فهرست مطالب

Theoretical and Applied Vibration and Acoustics - Volume:4 Issue:2, 2018
  • Volume:4 Issue:2, 2018
  • 100 صفحه،
  • تاریخ انتشار: 1397/04/10
  • تعداد عناوین: 6
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  • Mahya Mohammadi *, Seyed Esmail Razavi, Cyrus Aghanajafi, Hadi Khorand Pages 125-140
    Transmission loss and noise reduction of reactive mufflers are determined by linear acoustic theory and unsteady flow field study, respectively. The effects of extending the inlet tube of the muffler, adding holes to the extension, and the number of holes on both transmission loss and noise reduction are investigated. In noise reduction analysis, the Navier-Stokes equations and k-ε model are used to study the unsteady turbulent flow. Helmholtz equation is solved for transmission loss analysis. The present study is validated with experimental data. Numerical results show a rise in noise reduction by extending the inlet tube of the muffler and increasing its length. Moreover, the extended mufflers cause more transmission loss and broadband behavior at some frequencies due to the resonances. According to the results, different points of view in the investigation of acoustic attenuation performance of mufflers can be helpful to understand more and better about the effect of geometrical parameters.
    Keywords: Transmission loss, Noise reduction, Reactive muffler, k-ε model, Helmholtz equation
  • Maryam Shafiei Alavijeh, Mohammad Hossein Soorgee * Pages 141-152
    Reliable damage detection is one of the most critical tasks in composite plate structures. Ultrasonic guided waves are acknowledged as an effective way of structural health monitoring (SHM). In this research, ABAQUS FE package is employed in order to develop a 3D finite element (FE) model to investigate the wave propagating features in a four-layer Woven Glass fiber (WGF) /epoxy composite plate. Dispersion curves have been extracted using semi-analytical finite element (SAFEM) in MATLAB. An experimental study has been done to obtain the sensitivity of the excitation frequency on the delamination detection problem. The Fast Fourier Transform (FFT), Butterworth filtering and the Continuous Wavelet Transform (CWT) signal processing methods have been utilized to extract a more accurate damage sensitive feature from experimental signals.Calculations of amplitude reduction ratio (ARR) for both raw and filtered signals shows that increasing the excitation frequency, which means decreasing the wavelength, leads to increase in the ARR in an approximately linear manner for raw signals, while using the filtered signals for ARR extraction yields higher ARR, peaked at the tuned Lamb mode, which is F=330 kHz in the study. The Butterworth filtering provides larger damage sensitive feature compared to CWT method. Consequently, the ARR is a reliable and enough sensitive feature for delamination detection in composite plates, especially when extracted from filtered signals
    Keywords: Delamination Detection, Composite plate, Ultrasonic guided waves, Amplitude Reduction Ratio
  • Hassan Nahvi *, Hosein Amani, Reza Tikani Pages 153-170
    In this study, nonlinear vibration of axially moving strings in thermal environment is investigated. The vibration haracteristics of the system such as natural frequencies, time domain response and stability states are studied at different temperatures. The velocity of the axial movement is assumed to be constant with minor harmonic variations. It is presumed that the system and the environment are in thermal equilibrium. Using Hamilton’s principle, the system equation of motion, and t[1]he boundary conditions are derived and then solved by applying Multiple Time Scales (MTS) method. The effect of temperature on the vibration characteristics of the system such as linear and nonlinear natural frequencies, stability, and critical speeds is investigated. Considering ideal and non-ideal boundary conditions for the supports, nonlinear vibration of the system is discussed for three different excitation frequencies. The bifurcation diagrams for ideal and non-ideal boundary conditions are presented under the influence of temperature at various speeds.
    Keywords: Axially moving string, Nonlinear vibration, Multiple time scales method, Critical speed
  • Vahid Khalafi, Jamshid Fazilati * Pages 171-188
    A NURBS-based isogeometric finite element formulation is developed and adopted to the free vibration analysis of finite square and skew laminated plates. Variable stiffness plies are assumed due to implementation of curvilinear fiberreinforcements. It is assumed due to employment of tow placement technology, in each ply of variable stiffness composite laminated plate the fiber reinforcement orientation angle is changed linearly with respect to longitudinal geometry coordinate. The classic plate theory is utilized for structural model description. The cubic NURBS basis functions are employed to approximate the geometry of the plate while simultaneously serve as the shape functions for solution field approximation in the analysis. To show the effectiveness and accuracy of the developed formulation, some representativeresults are extracted and compared to similar items available in the literature. The effects of curvilinear fiber angles, different geometries and various end constraints are evaluated on the variable stiffness composite laminated skew panel behavior.
    Keywords: Isogeometric Analysis, curvilinear fibers laminate, skew plate, Free vibration
  • Bakhtiyar Ghanbari *, Amir Huoshang Akhavessiy Pages 189-204
    Dynamic Ratcheting (DR) is a nonlinear dynamic phenomenon occurring in hysteretic damping systems. It means the structural plastic deformation increases asymmetrically in successive cycles under an earthquake excitation. Although it is generally recognized that DR is closely related to the frequency contents of an earthquake excitation applied to thestructure, no targeted analysis has been conducted on the influence of time- varying frequency content on occurrence and development of DR. This manuscript aims to analyze the influence of time evolution of DR-inducing ground motion frequency content on developed DR phenomenon in the Single Degree-Of-Freedom (SDOF) system with the Elastic-Perfectly-Plastic (EPP) hysteretic behavior. To survey the influence of time evolution of ground motion frequencycontent on the developed DR: In the first step, the three DR-inducing ground motion records were selected as excitations input of EPP SDOF systems. In the second step, time-varying frequency of ground motions were changed by shifting theirfrequency content  forward or backward in time using wavelet transform to produce altered versions records. In the final step, the displacement responses of EPP SDOF systems under selected records and their altered versions were compared. By analyzing the displacement response of EPP SDOF systems excited by selected ground motions and their altered versions, it can be found that the time-varying frequency content considerably influences the developed DR behavior in SDOF systems. In the selection of records for dynamic analysis of structures, time-varying frequency content could be further considered as an important characteristic variable for ground motion records.
    Keywords: Dynamic Ratcheting, Time-varying frequency content, EPP SDOF system, Wavelet transform
  • Abbas Rahi *, Hamed Petoft Pages 205-222
    In this article, the size effect on the dynamic behavior of a simply supported multi-cracked microbeam is studied based on the Modified Couple Stress Theory (MCST).  At first, based on MCST, the equivalent torsional stiffness spring for every open edge crack at its location is calculated; in this regard, the Stress Intensity Factor (SIF) is also considered for all open edge cracks. Hamilton’s principle has been used in order to achieve the governing equations of motion of the system and associated boundary conditions are derived based on MCST. Then the natural frequencies of multi-cracked microbeamare analytically determined. After that, the Numerical solutions have been presented for the microbeam with two open edge cracks. Finally, the variation of the first three natural frequencies of the system is investigated versus different values of the depth and the location of two cracks and the material length scale parameter. The obtained results express that the natural frequencies of the system increase by increasing the material length scale parameter and decrease by moving away from the simply supported of the beam and node points, in addition to increasing the number of cracks and cracks depth.
    Keywords: multi-cracked, microbeam, MCST, natural frequency