Coherent-vortex dynamics in large-eddy simulations of turbulence

Abstract

We present a review of coherent-vortex dynamics obtained thanks to large-eddy simulations (LES) associated with simple and effective vortexidentification and animation techniques. LES of a large class of constant-density or weakly compressible three-dimensional flows have been carried out. In isotropic turbulence, we present the formation and evolution of spaghetti-type vortices, seen thanks to Q, vorticity and pressure, together with the time evolution of the kinetic energy, enstrophy and skewness. In a spatially growing boundary layer on a flat plate, one observes during transition big Λ vortices lying on the wall (with very well correlated oblique induced low-and high-speed streaks) shedding smaller hairpin vortices around their tips. In the developed boundary layer, we show animations of the purely longitudinal low-and high-speed streaks, as well as animations of low-pressure regions. In a backwards-facing step, we examine the influence of upstream conditions upon the flow structure, by comparing two inflow conditions: a white noise superposed on a mean velocity profile and a realistic turbulent boundary layer. The latter three-dimensionalizes the flow downstream of the step and reduces the reattachment length. In both cases big staggered arch vortices form, impinge the lower wall and are carried away downstream. In a two-dimensional (2D) square cavity, spanwisely oriented vortices are shed behind the upstream edge, and impinge the downstream edge, transforming into arch vortices very similar to the back-step case. These arch vortices are also found behind a 2D rectangular obstacle with wall effect. We discuss the relevance of the vortices found with respect to reality. All these eddies are very important in terms of drag and noise reduction in aerodynamics and aeroacoustics.

Vortex identification: New requirements and limitations

“…The identification of coherent vortices on the basis of direct numerical simulation (DNS) and large-eddy simulation (LES) of turbulent flows has become particularly important (e.g. Dubief and Delcayre, 2000;Lesieur et al, 2003;García-Villalba et al, 2006). In the following section the most popular identification criteria are shortly described.…”

On the Lamb vector and the hydrodynamic charge

“…Several criteria to detect coherent structures (different from the vorticity one) were introduced in the literature and no consensus was reached so far [1]. Recently the notion of "Q criterion" (or Weiss determinant) was introduced in order to characterize turbulent flows mainly in numerical simulations: one calculates the Laplacian of the dynamic pressure alone to visualize the coherent structures [2,3]. On the other hand, a vortex is characterized by a low pressure in its centre and its core exhibits a local velocity minimum.…”

A Raman lidar to measure water vapor in the atmospheric boundary layer

“…[31,63,54]) or LES (i.e. [57,47,39,64,69,13]). Observations of such structures have been obtained in wind tunnel facilities [6,32,43,20].…”

“…High resolution numerical approaches such as LES have been used to simulate the interactions of the land surface with the atmosphere over complex terrain [57,47,3,39,64,69,13]. These simulations require land surface boundary and atmospheric initial conditions [2], as well as new means of validation.…”