Transition to turbulence in the bottom boundary layer under a solitary wave.

Se estudia la transición a la turbulencia en una capa límite oscilatoria.Se estudia la transición a la turbulencia en una capa límite oscilatoria.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Self-preservation in stratified momentum wakes

A general model is described for drag wakes in a linearly stratified fluid, based on the self-preservation of the flow. It is assumed that the buoyancy-controlled self-similar wake expands in the horizontal direction due to turbulent diffusion and in the vertical direction due to viscous diffusion. The mean characteristics of the wake (height, width and velocity defect) are analytically derived and show good agreement with existing data from experimental and numerical results. Moreover, the three regimes previously found in the literature that characterise different dynamical phases of the wake evolution are recovered, and two new regimes are found. The model allows for prediction of characteristic length and velocity scales at the high Reynolds numbers of large-scale applications of geophysical and naval origin

Nonlinear evolution of harmonically forced perturbations on a wingtip vortex

Wingtip vortices are created by flying airplanes due to lift generation. The vortex interaction with the trailing aircraft has sparked researchers’ interest to develop an efficient technique to destroy these vortices. Different models have been used to describe the vortex dynamics and they all show that, under real flight conditions, the most unstable modes produce a very weak amplification. Another linear instability mechanism that can produce high energy gains in short times is due to the non-normality of the system. Recently, it has been shown that these non-normal perturbations also produce this energy growth when they are excited with harmonic forcing functions. In this study, we analyze numerically the nonlinear evolution of a spatially, pointwise and temporally forced perturbation, generated by a synthetic jet at a given radial distance from the vortex core. This type of perturbation is able to produce high energy gains in the perturbed base flow (10^3), and is also a suitable candidate for use in engineering applications. The flow field is solved for using fully nonlinear three-dimensional direct numerical simulation with a spectral multidomain penalty method model. Our novel results show that the nonlinear effects are able to produce locally small bursts of instability that reduce the intensity of the primary vortex.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Automated Tracking of 3-D Overturn Patches in Direct Numerical Simulation of Stratified Homogeneous Turbulence

Abstract. Direct numerical simulation is a valuable tool for modeling turbulence, but like "wet lab" simulation, it does not solve the problem of how to interpret the data. Manual analysis, accompanied by visual aids, is a time consuming, error prone process due to the elaborate timedependent structures appearing in simulations. We describe a technique based on volume tracking, that enables the worker to identify and observe evolving coherent flow structures, eliminating uninteresting background data. Using our techniques we were able to investigate 3-D density overturns in stably stratified homogeneous turbulence, understand entangled physical structures and their dynamical behavior. We describe our technique, which improves on past work by incorporating application-specific knowledge into the identification process. Such knowledge was vital in filtering out spurious information that would have interfered with the experimental method. Representative results are shown which summarize the physical insight gained by the application of the above identification/tracking method

Large-scale characteristics of stratified wake turbulence at varying Reynolds number

We analyze a large-eddy simulation data set of wakes of a towed sphere of diameter D at speed U in a uniformly stratified Boussinesq fluid with buoyancy frequency N and kinematic viscosity ν. These temporally evolving wakes are simulated using a spectral multidomain penalty-method-based incompressible Navier-Stokes solver for Fr≡2U/ND∈{4,16,64} and Re≡UD/ν∈{5×103,105,4×105, enabling a systematic examination of stratified wakes at three different values of Re sufficiently separated in magnitude. As such, particular attention is paid to the effects of varying Re on the evolution of large-scale characteristics of stratified wake turbulence. We examine the evolution of horizontal and vertical integral length scales (ℓh and ℓv), horizontal and vertical fluctuation velocities (U and W), local vertical shear, as well as the resulting dimensionless parameters based on the above quantities. In particular, the vertical turbulent Froude number Fr★v≡2πU/Nℓv is found to be of order unity, a signature of the dynamics in the strongly stratified regime where shear instabilities develop between anisotropic flow layers. The horizontal turbulent Reynolds number Reh≡Uℓh/ν stays approximately constant in time and the horizontal turbulent Froude number Frh≡U/Nℓh decays in time as (Nt)−1, consistent with scaling analysis of freely decaying turbulence. We characterize the transitions between distinct stratified flow regimes and examine the effects of body-based parameters Re and Fr on these transitions. The transition from the weakly to the strongly stratified regime, which is marked by Fr★v decaying to unity, occurs when Frh≃O(0.01). We further show that the initial value of Reh at which the flow completes the above transition scales as ReFr−2/3, which provides a way to predict the possibility of accessing the strongly stratified regime for a wake of given Re and Fr. The analysis reported here constitutes an attempt to obtain the predictive capability of stratified wake turbulence in terms of Reynolds number Re, applying select elements of strongly stratified turbulence theory, so far typically utilized for homogeneous turbulence, to a canonical inhomogeneous turbulent free-shear flow.Natural Sciences and Engineering Research Council - Collaborative Research & Development Gran