Extreme fluctuations and the finite lifetime of the turbulent state

We argue that the transition to turbulence is controlled by large amplitude events that follow extreme distribution theory. The theory suggests an explanation for recent observations of the turbulent state lifetime which exhibit super-exponential scaling behaviour with Reynolds number.Comment: Change log: Universality of c2/c1 argument has been removed, scaling with size of puff added. To appear in Phys. Rev. E Rapid Communications

Un modelo de hormigón basado en plasticidad no asociada y fractura

Se presenta un modelo para el análisis de estructuras de hormigón mediante el método de elementos finitos. El modelo se basa en la teoría de plasticidad no asociada y en la teoría de la fisura difusa. La implantación numérica en un programa general no lineal de elementos finitos motivó desarrollos específicos en el esquema de integración de las relaciones constitutivas y en el uso de matrices de rigidez simétricas.A model for the analysis of concrete structures by the finite element method is presented. The model is based on the non-associated platicity theory and the smeared crack approach. The numerical implantation in a general purpose nonlinear finite element program led to developments in the scheme for the integration of the constitutive relation and to the use of symmetric stiffness matrices.Peer Reviewe

Un modelo de hormigón basado en plasticidad no asociada y fractura

Se presenta un modelo para el análisis de estructuras de hormigón mediante el método de elementos finitos. El modelo se basa en la teoría de plasticidad no asociada y en la teoría de la fisura difusa. La implantación numérica en un programa general no lineal de elementos finitos motivó desarrollos específicos en el esquema de integración de las relaciones constitutivas y en el uso de matrices de rigidez simétricas.A model for the analysis of concrete structures by the finite element method is presented. The model is based on the non-associated platicity theory and the smeared crack approach. The numerical implantation in a general purpose nonlinear finite element program led to developments in the scheme for the integration of the constitutive relation and to the use of symmetric stiffness matrices.Peer Reviewe

Marangoni shocks in unobstructed soap-film flows

It is widely thought that in steady, gravity-driven, unobstructed soap-film flows, the velocity increases monotonically downstream. Here we show experimentally that the velocity increases, peaks, drops abruptly, then lessens gradually downstream. We argue theoretically and verify experimentally that the abrupt drop in velocity corresponds to a Marangoni shock, a type of shock related to the elasticity of the film. Marangoni shocks induce locally intense turbulent fluctuations and may help elucidate the mechanisms that produce two-dimensional turbulence away from boundaries.Comment: 4 pages, 5 figures, published in PR

Spectral derivation of the classic laws of wall-bounded turbulent flows

We show that the classic laws of the mean-velocity profiles (MVPs) of wall-bounded turbulent flows—the ‘law of the wall,’ the ‘defect law’ and the ‘log law’—can be predicated on a sufficient condition with no manifest ties to the MVPs, namely that viscosity and finite turbulent domains have a depressive effect on the spectrum of turbulent energy. We also show that this sufficient condition is consistent with empirical data on the spectrum and may be deemed a general property of the energetics of wall turbulence. Our findings shed new light on the physical origin of the classic laws and their immediate offshoot, Prandtl’s theory of turbulent friction

Small-scale universality in the spectral structure of transitional pipe flows

Turbulent flows are not only everywhere, but every turbulent flow is the same at small scales. The extraordinary simplification engendered by this "small-scale universality" is a hallmark of turbulence theory. However, on the basis of the restrictive assumptions invoked by A. N. Kolmogorov to demonstrate this universality, it is widely thought that only idealized turbulent flows conform to this framework. Using experiments and simulations that span a wide range of Reynolds number, we show that small-scale universality governs the spectral structure of a class of flows with no apparent ties to the idealized flows: transitional pipe flows. Our results not only extend the universality of Kolmogorov\u27s framework beyond expectation but also establish an unexpected link between transitional pipe flows and Kolmogorovian turbulence

Macroscopic effects of the spectral structure in turbulent flows

Two aspects of turbulent flows have been the subject of extensive, split research efforts: macroscopic properties, such as the frictional drag experienced by a flow past a wall, and the turbulent spectrum. The turbulent spectrum may be said to represent the fabric of a turbulent state; in practice it is a power law of exponent \alpha (the "spectral exponent") that gives the revolving velocity of a turbulent fluctuation (or "eddy") of size s as a function of s. The link, if any, between macroscopic properties and the turbulent spectrum remains missing. Might it be found by contrasting the frictional drag in flows with differing types of spectra? Here we perform unprecedented measurements of the frictional drag in soap-film flows, where the spectral exponent \alpha = 3 and compare the results with the frictional drag in pipe flows, where the spectral exponent \alpha = 5/3. For moderate values of the Reynolds number Re (a measure of the strength of the turbulence), we find that in soap-film flows the frictional drag scales as Re^{-1/2}, whereas in pipe flows the frictional drag scales as Re^{-1/4} . Each of these scalings may be predicted from the attendant value of \alpha by using a new theory, in which the frictional drag is explicitly linked to the turbulent spectrum. Our work indicates that in turbulence, as in continuous phase transitions, macroscopic properties are governed by the spectral structure of the fluctuations.Comment: 6 pages, 3 figure

Ray Systems in Granular Cratering

In classical experiments of granular cratering, a ball dropped on an evened-out bed of grains ends up within a crater surrounded by a uniform blanket of ejecta. In this Letter, we show that the uniform blanket of ejecta changes to a ray system, or set of radial streaks of ejecta, where the surface of the granular bed includes undulations, a factor that has not been addressed to date. By carrying out numerous experiments and computational simulations thereof, we ascertain that the number of rays in a ray system proportional, variantD/lambda, where D is the diameter of the ball and lambda is the wavelength of the undulations. Further, we show that the ejecta in a ray system originates in a narrow annulus of diameter D with the center at the site of impact. Our findings may help shed light on the enigmatic ray systems that ring many impact craters on the Moon and other planetary bodies