces between the components. The presence of an interface can induce microslip, which generates internal friction that may cause instability and machinery failure. A novel approach of modeling the interface viscous damping effect on rotordynamics is proposed by combining a GW (Greenwood and Williamson) contact model with the Yoshimura damping model. All structural components are modeled using 3D solid finite elements. Modal damping ratio is utilized to identify the instability onset speed (IOS). The results show that internal friction has a destabilizing effect on whirl motion above the first critical speed, but has a stabilizing effect on the motion below the first critical speed. The destabilizing effect can be reduced by increasing the bearing damping, however excessive bearing damping can drive the effective damping towards negative values. Increasing the number of interfaces reduces stability while increasing an interface preload prevents microslip, and increases stability. Lastly, smoother surfaces at the interfaces increase the IOS.

This paper presents an absolute band gap (BG) by overlapping coupled band gaps based on frequency locking mechanism within a wavy structure. We first introduce coupled BG’s trigger conditions, including weak coupling, and then utilize two coupling in a wavy structure to lock three wave modes into two overlapped coupled BGs. A wavy specimen is determined for experimental validation, by selecting the geometric parameters based on their individual effects on the coupled BG. Four testing loads with the capability to excite all guided waves are applied numerically and experimentally to verify the performance of the absolute BG. The response signals validate the existence of a wide attenuation band introduced by the absolute BG. The results exhibit a great potential for utilizing the frequency locking mechanism to construct an absolute BG for applications such as vibration control.

In this manuscript, the temporal rainbow effect for surface acoustic waves (SAWs) is illustrated through a temporal analog of space metagradings. We show that a time-modulated array of mechanical resonators induces a wavenumber-preserving frequency transformation which, in turn, dictates Rayleigh-to-Shear wave conversion. The process is unfolded through the adiabatic theorem, which allows us to delineate the transition between a solely frequency-converted wave packet and a temporally-driven mode conversion. In other words, our paper explores the role of time modulation in the context of elastic metasurfaces, and we envision our implementation to be suitable for designing a new family of SAW devices with frequency conversion, mode conversion, and unusual transport capabilities.

A hybrid approach composed of the radiative energy transfer method (RETM) and image source method (ISM) is proposed in this study for estimating the vibrational energy of coupled plates loaded by high-frequency point excitation. The vibrational energy at a receiver is represented by energy density and intensity, which are superposed by incoherent rays emitted by the load point in the analyzed domain and reflected/transmitted by the domain boundaries. The energy rays reflected/transmitted at the boundaries are described by two modes: diffuse reflection/transmission, which is a basic assumption in RETM, and the other obeys Snell’s law of reflection/refraction, which is deduced by Fermat’s principle and often applied in ISM. The local energy response distribution in the loading subsystem is described by ISM. The energies in other subsystems are described by RETM. The energy transfer relationship between subsystems is expressed by the energy transfer coefficient. At the boundary of coupled plates, a Fredholm equation of the second type is established through the balance among the outgoing energy of the diffuse reflection fictitious sources and the incident energy of actual sources, other diffuse reflection sources, and specular reflection image sources, which can be used to determine the intensity of the diffuse reflection fictitious sources. The average energy transfer coefficient in the transmission direction half-space is used to express the energy transfer relationship associated with diffuse reflection sources, while the energy transfer relationship associated with specular reflection sources is expressed directly by the energy transfer coefficient. Numerical tests show that the energy distributions and energy flow fields of typical coupled plates are well predicted by the proposed hybrid RETM-ISM approach. Compared with RETM, hybrid RETM-ISM is more consistent with the FEM solution.