含裂纹一维欧拉梁的裂纹无效位置分析

基于集中柔度模型,建立了含裂纹一维欧拉梁的频率方程,以此为基础探讨了裂纹无效位置的求解方法。数值计算结果显示,裂纹无效位置和位移振型节点并不一致。进一步的理论推导证明裂纹无效位置就是曲率模态振型的零点位置,从曲率和力学性能基本参数的关系分析,这一结论是合理的。本文结论对于实验、测试方案设计有指导意义。     

Existence of Rotating Spots with Spatially Dependent Feedback in the Plane in a Wave Front Interaction Model

In this paper, we study the localized rotating spots created by the spatially inhomogeneous feedback. We adopt the wave front interaction model proposed by Zykov and Showalter in 2005. The existence of rotating spots in the plane are shown by choosing the spatially dependent feedbacks appropriately.     

Intensity Capping: a simple method to improve cross-correlation PIV results

A common source of error in particle image velocimetry (PIV) is the presence of bright spots within the images. These bright spots are characterized by grayscale intensities much greater than the mean intensity of the image and are typically generated by intense scattering from seed particles. The displacement of bright spots can dominate the cross-correlation calculation within an interrogation window, and may thereby bias the resulting velocity vector. An efficient and easy-to-implement image-enhancement procedure is described to improve PIV results when bright spots are present. The procedure, called Intensity Capping, imposes a user-specified upper limit to the grayscale intensity of the images. The displacement calculation then better represents the displacement of all particles in an interrogation window and the bias due to bright spots is reduced. Four PIV codes and a large set of experimental and simulated images were used to evaluate the performance of Intensity Capping. The results indicate that Intensity Capping can significantly increase the number of valid vectors from experimental image pairs and reduce displacement error in the analysis of simulated images. A comparison with other PIV image-enhancement techniques shows that Intensity Capping offers competitive performance, low computational cost, ease of implementation, and minimal modification to the images.     

Visualisation of turbulent spots under the influence of adverse pressure gradients

In this paper, a method of using temperature sensitive liquid crystal to visualize the thermal footprints of turbulent spots is described. It is employed to study the nature and development of artificially created turbulent spots in a laminar boundary layer over a heated surface under the influence of adverse pressure gradients in a water flow experiment. This method not only provides qualitative information about the structures and growth of the spot but also contours of heat flux beneath an entire spot as it propagates downstream. Furthermore, the lateral spreading angle and propagation rate of turbulent spots are obtained by analysing the colour images and heat transfer contours. The results are compared with those obtained by other researchers using different experimental methods. Received: 17 June 1998/Accepted: 7 June 1999     

Statistical modeling of the turbulent transition in the boundary layer

A method of statistical modeling the flow in the boundary-layer transition region is proposed on the basis of experimental data on kinematics and dynamics of turbulent spots (Emmons spots) on a flat plate in an incompressible fluid. This method allows one to determine the intermittency with allowance for overlapping of the spots, the forces on the plate surface, and the flow field in the vicinity of the transition region if the field of the streamwise component of the mean velocity in the developed turbulent boundary layer is known as a function of the Reynolds number. In contrast to multi-parameter models of the transition, this approach makes it possible to avoid the use of physically meaningless parameter values.     

New experimental methods for turbulent spots and turbulent spirals in the Taylor-Couette flow

 The flow between concentrically counter-rotating circular cylinders is investigated experimentally with respect to the appearance of turbulent spots, combining two new methods, based on digital image processing. The simultaneous visualization of the transition to turbulence in the whole flow field in the gap between the two cylinders leads to a qualitative understanding of the phenomenon of turbulent spots. Quantitative results about spiral turbulence are obtained from measurements based on a time-resolved technique introducing a special method of image processing for long sequences of video frames. Variations of the gap width between the cylinders and the investigation of different boundary conditions at the end plates of the rotating cylinders allow conclusions concerning the importance of locally defined parameters of the flow field. Received: 4 December 1995/Accepted: 15 May 1997     

Turbulent Spot in Linearly Stable Taylor Couette Flow

The transition from the laminar to the turbulent regime in linearly stable shear flows, for example, pipe and plane Couette flows, occurs abruptly with no precursor. The evolution of turbulent spots has been studied to better understand the dynamics of this transition and the onset of turbulence. These studies have mostly focused on pipe flows for which a finite lifetime of spots was proven. The same conclusion was drawn in the only available study performed in a Taylor Couette setup. Here, the spot lifetime is measured in a different size TC setup. It is shown that the lifetime is indeed finite and also very sensitive to boundary conditions, but not much to perturbation mechanisms. A scaling approach is provided which suggests in addition to the Reynolds number, the aspect and radius ratios are influential parameters on the lifetime. It is found that the spot size varies during its lifetime and increases with the Reynolds number that confirms the rise in turbulence proliferation by approaching the transitional point.     

Strain measurement using heterodyne photogrammetry of white-light specklegrams

When an object coated with retroreflective paint is illuminated and imaged through a beamsplitter, a good lens system can resolve the individual beads in the paint to form high-contrast spots in a nearly black field. When the resulting photographs are converted to dichromated gelatin, the spots form microlenses that scatter light over a much wider angle than defined by the focal ratio of the lens. Treated as specklegrams, these photographs allow strain resolutions almost equivalent to laser-speckle photographs.Paper was presented at 1984 SEM Fall Conference on Experimental Mechanics held in Milwaukee, WI on November 4–7, 1984.     

Measurements of admixture concentration fluctuations in a turbulent shear flow using an averaged Talbot image

The present study is concerned with adopting of a Talbot effect-based technique for analyzing flows with random phase inhomogeneities. It is shown that this method is a powerful tool for diagnostics of turbulent flows. The potential of the technique is illustrated by measuring mean and fluctuating values of admixture concentration of two-dimensional turbulent helium jet issuing into the ambient air. Averaged air and helium concentrations throughout the flow field are determined using local light refraction measurements with a high spatial resolution from a long-exposed Talbot image of the jet. The analysis of light intensity distributions in light spots of a Talbot-image shows that the jet turbulence is inhomogeneous and anisotropic. Quantitative information on rms fluctuations of concentration gradients throughout the flow field is obtained from local photometric measurements at the Talbot light spots.     

Numerical modeling of the receptivity of a supersonic boundary layer to entropy disturbances

Numerical modeling of the receptivity of a two-dimensional flat-plate boundary layer to entropy disturbances is carried out at the freestream Mach number M_∞ = 6. Low-intensity perturbations considered are in the form of temperature spots of various shapes and with different initial positions downstream of the shock. They are shown to be able to generate unstable disturbances in the boundary layer. This receptivity mechanism is relatively weak as compared with the receptivity to acoustic waves. When the entropy perturbations are introduced upstream of the bow shock, they first pass across the shock. Downstream of the shock this interaction generates acoustic waves which, in turn, penetrate into the boundary layer thus exciting unstable disturbances of a considerably greater amplitude than the temperature spots. Thus, the bow shock can change the receptivity mechanism.     

Wave forerunners of streamwise structures in a swept wing boundary layer

In a model experiment wave packets (forerunners) have been detected for the first time in the flow regions preceding the fronts of streamwise structures in a swept wing boundary layer. The characteristics of the wave packets and the generating streamwise structures and the dynamics of their downstream development are investigated. It is shown that the forerunners can transform into turbulent spots thus leading to laminar-turbulent transition. Certain components of the forerunners are compared with a periodic instability wave.     

The Effect of wake Turbulence Intensity on Transition in a Compressor Cascade

Direct numerical simulations of separating flow along a section at midspan of a low-pressure V103 compressor cascade with periodically incoming wakes were performed. By varying the strength of the wake, its influence on both boundary layer separation and bypass transition were examined. Due to the presence of small-scale three-dimensional fluctuations in the wakes, the flow along the pressure surface undergoes bypass transition. Only in the weak-wake case, the boundary layer reaches a nearly-separated state between impinging wakes. In all simulations, the flow along the suction surface was found to separate. In the simulation with the strong wakes, separation is intermittently suppressed as the periodically passing wakes managed to trigger turbulent spots upstream of the location of separation. As these turbulent spots convect downstream, they locally suppress separation.     

Patterned turbulence in spatially evolving magnetohydrodynamic duct and pipe flows

Laminar-turbulent transition in spatially evolving magnetohydrodynamic pipe and duct flows is investigated numerically. The results are in good agreement with the classical 1937 experiments of J. Hartmann and F. Lazarus. It is found that the recently discovered flow regimes with turbulent spots near sidewalls are realized in spatially developing flows and must be detectable in experiments.     

Ignition and detonation of solid explosives: a micromechanical burn model

This paper describes a new computational framework for modeling splid explosives and proof-of-concept calculations. Our goal is to expand predictive model capability through the inclusion of various micro-mechanical burn processes. We propose a model which is complicated enough to represent underlying physics, but simple enough for engineering scale computations. Key components of the model include energy localization, the growth of hot spots, micro-mechanics in/around hot spots, and a phase-averaged mixture equation of state. The nucleation and growth of locally heated regions is treated by a statistical model based on an exponential size distribution. Proof-of-concept calculations are limited to shock loading, but show the capability of simulating Pop-plots, initial temperature effect, detonation waves in 2D, detonation shock confinement test, and multi-dimensional effects in a unified fashion based on micro-physics.

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宝顶山石窟岩体风化破坏的作用因素分析

宝顶山石窟石雕风化破坏日趋严重，这是石刻区普遍存在的问题。本文在现场工作及试验研究的基础上，从岩体内部作用因素和外部环境因素两方面，详尽论证岩体风化破坏的形成原因，作为石刻区制定防风化措施的依据。     

Statistical description of laminar-turbulent transition in a boundary layer at high freestream turbulence degree

The statistical approach is applied to calculate the intermittence coefficient in boundarylayer laminar-turbulent transition due to external turbulence. It is assumed that turbulent regions in the boundary layer are associated with the appearance of turbulent spots generated by the secondary instability of streaky structures that have reached a threshold amplitude. An universal dependence of the intermittence coefficient in the transition region on the velocity fluctuation amplitude and the Reynolds number is obtained. It describes well the experimental results.     

DNS of heat transfer in transitional, accelerated boundary layer flow over a flat plate affected by free-stream fluctuations

Direct numerical simulations (DNS) of flow over and heat transfer from a flat plate affected by free-stream fluctuations were performed. A contoured upper wall was employed to generate a favourable streamwise pressure gradient along a large portion of the flat plate. The free-stream fluctuations originated from a separate LES of isotropic turbulence in a box. In the laminar portions of the accelerating boundary layer flow the formation of streaks was observed to induce an increase in heat transfer by the exchange of hot fluid near the surface of the plate and cold fluid from the free-stream. In the regions where the streamwise pressure gradient was only mildly favourable, intermittent turbulent spots were detected which relaminarised downstream as the streamwise pressure gradient became stronger. The relaminarisation of the turbulent spots was reflected by a slight decrease in the friction coefficient, which converged to its laminar value in the region where the streamwise pressure gradient was strongest.     

铸装TNT炸药表面热点温度的测量

非均匀炸药冲击起爆的热点理论一直是爆轰物理中,人们普遍关心的课题之一。本文对热点温度实验测量工作进行了尝试,叙述了用多通道光学高温计测量非均匀炸药表面热点温度的原理及方法。用实验方法得到在3.44GPa左右的冲击压力下,铸装TNT炸药表面热点温度约为3640K。     

The fundamental solution in the theory of shallow shells

A series representation for the fundamental solution of the shallow shell equations is obtained by means of a plane-wave decomposition of the Dirac δ-function. From this solution we can produce the singular solutions which correspond to concentrated forces, couples and thermal hot spots applied to a shallow shell with an arbitrary quadratic middle surface. The solutions converge for the entire range of the Gaussian curvature. Numerical results are presented for the case of a concentrated normal force acting on infinite shells having positive, zero or negative Gaussian curvature.