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.
In our experiment two laminar round jets collide in water forming a zero-momentum toroidal vortex and this flow is modeled theoretically. First, the linearized time dependent basic solution for the starting round jet is derived in a straightforward manner. Then a superposition of these solutions is used to model the frontal collision of two round jets. The resulting flow patterns are calculated and compared with the experiments. The comparison shows good qualitative agreement.
We hypothesize that a formation mechanism of anticyclonic eddies (lenses) is the outflow of intermediate waters down the canyons of the continental shelf. We reproduced in the laboratory the horizontal injection of fluid into the rotating stratified surroundings at the equilibrium density level. The experiments demonstrate that such an injection forms an anticyclonic eddy. Measurements of the velocity field show that the core of the eddy is almost axisymmetric and is in solid body rotation. The periphery of the eddy is formed by the jet flow. Cyclonic satellites were also observed at the periphery of the eddy. The main features of the laboratory flow are consistent with those of the “young” eddy observed recently in the Gulf of Cadiz.
The results of experiments with jet-like flows, induced by a horizontally moving momentum source in a density-stratified fluid are presented. The jet acts either impulsively or continuously. In the latter case both the counter-and co-flowing motions are considered. It is shown that large eddies and vortex streets, similar to those observed behind bluff bodies, are formed in the flow, where the effect of solid boundary is negligible. The conditions under which large eddies and vortex streets appear in the flow are determined. Possible applications include stratified wakes behind maneuvering self-propelled bodies. IntroductionWhen a self-propelled body makes a maneuver (e.g., accelerates), the drag force is not identically equal to the thrust during the maneuver, which lasts for a time interval At. Experiments in a stratified fluid show (Voropayev et al. 1999) that in this case the formation of unusually large eddies is possible. The late flow consists either of a large dipolar eddy or a system of smaller eddies organized in a vortex street (similar to that behind a sphere in a stratified fluid, see, e.g., Pao & Kao, 1977, Lin et al., 1992Spedding, 1997). The direction of the vortex street depends on the correlation between the drag and the thrust. In a still fluid the lifetime of the formed structures is several orders of magnitude larger than the time interval At, but the background shear may reduce this time (Voropayev et al. 2000). This pronounced feature of the stratified wakes was explained qualitatively as follows (Voropayev et al., 1999): during the time interval At the self-propelled body applies a force to the fluid, so that the wake behind it acquires a momentum, which is an integral of motion. The organization of eddies in the late flow is governed by this quantity and does not depend strongly on the particular distribution of vorticity at the surface of the body. It is also plausible that the amount of momentum determines the formation and evolution of the late wake eddies and their characteristics. In order to verify this idea, we conducted a series of experiments with a moving source, which produces a controllable momentum while the effect of a solid boundary is negligible. For this purposes a moving momentum source (jet), for which solid body drag is negligibly small compare to the momentum flux, was used in the experiments. Two basic configurations are considered, namely, continuously and impulsively acting jets. In the former case counter-and co-flows were reproduced. Experimental set-upThe experiments were conducted in a long rectangular tank 400x30x40 cm filled with a stratified by salt water (buoyancy frequency N). Vertical profiles of conductivity (density) were measured using standard four-electrode micro-scale conductivity probe. The momentum flow with controllable intensity J was generated using a thin vertical L-shaped tube (diameter d = 0.13 cm), which was driven with the velocity U along the tank by a computer controlled traverse mechanism. By measuring the amount of dyed fluid ...
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