Stratified turbulence

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

Abstract:

Aeronautical technology has evolved substantially from the first human-made airplane, back in 1903. Yet, flying animals (e.g., birds), which represent one of nature’s finest locomotion methods, still feature superiority on any current technology, in respect to flight efficiency. Today, engineers mimic nature in order to take advantage of such unconventional propulsion and lift generation methods, used by flying animals, to enhance performances or supplant traditional methods (fixed wings aircraft). One of the remaining puzzles is the role of unsteady motion in the flow due to the wing flapping and its contribution to the forces acting on the animal during the flapping flight. The wake of a freely flying European Starling was investigated as a case study of unsteady wing aerodynamics. Measurements of the near-wake downstream of the bird have been taken in a hypobaric wind tunnel specially designed for avian research. The experiments were performed using long duration high-speed PIV system, which enables a continuous acquisition of images for 20 minutes using two cameras. High-speed videos recorded the wing and body motion simultaneously with the PIV. The wake region has been characterized through the calculation of velocity and vorticity fields. Mean velocity profiles provide evidence of three different flight modes. Drag and lift have been estimated by means of mean velocity deficit and circulation at the wake region. Time evolution of the velocity field depicts the unsteady motion in the flow due to the presence of the flapping wings. Consequently, the wake topography has been reconstructed emphasizing flow features as function of time. Correlations between the wing kinematics and the flow field characteristics are presented. It is shown that the unsteady aerodynamics plays a major role in the formation of drag and lift during flapping flight.


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

Two-layer system

Lagrangian framework

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.