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Turbulent entrainment in stably stratified systems


Turbulent flows with stratification

Stratified (layered by density) turbulence is a special case of turbulent flows. The main difference is due to the additional effect through the body force (density times gravity) that enters the equations of motion and modifies the balances in the Navier Stokes equations. The differences are very peculiar and stratified turbulent flows are many times counter intuitive to our perception and analysis.

We study various aspects of stratified flows. First we tried to measure the force on a lifting airfoil that accelerates from rest. Another project is a turbulent patch that grows in a stably stratified environment and we study the way its very early, initial growth is modified due to buoyancy. Very detailed local information around the propagating turbulent interface has been obtained using PIV. The third project is an attempt to study the motion of particles in Lagrangian framework across the turbulent/non-turbulent interface. For this project we had to design a steady-state experiment and we therefore work with a two-layer system.

Force on an accelerating hydrofoil

M.Sc.thesis abstract

Video


Presentation at the M.Sc. seminar

Conferences

Israel Annual Conference on Aerospace Sciences

Near-Wake Characteristics of a Freely Flying European Starling Ben-Gida, Hadar Tel-Aviv Univ. Gurka, Roi Ben-Gurion Univ. Kopp, Gregory Univ. of Western Ontario Kirchhefer, Adam Jonathon The Univ. of Western Ontario Keywords: Aerodynamics, CFD, Fluid Dynamics, Propulsion

6th annual European Postgraduate Fluid Dynamics Conference

American Physical Society, 64th Annual Meeting of the APS Division of Fluid Dynamics, November 20-22, 2011, abstract #R28.007

Turbulent patch

We present a laboratory experiment of the growth of a turbulent patch in a stably stratified fluid, due to a localized source of turbulence, generated by an oscillating grid. Synchronized and overlapping particle image velocimetry and planar laser induced fluorescence measurements have been conducted capturing the evolution of the patch through its initial growth until it reached a maximum size, followed by its collapse. The simultaneous measurements of density and velocity fields allow for a direct quantification of the degree of mixing within the patch, the propagation speed of the turbulent/non-turbulent interface and its thickness. The velocity measurements indicate significant non-equilibrium effects inside the patch which are not consistent with the classical used grid-action model. A local analysis of the turbulent/non-turbulent interface provides direct measurements of the entrainment velocity we as compared to the local vertical velocity and turbulent intensity at the proximity of the interface. It is found that the entrainment rate E is constrained in the range of 0÷0.1 and that the local, gradient Richardson number at the interface is (100). Finally, we show that the mean flow is responsible for the patch collapse. https://arxiv.org/abs/1603.06516

Patch




Two layer system




closer zoom into the stratified two layer system interface from Alex Liberzon on Vimeo.


Visualization of entrainment above a two-layer interface in a stably stratified turbulent flow under an oscillating grid from Alex Liberzon on Vimeo.

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