Journal of Sound and Vibration最新文献

Rotating blades operating under high-temperature experience intense mechanical and thermal stresses induced by centrifugal forces and thermal shock. In this research, to address this critical yet understudied topic, coupled thermo-elastic dynamics of rotating blades incorporating precise rotational effects is investigated. A comprehensive modeling approach, characterizing blade geometry in terms of pre-twist and pre-set angles is employed based on the theory of surfaces. Thermal strain–displacement field is defined based on third-order shear deformation theory in the model integrated with a third-order expansion of temperature change distribution within the blade. Rotational factors related to Coriolis, centrifugal stiffening, and rotational softening effects are considered. In the case of stiffening effects, two different modeling approaches including direct integration of centrifugal forces (DICFs), and pre-stressed dynamics about steady-state equilibrium deformations (SSEDs) are employed. When incorporating large-amplitude SSEDs in the latter, nonlinear rotational effects are incorporated into the model. To overcome the complexity arising from coupled thermo-elasticity, and rotational motion, the spectral Chebyshev technique is applied to the derived integral boundary value problems and coupled energy equations. Finally, natural frequencies, and thermal and centrifugal deformations are investigated comprehensively by including DICFs and pre-stressed dynamics. Results show that thermo-elastic dynamics of the system is highly affected by the modeling approaches used for centrifugal stiffening effects.

Thermoacoustic instabilities arise from the feedback between an acoustic field and the unsteady heat released in a burner, yielding self-sustained oscillations. A fundamental framework for modelling thermoacoustic instabilities in systems where a mean flow is present is introduced, based on the definition of the adjoint Green’s function which permits to convert the acoustic analogy equation into an integral equation. The adjoint Green’s problem produces sensitivity functions which quantify the response of the system to initial, boundary or other forcing terms. A simple one-dimensional system is examined; it includes a steady uniform mean flow and a nonlinear heat source with an amplitude-dependent time-delay heat release model. The versatility of the approach is demonstrated by applying it to two resonators characterized by different acoustic boundary conditions: a Rijke tube and a quarter-wave resonator. The control parameters are: heat source position, heater power and tube length. The results reveal that the proposed analytical framework successfully captures the limit cycles, triggering phenomena, hystereses, and Hopf bifurcations observed in experiments. We show that the mean flow velocity cannot be discarded in the study of such systems; by increasing it, a stabilization generally ensues, with a modification of the bistability characteristics of the system.

It is well documented that the static and dynamic performance of foil air bearings (FABs) can be improved through the alteration of the radial clearance profile. One way of achieving such controllability is through piezoelectric actuation of the top foil. Such a piezoelectric foil air bearing (PFAB) avoids the dynamics complications introduced by alternative actuating mechanisms. So far, theoretical analysis of a PFAB pad in isolation has shown that top foil actuation can raise the static load capacity. This paper theoretically investigates the potential for improving the dynamic performance. A computational approach that uses Galerkin Reduction (GR) to model the air film and considers the detachment of the top foil from the bump foil is upgraded for multi-pad PFABs, thus facilitating parameter optimization. The investigation considers two conventional FAB rotor systems from the literature that exhibit strong sub-synchronous vibrations under high unbalance. It is found that the optimal voltage and placement of piezoelectric patches can result in total suppression of the strong sub-synchronous frequencies, as well as a rise in the onset of instability speed. The polarity of the voltage required for suppression of the sub-synchronous frequencies is found to be opposite to that required to raise the load capacity.

Vibrations generated by mechanical equipment are the main source of radiation noise for ships. In actual systems, there are multiple transmission paths for mechanical vibrations from the equipment to shell. Common paths include vertical support structures, such as vibration isolators, and lateral attachment structures, such as pipes. However, most existing models only consider a single-path scenario. Multi-path systems have more complex modal and vibration transmission characteristics and introduce more complex coupling problems. There is a lack of theoretical modeling and physical analyses of common multiple transmission systems in ships. In this study, a theoretical model of an equipment–shell multiple-transmission-path system with an attached liquid-filled pipe is established as a typical multiple-transmission-path system for ships. In this model, the liquid-filled pipe is solved using a newly proposed traveling-wave method based on Kennard shell theory. Based on the theoretical model, the effects of each type of traveling wave on the pipe on the system modal characteristics, the mechanism behind each path on the system coupling characteristics, the influence of structural parameters on system modal and vibration transmission characteristics, and the proportion of vibration transmitted by the two transmission paths are analyzed. This study provides theoretical insights and guidance for practical vibration and noise reduction engineering.

This study investigates the effects of serration angle on the three-dimensional flow field and noise reduction mechanism of a NACA6512-63 airfoil using large eddy simulation (LES) and spectral proper orthogonal decomposition (SPOD) techniques. The serration angle is defined as the ratio of serration wavelength ($\lambda$) to serration amplitude (h). The mean flow field analysis on suction side of serration reveals the outward flow towards the serration edges and the downwash flow between the valleys that generate the counter-rotating vortex pair. This flow alteration changes the serration effective angle of the serration, consequently affecting the noise reduction performance. Additionally, by comparing the general solution of Lyu’s semi-analytical equation with the SPOD mode results, the noise reduction mechanism is examined, and the frequency range in which noise reduction occurs is analyzed. The linearly distributed multipole-like pressure patterns, similar to those in the semi-analytical solution, are observed in the low-frequency range for all three cases, with the $\lambda$/h = 0.3 case exhibiting the most effective noise reduction. The findings suggest that the local deviation of the turbulence axis, caused by the strong outward flow near the serration edges, can hinder the formation of the optimal pressure field distribution required for effective noise reduction.

In order to break through the limitation of the "single-point" design of Quasi-Zero Stiffness (QZS) property and satisfy multiple performance objectives, this study proposes a novel bio-inspired vibration isolator integrating the biological leg configuration and flexible joints, and conducts nonlinear analysis and experimental verification. Bio-inspired by the frog legs, the proposed isolator is a multilink structure consisting of two symmetrical bionic legs with long links (as bones) and scissor-like units (as joints). It is mounted on the lower level of the floating slab isolation track, which is simplified as a two-degree-of-freedom isolation system. For the actual requirement in the floating slab track, three performance indexes, including load carrying capacity, dynamic stiffness, and stability of isolation performance, are considered in structural optimization. Based on these optimization objectives, we carry out the multi-objective Pareto design criterion to achieve a collaborative optimization of the mechanical properties. Additionally, the experimental prototypes and low-frequency isolation experiments are carried out to verify the effectiveness of the multi-objective Pareto optimization framework. This bio-inspired isolator can effectively coordinate large loads and low-frequency vibration isolation problems and has promising applications in the field of rail transportation.

Many media (such as geological formations or osseous tissues) are composites made up of at least two basic materials which can themselves obey complex constitutive laws. Such composites exhibit a wide range of mechanical properties that are essential to estimate using relatively simplified formulas. Here, wave propagation in a random heterogeneous medium (the composite) consisting of a distribution of parallel poroelastic cylinders in a poroelastic matrix is considered. The cylinders and the matrix are Biot’s porous media saturated with fluid. Expressions for the three effective wavenumbers (fast, slow and shear) are provided in the low frequency range using the Conoir and Norris multiple scattering formula accounting for wave conversions, up to the order of $n_0^2$ ($n_0$ is the number of cylinders per unit area). Basing upon this effective wavenumbers, quantities such as mass densities, bulk and shear poroelastic moduli, and diffusion coefficient are estimated at the static limit.

The computational cost of predicting sound radiated from rotating sources is mainly affected by three parameters, i.e., the number of frequencies, source points and field points. The key of reducing the prediction cost of sound radiated from rotating sources is to reduce the times of integral calculation invoked in multi-frequency and combination of multi-source-field points. In view of the above engineering background, this paper employs a frequency-domain integral prediction method combined with a Kriging interpolation model to accelerate acoustic prediction. Three numerical cases are performed to validate the effectiveness of the above method in accelerating prediction of the frequency spectrum and acoustic field around rotating sources. The research also provides recommendations on the distribution characteristics of the initial sampling points and the method of adding sampling points.

Considering stiffness mistuning, position mistuning, and arc-length mistuning, a unified modeling approach is presented for the first time to investigate the traveling wave vibration of double thin-walled cylindrical shells coupled structure with multi-ring hard coating in a rotation state. Initially, a discontinuous arc connection is constructed to simulate a fault of bolt connection mistuning by improving the whole-circumference continuous artificial spring method. The elastic boundary conditions are simulated by using the artificial spring method. Meanwhile, a hard coating multi-ring strategy is more suitable for vibration suppression compared to the full coating strategy in practical applications, so the parametric multi-partitioning approach is utilized to model the multi-ring hard coating. Then, the dynamical equation is derived according to Sanders' shell theory and the Rayleigh-Ritz method. Finally, an efficient state space method is adopted to solve the traveling wave frequencies, and the validity of the theoretical method is verified through the literature. Furthermore, the influences of the mistuning Model, mistuning degree, and mistuning number on the traveling wave frequencies are analyzed. The analysis reveals that the influences of mistuning Models on the traveling wave frequencies are significantly different among the three mistuning types. The increase in mistuning degree and mistuning number leads to a decrease in traveling wave frequency, and the decrease range in the forward traveling wave frequency is greater compared to the backward traveling wave frequency.

Gearboxes are widely used for effective transmission of power and motion. With continuous running of gear systems, faults such as chipped tooth, broken tooth, etc. are developed and further lead to their breakdown. An economical solution to analyze health of the gear system with different geometrical parameters and operating conditions is to develop an analytical/multibody dynamic model. In the present work, experiments are performed using gear fault simulator where prefabricated fault is given to the pinion of the gear system. Two different models of bevel gear system (8-DOF nonlinear dynamic model and a multibody dynamic model) are developed to compare the usefulness of the model with experiments and the model can incorporate change in operating conditions to know the health of the system. This article gives a path to practicing engineers to develop more reliable and robust gear systems and to identify incipient fault in the gear systems.