1,721,034 research outputs found
Self-sustained double-diffusive interleaving
No full textThe formation and evolution of double-diffusive interleaving is experimentally investigated with the purpose of analysing the influence of the convective flow structures, at different scales, on the mean flow. Recently, Krishnamurti (J. Fluid Mech., vol. 558, 2006, p. 113) has shown that, in the case of a continuous stratification experiment, the Reynolds stresses, due to convective flow patches, are able to vertically transport horizontal momentum, maintaining the mean flow. This mechanism is similar to the turbulent wind observed in thermal convection. In this study, the interleaving is produced using the classical set-up of Ruddick & Turner (Deep-Sea Res., vol. 558, 1979, p. 903). The dam-break experiments better resemble the case of oceanic fronts, where interleaving is commonly observed. The flow structures are investigated by measuring the two-dimensional flow fields using the particle image velocimetry technique. The resulting two-dimensional vector fields reveal complex fine-scale flow structures, and convective patterns are observed inside the finger-favourable layers. Vortical structures at scales comparable with the layer thickness are embedded in these regions and seem to be responsible for sustaining the horizontal mean flow against the viscous dissipations, especially in a region close to the layer nose. A spectral analysis of the flow fields suggest that the energy balance is governed by an inverse energy cascade, which implies a transfer of energy from the smaller scales to the larger scales (mean flow).</jats:p
Horizontal mixing of quasi-uniform, straight, compound channel flows
No full textThe generation and evolution of large-scale vortices with vertical axis (macro-vortices) in a straight compound channel under quasi-uniform flow conditions is investigated.
We discuss possible similarities and clear differences with free shear layer flows induced by the meeting of shallow streams of different speeds. An experimental investigation based on particle image velocimetry (PIV) measurements of free-surface velocities
forms the basis for an analysis of both the specific features of macro-vortices and of the related mean flow characteristics. Dynamical properties strongly depend on the ratio rh between the main channel flow depth (hmc) and the floodplain depth (hfp), and three flow classes can be identified. ‘Shallow flows’ (rh >3) are dominated by strong shearing and large macro-vortices populating the transition region between the main channel and the floodplains. The mean streamwise velocity induced in intermediate flows’(2<rh <3) is characterized by a dip in the transition region, while it closely resembles that occurring in a rectangular channel in the case of ‘deep flows’ (rh <2). For both
the latter cases the shear in the transition region decreases and the macro-vortices are also generated in the wall boundary layer of the floodplains. The typical size of the quasi-two-dimensional macro-vortices, generated at the transition region, is found to
be independent of the streamwise coordinate. This and the non-monotonic behaviour of the mean streamwise velocity suggest that in straight compound channels the
topographic forcing is so dominant that conceptual models interpreting these flows as free shear layers may largely fail to describe the physics of compound channels flows
Detection of Lagrangian Coherent Structures due to shallow water macrovortices in compound channels
No full textWater quality management is a crucial aspect for a riverine environment, since it hosts a great variety of wildlife that can be dramatically influenced by the controlled and uncontrolled release of pollutants in the rivers themselves. All strategies to monitor the water quality must take into account the complex processes involved in the mass transport related to the river flows. These processes should be studied in a Lagrangian framework more than from an Eulerian point of view. Several techniques are now available to describe and quantify the relevant characteristics of the mixing processes, i.e. Lagrangian statistics in terms of single and multiple particles dynamics. However, applying the standard Taylor theory of the dispersion, which leads to the evaluation of the averaged dispersion coefficients, has some intrinsic limitations that are related to the average over a great number of trajectories in the entire domain under investigation. In the present study, we apply the techniques that derive from the nonlinear dynamical system theory to a riverine situation, in which the natural shape of the river cross-section is accounted for. These techniques allow for the detection of the so-called Lagrangian Coherent Structures (LCS), which are material elements embedded in the flow that strongly influence the mixing processes. Indeed LCSs represents material barriers that can separate the domain in distinct regions with no mass transport among them. We investigate the possibility of formation of LCSs in a compound channel under uniform flow conditions depending on the main physical parameters
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