A novel type of transverse surface wave propagating in a layered structure consisting of a piezoelectric layer attached to an elastic half-space

The existence and propagation of transverse surface waves in piezoelectric coupled solids is investigated, in which perfect bonding between a metal/dielectric substrate and a piezoelectric layer of finite-thickness is assumed. Dis- persion equations relating phase velocity to material con- stants for the existence of various modes are obtained in a simple mathematical form for a piezoelectric material of class 6mm. It is discovered and proved by numerical examples in this paper that a novel Bleustein-Gulyaev （B-G） type of transverse surface wave can exist in such piezoelectric cou- pled solid media when the bulk-shear-wave velocity in the substrate is less than that in the piezoelectric layer but greater than the corresponding B-G wave velocity in the same pie- zoelectric material with an electroded surface. Such a wave does not exist in such layered structures in the absence of pie- zoelectricity. The mode shapes for displacement and electric potential in the piezoelectric layer are obtained and discussed theoretically. The study extends the regime of transverse sur- face waves and may lead to potential applications to surface acoustic wave devices.     

Effect of initial stress on the propagation behavior of Love waves in a layered piezoelectric structure

The propagation behavior of Love waves in a layered piezoelectric structure with an initial stress is investigated in this article. It involves a thin piezoelectric layer bonded perfectly to an elastic substrate. Solutions of the mechanical displacement and electrical potential function are obtained for the piezoelectric layer and elastic substrate by solving the coupled electromechanical field equations. The phase velocity equations of the Love wave propagation and the stress fields in the layered piezoelectric structure are obtained for electrical open and short cases on the free surface, respectively. The effect of the initial stress on the phase velocity, the stress fields and the coupled electromechanical factor are discussed, respectively. Three sets of piezoelectric layer–elastic substrate systems are considered, i.e. BaTiO₃ ceramic layer–borosilicate glass substrate, PZT-5H ceramic layer–borosilicate glass substrate, and PZT-5H ceramic layer–SiO₂ glass substrate. It is seen that the phase velocity of the Love wave propagation decreases with the increase of the magnitude of the initial stress. The coupled electromechanical factor increases remarkably, as the magnitude of the initial the stress is greater than 100 MPa. This is useful for the design of acoustic surface wave devices.     

ON THE MECHANISM OF TURBULENT COHERENT STRUCTURE(II)──A PHYSICAL MODEL OF COHERENT STRUCTURE FOR THE ROUGH BOUNDARY LAYER

In this paper, the differences of turbulent coherent structure between the smooth and rough boundary layers are analysed. Based on the discussing the transient properties from the smooth wall to the rough wall, the physical model of coherent structure for the rough boundary layer are established. The width of slowly-moving turbulent spot and the bursting time are obtained, which are in agreement with experimental results.The project supported by the National Nature Science Foundation of China     

Influence of drag reducing additives on the structure of turbulence in a mixing layer

The structure of a free turbulent mixing layer in Newtonian and drag reducing fluids is discussed using results from visualization and LDA-techniques. Particular emphasis is placed on results for the turbulent shear stress and the time correlation function.     

A theoretical model of the coherent structure in the wall region of a turbulent boundary layer

In this paper,the formation of the coherent structures in the wall region of aturbulent boundary layer was studied,using the nonlinear theory of the hydrodynamicstability.The spanwise and streamwise wavelengths of the most amplified unstablewave obtained by this study were found in good agreement with the experiments,whichmakes the distinct feature of this study in the present paper,as the basis of thestability analysis,a more rational velocity profile has been used,which is different fromthat of the turbulent mean flow.And also,the new nonlinear theory was used.Theresult is useful in understanding of the quasi-periodicity of the coherent structure in theturbulent boundary layer.     

A correction for Reynolds-averaged-Navier–Stokes turbulence model under the effect of shock waves in hypersonic flows

Studies on the unphysical increase of turbulent quantities for RANS simulation induced by shock waves in hypersonic flows are carried out. Numerical experiments on the hypersonic flow over a blunt body reveal that the phenomenon of unphysical increase of turbulent quantities across the detached shock wave is induced by the strain-rate-based production terms of the k-$\omega$ and k-$\omega$ SST turbulence models, which leads to the over-prediction of aerothermal prediction. While this phenomenon does not occur for Spalart–Allmaras (S–A) turbulence model because of its vorticity-based production term. In order to eliminate this unphysical phenomenon, and to maintain the accuracy of the original models for boundary layer and separation flows, a new correction method for the k-$\omega$ and k-$\omega$ SST models is proposed: by comparing the orders of magnitude between the strain-rate-based and vorticity-based production terms, the vorticity-based production term is used near the shock waves, while the original strain-rate-based production term is still used in other regions. Finally, the correction method is applied to turbulence and transition flows over blunt bodies, and the numerical results show that the correction method effectively eliminates the unphysical increase of turbulent quantities across shock waves and improves the accuracy of aerothermal and transition onset location prediction.     

基于梁平截面假设的复杂细长结构动力模型简化方法

针对纵向尺度显著大于横向的复杂细长结构,提出了一个基于梁平截面假设的模型简化方法,建立了具有物理意义的基向量。同时考虑这类结构截面变形的翘曲模式,增加了翘曲基向量,得到精度较高的简化动力模型。这样的简化模型可以用于原结构在不同边界条件的动力分析。结构频率计算的两个具体算例表明了简化方法的有效性。     

A simple model for the effect of peak-locking on the accuracy of boundary layer turbulence statistics in digital PIV

A simple model was constructed to study the effect of peak-locking on the accuracy of particle image velocimetry (PIV) turbulence statistics. A crucial parameter is the ratio between the root-mean-square (rms) velocity and the discretization velocity, which reflects the number of peaks distributed over the velocity probability density functions. When the ratio of the discretization velocity, which is set by the PIV setup parameters, to the rms, given by the flow, is larger than two, the maximum errors introduced in the mean and rms values become significant (larger than 1%). The errors introduced also depend on the amplitude, or severity, of the peak-locking, and whether the mean displacement corresponds to an integer or a fractional number of pixels. The peak-locking affects the statistical moments of different order in such a way that the errors are phase shifted. The proposed model can be used to predict errors in the turbulence statistics in a laboratory PIV experiment. According to our model predictions, the most significant influence of peak-locking in a boundary layer type of flow is an overall underestimation of the wall-normal rms. Our predictions are in good agreement with our experimental results from turbulent boundary layers and the recent experimental results from a turbulent channel flow by Christensen (Exp Fluids 36:484–497, 2004) for a case of moderate peak-locking.

---

战斗部近距爆炸下夹芯复合舱壁结构防护能力的理论评估模型

为改善当前战斗部近距爆炸下基于单纯抗爆或抗穿甲载荷开展防护结构设计的不足,本文中建立了战斗部近距爆炸下夹芯复合舱壁结构防护能力的理论评估模型,提出了联合作用下夹芯复合舱壁结构的防护能力需同时满足抗弹性能和整体变形破坏两方面要求。具体步骤为:首先计算战斗部爆炸后的联合毁伤载荷,然后基于抗弹理论模型评估夹芯复合舱壁结构的抗弹性能。若满足要求,则进一步根据联合作用理论模型校核夹芯复合舱壁结构在冲击波和破片群联合作用下是否满足整体变形破坏要求,判据为后面板是否产生撕裂、破口破坏。与有关实验结果进行了计算比较,结果吻合良好,证明了此理论评估模型的合理性。