Hydrodynamics of a Laterally Confined Density Current

A wide range of density currents can be found in natural and man-made environments. Such flows have been the focus of a large area of scientific research and engineering applications, including the propagation of effluents, turbidity or ashes in reservoirs and other water bodies. The majority of the studies regarding these currents have been focused mainly on the intrinsic mechanisms and non-conservative properties of the flow, such as entrainment and self-acceleration of density and turbidity currents. Hence, the employed geometries and setups are usually simple, in order to control and isolate the different driving mechanisms.

The propagation of these currents can become more complex, owing to factors such as bed roughness and physical environment geometry. In particular, internal waves can be generated as a result of interaction with domain boundaries and additional dissipative effects can be brought about by boundary roughness.

The structure and behaviour of the flow produced by a constant flux density current, with the superimposition of the effects from internal waves, are herein studied experimentally. For that, a set up was established in order to produce a continuous and asymmetrical inflow, aiming at the generation of a complex 2D flow with reflected waves. The transport and the mixing processes are recorded with high-speed video, with mass and velocity distributions being computed with photometric and Particle Image Velocimetry methodologies, respectively.

In the near-field, the results show that the interaction between density currents and physical boundaries originate the development of internal waves that propagate with nearly constant shape and a celerity compatible with that of a solitary wave. The results for the velocity field suggest that there is a convergent surface that correlates well with higher mass values, predominantly at the crest of the wave. In the far-field, the current front behaves mostly as a 1D flow, with streaks that oscillate in direction and intensity, as the internal waves merge with the front of the current.

This work was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 and by the grant PD/BD/113620/2015.