Decaying grid turbulence in a strongly stratified fluid

Abstract

Grid turbulence experiments have been carried out in a stably stratified fluid at moderately large Reynolds numbers (160 based on the Taylor microscale). A scanning particle image velocimetry technique is used to provide time-resolved velocity fields in a relatively large volume. For late times, in the low-Froude-number regime, the flow consists of quasi-horizontal motion in a sea of weak internal gravity waves. Here the dynamics of the flow is found to be independent of the ambient stratification. Fundamental differences with two-dimensional turbulence, due to the strong vertical shearing of horizontal velocity, are observed. In this regime, a self-similar scaling law for the energy decay and the length-scale evolution is observed. This behaviour reflects a process of adjustment of the eddy aspect ratio based on a balance between the horizontal advective motion which tends to vertically decorrelate the flow and the dissipation due to strong vertical shear. The characteristic vertical size of the eddies grows according to a diffusion law and is found to be independent of the turbulence generation. The organization of the flow into horizontal layers of eddies separated by intense shear leads to a strong anisotropy of the dissipation: this has been checked by direct measurement of the different tensorial components of the viscous dissipation.

Self-preservation in stratified momentum wakes

“…In this respect, the approach is analagous to stratified dipole and turbulence evolution experiments where experimental data were quite closely matched by numerical models with turbulent and viscous diffusion in the horizontal and vertical, respectively. [28,29] The averaged equations of motion show that a Gaussian solution can be found and allow an analytical determination of the wake height, the wake width and the velocity defect.…”

Direct Numerical Simulation of Turbulence with Scalar Transfer around Complex Geometries Using the Immersed Boundary Method and Fully Conservative Higher-Order Finite-Difference Schemes

“…For instance, the direct measurement of fundamental statistics such as those including pressure fluctuation and/or spatial derivatives is difficult; these statistics are usually estimated from limited measurable quantities using certain hypotheses. Recently, the measurements of grid-generated turbulence have been conducted using particle image velocimetry (PIV) (Proud et al 2005;Suzuki et al 2010a), and more detailed information on the flow field has been obtained. However, it is still difficult to elucidate three-dimensional structures in the aforementioned flows.…”

Robust identification of dynamically distinct regions in stratified turbulence

“…Praud et al 2005;Kimura & Herring 2012;Almalkie & de Bruyn Kops 2012b;de Bruyn Kops 2015). It is also of inherent interest to understand the dynamics of turbulence in stratified flow at a range of parameters, in particular to identify in what ways flows not in the formally asymptotic LAST regime differ from or resemble flows actually in the LAST regime.…”