Comparison of Piezoelectric Structural Damping Based on Velocity Controlled Switching and Pulse Width Modulation Switching Circuits

Two novel piezoelectric damping techniques (VSD and PWMD) are compared in this paper to the traditional resonant shunt damping technique and SSDV technique. In VSD, the switching shunt circuit turns ON or OFF according to the polarity of the vibration velocity of the host structure to shift the piezoelectric voltage phase. An external voltage source is connected to enlarge the voltage amplitude across the piezoelectric element and to optimize the dissipated power. The PWM shunt technique not only can decrease the audible noises more efficiently but also ensure the stability of the control system with a constant voltage source. The theoretical and the experimental results show that the piezoelectric voltage can be adaptive to the vibration displacement by the pulse widths variation, so the PWMD can stay in stable state with a constant voltage source and can still provide a very good performance.

A Numerical Study of the Blade Passing Frequency Noise of a Centrifugal Fan

Tonal noise constitutes the major part of the overall fan noise, especially the blade passing frequency (BPF) noise which is generally the most dominant component. This paper studies the BPF tonal noise of a centrifugal fan, including the blade noise, casing aerodynamic noise, and casing structural noise caused by the flow-induced casing vibration. Firstly, generation mechanism and propagation process of fan noise were discussed and the measured spectra of fan noise and casing vibration were presented. Secondly, a fully 3-D transient simulation of the internal flow field of the centrifugal fan was carried out by the computational fluid dynamics (CFD) approach. The results revealed that the flow interactions between the impeller and the volute casing caused periodic pressure fluctuations on the solid walls of the impeller and casing. This pressure fluctuation induces aerodynamic noise radiation as dipole sources, as well as structural vibration as force excitations. Thirdly, using the acoustic analogy theory, the aeroacoustic dipole sources on the casing and blade surface were extracted. The BPF casing and blade aerodynamic sound radiation were solved by the boundary element method (BEM) taking into account the scattering effect of the casing structure. Finally, the casing structural noise was studied. The casing forced vibration and sound radiation under the excitation of BPF pressure fluctuation were calculated by finite element method (FEM) and BEM, respectively. The result indicates that at the studied flow rate, the sound power levels of the casing aerodynamic noise, blade aerodynamic noise and casing structural noise are 103 dB, 91 dB and 79 dB with the reference sound power of 1×10−12 W, respectively.

Un-Shocked High Amplitude Standing Waves in Wave-Shaped Resonators

A numerical study of non-linear, high amplitude standing waves in non-cylindrical circular resonators is reported here. These waves are shock-less and can generate peak acoustic overpressures that can exceed the ambient pressure by three/four times its nominal value. A high fidelity compressible computational fluid dynamic model is used to simulate the phenomena in cylindrical and arbitrarily shaped axisymmetric resonators. A right circular cylinder and frustum of cone are the two geometries studied. The model is validated using past numerical and experimental results of standing waves in cylindrical resonators. The non-linear nature of the harmonic response of the frustum of cone resonator system is investigated for two different working fluids (carbon dioxide and argon) operating at various values of piston amplitude. The high amplitude non-linear oscillations demonstrated can be used as a prime mover in a variety of applications including thermoacoustic cryocooling.

High-Power Operation of Acoustic-Electric Power Feedthroughs Through Thick Metallic Barriers

Throughout the last few years there has been a significant push to develop a means for the transmission of electrical power through solid metallic walls using ultrasonic means. The bulk of this effort has been focused on using two coaxially aligned piezoelectric transducers on opposite sides of a thick metallic transmission barrier, where one transducer serves as the “transmit” transducer and the other as the “receive” transducer. Previous modeling has predicted reasonably high power transfer efficiencies through the wall using this type of “acoustic-electric channel” to be possible at low power levels, which implies that channel component operates in a linear range with little concern of failure. High-power testing of two acoustic-electric channels has been done in an effort to determine power limits on such channels and to determine levels at which non-linear effects on the piezoelectrics become non-negligible. The tested channels are characterized by the “power density” imposed on the transmit transducer, that is, the power applied per unit area, as the values found for maximum power density are considered to be independent of transducer radii. The constructed channels are shown to be capable of transmitting large amounts of power (over 100 watts) without failure; and further, extrapolation of the results to channels with larger diameter transducers predicts power transfer of 1 kW to be highly feasible.

Testing of Sonic-Absorbent Panels’ Behavior in Open-Air Environment

One wildly used method to reduce and control the noise pollution in green city’s buildings is using sonic-absorbent panels. Their applications can be multiple, such as the insulation of buildings, acoustic barriers and fences along the highway or in front of supermarkets, hospitals and other public buildings. This paper presents a method for testing the behavior of sonic-absorbent panels in open-air environment. The work represents a carrying on of previous research about absorbent materials from gypsum family, tested in lab conditions. The experiment setup used a dynamic installation and as a sample a stand formed by six sonic-absorbent panels from special modeling alpha-gypsum plaster. This installation has been composed of two loudspeakers for emitting the sound at a well-defined frequency by the first laptop, the microphone for detecting and transmitting the signal to the second laptop for analyzing and processing the data. All operations were performed using MATLAB Programs, while a Data Logger Sound Level Meter type CENTER 332 was put on near the microphone to compare both results. The first experiment of acoustic stand has been realized by setting up the installation at a frequency from 50 Hz to 1250 Hz and altering the distance between loudspeakers and stand at 0.5m to 1m and 1.5m, respectively. The second experiment kept the same test’s conditions, while two and three layers of sonic-absorbent panels formed the stand, respectively, but at same distance from source of 0.5 m. In both tests, the results underlined the good sonic-absorbent properties of these panels, especially at medium and high frequency, which can recommend using the panels for multiple outside applications.

Experimental Comparison of GCC-Based Time Delay Estimation Methods for Ultrasonic Leak Detection in Compressed Air System

Air leakage is one of the most significant energy waste factors in compressed air systems which account for about 10% of total industrial energy consumption. It is estimated that about 10%∼40% of the compressed air is wasted through leakage in most plants. A new ultrasonic leak detection method based on time delay estimation (TDE) is proposed to locate the compressed air leak for preventing energy waste in pneumatic systems. The accuracy of detection is highly dependent on the performance of the TDE method. Performances of six typical TDE methods based on generalized cross correlation (GCC) are compared, and these methods are the basic cross correlation (BCC), the Roth impulse response, the phase transform (PHAT), the smoothed coherence transform (SCOT), the WEINER processor, and the Hannan-Thomson (HT) processor. The experimental results show that: Firstly, the accuracy and precision of time delay estimation increases with the observation interval for all these methods. Secondly, the success rates of Roth, PHAT, SCOT and HT are much higher than that of BCC and WEINER, among which the HT processor performs best with a highest success rates closely followed by the PHAT processor. Thirdly, the HT processor which is a maximum likelihood estimator gives the minimum standard deviation of the time delay estimate; however, the standard deviations of all these GCC methods are very small. The HT processor outperforms other GCC methods in terms of success rate and standard deviation. Consequently, it is preferable to apply the HT processor for this particular purpose.

Investigation of Rub-Impact Setup Supported on Flexible Multi-Bearing Rotor

Vibration system investigation of the chaotic response of full annular rub impact rotor system supported on two ball bearings is investigated. Modeling and experiments of nonlinear dynamics on flexible multi-bearing rotor test rig is presented in this work. The test rig has two balancers that are assembled on rotor shaft, a rub impact device at the center, and ball bearings at both ends of the shaft. A 12-degree-of-freedom (DOF) linear model was developed for this test rig. The mathematical model was developed in the beginning without considering the rub impact part in order to validate the model with the experiment results. Then, then experimentally chaotic response and bifurcation diagram of the rub impact system were investigated.

Chip Design for Coupled Nonlinear Structronic Thermo-Electro-Elastic/Control Systems

The objective of this study is to demonstrate the feasibility that a fully-coupled nonlinear piezo(electric)-thermoelastic/control structronic systems can be represented by a single micro-electronic chip. This non-volatile chip is a poTable.lle miniature hardware that serves as a design standard for future calibration and diagnosis of the original “large-scale” structronic system and it can be used anywhere after any catastrophic disruption in extreme hostile environments. Distributed control of a nonlinear structronic beam system (i.e., an elastic beam laminated with distributed sensors/actuators and coupled with control electronics) subjected to mechanical and temperature excitations has been investigated recently. This study is to design an integrated electronic circuit chip encompassing the complete piezothermoelastic and control behavior of the nonlinear structronic beam system. The fully coupled nonlinear beam equations are first discretized into a number of “elements” and each element can be implemented by an active circuit block including operational amplifiers, resistors, capacitors, and other nonlinear multipliers. Signals from the integrated circuit chip of the coupled nonlinear piezothermoelastic beam system are favorably compared with analytical solutions.

Trigonometric Collocation for Computation of Steady State Responses of Cracked Structures

Development of vibration-based structural health monitoring techniques requires the use of various computational methods to predict dynamic responses of damaged structures. The method described in this work can be used for prediction of steady state harmonic responses for structures with fatigue cracks and may have several advantages over alternative techniques. The method appears to be relatively easy to implement and computationally inexpensive. The steady state response of the system at a given number of time points distributed over one vibration period is represented in terms of Fourier series containing higher frequency harmonics. Equations of motion are formulated in the form that allows for easy computation of Fourier coefficients for all terms in the series. Iterative procedure is used for determining the time of stiffness change in order to capture bilinear dynamic behavior. We present results of initial investigation by applying the method to a model of a cantilever beam with a crack.

Experimental Study on Dynamic Behavior of a Rolling Agricultural Tire

When agricultural machines are operated on pavements, the vibration and noise caused by the interaction between the tire lugs and the road surface are inevitable. In conventional studies, it is considered that the dynamic behavior of a rolling agricultural tire is influenced by the vibration characteristics of the tire. Resonance occurs when the lug excitation frequency of the tire, which is defined as the lug number multiplied by the number of revolutions of the tire, becomes equal to the natural frequency of the tire. In other words, the rolling tire shows large vibrations in the direction of the natural mode corresponding to the natural frequency of the tire. However, the vibration mode of the rolling tire in resonance state has not yet been clarified. In this study, it is confirmed that the dynamic behavior of the rolling tire can be evaluated by performing sound pressure measurements using closely located microphones to the tire. Further, the vibration mode in the resonance state is identified by performing simultaneous measurements of the sound pressure, and the vibration mode corresponds to the natural mode of the tire is confirmed as well.