Showing papers by "Yiu-Yin Lee published in 2003"

Vibration of Timoshenko Beams with Internal Hinge

Abstract: This paper is concerned with the free vibration problem of Timoshenko beams with an internal hinge Exact vibration frequencies for axially loaded, clamped-clamped beams and clamped-simply supported beams are determined The effects of axial force, transverse shear deformation, rotary inertia, and the location of the internal hinge on the fundamental frequency of vibration are investigated A necessary condition for the optimal location of internal hinge that maximizes the fundamental frequency is also presented We establish that (a) the optimal location of the hinge is at the point of inflection of an equivalent beam and (b) the maximum frequency value possible for a beam with an optimally positioned internal hinge is equal to the frequency of an equivalent beam without a hinge

Flow Angle, Temperature, and Aerodynamic Damping on Supersonic Panel Flutter Stability Boundary

Abstract: The effects of flow yaw angle, temperature, and aerodynamic damping on supersonic flutter of plates are investigated. Quasisteady, first-order piston theory is employed for formulation of aerodynamic forces. The von Karman large-deflection plate theory is adapted for the aerothermal deflection. Two types of thermal effects are considered: 1) plate expansion by uniform temperature and 2) thermal moment induced by temperature gradient across the plate thickness. A finite element modal formulation and a two-step procedure are presented for the predictions of stability boundaries and nonlinear aerothermal deflection and shown to be efficient in solution. Results have shown that flow angle has lesser effect on stability boundaries as compared with temperature for isotropic square plates. However, both flow angle and temperature have a large influence on stability boundaries for rectangular isotropic and laminated composite plates. The presence of the ripple characteristics of stability boundaries for composite plates caused by the frequency coalescence of higher modes and the smaller effect of aerodynamic damping is investigated. The stabilization effects on panel motions induced by variations of flow angle, temperature, and aerodynamic damping are discussed.

Chaotic responses of curved plate under sinusoidal loading

Abstract: In the present investigation, the nonlinear dynamic buckling of a curved plate subjected to sinusoidal loading is examined. By the theoretical analyses, a highly nonlinear snap-through motion of a clampedfreeclampedfree plate and its effect on the overall vibration response are investigated. The problem is reduced to that of a single degree of freedom system with the Rayleigh-Ritz procedure. The resulting nonlinear governing equation is solved using Runge-Kutta (RK.-4) numerical integration method. The snap-through boundaries, which vary with different damping coefficient and linear circular frequency of the flat plate are studied and given in terms of force and displacement. The relationships between static and dynamic responses at the start of a snap-through motion are also predicted. The analysis brings out various characteristic features of the phenomenon, i.e. l) small oscillation about the buckled position-softening spring type motion, 2) chaotic motion of intermittent snap-through, and 3) large oscillation of continuous snap-through motion crossing the two buckled positions-hardening spring type. The responses of buckled plate were found to be greatly affected by the snap-through motion. Therefore, better understanding of the snap-through motion is needed to predict the full dynamic response of a curved plate.

Structural Vibration Suppression Using the Piezoelectric Sensors and Actuators

Abstract: An experimental study for the active vibration control of structures subject to external excitations using piezoelectric sensors and actuators is presented. A simply supported plate and a curved panel are used as the controlled structures in two experiments, respectively. The Independent Modal Space Control (IMSC) approach is employed for the controller design. In order to increase the adaptability, the time-domain modal identification technique is incorporated into the controller to real-time update the system parameters. The adaptive effectiveness of the time-domain modal identification technique is tested by fixing an additional mass on the simply supported plate to change its structural properties. The vibration suppression performances of the controller are 5.7 dB and 10.8 dB for the simply-supported plate with/without the mass subject to a chirp sine excitation, respectively. For the experiment of the curved panel subject to a sinusoidal excitation, the vibration attenuation of the control scheme is 5.0 dB even the control circuit is subject to some noise generated by electrical and magnetic interferences.

Nonlinear Random Response of Cylindrical Panels to Acoustic Excitations Using Finite Element Modal Method

Abstract: This paper investigates large amplitude multi-mode free vibration andrandom response of thin cylindrical panels of rectangular planform usinga finite element modal formulation. A thin laminated composite doublycurved element is developed. The system equation in structural nodal DOFis transformed into the modal coordinates by the using the modes of theunderlying linear system. The nonlinear stiffness matrices are alsotransformed into nonlinear modal stiffness matrices. Numericalintegration is employed to determine free vibration and random response.Single-mode free vibration results are compared with existing classicalanalytical solutions to validate the nonlinear modal formulation.Nonlinear random analysis results for cylindrical panels have shown thatthe root mean square of panel deflections could be larger than thoseobtained using the linear structure theory. Time histories, probabilitydistribution functions, power spectral densities, and phase plane plotsare also presented.

Active vibration control of an aircraft cabin panel using piezoelectric sensors and actuators

Abstract: In this paper, the active vibration suppression of an aircraft cabin panel embedded with piezoelectric sensors and actuators under sinusoidal or random excitation is studied experimentally. The Independent Modal Space Control (IMSC) approach is employed in the controller design. The piezoelectric sensors and actuators associated with the IMSC technique have been applied to the active vibration control of the aircraft panel, and shown to be effective in vibration control. A second order controller is selected in the control scheme to suppress the fundamental modal vibration response of the aircraft cabin panel. The mode shapes of the panel are experimentally obtained, and used as the parameters of the objective functions for minimizing the unwanted vibration responses by appropriately selecting the sensor and actuator gains. Based on the experimental results, it is found that the vibration levels of the open and closed loop systems differ by up to 5.0 dB (for sinusoidal excitation) and 7.4 dB (for random excitation), even when the control circuit is interfered by electrical and magnetic noises.

Noise reduction techniques for close-fitting enclosures at structural/acoustic resonance

Abstract: In this paper, a close-fitting enclosure model is used to prove the existence of a common structural-acoustic coupling experimentally, which deteriorates the insertion loss performance but was not considered in the formulas developed by Refs. (1) - ( 3) and the classical formula used in Refs. (4) - (6). The experimental results show that an enclosure, which first anti-symmetric structural and acoustic resonant frequencies do not coincide or nearly coincide, can avoid strong structural-acoustic couplings within the air cavity. At the acoustic resonance frequencies, the insertion loss performance of an enclosure can be improved by using the aluminum fiberboard, porous aluminum, and perforated aluminum panel to absorb the sound energy. Stiffener is useful to enhance the insertion loss ability of an enclosure if the dominant frequency bands of the noise are below the first structural resonant frequency. For having good damping effect to suppress the noise radiation at structural resonant frequencies, an enclosure should be mounted with both stiffeners and damping treatments.

An Application of a New Roadside Barrier to Traffic Noise Mitigation

Abstract: In this paper, the noise reduction performance of a new roadside barrier, which is called double screen barrier, is presented and studied theoretically and experimentally. The theoretical predictions are developed and based on Macdonald's diffraction theory. A case study of traffic noise mitigation is presented to show the effectiveness of the different barriers, including vertical barrier, cantilever barrier, double screen barrier, and partial enclosure. It has been found that the double screen barrier gives the best noise reduction performance among the four mitigation measures. A 1: 4 scale model of the proposed double screen barrier has been developed. The experimental results have good agreements with the predictions, provided that absorption material must be fully filled into the trough of the V-shape panel of the double screen barrier to reduce the sound reflection, and the gap width between the V-shape panel edges must be large when compared with the wavelengths of frequencies of interest.