Advances in Acoustics and Vibration The latest articles from Hindawi Publishing Corporation © 2014 , Hindawi Publishing Corporation . All rights reserved. Surface Wave Propagation in a Microstretch Thermoelastic Diffusion Material under an Inviscid Liquid Layer Mon, 04 Aug 2014 09:58:10 +0000 The present investigation deals with the propagation of Rayleigh type surface waves in an isotropic microstretch thermoelastic diffusion solid half space under a layer of inviscid liquid. The secular equation for surface waves in compact form is derived after developing the mathematical model. The dispersion curves giving the phase velocity and attenuation coefficients with wave number are plotted graphically to depict the effect of an imperfect boundary alongwith the relaxation times in a microstretch thermoelastic diffusion solid half space under a homogeneous inviscid liquid layer for thermally insulated, impermeable boundaries and isothermal, isoconcentrated boundaries, respectively. In addition, normal velocity component is also plotted in the liquid layer. Several cases of interest under different conditions are also deduced and discussed. Rajneesh Kumar, Sanjeev Ahuja, and S. K. Garg Copyright © 2014 Rajneesh Kumar et al. All rights reserved. In Situ Measurement of Discomfort Curves for Seated Subjects in a Car on the Four-Post Rig Mon, 04 Aug 2014 06:45:04 +0000 The aim of this study is to measure and quantify perceived intensity of discomfort due to vibration in a vehicle in situ considering complete vehicle dynamic behaviour. The shaker table based discomfort curves or the road test results may not accurately and universally indicate the true level of human discomfort in a vehicle. A new experimental method, using a seated human in a car on the four-post rig simulator, is proposed to quantify discomfort. The intensity of perception to vibration decreased with decreasing input and increasing frequency; the rate of change is different from the published literature; the difference is large for angular modes of inputs. Vehicle dynamic response is used to inform and analyse the results. The repeatability of the method and the fact that they are in situ measurements may eventually help reduce reliance on the road tests. Furthermore, discomfort curves obtained, subsequently, can be used in predictive models. T. Ibicek and A. N. Thite Copyright © 2014 T. Ibicek and A. N. Thite. All rights reserved. Prediction of Vibrational Behavior of Grid-Stiffened Cylindrical Shells Thu, 10 Jul 2014 14:16:02 +0000 A unified analytical approach is applied to investigate the vibrational behavior of grid-stiffened cylindrical shells with different boundary conditions. A smeared method is employed to superimpose the stiffness contribution of the stiffeners with those of shell in order to obtain the equivalent stiffness parameters of the whole panel. Theoretical formulation is established based on Sanders’ thin shell theory. The modal forms are assumed to have the axial dependency in the form of Fourier series whose derivatives are legitimized using Stoke's transformation. A 3D finite element model is also built using ABAQUS software which takes into consideration the exact geometric configuration of the stiffeners and the shell. The achievements from the two types of analyses are compared with each other and good agreement has been obtained. The Influences of variations in shell geometrical parameters, boundary condition, and changes in the cross stiffeners angle on the natural frequencies are studied. The results obtained are novel and can be used as a benchmark for further studies. The simplicity and the capability of the present method are also discussed. G. H. Rahimi, M. Hemmatnezhad, and R. Ansari Copyright © 2014 G. H. Rahimi et al. All rights reserved. A Mathematical Images Group Model to Estimate the Sound Level in a Close-Fitting Enclosure Thu, 12 Jun 2014 09:33:29 +0000 This paper describes a special mathematical images model to determine the sound level inside a close-fitting sound enclosure. Such an enclosure is defined as the internal air volume defined by a machine vibration noise source at one wall and a parallel reflecting wall located very close to it and acts as the outside radiating wall of the enclosure. Four smaller surfaces define a parallelepiped for the volume. The main reverberation group is between the two large parallel planes. Viewed as a discrete line-type source, the main group is extended as additional discrete line-type source image groups due to reflections from the four smaller surfaces. The images group approach provides a convergent solution for the case where hard reflective surfaces are modeled with absorption coefficients equal to zero. Numerical examples are used to calculate the sound pressure level incident on the outside wall and the effect of adding high absorption to the front wall. This is compared to the result from the general large room diffuse reverberant field enclosure formula for several hard wall absorption coefficients and distances between machine and front wall. The images group method is shown to have low sensitivity to hard wall absorption coefficient value and presents a method where zero sound absorption for hard surfaces can be used rather than an initial hard surface sound absorption estimate or measurement to predict the internal sound levels the effect of adding absorption. Michael J. Panza Copyright © 2014 Michael J. Panza. All rights reserved. Effect of Velocity and Time-Step on the Continuity of a Discrete Moving Sound Image Thu, 05 Jun 2014 07:46:47 +0000 As a basic study into 3-D audio display systems, this paper reports the conditions of moving sound image velocity and time-step where a discrete moving sound image is perceived as continuous motion. In this study, the discrete moving sound image was presented through headphones and ran along the ear-axis. The experiments tested the continuity of a discrete moving sound image using various conditions of velocity (0.25, 0.5, 0.75, 1, 2, 3, and 4 m/s) and time-step (0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, and 0.14 s). As a result, the following were required in order to present the discrete moving sound image as continuous movement. (1) The 3-D audio display system was required to complete the sound image presentation process, including head tracking and HRTF simulation, in a time shorter than 0.02 s, in order to present sound image movement at all velocities. (2) A processing time longer than 0.1 s was not acceptable. (3) If the 3-D audio display system only presented very slow movement (less than about 0.5 m/s), processing times ranging from 0.04 s to 0.06 s were still acceptable. Yoshikazu Seki Copyright © 2014 Yoshikazu Seki. All rights reserved. Subband DCT and EMD Based Hybrid Soft Thresholding for Speech Enhancement Tue, 20 May 2014 05:19:01 +0000 This paper presents a two-stage soft thresholding algorithm based on discrete cosine transform (DCT) and empirical mode decomposition (EMD). In the first stage, noisy speech is decomposed into eight frequency bands and a specific noise variance is calculated for each one. Based on this variance, each band is denoised using soft thresholding in DCT domain. The remaining noise is eliminated in the second stage through a time domain soft thresholding strategy adapted to the intrinsic mode functions (IMFs) derived by applying EMD on the signal obtained from the first stage processing. Significantly better SNR improvement and perceptual speech quality results for different noise types prove the superiority of the proposed algorithm over recently reported techniques. Erhan Deger, Md. Khademul Islam Molla, Keikichi Hirose, Nobuaki Minematsu, and Md. Kamrul Hasan Copyright © 2014 Erhan Deger et al. All rights reserved. Experimental Study for Vibration Behaviors of Locally Defective Deep Groove Ball Bearings under Dynamic Radial Load Sun, 18 May 2014 12:40:23 +0000 Rolling element bearings are used in many mechanical systems at the revolute joints for sustaining the dynamic loads. Thus, the reliable and efficient functioning of such systems critically depends on the good health of the employed rolling bearings. Hence, health monitoring of rolling bearings through their vibration responses is a vital issue. In this paper, an experimental investigation has been reported related to the vibration behaviours of healthy and locally defective deep groove ball bearings operating under dynamic radial load. The dynamic load on the test bearings has been applied using an electromechanical shaker. The vibration spectra of the healthy and defective deep groove ball bearings in time and frequency domains have been compared and discussed. Overall vibration increases in presence of local defects and dynamic radial load. V. N. Patel, N. Tandon, and R. K. Pandey Copyright © 2014 V. N. Patel et al. All rights reserved. Vibration Analysis of Hollow Tapered Shaft Rotor Mon, 28 Apr 2014 07:17:40 +0000 Shafts or circular cross-section beams are important parts of rotating systems and their geometries play important role in rotor dynamics. Hollow tapered shaft rotors with uniform thickness and uniform bore are considered. Critical speeds or whirling frequency conditions are computed using transfer matrix method and then the results were compared using finite element method. For particular shaft lengths and rotating speeds, response of the hollow tapered shaft-rotor system is determined for the establishment of dynamic characteristics. Nonrotating conditions are also considered and results obtained are plotted. P. M. G. Bashir Asdaque and R. K. Behera Copyright © 2014 P. M. G. Bashir Asdaque and R. K. Behera. All rights reserved. Normal Incidence of Sound Transmission Loss from Perforated Plates with Micro and Macro Size Holes Thu, 17 Apr 2014 11:38:19 +0000 This paper studies the sound transmission loss of perforated panels and investigates the effect of the hole diameter on the sound insulation performance under normal incidence of acoustic loading. The hole diameters are distinguished into micro (submillimeter) and macro (millimeter) sizes. In general, the transmission loss reduces as the perforation ratio is increased. However, by retaining the perforation ratio, it is found that the transmission loss increases as the hole diameter is reduced for a perforate with micro holes due to the effect of resistive part in the hole impedance, which is contrary to the results for those with the macro holes. Both show similar trend at high frequency where the fluid behavior inside the hole is inertial. Simple analytical formulae for engineering purpose are provided. Validation of the models with measurement data also gives good agreement. A. Putra and A. Y. Ismail Copyright © 2014 A. Putra and A. Y. Ismail. All rights reserved. Monitoring Machines by Using a Hybrid Method Combining MED, EMD, and TKEO Thu, 20 Mar 2014 10:06:03 +0000 Amplitude demodulation is a key for diagnosing bearing faults. The quality of the demodulation determines the efficiency of the spectrum analysis in detecting the defect. A signal analysis technique based on minimum entropy deconvolution (MED), empirical mode decomposition (EMD), and Teager Kaiser energy operator (TKEO) is presented. The proposed method consists in enhancing the signal by using MED, decomposing the signal in intrinsic mode functions (IMFs) and selects only the IMF which presents the highest correlation coefficient with the original signal. In this study the first IMF1 was automatically selected, since it represents the contribution of high frequencies which are first excited at the early stages of degradation. After that, TKEO is used to track the modulation energy. The spectrum is applied to the instantaneous amplitude. Therefore, the character of the bearing faults can be recognized according to the envelope spectrum. The simulation and experimental results show that an envelope spectrum analysis based on MED-EMD and TKEO provides a reliable signal analysis tool. The experimental application has been developed on acoustic emission and vibration signals recorded for bearing fault detection. Mourad Kedadouche, Marc Thomas, and Antoine Tahan Copyright © 2014 Mourad Kedadouche et al. All rights reserved. Vibrational Suspension of Light Sphere in a Tilted Rotating Cylinder with Liquid Sun, 16 Mar 2014 09:10:38 +0000 The dynamics of a light sphere in a quickly rotating inclined cylinder filled with liquid under transversal vibrations is experimentally investigated. Due to inertial oscillations of the sphere relative to the cavity, its rotation velocity differs from the cavity one. The intensification of the lagging motion of a sphere and the excitation of the outstripping differential rotation are possible under vibrations. It occurs in the resonant areas where the frequency of vibrations coincides with the fundamental frequency of the system. The position of the sphere in the center of the cylinder could be unstable. Different velocities of the sphere are matched with its various quasistationary positions on the axis of rotating cavity. In tilted rotating cylinder, the axial component of the gravity force appears; however, the light sphere does not float to the upper end wall but gets the stable position at a definite distance from it. It makes possible to provide a vibrational suspension of the light sphere in filled with liquid cavity rotating around the vertical axis. It is found that in the wide range of the cavity inclination angles the sphere position is determined by the dimensionless velocity of body differential rotation. Victor G. Kozlov and Stanislav V. Subbotin Copyright © 2014 Victor G. Kozlov and Stanislav V. Subbotin. All rights reserved. Numerical Study on Energy Dissipation of Steel Moment Resisting Frames under Effect of Earthquake Vibrations Thu, 13 Mar 2014 12:22:25 +0000 In the regions near to active faults, if the fault rupture propagation is towards the site and the shear wave propagation velocity is near the velocity of fault rupture propagation, the forward directivity effect causes pulse-like long-period large-amplitude vibrations perpendicular to the fault plane which causes a large amount of energy to be imposed to structures in a short time. According to previous investigations, the amounts of input and dissipated energies in the structure represent the general performance of the structure and show the level of damage and flexibility of the structure against earthquake. Therefore, in this study, the distribution of damage in the structure height and its amount at the height of steel moment frames under the pulse-like vibrations in the near fault region has been investigated. The results of this study show that the increase rate of earthquake input energy with respect to increase in the number of stories of the structure in the near field of fault is triple that in the far field of fault which then leads to a 2–2.5 times increase in the earthquake input energy in the high rise moment frames in the near field of fault with respect to that in the far field of fault. Mohsen Gerami and Davood Abdollahzadeh Copyright © 2014 Mohsen Gerami and Davood Abdollahzadeh. All rights reserved. Optimization of Natural Frequencies and Sound Power of Beams Using Functionally Graded Material Thu, 20 Feb 2014 06:49:02 +0000 This paper presents a design method to optimize the material distribution of functionally graded beams with respect to some vibration and acoustic properties. The change of the material distribution through the beam length alters the stiffness and the mass of the beam. This can be used to alter a specific beam natural frequency. It can also be used to reduce the sound power radiated from the vibrating beam. Two novel volume fraction laws are used to describe the material volume distributions through the length of the FGM beam. The proposed method couples the finite element method (for the modal and harmonic analysis), Lumped Parameter Model (for calculating the power of sound radiation), and an optimization technique based on Genetic Algorithm. As a demonstration of this technique, the optimization procedure is applied to maximize the fundamental frequency of FGM cantilever and clamped beams and to minimize the sound radiation from vibrating clamped FGM beam at a specific frequency. Nabeel T. Alshabatat and Koorosh Naghshineh Copyright © 2014 Nabeel T. Alshabatat and Koorosh Naghshineh. All rights reserved. Effect of Two Temperatures on Reflection Coefficient in Micropolar Thermoelastic with and without Energy Dissipation Media Sun, 16 Feb 2014 14:34:13 +0000 The reflection of plane waves at the free surface of thermally conducting micropolar elastic medium with two temperatures is studied. The theory of thermoelasticity with and without energy dissipation is used to investigate the problem. The expressions for amplitudes ratios of reflected waves at different angles of incident wave are obtained. Dissipation of energy and two-temperature effects on these amplitude ratios with angle of incidence are depicted graphically. Some special and particular cases are also deduced. Rajneesh Kumar, K. D. Sharma, and S. K. Garg Copyright © 2014 Rajneesh Kumar et al. All rights reserved. Analysis of Excitation and Dead Vibration Modes of Quartz Resonators Mon, 10 Feb 2014 13:15:40 +0000 This study uses the finite element method (FEM) to analyze the excitation and dead vibration modes of two-dimensional quartz plates. We first simplify three-dimensional quartz plates with plane strain simplification and then compare the modes of the simplified three-dimensional plates to those of two-dimensional plates. We then analyze quartz vibrating elements of AT-cut plates and SC-cut plates. To understand the regularity of the resonance frequency of plates that are excitable by voltage loading, we compare the natural vibrations of quartz plates with the excitation frequency generated after the plates are excited by voltage loading. Zi-Gui Huang and Zheng-Yu Chen Copyright © 2014 Zi-Gui Huang and Zheng-Yu Chen. All rights reserved. Erratum to “Anomalous Dispersion of the Lamb Mode” Tue, 04 Feb 2014 12:02:07 +0000 Faiz Ahmad and Takasar Hussain Copyright © 2014 Faiz Ahmad and Takasar Hussain. All rights reserved. Energy Harvesting with Piezoelectric Element Using Vibroacoustic Coupling Phenomenon Mon, 23 Dec 2013 08:57:42 +0000 This paper describes the vibroacoustic coupling between the structural vibrations and internal sound fields of thin structures. In this study, a cylindrical structure with thin end plates is subjected to the harmonic point force at one end plate or both end plates, and a natural frequency of the end plates is selected as the forcing frequency. The resulting vibroacoustic coupling is then analyzed theoretically and experimentally by considering the dynamic behavior of the plates and the acoustic characteristics of the internal sound field as a function of the cylinder length. The length and phase difference between the plate vibrations, which maximize the sound pressure level inside the cavity, are clarified theoretically. The theoretical results are validated experimentally through an excitation experiment using an experimental apparatus that emulates the analytical model. Moreover, the electricity generation experiment verifies that sufficient vibroacoustic coupling can be created for the adopted electricity generating system to be effective as an electric energy-harvesting device. Hiroyuki Moriyama, Hirotarou Tsuchiya, and Yasuo Oshinoya Copyright © 2013 Hiroyuki Moriyama et al. All rights reserved. Optimal Placement of Piezoelectric Plates to Control Multimode Vibrations of a Beam Tue, 17 Dec 2013 15:56:41 +0000 Damping of vibrations is often required to improve both the performance and the integrity of engineering structures, for example, gas turbine blades. In this paper, we explore the possibility of using piezoelectric plates to control the multimode vibrations of a cantilever beam. To develop an effective control strategy and optimize the placement of the active piezoelectric elements in terms of vibrations amplitude reduction, a procedure has been developed and a new analytical solution has been proposed. The results obtained have been corroborated by comparison with the results from a multiphysics finite elements package (COMSOL), results available in the literature, and experimental investigations carried out by the authors. Fabio Botta, Daniele Dini, Christoph Schwingshackl, Luca di Mare, and Giovanni Cerri Copyright © 2013 Fabio Botta et al. All rights reserved. Static and Dynamic Characteristics of Composite Conoidal Shell Roofs Tue, 03 Dec 2013 15:15:01 +0000 A thorough review of the existing literature reflects that forced vibration studies of laminated composite conoidal shells with complicated boundary conditions are missing. Hence, the present paper aims to fill the lacuna. A finite element code utilizing eight-noded doubly curved elements together with modified Sanders’ first approximation theory for thin shells is used to study the forced vibration behavior of moderately thin laminated composite conoidal shells subjected to three different uniformly distributed time-dependent forces. Newmark’s direct time integration method is used to solve the dynamic problem. Results obtained using the present code are compared with the values available in the literature, and a good agreement of the results confirms the accuracy of the proposed code. The transient responses of the laminated shell are studied meticulously for parametric variations like boundary conditions and stacking orders of cross and angle-ply laminates and are compared with bending responses of the shell to conclude on the necessity of the dynamic study. Kaustav Bakshi and Dipankar Chakravorty Copyright © 2013 Kaustav Bakshi and Dipankar Chakravorty. All rights reserved. Differential Evolution: An Inverse Approach for Crack Detection Wed, 13 Nov 2013 15:51:50 +0000 This paper presents a damage detection technique combining analytical and experimental investigations on a cantilever aluminium alloy beam with a transverse surface crack. Firstly, the first three natural frequencies were determined using analytical methods based on strain energy release rate. Secondly, an experimental method was adopted to validate the theoretical findings. The damage location and severity assessment is the third stage and is formulated as a constrained optimisation problem and solved using the proposed differential evolution (DE) algorithm based on the measured and calculated first three natural frequencies as inputs. Numerical simulation studies indicate that the proposed method is robust and can be used effectively in structural health monitoring (SHM) applications. Prabir Kumar Jena, Dhirendra Nath Thatoi, and Dayal R. Parhi Copyright © 2013 Prabir Kumar Jena et al. All rights reserved. Biomass from Paddy Waste Fibers as Sustainable Acoustic Material Thu, 07 Nov 2013 13:19:11 +0000 Utilization of biomass for green products is still progressing in the effort to provide alternative clean technology. This paper presents the utilization of natural waste fibers from paddy as acoustic material. Samples of sound absorbing material from paddy waste fibers were fabricated. The effect of the fiber density, that is, the fiber weight and the sample thickness, and also the air gap on the sound absorption coefficient is investigated through experiment. The paddy fibers are found to have good acoustic performance with normal incidence absorption coefficient greater than 0.5 from 1 kHz and can reach the average value of 0.8 above 2.5 kHz. This result is comparable against that of the commercial synthetic glass wool. Attachment of a single layer of polyester fabric is shown to further increase the absorption coefficient. A. Putra, Y. Abdullah, H. Efendy, W. M. F. W. Mohamad, and N. L. Salleh Copyright © 2013 A. Putra et al. All rights reserved. Autocorrelation Analysis in Time and Frequency Domains for Passive Structural Diagnostics Sun, 27 Oct 2013 13:48:45 +0000 In this paper, modal frequency estimation by using autocorrelation functions in both the time and frequency domains for structural diagnostics is discussed. With popular structural health monitoring methods for periodic inspections such as with the “hammering test,” hearing is very useful for distinguishing differences between structural conditions. Hearing detects pitch and tone, and it is known that the auditory process is related to wave periodicity calculated from autocorrelation functions. Consequently, on the basis of the hammering test, modal frequencies can be estimated by autocorrelation, the same as hearing. In this paper, modal frequencies were estimated by using autocorrelation for constant structural health monitoring under a nonstationary noise condition. First, fundamental modal frequencies were estimated by using the autocorrelation of the time domain which was inspired by pitch detection of hearing. Second, higher modal frequency compositions were also analyzed by using autocorrelation in the frequency domain as with tones discrimination. From the results by conducting scale-model experiments under unknown nonstationary noise conditions, periods of fundamental modal frequency were derived by using periods histogram of autocorrelation functions. In addition, higher modal frequency estimation under nonstationary noises was also discussed. Satoru Goto, Yoshinori Takahashi, and Mikio Tohyama Copyright © 2013 Satoru Goto et al. All rights reserved. Diffraction of a Plane Elastic Wave by a Gradient Transversely Isotropic Layer Sat, 26 Oct 2013 13:33:21 +0000 The problem of diffraction of a plane elastic wave by a gradient transversely isotropic layer is considered. Using the method of overdetermined boundary value problem in combination with the Fourier transform method, the system of ordinary differential equations of the second order with boundary conditions of the third type is obtained which is solved by the grid method. Results of calculations obtained using the above-mentioned technique for the case of piecewise linear profiles for the Young modulus of the layer are given. Anastasiia Anufrieva and Dmitrii Tumakov Copyright © 2013 Anastasiia Anufrieva and Dmitrii Tumakov. All rights reserved. Three-Dimensional Investigation of the Stokes Eigenmodes in Hollow Circular Cylinder Tue, 08 Oct 2013 08:18:01 +0000 This paper studies the influence of boundary conditions on a fluid medium of finite depth. We determine the frequencies and the modal shapes of the fluid. The fluid is assumed to be incompressible and viscous. A potential technique is used to obtain in three-dimensional cylindrical coordinates a general solution for a problem. The method consists in solving analytically partial differential equations obtained from the linearized Navier-Stokes equation. A finite element analysis is also used to check the validity of the present method. The results from the proposed method are in good agreement with numerical solutions. The effect of the fluid thickness on the Stokes eigenmodes is also investigated. It is found that frequencies are strongly influenced. Adil El Baroudi and Fulgence Razafimahery Copyright © 2013 Adil El Baroudi and Fulgence Razafimahery. All rights reserved. An Investigation of Structural Damage Location Based on Ultrasonic Excitation-Fiber Bragg Grating Detection Mon, 23 Sep 2013 14:43:42 +0000 With the continuous development of mechanical automation, the structural health monitoring techniques are increasingly high requirements for damage detection. So structural health monitoring (SHM) has been playing a significant role in terms of damage prognostics. The main contribution pursued in this investigation is to establish a detection system based on ultrasonic excitation and fiber Bragg grating sensing, which combines the advantages of the ultrasonic detection and fiber Bragg grating (FBG). Differencing from most common approaches, a new way of damage detection is based on fiber Bragg grating (FBG), which can easily realize distributed detection. The basic characteristics of fiber Bragg grating sensing system are analyzed, and the positioning algorithm of structural damage is derived in theory. On these bases, the detection system was used to analyze damage localization in the aluminum alloy plate of a hole with diameters of 6 mm. Experiments have been carried out to demonstrate that the sensing system was feasible and that the estimation method of the location algorithm was easy to implement. Yuegang Tan, Li Cai, Bei Peng, and Lijun Meng Copyright © 2013 Yuegang Tan et al. All rights reserved. Normal Incidence of Sound Transmission Loss of a Double-Leaf Partition Inserted with a Microperforated Panel Wed, 04 Sep 2013 10:10:51 +0000 A double-leaf partition in engineering structures has been widely applied for its advantages, that is, in terms of its mechanical strength as well as its lightweight property. In noise control, the double-leaf also serves as an effective noise barrier. Unfortunately at low frequency, the sound transmission loss reduces significantly due to the coupling between the panels and the air between them. This paper studies the effect of a microperforated panel (MPP) inserted inside a double-leaf partition on the sound transmission loss performance of the system. The MPP insertion is proposed to provide a hygienic double-leaf noise insulator replacing the classical abrasive porous materials between the panels. It is found that the transmission loss improves at the troublesome mass-air-mass resonant frequency if the MPP is located closer to the solid panel. The mathematical model is derived for normal incidence of acoustic loading. A. Putra, A. Y. Ismail, R. Ramlan, Md. R. Ayob, and M. S. Py Copyright © 2013 A. Putra et al. All rights reserved. The Effect of Uncertainty in the Excitation on the Vibration Input Power to a Structure Tue, 27 Aug 2013 08:57:32 +0000 In structural dynamic systems, there is inevitable uncertainty in the input power from a source to a receiver. Apart from the nondeterministic properties of the source and receiver, there is also uncertainty in the excitation. This comes from the uncertainty of the forcing location on the receiver and, for multiple contact points, the relative phases, the force amplitude distribution at those points, and also their spatial separation. This paper investigates quantification of the uncertainty using possibilistic or probabilistic approaches. These provide the maximum and minimum bounds and the statistics of the input power, respectively. Expressions for the bounds, mean, and variance are presented. First the input power from multiple point forces acting on an infinite plate is examined. The problem is then extended to the input power to a finite plate described in terms of its modes. The uncertainty due to the force amplitude is also discussed. Finally, the contribution of moment excitation to the input power, which is often ignored in the calculation, is investigated. For all cases, frequency band-averaged results are presented. A. Putra and B. R. Mace Copyright © 2013 A. Putra and B. R. Mace. All rights reserved. Anomalous Dispersion of the Lamb Mode Tue, 06 Aug 2013 09:01:40 +0000 The mode of the Lamb spectrum of an isotropic plate exhibits negative group velocity in a narrow frequency domain. This anomalous behavior is explained analytically by examining the slope of each mode first in its initial state and then near its turning points. Faiz Ahmad and Takasar Hussain Copyright © 2013 Faiz Ahmad and Takasar Hussain. All rights reserved. Experimental Analysis of a Piezoelectric Energy Harvesting System for Harmonic, Random, and Sine on Random Vibration Sun, 04 Aug 2013 09:32:37 +0000 Harvesting power with a piezoelectric vibration powered generator using a full-wave rectifier conditioning circuit is experimentally compared for varying sinusoidal, random, and sine on random (SOR) input vibration scenarios; the implications of source vibration characteristics on harvester design are discussed. The rise in popularity of harvesting energy from ambient vibrations has made compact, energy dense piezoelectric generators commercially available. Much of the available literature focuses on maximizing harvested power through nonlinear processing circuits that require accurate knowledge of generator internal mechanical and electrical characteristics and idealization of the input vibration source, which cannot be assumed in general application. Variations in source vibration and load resistance are explored for a commercially available piezoelectric generator. The results agree with numerical and theoretical predictions in the previous literature for optimal power harvesting in sinusoidal and flat broadband vibration scenarios. Going beyond idealized steady-state sinusoidal and flat random vibration input, experimental SOR testing allows for more accurate representation of real world ambient vibration. It is shown that characteristic interactions from more complex vibration sources significantly alter power generation and processing requirements by varying harvested power, shifting optimal conditioning impedance, inducing voltage fluctuations, and ultimately rendering idealized sinusoidal and random analyses incorrect. Jackson W. Cryns, Brian K. Hatchell, Emiliano Santiago-Rojas, and Kurt L. Silvers Copyright © 2013 Jackson W. Cryns et al. All rights reserved. Dynamic Stability of Plane Free Surface of Liquid in Axisymmetric Tanks Sun, 28 Jul 2013 09:11:32 +0000 When liquid filled containers are excited vertically, it is known that, for some combinations of frequency and amplitude, the free surface undergoes unbounded motion leading to instability, called parametric instability or parametric resonance, while for other combinations the free surface remains plane. In this paper, the stability of the plane free surface is investigated theoretically when the vessel is a vertical axisymmetric container. The effect of coupled horizontal excitation on the stability is examined. The dynamics of sloshing flows under specified excitations are simulated numerically using fully nonlinear finite element method based on non-linear potential flow theory. A mixed Eulerian-Lagrangian technique combined with 4th-order Runge-Kutta method is employed to advance the solution in time. A regridding technique based on cubic spline is applied to the free surface for every finite time step to avoid possible numerical instabilities. Siva Srinivas Kolukula and P. Chellapandi Copyright © 2013 Siva Srinivas Kolukula and P. Chellapandi. All rights reserved.