Open loop control of vortex-induced vibration of a circular cylinder

In this paper both numerical and experimental investigations have been carried out to suppress the vortex-induced vibration (VIV) of a circular cylinder in an electrically low-conducting fluid. The electromagnetic forces (Lorentz forces) in the azimuthal direction were generated through the mounted electrodes and magnets locally on the surface of the cylinder, which have been proved having an accelerating effect to the fluid on the surface of the cylinder. Results of computations are presented for synchronous vibration phenomenon of a cylinder at Re=200, which are in good agreement with previous computational results. With the Lorentz forces loaded, the VIV of the cylinder has been suppressed successfully. Experimental results have also shown the same tendency and are in reasonable agreement with the numerical results.     

Transmission properties of frequency selective structures with air gaps

The transmission properties of compound frequency selective structures with dielectric slab and air gaps were investigated by computation and experimentation.Mechanism analyses were also carried out.Results show that the air gaps have a distinct influence on the transmission properties.Resonant frequency of the structure would increase rapidly when the air gap appears.After the gap gets larger to a specific value,generally 1/5 wavelength corresponding to the resonant frequency,the transmission properties would change periodically with the gap thickness.The change of transmission properties in one period has a close relationship with the dielectric thickness.These results provide a new method for designing a bandpass radome of large incidence angle and low loss with the concept of stealth shield radome.     

Propagation of thickness-twist waves in a piezoelectric ceramic plate with unattached electrodes

We analyze the propagation of thickness-twist waves in an unbounded piezoelectric ceramic plate with air gaps between the plate surfaces and two electrodes. These waves are also called anti-plane or shear-horizontal waves with one displacement component only. An exact solution is obtained from the equations of the linear theory of piezoelectricity. Dispersion relations of the waves are obtained and plotted. Results show that the wave frequency or speed is sensitive to the air gap thickness. This effect can be used to manipulate the behavior of the waves and has implications in acoustic wave devices.     

Flexural vibration band gaps in a thin plate containing a periodic array of hemmed discs

Using a periodic expansion by means of the Bloch theorem, the flexural vibration band gaps are studied in a thin plate with two-dimensional ternary locally resonant structures, i.e. a thin epoxy plate containing a periodic square array of lead discs hemmed around by rubber. The full band gaps of flexural vibration in the thin plate are obtained within which sound and vibration will be forbidden. The numerical results are used to show how the width of the first full band gap depends on the radius ratio of lead disc to hemmed disc, filling fraction, lattice constant (distance between the centers of the nearest lead discs) and thickness of the thin plate. It is observed that the gap width can be changed a lot by modulating these physical parameters.     

一种多频局域共振型声子晶体板的低频带隙与减振特性

设计了一种多频局域共振型声子晶体板结构, 该结构由一薄板上附加周期性排列的多个双悬臂梁式子结构而构成. 由于多个双悬臂梁式子结构的低频振动与薄板振动的相互耦合作用, 这种局域共振型板结构可产生多个低频弯曲波带隙(禁带); 带隙频率范围内的板弯曲波会被禁止传播, 利用带隙可以实现对薄板的多个目标频率处低频减振. 本文针对这种局域共振型板结构进行了简化, 并基于平面波展开法建立了其弯曲波带隙计算理论模型; 基于该模型, 结合具体算例进行了带隙特性理论分析. 设计、制备了一种存在两个低频弯曲波带隙的局域共振型板结构样件, 通过激光扫描测振仪测试证实该结构存在两个低频带隙, 在带隙频率范围的板弯曲振动被显著衰减.     

多振子梁弯曲振动中的局域共振带隙

从梁的弯曲振动方程出发,利用传递矩阵法,给出了无限周期结构的一维多振子声子晶体梁的弯曲振动能带结构,并利用有限元方法计算了有限周期结构梁的弯曲振动频率响应.建立了多振子声子晶体梁的简化模型,推导出带隙起始截止频率公式.结果表明:一维多振子声子晶体梁具有比单振子声子晶体梁更宽更丰富的振动带隙,可应用于呈倍频关系的减振降噪中;振动在带隙频率范围内频率响应具有明显的衰减;所建立的简化模型与理论模型结果符合较好.研究工作为梁类结构的减振提供一种新的思路.     

用DIS位移传感器研究气垫导轨上的阻尼振动

在简述阻尼振动实验原理的基础上,阐述了用DIS位移传感器研究气垫导轨上阻尼振动的方法,通过对比实验进一步论述了该方法的可行性.     

Stability and vibration of a helical rod with circular cross section in a viscous medium

The stability and vibration of a thin elastic helical rod with circular cross section in a viscous medium are discussed. The dynamical equations of the rod in the viscous medium are established in the Frenet coordinates of the centreline with the Euler angles describing the attitudes of the cross section as variables. We have proved that the Lyapunov and Euler conditions of stability of a helical rod in the space domain are the necessary conditions for the asymptotic stability of the rod in the time domain. The free frequencies and damping coefficients of torsional and flexural vibrations of the helical rod in the viscous medium are calculated.     

Investigation of turbulent flow past a flagged short finite circular cylinder

This study was conducted to investigate the flow structures of turbulent flow passing over a short finite cylinder in which a rigid flag is attached to the rear of the cylinder, in wake region. The length-to-diameter ratio of the cylinder was chosen to be L/D = 2, whereas the rigid flag had a width-to-diameter ratio of W/D = 1.5. Wall-adapted large-eddy simulation (LES-WALE) was used to resolve unsteady turbulent flow structures. The far field Reynolds number based on cylinder diameter was chosen to be 20,000. The results were compared with the regular case wherein no flag was attached to the cylinder. Results revealed that the flow pattern behind the cylinder with flag was totally different in comparison with the regular case one. However, top free end of the cylinder was not influenced by the flag in contrast with the wake region. At far downstream from the cylinder, most of the flow structures in both cases appeared the same. The horseshoe vortices in both cases appeared to be an unsteady phenomenon, with slightly different patterns. Moreover, in the case of flag attachment, the pressure coefficient was smaller than that of with no flag. Finally, it was shown that the main and secondary Strouhal numbers locations were different in both cases.     

空气环型二维光子晶体完全带隙特性研究

采用平面波展开法, 系统研究了空气环型二维光子晶体的完全光子带隙随结构参数变化而改变的规律, 并将其与普通的空气孔型和介质柱型二维光子晶体的完全带隙进行了比较. 研究表明: 空气环型二维光子晶体不仅可以获得更宽的完全带隙, 而且, 当介质折射率较低时, 其可以获得普通空气孔型和介质柱型二维光子晶体在低折射率条件下所无法获得的完全带隙. 关键词： 空气环型二维光子晶体 完全带隙 平面波展开法     

Flow structure of the entrance of a T-junction duct without/with a circular cylinder

The flow characteristics of the trailing edge of vertical vanes installed at the intersection of a T-junction duct were experimentally investigated using particle image velocimetry. The measured velocity field in the branch duct with/without single circular cylinder was studied under different cross velocities and velocity ratios. Additionally, the effect of the locations of cylinder on the flow field was discussed. The positive velocity region, the unsteady flow region and the trailing edge flow region of the vane, have been observed. The positive velocity region existed in almost one half of the measured area. As for the unsteady flow region, the unstable double-vortex structure transformed into a single-vortex structure as the velocity ratio increased. As for the trailing edge flow region of the vanes, the vortex streets could be visualised. Furthermore, the location of cylinder has revealed significant influence on the flow distributions in the trailing edge flow regions of the vanes. The flow structure without cylinder in the measured area is dependent on combinations of the cross velocity and velocity ratio, whereas that with cylinder is dependent on the velocity ratio. The vorticity fields were analysed in each region, and the velocity components revealed the cause of airflow trajectory.     

Scattering of an arbitrarily oriented dipole field by an infinite and a finite length PEMC circular cylinder

Electromagnetic scattering from an infinite and a finite length PEMC circular cylinder, illuminated by an arbitrarily oriented dipole, is investigated theoretically. An electric dipole as a source of excitation is considered first, and then a magnetic dipole as a source of excitation is treated. In contrast to the case of an axially directed dipole, it is shown that no additional terms are needed to incorporate the cross-polarized component of the field for the case of radial and circumferential dipoles. Numerical verifications are presented to verify the validity of derived results and numerical code by comparing results with the published literature.      

新型柱孔式模式转换型纵-扭复合模态超声振动系统*

为了改善基于螺旋槽结构和斜槽结构的模式转换型纵-扭复合模态超声振动系统存在的结构复杂、扭转分量较小等问题,论文提出了基于新型柱孔式复合变幅杆的模式转换型纵-扭复合模态超声振动系统,并利用有限元法和数据分析对其进行了仿真分析,结果表明,引入新型柱孔式复合变幅杆的系统的输出端面的剪切应力、旋转角度得到了大幅提升,能够有效地提高纵、扭振动的转换效率。     

黏性介质中圆截面弹性细杆的平面振动

讨论圆截面弹性细杆在黏性介质中的平面振动. 基于Kirchhoff理论，以杆中心线的Frenet坐标系为参考系，建立其动力学方程，杆中心线为任意平面曲线时，其扭转振动与弯曲振动解耦. 讨论两端固定条件下任意形状杆的平面扭转振动，以及无扭转的轴向受压直杆和圆环杆的平面弯曲振动，导出其自由振动频率和阻尼系数. 证明空间域内压杆的Lyapunov稳定性和欧拉稳定性条件为时域内渐近稳定性的充分必要条件，或无阻尼压杆的稳定性必要条件. 圆环杆平衡恒满足渐近稳定性条件. 关键词： 弹性细杆 黏性介质 扭转振动 弯曲振动     

X形超阻尼局域共振声子晶体梁弯曲振动带隙特性

以声子晶体理论为基础,设计了一种具有超阻尼特性的X形局域共振结构,分析了周期性附加X形局域共振的梁弯曲振动传播特性.利用拉格朗日方程分析了X形局域共振结构动力学等效特性,揭示了该结构的阻尼放大的机理,分析了几何结构参数对于带隙特性的影响,并利用有限元法验证了X形局域共振结构的超阻尼特性.研究结果表明,周期性附加X形局域结构能够有效地抑制低频弯曲振动在梁中的传播,产生超阻尼特性,实现低频、宽带的减振效果,为结构的低频减振提供了一个新的设计方案.     

Geometrically distributed aperiodic circular photonic crystals with broad and isotropic photonic band gaps

In this paper, a new type of circular photonic crystal (CPC) with a geometrical distribution of concentric layers is presented. A broad and isotropic photonic band gap is achieved using this geometrically distributed CPC (GCPC). Also, the influence of the number of concentric layers to the overall band gap of GCPC is studied. It is demonstrated that the band gap broadens with a red shift when the number of concentric layers is increased. The reason for the red shift of the band gap is further investigated.     

Electrostatic field calculation in air gaps with a transverse layer of dielectric barrier

In this paper the influence of transverse layer of dielectric barrier to the electrostatic field strength of air gaps using a hyperbolic needle-to-plane configuration is investigated. A three dimensional field problem is presented for simulating the electrostatic field in the air gap. This is achieved by using the charge simulation method. Group of ring charges are used to simulate the needle surface. A genetic algorithm is used for optimum number, location, dimension and value of simulating ring charges. The used needle tip radius was 8 μm while the gap spacing varied from 0.3 cm to 50 cm. The optimum barrier position minimizes the maximum electrostatic field and the electric energy density at the needle tip and consequently maximizes the required breakdown value. The simulation result has been compared with available experimental observations. A good agreement is found between the numerical and experimental data.     

圆柱电极激光触发开关击穿前的模拟计算

在加入SF6气体与电子的碰撞反应截面后,利用基于PIC-MCC方法的XOOPIC程序,对天光Ⅰ号激光装置预放大器上的圆柱电极激光触发开关进行击穿前流柱形成过程的模拟计算。详述了模拟条件的选择及激光触发条件的等效,并给出了模拟结果。模拟结果表明,在弧道初始半径约为250 μm的条件下,经过0.15 ns后正离子到达阴极,开关导通。因此验证了通过PIC-MCC方法模拟,可以为开关弧道的电阻的动态变化的电路模拟给出初始条件,使之能够更为准确的预测开关的导通行为,为开关设计提供了一个新的工具。     

再论端面受到空气阻力的弹性杆的振动

重新研究了端面受到空气阻力的弹性杆振动的分离变量法.建立了复本征函数的广义正交性.用复数形式的本征振动展开一般解.利用正交性求出了一般解中的系数以满足任意给定的初始条件.求解了多种边界条件下的振动,包括两端均受到空气阻力、一端有集中质量物体和弹性连接并受到空气阻力等情况.     

圆柱电极激光触发开关击穿前的模拟计算

在加入SF6气体与电子的碰撞反应截面后,利用基于PIC-MCC方法的XOOPIC程序,对天光Ⅰ号激光装置预放大器上的圆柱电极激光触发开关进行击穿前流柱形成过程的模拟计算。详述了模拟条件的选择及激光触发条件的等效,并给出了模拟结果。模拟结果表明,在弧道初始半径约为250μm的条件下,经过0.15 ns后正离子到达阴极,开关导通。因此验证了通过PIC-MCC方法模拟,可以为开关弧道的电阻的动态变化的电路模拟给出初始条件,使之能够更为准确的预测开关的导通行为,为开关设计提供了一个新的工具。