Horizontal jets and vortex dipoles in a stratified fluid

Abstract

When a horizontal force is applied locally to some volume of a viscous densitystratified fluid, flows with high concentration of vertically oriented vorticity (vortex dipoles) are generated. The processes of generation and evolution with time of these unsteady flows in a stratified fluid are studied. A convenient way to produce and study these flows in the laboratory is to use a submerged horizontal jet as a ‘point’ source of momentum. The main governing parameter (the ‘force’) is easily controlled in this case. Two regimes were studied: starting jets with dipolar vortex fronts (the force acts continuously) and impulsive vortex dipoles (the force acts for a short period of time). A conductivity microprobe, aluminium powder, shadowgraph, thymol-blue and other techniques have been used to measure the velocity and density distributions in the flows. It is found that in both regimes the flows are self-similar: the lengthscale of the flows increases with time as t½ for starting jets and as t1/3 for vortex dipoles. Detailed information about the generation mechanism, kinematics and dynamics of the flows is obtained. On the basis of similarity principles a theoretical explanation of the experimental results is given. The theory is in good agreement with the results obtained.

Modeling and Control of Large Eddies Generated by Maneuvering Self-Propelled Bodies in Stratified Fluids

“…11. As can be seen, the flow k> development is qualitatively similar to the case of zero shear [22] with the resulting planar dipolar eddy. The main difference here is that the initial turbulent blob (Fig.…”

Vortex streets generated by a moving momentum source in a stratified fluid

“…In many situations Re* and Fr* can be very large initially, but at the late stages of the flow development, with typical flow velocity and length scales U 0 and D 0 , the flow parameters, Re 0 ϭU 0 D 0 / and Fr 0 ϭU 0 /ND 0 , become small ͑because U 0 decreases and D 0 increases with time and U 0 usually decreases faster than D 0 increases͒ and so-called flow laminarization occurs with time. 12 At small Fr 0 stratification suppresses the vertical component of the velocity and the flow becomes quasi-twodimensional. Indeed, observations show that the far wake consists of long lasting patches of vertical vorticity of opposite signs ͑''pancake'' eddies͒ organized into a vortex street, the vertical length and velocity scales of which are much smaller than those in the horizontal direction.…”

Experimental characterization of the 3D dynamics of a laminar shallow vortex dipole

“…Many laboratory experiments have shown that quasitwo-dimensional vortex dipoles are generically produced by the collapse of initially three-dimensional turbulent jetlike flows when submitted to specific constraints such as a density stratification (Voropayev et al 1991;Flór et al 1995) or a small fluid-layer thickness (Sous et al 2004;Voropayev et al 2007). In Voropayev et al (2007) and Sous et al (2004), an impulsive jet in a shallow layer of water is used to mimic the aforementioned geophysical flow configurations.…”