Anisotropy of the Reynolds stresses in a turbulent boundary layer on a rough wall

The measured anisotropy invariants of the Reynolds stress tensor in a self-preserving rough wall turbulent boundary layer indicate that the anisotropy is significantly smaller than in a smooth wall layer.RAA is grateful to Dr. P. Spalart for the DNS data. The support of the Australian Research Council is acknowledged.     

Anisotropy measurements in the boundary layer over a flat plate with suction

Laser Doppler velocity measurements are carried out in a turbulent boundary layer subjected to concentrated wall suction (through a porous strip). The measurements are taken over a longitudinal distance of 9× the incoming boundary layer thickness ahead of the suction strip. The mean and rms velocity profiles are affected substantially by suction. Two-point measurements show that the streamwise and wall-normal autocorrelations of the streamwise velocity are reduced by suction. It is found that suction alters the redistribution of the turbulent kinetic energy k between its components. Relative to the no-suction case, the longitudinal Reynolds stress contributes more to k than the other two normal Reynolds stresses; in the outer region, its contribution is reduced which suggests structural changes in the boundary layer. This is observed in the anisotropy of the Reynolds stresses, which depart from the non-disturbed boundary layer. With suction, the anisotropy level in the near-wall region appears to be stronger than that of the undisturbed layer. It is argued that the mean shear induced by suction on the flow is responsible for the alteration of the anisotropy. The variation of the anisotropy of the layer will make the development of a turbulence model quite difficult for the flow behind suction. In that respect, a turbulence model will need to reproduce well the effects of suction on the boundary layer, if the model is to capture the effect of suction on the anisotropy of the Reynolds stresses.     

Critical conditions for thermal explosion in conductive heat transfer in the reaction zone and the surrounding medium

The critical conditions for thermal explosion are examined for plane, sphere, and cylinder in an unbounded medium with convective heat transfer in the internal and external regions. Criteria are used in integro-differential equations in the form of a Duhamel integral, which allow the conjugate problem to be reduced to a boundary one. Critical conditions are calculated by computer, with analysis for the cases of greatest practical interest.Notation c thermal capacity - density - thermal conductivity - a thermal diffusivity - Q heat of reaction - E activation energy - k₀ the preexponential factor - extent of reaction - T temperature - T₀ temperature of surrounding medium - r₀ characteristic dimension of body We are indebted to B. I. Khaikin and V. V. Barzykin for valuable advice.     

Inverse thermal analysis of the drying zone of the evaporator of an axially grooved heat pipe

An inverse approach is performed to characterize the thermal behaviour of an axially grooved heat pipe, in steady state, for various operating conditions. For this purpose, an experimental set up, as well as a network conduction model, are developed to simulate the heat transfer in the wall at the evaporator section. The minimization of an objective function, taking into account the discrepancy between measured temperatures and computed ones, allows then the estimation of a heat transfer coefficient as well as the drying out front positions for all the axial grooves. Hence, at the burnout point, the significant temperature increase in the evaporator extremity is considered to be a direct consequence of the restriction of the evaporative zone. Therefore, the distribution of liquid phase in the capillary structure of the heat pipe can be obtained through the analysis of the measured temperature gradient in the evaporator section where the dry out front was expected to occur. Furthermore, the dry out front expansion can be observed when the input heat load is increased or when the adiabatic temperature is decreased. Introducing an adverse tilt angle also shows the effect of the puddle.     

Anisotropy effect on the convection of a heat-conducting fluid in a porous medium and cosymmetry of the Darcy problem

The onset of convection in a porous rectangle is analyzed with account for the anisotropy of the thermal parameters and the permeability. For the Darcy–Boussinesq equations the conditions under which the problem pertains to the class of cosymmetric systems are established and explicit formulas for the critical Rayleigh numbers corresponding to the loss of stability of the mechanical equilibrium are derived. The critical numbers and the branching stationary convection regimes are calculated using a finite difference method conserving the problem cosymmetry.     

An electrically impermeable and magnetically permeable interface crack with a contact zone in magnetoelectroelastic bimaterials under a thermal flux and magnetoelectromechanical loads

An interface crack with a frictionless contact zone at the right crack-tip between two dissimilar magnetoelectroelastic materials under the action of a thermal flux and remote magnetoelectromechanical loads is considered. The open part of the crack is assumed to be electrically impermeable and magnetically permeable, and the crack faces are assumed to be heat insulted. The inhomogeneous combined Dirichlet–Riemann and Hilbert boundary value problems are, respectively, formulated and solved analytically. Stress, electrical displacement intensity factors as well as energy release rate are found in analytical forms, and analytical expressions for the contact zone length have been obtained for both the general case and the case of small contact zone length. Some numerical results are presented, which show clearly the effects of thermal and magnetoelectromechanical loads on the contact zone length, stress intensity factor and energy release rate. Results presented in this paper should have potential applications to the design of multilayered magnetoelectroelastic structures and devices.     

Anisotropy of creep of materials

It has been shown that when ordinary stress-strain (-) diagrams are constructed for 20-mm thick rolled duralumin plate at various temperatures, the alloy behaves like an isotropic material, while a considerable degree of anisotropy is observed in creep testing.Thanks are due to A. F. Nikitenko for his assistance in obtaining and processing the experimental data.     

Shearing effects on the breathing mechanism of a cracked beam section in bi-axial flexure

The main purpose of this paper is to complete the works presented by Andrieux and Varé (2002) and El Arem et al. (2003) by taking into account the effects of shearing in the constitutive equations of a beam cracked section in bi-axial flexure. The paper describes the derivation of a lumped cracked beam model from the three-dimensional formulation of the general problem of elasticity with unilateral contact conditions on the crack lips. Properties of the potential energy and convex analysis are used to reduce the three-dimensional computations needed for the model identification, and to derive the final form of the elastic energy that determines the nonlinear constitutive equations of the cracked transverse section. We aim to establish a relation of behavior between the applied forces and the resulting displacements field vectors, which is compatible with the beams theory in order to allow the model exploitation for shafts dynamics analysis. The approach has been applied to the case of a cracked beam with a single crack covering the half of its circular cross section.     

砂岩岩石力学参数各向异性研究

对胜利油田不同深度的四口井的砂岩岩芯进行了三轴压缩试验和动态岩石力学参数试验。根据获得的垂直地层方向和平行地层方向的试验数据,计算了各岩石力学参数的各向异性系数。通过分析围压以及深度对各向异性系数的影响,给出了砂岩峰值强度各项异性系数、静泊松比各向异性系数、纵波波速各向异性系数、纵横波速比和动泊松比各向异性系数与深度的拟合公式。发现峰值强度、纵波波速和纵横波速比的各向异性系数都随着深度的增加而减小,而泊松比各向异性系数随着深度的增加而增大。动、静弹性模量各向异性系数与深度相关性都很差,动弹性模量各向异性较弱,静弹性模量相对较强。     

Plane Anisotropy by the Polar Method*

This paper is a critical review of the so-called polar method, a mathematical technique based upon tensor invariants introduced as early as 1979 by Professor G. Verchery to treat anisotropic plane problems. The paper contains a brief presentation of the method and a summary of some results obtained by the author and co-workers in some problems mainly concerning elasticity and the elastic design of composite laminates. This paper is dedicated to Professor Georges Verchery, on the occasion of his 60th birthday.     

Anisotropy degree of nonlinear solids

This research aims at the generalization of the concept of anisotropy degree of linearly elastic solids which has been defined and investigated in detail by Zhang [1988] to that of nonlinear and non-elastic solids. The properties of the anisotropy degrees defined here show that they are reasonable.     

Interacting micro-cracks near the tip in the process zone of a macro-crack

For a linearly elastic and isotropic solid containing two or more cracks, cavities and other interacting defects of complex geometries, a method called “the method of pseudo-tractions” has been recently proposed by Hori and Nemat-Nasser (1983, 1985a), which can effectively solve two-dimensional problems of this kind, when cracks or cavities with sharp corners are suitably far apart. The method, however, breaks down when a crack or cavity is situated very close to the tip of another crack, which is the case when the process zone at the tip of a crack contains many micro-cracks.In this work, modifications of the method of pseudo-tractions are introduced, which will permit effective calculation of the stress intensity factors when a large crack interacts with small cracks which are situated very close to its tip in its process zone. Explicit asymptotic expressions are obtained for the stress intensity factors of the macro-crack, as well as those of the micro-cracks. It is shown that the presence of the microcracks in the process zone of a macro-crack may induce out-of-plane crack growth even under far-field hydrostatic tension. Several illustrative examples are worked out, including two collinear cracks for which an exact solution exists, arriving at an excellent correlation.     

Contact zone during the noninstantaneous opening of a diaphragm in a shock tube

The structure and nature of motion of the contact zone during the noninstantaneous opening of a diaphragm in a shock tube are analyzed for the case of Re and equal adiabatic indices of the propelling and propelled gases. It is shown that the temperature profiles in the contact zone are self-similar, and an expression is obtained for the trajectory of the interface of the gases in the contact zone.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 159–161, November–December, 1976.     

Large Eddy Simulation of a thermal mixing tee in order to assess the thermal fatigue

The present paper deals with thermal fatigue phenomenon, and more particularly with the numerical simulation using Large Eddy Simulation technique of a mixing tee, for which experimental thermal statistics are available. The sensitivity to the sub-grid scale closure is first evaluated by comparing the experimental statistics with the numerical results obtained via both the Smagorinsky and the structure-function models. Because of a difference of temporal resolution between the experiment and the simulation, the direct comparison of the fluctuations is not possible. Therefore, a methodology based on filtering the numerical results is proposed in order to achieve a proper comparison. The comparison of the numerical results with the experiment suggests that slight better predictions are obtained with the structure-function model even if the dependency of the results to the sub-grid scale model is low. Then, the possibility to reduce the fluid computational domain by prescribing synthetic turbulence at the inlet is tested. First results are encouraging and underline the advantage of considering this technique instead of a standard noise at the entrance of the domain. All the simulations are conducted with the commercial CFD code STAR-CD.