Experimental investigation of fluidized bed dynamics under resonant frequency of sound waves

Sound-assisted fluidization has of late gained a significant research focus as a potential assisted fluidization technique for improving the hydrodynamics of solids that exhibit cohesive and non-homogeneous fluidization behavior. This study investigated the dynamics of a bed subjected to acoustic perturbations at different frequencies during the sound-assisted fluidization of a hydrophilic nanopowder with strong agglomeration behavior. The bed pressure transients were carefully monitored using sensitive pressure transducers in different sections of the bed over a wide range of velocities using ambient air as the fluidizing gas. Both fluidization and defluidization dynamics were investigated by varying the velocity in small steps using electronic mass flow controllers connected to a data acquisition system. In addition to the resonance frequency of 220 Hz, acoustic vibrations of 200 and 150 Hz frequency were also investigated to clearly delineate the effect of resonant frequency on the bed response. Our results clearly suggest that operation of sound-assisted fluidization at the resonant frequency greatly enhances its effectiveness.     

Natural frequencies of vibration of a class of solids composed of layers of isotropic materials

In a recent paper, the Ritz method with simple algebraic polynomials as trial functions was used to obtain an eigenvalue equation for the free vibration of a class of homogeneous solids with cavities. The method presented is here extended to the study of a class of non-homogeneous solids, in which each solid is composed of a number of isotropic layers with different material properties. The Cartesian coordinate system is used to describe the geometry of the solid which is modelled by means of a segment bounded by the yz, zx and xy orthogonal coordinate planes and by two curved surfaces which are defined by fairly general polynomial expressions in the coordinates x, y and z. The surface representing the interface between two material layers in the solid is also described by a polynomial expression in the coordinates x, y and z. In order to demonstrate the accuracy of the approach, natural frequencies are given for both a two- and three-layered spherical shell and for a homogeneous hollow cylinder, as computed using the present approach, and are compared with those obtained using an exact solution. Results are then given for a number of two- and three-layered cylinders and, to demonstrate the versatility of the approach, natural frequencies are given for a five-layered cantilevered beam with a central circular hole as well as for a number of composite solids of more general shape.     

Improved Modal Dynamics of Wind Turbines to Avoid Stall‐induced Vibrations

Stall‐induced edgewise blade vibrations have occasionally been observed on three‐bladed wind turbines over the last decade. Experiments and numerical simulations have shown that these blade vibrations are related to certain vibration modes of the turbines. A recent experiment with a 600 kW turbine has shown that a backward whirling mode associated with edgewise blade vibrations is less aerodynamically damped than the corresponding forward whirling mode. In this article the mode shapes of the particular turbine are analysed, based on a simplified turbine model described in a multi‐blade formulation. It is shown that the vibrations of the blades for the backward and forward edgewise whirling modes are different, which can explain the measured difference in aerodynamic damping. The modal dynamics of the entire turbine is important for stability assessments; blade‐only analysis can be misleading. In some cases the modal dynamics may even be improved to avoid stall‐induced vibrations. Copyright © 2003 John Wiley & Sons, Ltd.     

Fast frequency sweeps with many forcing vectors through adaptive interpolatory model order reduction

The goal of this work is to examine the efficacy of interpolatory model order reduction on frequency sweep problems with many forcing vectors. The adaptive method proposed relies on the implicit interpolatory properties of subspace projection with basis vectors spanning the forced response of the system and its derivatives. The algorithm is similar to a recently proposed adaptive scheme in that it determines both interpolation location and order within the frequency domain of interest. The bounds of efficiency of the approach as the number of forcing vectors grows are explored through the use of rough floating operation counts. This, in turn, guides choices made in the adaptive algorithm, including a new technique that prevents excessive subspace growth by capitalizing on subspace direction coupling. In order to demonstrate the algorithm's utility, a series of frequency sweep problems drawn from the field of structural acoustics is analyzed. It is demonstrated that increased order of interpolation can actually degrade the efficiency of the algorithm as the number of forcing vectors grows but that this can be limited by the subspace size throttling procedure developed here. Copyright © 2014 John Wiley & Sons, Ltd.     

地铁振动的地表低频响应预测研究

地铁振动引起的环境问题备受人们的关注,特别是近年来,低频(<20Hz)振动对高精密仪器和设备、古建筑物等的影响已成为焦点。如何采取合理的减振措施将低频振动的影响降到最低是亟需解决的问题。在北京交通大学轨道减振与振动控制试验室,开展钢弹簧浮置板轨道和普通轨道低频振动试验;基于地表低频响应测试数据,结合"钢弹簧浮置板轨道-隧道-地层"和"普通轨道-隧道-地层"耦合的三维有限元数值模拟结果,采用回归分析的方法,研究地表低频响应受激振频率(1～20Hz)、距轨道水平距离(0～100m)和轨道埋深(10～30m)等因素的影响,推导出地表低频响应量分析公式。将公式预测的地表低频响应量的最大值分成II、II、II和IV四级响应带,进一步讨论响应带分布与轨道埋深、距轨道水平距离和轨道基频等关键因素的关系,从而为减少地铁振动的地表低频响应措施提供参考。     

Application of the continuous wavelet transform on the free vibrations of a steel-concrete composite railway bridge

In this article, the Continuous Wavelet Transform (CWT) is used to study the amplitude dependency of the natural frequency and the equivalent viscous modal damping ratio of the first vertical bending mode of a ballasted, single span, concrete-steel composite railway bridge. It is shown that for the observed range of acceleration amplitudes, a linear relation exists between both the natural frequency and the equivalent viscous modal damping ratio and the amplitude of vibration. This result was obtained by an analysis based on the CWT of the free vibrations after the passage of a number of freight trains. The natural frequency was found to decrease with increasing amplitude of vibration and the corresponding damping ratio increased with increasing amplitude of vibration. This may, given that further research efforts have been made, have implications on the choice of damping ratios for theoretical studies aiming at upgrading existing bridges and in the design of new bridges for high speed trains. The analysis procedure is validated by means of an alternative analysis technique using the least squares method to fit a linear oscillator to consecutive, windowed parts of the studied signals. In this particular case, the two analysis procedures produce essentially the same result.     

Determination of Natural Frequencies of Multilayered Plates by Mixed Finite Elements

Natural frequencies for multilayer plates are calculated by mixed finite element method. The main object of this paper is to use the mixed model for multilayer plates, analyzing each layer as an isolated plate, where the continuity of displacements is achieved by Lagrange multipliers （representing static variables）. This procedure allows us to work with any model for single plate （so as to ensure the proper behavior of each layer）, and the complexity of the multilayer system is avoided by ensuring the condition of displacements by the Lagrange multipliers （static variables）. The plate is discretized by finite element modeling based on a primary hybrid model, where the domain is divided by quadrilateral, both for the displacement field and static variables. This mixed element for plates was implemented and several examples of vibrations have been verified successfully by the results obtained by other methods in the literature.     

Spray characterization in air-assist atomizers using flow-induced acoustic vibrations and multivariate analysis

This feasibility study investigated a new non-intrusive approach employing acoustic chemometrics. The method includes acoustic/vibration data recording obtained utilizing two clamp-on piezoelectric accelerometers and two electret condensers-type microphones mounted on an arc. Principal Component Analysis (PCA) classification models were based on the acoustic FFT spectra from four sensors. The non-dimensional number (X) values correspond to the different breakup regimes comprising a range of air and liquid (water) flow rates in this air-assisted atomizer (one-analyte system). PCA classification model discerns the clusters belonging to similar non-dimensional number (X) values with the maximum variance in the first principal component (PC1) direction for both sensors combined. This study also assesses the utility of the acoustic chemometrics approach for predicting the flow parameter, such as Sauter mean diameter (SMD) based on the Partial Least Squares-Regression (PLS-R). The PLS-R prediction models work best for the 550 mm location with a low root mean square error of prediction (RMSEP) value of 5.443 and a high Pearson correlation coefficient (R²) value of 0.856 when validated using 50% independent data (test set validation). The comparison between the two sensor types demonstrated superior prediction performance for accelerometers for all the prediction models.     

Piezoelectric actuator design for ultrasonically assisted deep hole drilling

From different chipping machining processes it is known that a superposition of the cutting kinematics with additional vibration energy increases material removal rate and tool life. Concerning the deep drilling process in the scope of smallest diameters from 0.9 to 6 mm insights to this so called hybrid processes are still awaited. Preliminary investigations indicated that here is high, so far unused potential. The goal of current research is an increase in effectiveness of the deep hole drilling process by superimposing additional vibration energy in ultrasonic frequency range by means of a piezoelectric transducer and low-frequency vibrations in the range of acoustic frequencies as well. Positive effects can appear in a couple of areas, e.g. achievable surface quality, feeding force, drilling torque, shape and length of chips, feasibility of machining ceramic materials and tool wear. This paper describes mainly the ultrasound conform design of the vibration unit. Furthermore issues of contactless energy transfer into a rotating tool and model based design of piezoelectric transducers will be addressed.