Wake-induced vibrations of an elastically mounted cylinder located downstream of a stationary larger cylinder at low Reynolds numbers

Two-dimensional numerical simulations of flow past two unequal-sized circular cylinders in tandem arrangement are performed at low Reynolds numbers (Re). The upstream larger cylinder is stationary, while the downstream cylinder has both one (transverse-only) and two (transverse and in-line) degrees of freedom (1-dof and 2-dof, respectively). The Re, based on the free stream velocity U_∞ and the downstream cylinder diameter d, varies between 50 and 200 with a wide range of reduced velocities U_r. The diameter of the upstream cylinder is twice that of the downstream cylinder, and the center-to-center spacing is 5.5d. In general, for the 1-dof case, the calculations show that the wake-induced vibrations (WIV) of the downstream cylinder are greatly amplified when compared to the case of a single cylinder or two equal-sized cylinders. The transverse amplitudes build up to a significantly higher level within and beyond the lock-in region, and the U_r associated with the peak amplitude shifts toward a higher value. The dominant wake pattern is 2S mode for Re=50 and 100, while with the increase of Re to 150 and 200, the P+S mode can be clearly observed at some lower U_r. For the 2-dof vibrations, the transverse response characteristics are similar to those presented in the corresponding 1-dof case. The in-line responses are generally much smaller, except for several significant vibrations resulting from in-line resonance. The obvious in-line vibration may induce a C (chaotic) vortex shedding mode for higher Re (Re=200). With regard to the 2-dof motion trajectories, besides the typical figure-eight pattern, several odd patterns such as figure-double eight and single-looped trajectories are also obtained due to the wake interference effect.     

Stabilization of Parametric Vibrations of a Nonlinear Continuous System

The purpose of the present paper is to solve an active control problem of nonlinear continuous system parametric vibrations excited by the fluctuating force. The problem is solved using the concept of distributed piezoelectric sensors and actuators with a sufficiently large value of velocity feedback. The direct Liapunov method is proposed to establish criteria for the almost sure stochastic stability of the unperturbed (trivial) solution of the shell with closed-loop control. The distributed control is realized by the piezoelectric sensor and actuator, with the changing widths, glued to the upper and lower shell surface. The relation between the stabilization of nonlinear problem and a linearized one is examined. The fluctuating axial force is modeled by the physically realizable ergodic process. The rate velocity feedback is applied to stabilize the shell parametric vibrations.     

Subharmonic Response of a Quasi-Isochronous Vibroimpact System to a Randomly Disordered Periodic Excitation

A quasi-isochronous vibroimpact system is considered, i.e. a linear system with a rigid one-sided barrier, which is slightly offset from the system's static equilibrium position. The system is excited by a sinusoidal force with disorder, or random phase modulation. The mean excitation frequency corresponds to a simple or subharmonic resonance, i.e. the value of its ratio to the natural frequency of the system without a barrier is close to some even integer. Influence of white-noise fluctuations of the instantaneous excitation frequency around its mean on the response is studied in this paper. The analysis is based on a special Zhuravlev transformation, which reduces the system to one without impacts, or velocity jumps, thereby permitting the application of asymptotic averaging over the period for slowly varying inphase and quadrature responses. The averaged stochastic equations are solved exactly by the method of moments for the mean square response amplitude for the case of zero offset. A perturbation-based moment closure scheme is proposed for the case of nonzero offset and small random variations of amplitude. Therefore, the analytical results may be expected to be adequate for small values of excitation/system bandwidth ratio or for small intensities of the excitation frequency variations. However, at very large values of the parameter the results are approaching those predicted by a stochastic averaging method. Moreover, Monte-Carlo simulation has shown the moment closure results to be sufficiently accurate in general for any arbitrary bandwidth ratio. The basic conclusion, both of analytical and numerical simulation studies, is a sort of smearing of the amplitude frequency response curves owing to disorder, or random phase modulation: peak amplitudes may be strongly reduced, whereas somewhat increased response may be expected at large detunings, where response amplitudes to perfectly periodic excitation are relatively small.     

IR spectroscopic investigation of structural hydroxyl groups in W−Si heteropolycompound supported on Al2O3

Structural hydroxyl and deuteroxyl groups within the K₄[SiW₁₂O₄₀]/Al₂O₃ and K₆[SiW₁₁PdO₃₉]/Al₂O₃ systems were studied by diffuse-reflectance FTIR spectroscopy in a spectral range of fundamental stretching vibrations, first overtones, and combination bands of stretching and bending vibrations. For hydroxyl groups, the region of combination vibrations is the most informative. The calculated frequencies of bending vibrations of hydroxyl groups (865 and 730 cm⁻¹) are characteristic of acidic OH groups. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2017–2020, October, 1999.     

Manifestation of a Photorefractive Effect in the Raman Scattering Spectra of Lithium Niobate Crystals of Variable Composition

Using the Raman scattering spectra, we investigated the ordering of the structural units in the cation sublattice and the photorefractive properties of lithium niobate single crystals of variable composition, i.e., nominally pure ones with different Li/Nb ratios and those doped with the nonphotorefractive cations Mg²⁺, Gd³⁺, and Y³⁺. It is shown that at low concentrations of Mg²⁺, Gd³⁺, and Y³⁺ the magnitude of the photorefractive effect is substantially determined by the ordering of the structural units of the cation sublattice. It has been found for the first time that the intensity of the line corresponding to the bridge valence vibrations of oxygen atoms in the octahedrons of NbO₆ is sensitive to the dipole ordering of the cation sublattice of the crystal. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 611–614, September–October, 2005.     

锂锰尖晶石红外光谱的研究

本文对锂锰尖晶石的红外光谱进行了研究。由于锂锰尖晶石的晶体结构属于Fd3m空间群,锂离子占据四面体空隙(8a位置),锰离子占据八面体空隙(16d位置)。根据群论的知识,对锂锰尖晶石晶体中离子的振动方式与红外活性之间的内在关系进行了讨论。并列出了锂锰尖晶石的红外光谱实验数据。通过理论分析,我们推断:位于618.6和501.5cm~(-1)的红外吸收带分别来源于Mn(Ⅳ)-O和Mn(Ⅲ)-O键在晶体中的不对称伸缩振动(单元为Mn(Ⅳ)O_6和Mn(Ⅲ)O_6八面体),位于1124cm~(-1)的弱红外吸收带来源于Li-O键的不对称伸缩(单元为LiO_4四面体)。还有一些低于400cm~(-1)的可能吸收带在400～4000cm~(-1)范围内未能检测到。这一结论的可靠性通过锂锰尖晶石和掺杂的锂锰尖晶石的红外光谱实验数据得到证实。     

环形调液阻尼器振动控制中拍的研究

对环形调液阻尼器减振控制中的拍现象进行研究,分析了环形调液阻尼器对结构纯扭转振动控制中的拍现象,分别考虑无阻尼结构体系、主体结构有阻尼而CTLCD无阻尼的体系及主体结构和CTLCD中均有阻尼的体系,从数学角度对拍现象发生的机理进行解释.研究结构表明,当拍现象发生时,主体结构的振动不仅不会受到抑制,有时反而会加强;当环形调液阻尼器的阻尼增大到一定程度时,拍现象会消失;对于受廹振动,发生拍现象时,由于结构反应的瞬态部分不能得到迅速衰减,结构的瞬态响应将会占结构响应的很大一部分,如果仅考虑结构的稳态响应会带来较大的误差.     

Statistical energy analysis by finite elements for middle frequency vibration

Statistical energy analysis (SEA) is commonly used in industry to predict high-frequency vibrational response of structures. Since only local modes are used in SEA, only high-frequency responses can be predicted. This study extends SEA to the middle-frequency region by additionally using global modes. Methods using impedance matrices that can be found by NASTRAN are developed. Then the results are post processed to determine coupling loss factors.     

Giant forced volume magnetostriction in polycrystalline Tb0.3Dy0.7Fe1.95 alloys under magnetomechanical loading

The axial and transversal linear magnetostrictions (λ_∥ and λ_⊥) in [1 1 0] oriented polycrystalline Tb_0.3Dy_0.7Fe_1.95 alloys were measured simultaneously under uniaxial magnetomechanical loading to get the forced volume magnetostriction (ω=λ_∥+2λ_⊥). Despite the almost zero ω observed in Terfenol-D single crystals, it reaches up to 1000×10⁻⁶ in polycrystalline Tb_0.3Dy_0.7Fe_1.95 alloys near the saturation magnetic field under a stress above 50 MPa.