Inviscid,laminar and turbulent opposed flows

This paper attempts to reproduce numerically previous experimental findings with opposed flows and extends their range to quantify the effects of upstream pipes and nozzles with inviscid, laminar and turbulent flows. The choice of conservation equations, boundary conditions, algorithms for their solution, the degree of grid dependence, numerical diffusion and the validity of numerical approximations are justified with supporting calculations where necessary. The results of all calculations on the stagnation plane show maximum strain rates close to the annular exit from the nozzles and pipes for lower separations and it can be expected that corresponding reacting flows will tend to extinguish in this region with the extinction moving towards the axis. With laminar flows, the maximum strain rate increased with Reynolds number and the maximum values were generally greater than with inviscid flows and smaller than with turbulent flows. With large separations, the strain rates varied less and this explains some results with reacting flows where the extinction appeared to begin on the axis. The turbulent‐flow calculations allowed comparison of three common variants of a two‐equation first‐moment closure. They provided reasonable and useful indications of strain rates but none correctly represented the rms of velocity fluctuations on the axis and close to the stagnation plane. As expected, those designed to deal with this problem produced results in better agreement with experiment but were still imperfect. Copyright © 2004 John Wiley & Sons, Ltd.     

General Anisotropic Effective Medium Theory for the Effective Permeability of Heterogeneous Reservoirs

One of the techniques to calculate the effective property of a heterogeneous medium is the effective medium theory. The present paper presents a general mathematical formulation for the effective medium approximation using a self-consistent choice of the effective permeability, to apply it to the case of a general anisotropic 2D medium and to the case of a 3D isotropic medium with randomly oriented ellipsoidal inclusions. The 2D results are compared with analytical results and with a homogenization technique with good result. The 3D correlations are used to derive percolation thresholds in two-phase systems with a large permeability contrast, which are compared to numerical results from the literature, also with good results.     

微型四旋翼飞行器最佳旋翼间距布局研究

为研究悬停状态下不同翼间距对微型四旋翼飞行器气动性能的影响,结合整机试验和数值模拟,分析了不同旋翼间距下微型四旋翼飞行器拉力和功耗的变化规律。在样机试验中,通过搭建试验平台对间距比l/d范围1.1～2.0的微型四旋翼飞行器进行了拉力和功耗的测量,确定了相同功耗条件下具有较大拉力的最佳旋翼间距范围。为更直观地得到旋翼间气动干扰对整机气动性能的影响,通过CFD方法对微型四旋翼飞行器流场进行了仿真,得到了不同间距下的压力、流线和涡量分布情况,进而对四旋翼飞行器在不同旋翼间距下表现出的不同气动特性进行对比。结果表明,与无干扰状态下的孤立单旋翼相比,四旋翼间存在的气动干扰在合理的旋翼间距下可以保持涡流完整,并有助于提升四旋翼系统的拉力。最后,通过试验和仿真对比发现,在旋翼间距为1.8d时,四旋翼飞行器具有较大的功率载荷和良好的气动特性,是该四旋翼飞行器整机的最佳气动布局。     

智能结构密频系统振动控制及其摄动分析

提出一种智能结构密频控制规律的设计方法。其主要工作为：将密频子空间转化为重频子空间,把密频系统作为重频系统上的摄动;为了解决重频密频子空间相对应的特征向量选取的敏感性问题,通过摄动分析得到与重频密频子空间相对应特征向量的线性组合并利用闭环系统特征值极点配置的方法得到重频密频系统的振动规律;把所设计的振动控制规律作用于原系统和摄动系统上,讨论了结构参数改变后对系统的动态特性的影响;最后通过算例证明该方法的有效性。     

Thermal Stresses in Plane Strain of Porous Elastic Solids

This paper is concerned with the linear theory of thermoelastic materials with voids. We present a method to reduce the thermoelastic problem to an isothermal one with zero body loads and with certain known boundary data. The results are used to study the thermal stresses in a tube and the thermoelastic deformation of a cylinder subjected to a uniform temperature gradient.     

Influence of vertical internals on a bubbling fluidized bed characterized by X-ray tomography

An ultra-fast X-ray tomographic scanner is applied to study the hydrodynamics in a bubbling fluidized bed with and without vertical internals (e.g., heat exchanger tubes). The objective of this study is to understand the influence of vertical internals on hydrodynamic properties such as bubble volume, size and velocity and to provide measurement data for the design and scale-up of catalytic bubbling fluidized bed reactors with vertical internals. With these new measurements, correlations of bubble properties can be developed to reliably scale-up bubbling fluidized beds with vertical internals. For the investigated reactor with Geldart A/B particles, no relation between bubble size and velocity was observed for individual bubbles, i.e.; smaller bubbles tend to rise with higher velocities. A significant reduction in bubble size and sharpening of the bubble size distribution was generally obtained for a bed with vertical internals.     

摩擦学系统条件转化研究

通过对摩擦学系统条件转化的研究和转化实例分析 ,提出了解决摩擦学系统条件转化问题的新思路和方法 .人工智能技术为摩擦学系统条件转化研究提供了有效的手段 ,从而使得将某些摩擦系统条件下获得的摩擦学特性转化到其它系统条件下使用成为可能     

Ultrasound scattering from a turbulent round thermal pure plume

This research work brings about additional contribution to validate the ultrasound scattering technique as a nonintrusive probe in the Fourier space for measurements performed in unsteady flows. In particular, this work reports experimental evidence of scattering from a turbulent thermal plume utilized as a testing flow. This technique is based upon the scattering of an ultrasound wave hitting and interacting with an unstable flow. The coupling among the acoustic mode with vorticity and entropy modes is derived from nonlinear terms of Navier–Stokes and energy equations. Scattering mechanism occurs when characteristic length scales of flows are comparable with wavelength of sound. Thus, it is possible to probe the flow at different length scales by changing the incoming frequency. The results allow verifying some theoretical predictions, such as the existence of a nonscattering angle. It was also observed, that both the phase and the Doppler shift of the Fourier's signal are linear, respectively, with respect to the time and the frequency of the incident wave. The Doppler shift allowed us to determine the advection velocity and has proved to be sensitive to the direction of the wave vector, to the scattering angle and also, we show that it is possible to have both positive and negative angles. The advection velocity increases with temperature and its values are coherent with those obtained with traditional techniques. Broadening and Doppler shift of the scattering signal allowed us to define the turbulence intensity, whose values are in agreement with those found in thermal plumes, where well-known techniques are currently used. This study has shown that the turbulence intensity increases weakly with temperature, nevertheless it seems more sensitive to the size of the structure under observation.     

An application of Discontinuous Galerkin space and velocity discretisations to the solution of a model kinetic equation

An approach based on a Discontinuous Galerkin discretisation is proposed for the Bhatnagar–Gross–Krook model kinetic equation. This approach allows for a high-order polynomial approximation of molecular velocity distribution function both in spatial and velocity variables. It is applied to model one-dimensional normal shock wave and heat transfer problems. Convergence of solutions with respect to the number of spatial cells and velocity bins is studied, with the degree of polynomial approximation ranging from zero to four in the physical space variable and from zero to eight in the velocity variable. This approach is found to conserve mass, momentum and energy when high-degree polynomial approximations are used in the velocity space. For the shock wave problem, the solution is shown to exhibit accelerated convergence with respect to the velocity variable. Convergence with respect to the spatial variable is in agreement with the order of the polynomial approximation used. For the heat transfer problem, it was observed that convergence of solutions obtained by high-degree polynomial approximations is only second order with respect to the resolution in the spatial variable. This is attributed to the temperature jump at the wall in the solutions. The shock wave and heat transfer solutions are in excellent agreement with the solutions obtained by a conservative finite volume scheme.     

Application of GDQ Method for Study of Mixed Convection in Horizontal Eccentric Annuli

A generalized differential-integral quadrature (GDQ) discretization technique was used to solve a mixed heat convection problem in a body-fit coordinate system in its primitive variables form. A special treatment of the boundary condition to satisfy the continuity and momentum equations along the boundaries with the implementation of the GDQ method was investigated. Comparisons with the experimental and numerical results of other investigators are presented and discussed. In contrast with the existing published results, this highly accurate method was able to reveal extremely weak net circulation around the outer cylinder. In the horizontal annulus with the mixed heat convection problem the combination of unbalance of buoyancy and centrifugal forces causes net circulation. The net circulation decreases and approaches to zero with the rise of Rayleigh number, and it reaches its minimum value with high eccentricity when the inclination angle of eccentricity is π.     

A phenomenological modification of rheological models for concentrated two-phase systems: application to a thermoplastic/thermoset blend

This paper considers an improvement of the emulsion models to take into account concentrated emulsions with no coalescence but with significant interaction between particles. For this purpose, a term proportional to the volume fraction of material in excess to the percolation threshold is added to the dynamic modulus. Its usefulness was tested to model the viscoelastic behavior in oscillatory shear flow of concentrated and diluted blends of a thermoplastic polystyrene with an epoxy-amine thermoset. These blends experience phase separation upon polymerization and the volume fraction of separated phase varies continuously with time. At low volume fraction of dispersed phase, the behavior could be described with a simple emulsion model that takes into account the plastisizing, dilution, and phase separation mechanisms. However, for concentration in excess to the percolation threshold, the modification can cope with a larger increase in the modulus related to the mechanical percolation of the dispersed particles.     

Modeling low Reynolds number incompressible flows with curved boundaries using SPH

In this paper, a method that can be used to model low Reynolds number incompressible flows with curved boundaries using SPH was presented. In contrast to that usually used for the flows with flat and straight boundaries, the hydrostatic pressure gradient is treated as a variable body force in this method, and thus, it can be applied to simulate the flows with curved boundaries. Three numerical examples of low Reynolds number incompressible flows, including Poiseuille flow, flow in a section of blood vessel with a local expansion, and flow between inclined plates were calculated to test the method. The results obtained with the proposed method were in good agreement with the analytical solutions. It implies that the method presented in this paper can be successfully used to simulate low Reynolds number incompressible flows not only with flat and straight boundaries but also with curved boundaries. Copyright © 2011 John Wiley & Sons, Ltd.     

Transient flow around an impulsively started cylinder using a dynamic mesh method

The finite volume method (FVM) with a dynamic mesh method (DMM) to deal with the moving boundary was applied to the simulation of two-dimensional incompressible viscous flow past a circular cylinder that is impulsively started into rotation and translation. The non-dimensional rotating to translating speed ratio α is varied from 0.28 to 2.07, with the Reynolds number being 200 for the range of α. The computation covers a period, during which the cylinder translates seven times its diameter. The current scheme handles the impulsively moving boundary directly by DMM, which is implemented using both mesh deforming and local remeshing. The instantaneous asymmetrical flow configurations for various α are presented and compared with the experimental visualizations. Quantitatively, the velocity distributions with drag and lift coefficients are also compared with the experimental and numerical results. Results show that the flow is strongly influenced by the rotation. Comparisons are found to be satisfactory.     

Shape optimisation problem for stability of Navier–Stokes flow field

ABSTRACT

This paper presents the numerical results of a shape optimisation problem with regard to delaying the transition of a Navier–Stokes flow field from laminar to turbulent by using the theory developed by Nakazawa and Azegami. The theory was reviewed within the framework of functional analysis and updated with another expression of the shape derivative with respect to the objective function. A computer program was developed with the FreeFEM++. Numerical analyses were performed for two types of problems: a two-dimensional Poiseuille flow field with a sudden expansion and a two-dimensional uniform flow field around an isolated body. From the first example, two local minimum points of symmetric and asymmetric flow fields were determined, and the asymmetric flow field was found to be more stable. With regard to the second example, we reached the local minimum point of an elliptical shape, and infrequently determined a solution converging to an elliptical shape with the bluff in the leeward direction. By comparison, the superiority of the elliptical shape was obvious.     

Stress and Strain Analysis of Pipelines with Localized Metal Loss

This paper presents the analysis of stress and strain data acquired with the finite element method and with tests that used post-yielding strain gages bonded onto the external surface of pipes that suffered thickness metal loss and that had been loaded with internal pressure. These metal loss areas were produced by three different processes: actual internal corrosion, careful machining of external patches by spark-erosion, and milling of internal or external patches to simulate limited or extensive strip corrosion defects with depths up to 70% of the pipe’s thickness. Results show that: (1) the extensive longitudinal internal or external defect areas behave as extensive strips with a high degree of freedom to deform elastically and plastically in the circumferential and thickness directions, and (2) large restraints are offered to the longitudinal strains by the non-corroded thick walls parallel to the strip. Using the above experimental observation, a simple mathematical model was developed to predict the burst pressure of pipes with longitudinal extensive and reasonably constant depths of metal loss. This model employed thin-pipe-strength-of-material equations associated to a bulging correction factor, the material’s uniaxial ultimate strength and the von Mises criterion. The onset of plastic collapse predicted by the simple model was successfully compared with results determined from actual hydrostatic tests that were carried out with full scale pipe specimens and from finite element results generated by the use of a commercial program. The developed model was also helpful in showing that the yield and burst behaviors of new or corroded pipeline specimens under laboratory test conditions can be directly compared and extended to the yield and burst behaviors of buried pipeline in field operation.     

Robust control volume finite element methods for numerical wave tanks using extreme adaptive anisotropic meshes

Multiphase inertia-dominated flow simulations, and free surface flow models in particular, continue to this day to present many challenges in terms of accuracy and computational cost to industry and research communities. Numerical wave tanks and their use for studying wave-structure interactions are a good example. Finite element method (FEM) with anisotropic meshes combined with dynamic mesh algorithms has already shown the potential to significantly reduce the number of elements and simulation time with no accuracy loss. However, mesh anisotropy can lead to mesh quality-related instabilities. This article presents a very robust FEM approach based on a control volume discretization of the pressure field for inertia dominated flows, which can overcome the typically encountered mesh quality limitations associated with extremely anisotropic elements. Highly compressive methods for the water-air interface are used here. The combination of these methods is validated with multiphase free surface flow benchmark cases, showing very good agreement with experiments even for extremely anisotropic meshes, reducing by up to two orders of magnitude the required number of elements to obtain accurate solutions.     

Static analysis of Euler–Bernoulli beams with multiple unilateral cracks under combined axial and transverse loads

The paper deals with the static analysis of pre-damaged Euler–Bernoulli beams with any number of unilateral cracks and subjected to tensile or compression forces combined with arbitrary transverse loads. The mathematical representation of cracks with a bilateral behaviour (i.e. always open) via Dirac delta functions is extended by introducing a convenient switching variable, which allows each crack to be open or closed depending on the sign of the axial strain at the crack centre. The proposed model leads to analytical solutions, which depend on four integration constants (to be computed by enforcing the boundary conditions) along with the Boolean switching variables associated with the cracks (whose role is to turn on and off the additional flexibility due to the presence of the cracks). An efficient computational procedure is also presented and numerically validated. For this purpose, the proposed approach is applied to two pre-damaged beams, with different damage and loading conditions, and the results so obtained are compared against those given by a standard finite element code (in which the correct opening of the cracks is pre-assigned), always showing a perfect agreement.     

A frictional molecular model for the viscoelasticity of entangled polymer nanocomposites

The dynamics of polymer melts and concentrated solutions reinforced with nanoscale rigid spherical particles is analyzed. Nanocomposites with low filler volume fraction and strong polymer-filler interactions are considered. Entanglement effects are represented by requiring the diffusion in the chain contour direction to be more pronounced than in the direction transverse to the chain primitive path. Filler particles are treated as material points. They reduce the polymer mobility in both longitudinal and transverse tube directions due to short-range energetic filler-polymer interactions. Hence, the contribution to chain dynamics and stress production of both filler-polymer and polymer-polymer interactions is considered to be purely frictional in nature. In the model, the strain rate sensitivity is associated with the thermal motion of chains, with the convective relaxation of entanglement constrains and with the polymer-filler attachment/detachment process. The effect of model parameters is discussed and the predictions are compared with experimental data.     

喜马拉雅山地区重大滑坡灾害及其与地层岩性的关系研究

位于青藏高原南部的喜马拉雅山地区,是高原隆升最快的地方,这里内外动力作用异常活跃,是我国重大滑坡灾害最严重的地区之一,重大滑坡灾害对国民经济和社会发展带来了极大危害。本文在对研究区的地质、地理背景进行了详细分析的基础上,利用遥感解译和实地调查相结合的手段,研究了该区重大滑坡灾害的分布及其与地层岩性的关系,将本区地层岩性大致划分为13个工程地质岩组:松散岩组、软弱岩组、软弱岩夹较软弱岩组、软弱岩夹较坚硬岩组、较软弱岩组、较软弱岩夹软弱岩组、较软弱岩夹较坚硬岩组、较软弱岩夹坚硬岩组、较坚硬岩与软弱岩互层岩组、较坚硬岩夹软弱岩岩组、较坚硬岩夹较软弱岩组、较坚硬岩组、坚硬岩组,发现重大滑坡更容易发生在软弱岩组、较硬岩夹较软弱岩组以及坚硬岩组中,而软弱岩夹较软弱岩组、较软弱岩夹较坚硬岩组中无重大滑坡灾害分布。重大滑坡灾害的上述分布特征,反映出地层岩性与重大滑坡灾害之间的复杂关系,一方面岩性软弱,比较容易发生滑坡灾害; 另一方面,岩性越坚硬,地形越高陡,也容易发生重大滑坡灾害。     

On coupled gradient-dependent plasticity and damage theories with a view to localization analysis

Combinations of gradient plasticity with scalar damage and of gradient damage with isotropic plasticity are proposed and implemented within a consistently linearized format. Both constitutive models incorporate a Laplacian of a strain measure and an internal length parameter associated with it, which makes them suitable for localization analysis.The theories are used for finite element simulations of localization in a one-dimensional model problem. The physical relevance of coupling hardening/softening plasticity with damage governed by different damage evolution functions is discussed. The sensitivity of the results with respect to the discretization and to some model parameters is analyzed. The model which combines gradient-damage with hardening plasticity is used to predict fracture mechanisms in a Compact Tension test.