Yann Malecot

Intermittency and Reynolds number

Hot wire measurements of longitudinal and transverse increments are performed in three different types of flows on a large range of Reynolds numbers (100≲Rλ≲3000). An improved technique based on cumulant expansion of velocity structure functions is used to estimate the spreading of the pdfs and to study their scaling properties in the inertial range. Thus, the rate of intermittency depth through the scales of flow, called here β(Rλ), is experimentally introduced, and it is shown that β(Rλ) has a universal behavior on a very large Reynolds numbers range.

Reynolds dependence of third-order velocity structure functions

We study the experimental dependence of the third-order velocity structure function on the Taylor based Reynolds number, obtained in different flow types over the range 72⩽Rλ⩽2260. As expected, when the Reynolds number is increasing, the third-order velocity structure functions (plotted in a compensated way) converge very slowly to a possible −4/5 plateau value according to the Kolmogorov 41 theory. Actually, each of these normalized third-order functions exhibits a maximum, at a scale close to the Taylor microscale λ. In this Brief Communication, we show that experimental data are in good agreement with the recent predictions of Qian and Lundgren. We also suggest that, from an experimental point of view, a log-similar plot suits very well to study carefully the behavior of the third-order velocity structure functions with the flow Reynolds number.

Compression triaxial behavior of concrete: the role of the mesostructure by analysis of X-ray tomographic images

This paper is intended to assess the mesostructural damage mechanisms of concrete under hydrostatic and triaxial loadings. Such a study is possible thanks to two state-of-the-art laboratory instruments: a high-pressure triaxial press, and an X-ray computed tomography instrument. The laboratory protocol consists of scanning the concrete prior to the initial loading and after each cycle. An analysis of the resulting images indicates that under high hydrostatic loading, significant damage is visible in cement paste at the mesoscopic scale. For two triaxial tests conducted at 50 and 650 MPa of confining pressure, results reveal major differences in both damage and failure mechanisms. At the lower pressure, shear loading creates a localised failure mechanism characterised by sliding on an inclined plane, whereas at the higher pressure, the strain and damage mode are much more homogeneous with a failure localisation after unloading. Cet article a pour objectif d’estimer les mécanismes d’endommagement du béton présents à l’échelle mésoscopique sous chargements hydrostatique et triaxial. Une telle étude est possible grâce à deux instruments de laboratoire: une presse triaxiale de grande capacité, et un tomographe à rayons X. Le protocole de réalisation des essais consiste à scanner le béton à l’état vierge et après chaque cycle. L’analyse des images obtenues indique que sous fort chargement hydrostatique, l’endommagement est visible dans la pâte de ciment à l’échelle mésoscopique. Pour deux essais triaxiaux, réalisés à 50 et 650 MPa de pression de confinement, les résultats montrent des mécanismes d’endommagement et de rupture très différents. À basse pression, le cisaillement provoque un mécanisme de rupture qui apparait sous la forme d’un plan de glissement incliné. À forte pression, la déformation et le mode d’endommagement sont beaucoup plus homogènes avec néanmoins une localisation de la rupture lors de la décharge.

Accurate estimation of third-order moments from turbulence measurements

Abstract. Politano and Pouquets law, a generalization of Kolmogorovs four-fifths law to incompressible MHD, makes it possible to measure the energy cascade rate in incompressible MHD turbulence by means of third-order moments. In hydrodynamics, accurate measurement of third-order moments requires large amounts of data because the probability distributions of velocity-differences are nearly symmetric and the third-order moments are relatively small. Measurements of the energy cascade rate in solar wind turbulence have recently been performed for the first time, but without careful consideration of the accuracy or statistical uncertainty of the required third-order moments. This paper investigates the statistical convergence of third-order moments as a function of the sample size N . It is shown that the accuracy of the third-moment || ) 3 > depends on the number of correlation lengths spanned by the data set and a method of estimating the statistical uncertainty of the third-moment is developed. The technique is illustrated using both wind tunnel data and solar wind data.

Transverse Velocity Structure Functions in Developed Turbulence

Velocity structure functions based on the longitudinal increment δu ||(r) = u(x + r) − u(x) (in which the component u is aligned with the direction of the separation r) have been extensively measured to study the scaling laws of small scale intermittency of fully developed turbulence.

Ultimate strength of plain concrete under extreme combined stresses: Triaxial and proportional stress paths

ABSTRACT Concrete is a building material used for sensitive infrastructures (dams, nuclear reactors), however its behaviour under extreme dynamical loading (rock falls, explosions, ballistic impacts) remains poorly known. This is due both to the difficulty of experimentally reproducing such a loading and to the intrinsic complexity of concrete behaviour. In order to predict its response under dynamic loading the experimental characterization of its static behaviour in compression under very high confinement is needed. In this paper a new large capacity tri-axial press and the manufacturing and testing procedures developed to perform the tests are presented. Low strength plain concrete specimens were subjected to triaxial and proportional loading paths up to an ultimate state associated to failure. The influence of the loading path on the observed limit state of concrete subjected to multiaxial stress states will then be discussed.

Influence of the Saturation Ratio on Concrete Behavior under Triaxial Compressive Loading

When a concrete structure is subjected to an impact, the material is subjected to high triaxial compressive stresses. Furthermore, the water saturation ratio in massive concrete structures may reach nearly 100% at the core, whereas the material dries quickly on the skin. The impact response of a massive concrete wall may thus depend on the state of water saturation in the material. This paper presents some triaxial tests performed at a maximum confining pressure of 600 MPa on concrete representative of a nuclear power plant containment building. Experimental results show the concrete constitutive behavior and its dependence on the water saturation ratio. It is observed that as the degree of saturation increases, a decrease in the volumetric strains as well as in the shear strength is observed. The coupled PRM constitutive model does not accurately reproduce the response of concrete specimens observed during the test. The differences between experimental and numerical results can be explained by both the influence of the saturation state of concrete and the effect of deviatoric stresses, which are not accurately taken into account. The PRM model was modified in order to improve the numerical prediction of concrete behavior under high stresses at various saturation states.

Mesoscopic simulations of concrete strains incompatibilities under high creep stress level and consequences on the mechanical properties

Abstract The available models for estimating the creep strains of concrete generally assume that concrete is a homogeneous material. Since concrete is a composite, such models are incapable of taking into account incompatible strains between the cement paste and aggregates during creep loading. The main objective of this paper is to demonstrate that microcracks could increase the creep strain level and justify the use of a coupling between creep and damage at the macroscopic scale. To achieve this objective, a viscoelastic model has been adopted to compute the creep strains of a mesostructure composed of aggregates and cement paste under varying load levels, in both tension and compression. Creep tests drawn from the literature are used to show that the microcracks generated by incompatible strains significantly increase the creep strains under high stress/strength ratios. It is then proven that the mesoscopic approach is capable of predicting concrete failure during a ring test involving basic creep. Residual Concrete behavior is also predicted and modifications observed especially following creep in compression which is not consistent with the few experimental data available meaning that apparent non-linearity cannot be completely attributed to strains incompatibilities at the mesoscale.

Small Scale Passive Temperature Measurements in Fully Developed Turbulence

It is well accepted that the passive scalar field in fully developped turbulence is more intermittent than the velocity one. In particular, the probability density function (pdf) of the scalar increments δθ(r) (hereafter the temperature, Pr≈0.7) are more spread than the velocity pdf. However, as it is shown in figure 1, this temperature pdf becomes nearly gaussian when the temperature increments δθ(r) is conditionned to its local dissipation rate X r, whatever the scale r. Such an experimental feature suggests that the local variance transfer rate defined as: \(\frac{{\delta {\theta ^2}(r)}}{{r/\delta u(r)}}\) is also an appropriate quantity to study the small scale intermittency of the passive scalar (in agreement with [1]).

“Measurement” of Turbulence Intermittency in Physical and Numerical Experiments

In order to improve the modelling of turbulent flows, a better knowledge of the small scale intermittency is still needed. The main feature of the intermittency is that the velocity field has actually no scale invariance at finite Reynolds [1]. A few statistical models take into account a Reynolds number dependence [2], [3].