Flows around two airfoils performing fling and subsequent translation and translation and subsequent clap

The aerodynamic forces and flow structures of two airfoils performing “fling and subsequent translation“ and “translation and subsequent clap“ are studied by numerically solving the Navier-Stokes equations in moving overset grids. These motions are relevant to the flight of very small insects. The Reynolds number, based on the airfoil chord length c and the translation velocity U, is 17. It is shown that: (1) For two airfoils performing fling and subsequent translation, a large lift is generated both in the fling phase and in the early part of the translation phase. During the fling phase,a pair of leading edge vortices of large strength is generated; the generation of the vortex pair in a short period results in a large time rate of change of fluid impulse, which explains the large lift in this period. During the early part of the translation, the two leading edge vortices move with the airfoils;the relative movement of the vortices also results in a large time rate of change of fluid impulse, which explains the large lift in this part of motion. (In the later part of the translation, the vorticity in the vortices is diffused and convected into the wake.) The time averaged lift coefficient is approximately 2.4 times as large as that of a single airfoil performing a similar motion. (2) For two airfoils performing translation and subsequent clap, a large lift is generated in the clap phase. During the clap, a pair of trailing edge vortices of large strength are generated; again, the generation of the vortex pair in a short period (which results in a large time rate of change of fluid impulse) is responsible for the large lift in this period. The time averaged lift coefficient is approximately 1.6 times as large as that of a single airfoil performing a similar motion. (3) When the initial distance between the airfoils (in the case of clap, the final distance between the airfoils) varies from 0.1 to 0.2c, the lift on an airfoil decreases only slightly but the torque decreases greatly. When the distance is about lc， the interference effects between the two airfoils become very small.     

Connection between static electromagnetoelasticity and anisotropic elasticity and its applications

Connection between electromagnetoelasticity and anisotropic elasticity is explored in the state space setting. In the absence of electric charges and currents, the basic equations of static electromagnetoelasticity are formulated into a state equation and an output equation, which bear a remarkable resemblance to the corresponding equations of elasticity. Accordingly, the solutions for various steady-state problems of electromagnetoelasticity can be determined in parallel to their elastic counterparts. For illustration, the generalized plane problems are treated within the context. Exact solutions for the electromagnetoelastic fields in a half-space subjected to line loads and in an infinite plate with an elliptic notch under extension are determined in a simple way.     

Nonlinear Spherical Caps and Associated Plate and Membrane Problems

This paper deals with the existence and multiplicity problem of the equilibrium solutions of an elastic spherical cap within nonlinear strain theory. We pose the problem in the form of a three parameter bifurcation problem, one parameter being related to the load, the others to the geometry. When the geometrical parameters are different from zero, the so-called generic case, we revisit the nonuniqueness results, and explore the solutions in the parameter space. Then we analyze the formal limits as the geometrical parameters tend to zero. When the curvature tends to zero, we obtain from the nonlinear shell a von Kármán plate, a new, although natural, result. When the thickness parameter tends to zero, we get a nonlinear membrane problem. A study of the latter gives infinitely many solutions, and we discuss the construction, shapes, and stability in detail. This revised version was published online in August 2006 with corrections to the Cover Date.     

Calculation of friction and heat and mass exchange on rough permeable plane and axisymmetric bodies

The most effective methods of thermal shielding are characterized by the presence of a normal component of the velocity on the wall. They include porous cooling and the use of ablated coverings. If the heat-shielding materials have a granulated microstructure, their thermal expansion leads to roughness of the surface, as in the case of porosity of the wall. The present paper presents an integral method for calculating the drag and heat and mass transfer to a rough permeable surface in the case of a gradient flow of a compressible reacting gas.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 37–46, September–October, 1979.It is a pleasure to thank J. A. Schetz, who kindly sent tables of his experimental data, and also E. G. Zaulichny for discussing the work.     

Instability and friction

A review on the stability analysis of solids in unilateral and frictional contact is given. The presentation is focussed on the stability of an equilibrium position of an elastic solid in frictional contact with a fixed or moving obstacle. The problem of divergence instability and the obtention of a criterion of static stability are discussed first for the case of a fixed obstacle. The possibility of flutter instability is then considered for a steady sliding equilibrium with a moving obstacle. The steady sliding solution is generically unstable by flutter and leads to a dynamic response which can be chaotic or periodic. This dynamic response leads to the generation of stick–slip–separation waves on the contact surface in a similar way as Schallamach waves in statics. Illustrating examples and principal results recently obtained in the literature are reported. Some problems of friction-induced vibration and noise emittence, such as brake squeal for example, can be interpreted in this spirit. To cite this article: Q.S. Nguyen, C. R. Mecanique 331 (2003).     

Data-driven modeling and learning in science and engineering

In the past, data in which science and engineering is based, was scarce and frequently obtained by experiments proposed to verify a given hypothesis. Each experiment was able to yield only very limited data. Today, data is abundant and abundantly collected in each single experiment at a very small cost. Data-driven modeling and scientific discovery is a change of paradigm on how many problems, both in science and engineering, are addressed. Some scientific fields have been using artificial intelligence for some time due to the inherent difficulty in obtaining laws and equations to describe some phenomena. However, today data-driven approaches are also flooding fields like mechanics and materials science, where the traditional approach seemed to be highly satisfactory. In this paper we review the application of data-driven modeling and model learning procedures to different fields in science and engineering.     

Stability and folds

It is known that when one branch of a simple fold in a bifurcation diagram represents (linearly) stable solutions, the other branch represents unstable solutions. The theory developed here can predict instability of some branches close to folds, without knowledge of stability of the adjacent branch, provided that the underlying problem has a variational structure. First, one particular bifurcation diagram is identified as playing a special role, the relevant diagram being specified by the choice of functional plotted as ordinate. The results are then stated in terms of the shape of the solution branch in this distinguished bifurcation diagram. In many problems arising in elasticity the preferred bifurcation diagram is the loaddisplacement graph. The theory is particularly useful in applications where a solution branch has a succession of folds.The theory is illustrated with applications to simple models of thermal selfignition and of a chemical reactor, both of which systems are of Émden-Fowler type. An analysis concerning an elastic rod is also presented.     

Non-normality and bifurcation in plane strain tension and compression

The bifurcations of a rectangular block subject to plane strain tension or compression are investigated. The block material is taken to be incompressible and is characterized by an incrementally linear constitutive law for which “normality” does not necessarily hold. The consequences of non-normality regarding bifurcation are given primary emphasis here. The characteristic regimes of the governing equations (elliptic, parabolic and hyperbolic) are detennined. In each of these regimes both symmetric and antisymmetric diffuse bifurcation modes are available. Additionally, in the hyperbolic and parabolic regimes, bifurcation into a localized shear band mode is also possible. Particular attention is given to the limiting cases of long wavelength and soon wavelength diffuse bifurcation modes. The range of parameter values is identified for which bifurcation into some localized mode may precede bifurcation into a long wavelength diffuse mode. Some difficulties associated with employing a linear incremental solid in a bifurcation analysis, when primary interest is in the bifurcation of an underlying elastic-plastic solid, are also discussed.     

Displacement fields and self-energies of circular and polygonal dislocation loops in homogeneous and layered anisotropic solids

There are large classes of materials problems that involve the solutions of stress, displacement, and strain energy of dislocation loops in elastically anisotropic solids, including increasingly detailed investigations of the generation and evolution of irradiation induced defect clusters ranging in sizes from the micro- to meso-scopic length scales. Based on a two-dimensional Fourier transform and Stroh formalism that are ideal for homogeneous and layered anisotropic solids, we have developed robust and computationally efficient methods to calculate the displacement fields for circular and polygonal dislocation loops. Using the homogeneous nature of the Green tensor of order −1, we have shown that the displacement and stress fields of dislocation loops can be obtained by numerical quadrature of a line integral. In addition, it is shown that the sextuple integrals associated with the strain energy of loops can be represented by the product of a pre-factor containing elastic anisotropy effects and a universal term that is singular and equal to that for elastic isotropic case. Furthermore, we have found that the self-energy pre-factor of prismatic loops is identical to the effective modulus of normal contact, and the pre-factor of shear loops differs from the effective indentation modulus in shear by only a few percent. These results provide a convenient method for examining dislocation reaction energetic and efficient procedures for numerical computation of local displacements and stresses of dislocation loops, both of which play integral roles in quantitative defect analyses within combined experimental–theoretical investigations.     

Equations and closure models for material pulverization and debris flow

A transition from interacting continua to a disperse debris flow is numerically simulated using a multi-velocity formulation based on the ensemble phase average. The numerical calculation takes advantage of the recently developed dual domain material point method, which overcomes numerical instabilities, numerical diffusion and mesh distortion issues encountered in other numerical methods in cases of large material deformations. Comparisons with experiments show that the difference between the average of the velocity gradient and the gradient of the average velocity is important. To consider the transition from a continuum motion to a disperse flow, a model for this difference is expressed in terms of the effective plastic strain and the critical strain of the material. Although the model results in excellent comparisons to the experimental results, more work to study the difference and the model is needed for broader applications.     

Averaging and reduction

This paper deals with the averaging method which is the most useful technique for perturbation for a differential system. We show that in reality it is a reduction of the differential system, i.e. the initial system is replaced by a reduced system and a global connection between the two systems is emphasized. This connection is absolutely necessary in order to associate the solutions of the two systems in a structural way. In addition, this new approach (averaging as reduction) provides us with new reduced systems and the method of averaging is thus greatly extended.     

Periodic and Chaotic Motions of a Rotor-Active Magnetic Bearing with Quadratic and Cubic Terms and Time-Varying Stiffness

In this paper, we use the asymptotic perturbation method to investigate nonlinear oscillations and chaotic dynamics in a rotor-active magnetic bearings (AMB) system with 8-pole legs and the time-varying stiffness. The stiffness in the AMB is considered as the time varying in a periodic form. Because of considering the weight of the rotor, the formulation on the electromagnetic force resultants includes the quadratic and cubic nonlinearities. The resulting dimensionless equations of motion for the rotor-AMB system with the time-varying stiffness in the horizontal and vertical directions are a two-degree-of-freedom nonlinear system with quadratic and cubic nonlinearities and parametric excitation. The asymptotic perturbation method is used to obtain the averaged equations in the case of primary parametric resonance and 1/2 subharmonic resonance. It is found that there exist period-3, period-4, period-6, period-7, period-8, quasiperiodic and chaotic modulated amplitude oscillations in the rotor-AMB system with the time-varying stiffness. It is seen from the numerical results that there are the phenomena of the multiple solutions and the soft-spring type and the hardening-spring type in nonlinear frequency-response curves for the rotor-AMB system. The parametric excitation, or the time-varying stiffness produced by the PD controller is considered to be a controlling force which can control the chaotic response in the rotor-AMB system to a period n motion.     

Bifurcation and post-bifurcation analysis in plasticity and brittle fracture

A mathematical model for the bifurcation and post-bifurcation behaviour of rate independent systems governed by normality laws is presented. The considered framework encompasses a large class of phenomena, including simple models of brittle fracture, brittle damage and metal plasticity. An associated method of asymptotic development of the bifurcated solution by integer series is discussed and illustrated by simple examples in plastic stability and fracture, i.e. the buckling of some simple elastic-plastic structures and the bifurcation of systems of interacting cracks. The proposed method furnishes a rigorous framework for the study of the bifurcation problem in the context of rate-independent dissipative systems obeying normality laws, using a different approach than the one adopted so far.     

Plasmonic cloaking and scattering cancelation for electromagnetic and acoustic waves

One popular approach to cloaking objects from electromagnetic waves at moderately long wavelengths is the scattering cancelation technique. This mechanism is based on the use of a single homogeneous thin layer to cover an object of interest in order to provide scattering suppression in a given frequency band. This approach has also been recently extended to acoustic waves. This paper provides an investigation of the physical nature of scattering cancelation by a uniform thin layer for both electromagnetic and acoustic waves in inviscid fluids. Two distinct scattering cancelation regions are obtained within the available parameter space: a non-resonant plasmonic cloaking region and an anti-resonant cloaking region, which are identified and compared in both the electromagnetic and acoustic domains. Although both types of operations allow for the suppression of the dominant scattering orders, the resulting internal fields and physical functionality of the cloaks present distinct differences between the two domains. We discuss analogies and differences between these functionalities and their implications in electromagnetic and acoustic cloaking problems, with an insight into their practical implementation.     

Soliton-like thermal source forcing and singular response of atmosphere and oceans to it

IntroductionAsystemfrequentlyobservedingeophysicalfluidsthatislong_lived ,organized ,andalwaysofsteadylarge_amplitudeiscalledasolitonasaphenomenonwhichisseen ,forexample,asa“greatspot”intheJupiter’satmosphereandablockinghigh’ssituationintheatmosphereofourplanetary[1].Inthenonlinearforcing_dissipativesystemsoftheatmosphereandoceansthesolitonisbothaspecialandanessentialentity .Onadynamicbasis,itcanbeviewedapproximatelyasaproductwhenbalanceisreachedbetweenlineardispersionandnonlinearsteepness…     

Andrade and critical time-to-failure laws in fiber-matrix composites: Experiments and model

We present creep experiments on fiber composite materials. Recorded strain rates and acoustic emission (AE) rates exhibit both a power-law relaxation in the primary creep regime and a power-law acceleration before global failure. In particular, we observe time-to-failure power-laws in the tertiary regime for acoustic emissions over four decades in time. We also discover correlations between some characteristics of the primary creep (exponent of the power-law and duration) and the time to failure of the samples. This result indicates that the tertiary regime is dependent on the relaxation and damage processes that occur in the primary regime and suggests a method for predicting the time to failure based on the early time recording of the strain rate or AE rate. We consider a simple model of representative elements, interacting via democratic load sharing, with a large heterogeneity of strengths. Each element consists of a non-linear dashpot in parallel with a spring. This model recovers the experimental observations of the strain rate as a function of time.     

Rheology and microstructure of gluten and soya-based o/w emulsions

Highly concentrated oil-in-water (o/w) emulsion stabilised by means of gluten and soya protein isolate (SPI) at low pH have been characterized by means of linear dynamic viscoelasticity and droplet size distribution analysis (DSD). The microstructure of these emulsions has been characterized at a colloidal level by using confocal laser scanning microscopy (CLSM) and light microscopy (LM). These emulsions always exhibited a behaviour characteristic of highly flocculated emulsions with a mechanical spectrum showing a well-developed plateau region. DSD results generally showed log normal bimodal profiles. Microstructure images revealed occurrence of a close packing of droplets with a broad distribution of sizes participating in the formation of a three dimensional flocculated network. The Mason model of elasticity of compressed emulsions has been used to correlate viscoelastic and microstructural parameters giving adequate fitting but underestimating the elastic properties obtained for the highest concentration of gluten. These deviations may be explained in terms of an enhancement of the elastic network formed in the aqueous phase in which the glutenin fraction must play an important role. Paper presented at the Annual European Rheology Conference (AERC) 2005, April 21-23, Grenoble, France.     

Thermodynamics and evolutionary genetics

Thermodynamics and evolutionary genetics have something in common. Thus, the randomness of mutation of cells may be likened to the random thermal fluctuations in a gas. And the probabilistic nature of entropy in statistical thermodynamics can be carried over to a population of haploid and diploid cells without any conceptual change. The energetic potential wells in which the atoms of a liquid are caught correspond to selective advantages for some phenotype over others. Thus, the eventual stable state in a population comes about as a compromise in the universal competition between entropy and energy.     

重晶石和铁矿粉对套管/钻杆摩擦副摩擦磨损性能的影响

利用MG-200型高温高速摩擦磨损试验机对比考察了钻井液中所含的重晶石和铁矿粉对套管和钻杆试件摩擦学性能的影响，并采用扫描电子显微镜观察分析了磨损表面形貌，探讨了其磨损机理．结果表明，钻井液中的重晶石使得套管／钻杆磨损机理由严重塑性流动转变为磨粒磨损；而随着钻井液中铁矿粉含量的增大，磨损机理由磨粒磨损转变为粘着磨损和疲劳磨损；当铁矿粉含量较低时，采用较高的转速更有利于减小摩擦。