21 research outputs found
Relating statistics to dynamics in axisymmetric homogeneous turbulence
The structure and the dynamics of homogeneous turbulence are modified by the
presence of body forces such that the Coriolis or the buoyancy forces, which
may render a wide range of turbulence scales anisotropic. The corresponding
statistical characterization of such effects is done in physical space using
structure functions, as well as in spectral space with spectra of two-point
correlations, providing two complementary viewpoints. In this framework,
second-order and third-order structure functions are put in parallel with
spectra of two-point second- and third-order velocity correlation functions,
using passage relations. Such relations apply in the isotropic case, or for
isotropically averaged statistics, which, however, do not reflect the actual
more complex structure of anisotropic turbulence submitted to rotation or
stratification. This complexity is demonstrated in this paper by
orientation-dependent energy and energy transfer spectra produced in both cases
by means of a two-point statistical model for axisymmetric turbulence. We show
that, to date, the anisotropic formalism used in the spectral transfer
statistics is especially well-suited to analyze the refined dynamics of
anisotropic homogeneous turbulence, and that it can help in the analysis of
isotropically computed third-order structure function statistics often used to
characterize anisotropic contexts.Comment: Physica
Lagrangian evolution of velocity increments in rotating turbulence: The effects of rotation on non-Gaussian statistics
The effects of rotation on the evolution of non-Gaussian statistics of velocity increments in rotating turbulence are studied in this paper. Following the Lagrangian evolution of the velocity increments over a fixed distance on an evolving material element, we derive a set of equations for the increments which provides a closed representation for the nonlinear interaction between the increments and the Coriolis force. Applying a restricted-Euler-type closure to the system, we obtain a system of ordinary differential equations which retains the effects of nonlinear interaction between the velocity increments and the Coriolis force. A priori tests using direct numerical simulation data show that the system captures the important dynamics of rotating turbulence. The system is integrated numerically starting from Gaussian initial data. It is shown that the system qualitatively reproduces a number of observations in rotating turbulence. The statistics of the velocity increments tend to Gaussian when strong rotation is imposed. The negative skewness in the longitudinal velocity increments is weakened by rotation. The model also predicts that the transverse velocity increment in the plane perpendicular to the rotation axis will have positive skewness, and that the skewness will depend on the Rossby number in a non-monotonic way. Based on the system, we identify the dynamical mechanisms leading to the observations. (c) 2010 Elsevier B.V. All rights reserved
Working with young Afro-Caribbean offenders Some dilemmas and opportunities
SIGLELD:8318.226(20) / BLDSC - British Library Document Supply CentreGBUnited Kingdo