From discovery to recognition of periodic large scale vortices in rod bundles as source of natural mixing between subchannels—A review

Flood risk assessment in urban regions does not account for human behaviours during emergency. Understanding behaviours during floods and developing related simulators highlight critical phenomena for individuals' safety, during and after the flood, and suggest risk-reduction strategies aimed at helping evacuees. The proposed Flooding Pedestrians' Evacuation Dynamics Simulator (FlooPEDS) combines flood hydrodynamics (based on Nonlinear Shallow Water Equations) and individuals' evacuation (by modifying the Social Force Model). FlooPEDS capabilities are illustrated with reference to an important case study. Results focus on evacuees’ motion and path choices. This preliminary simulator implementation, still raw for practical use by planners and authorities, is already mature to guide the design of innovative resilience-increasing urban solutions (i.e.: architectural components like handrails, raised flooring systems) in specific hazardous urban areas, as support to traditional strategies (i.e.: early warning systems; evacuation plan communication).

A preliminary combined simulation tool for the risk assessment of pedestrians’ flood-induced evacuation

This is the second part of a series examining the behaviour of breaking-wave-induced macrovortices. The first part examined theoretically general behaviours during startup conditions, and gave computational results for macrovortex generation and their evolution on widely spaced breakwaters. In this paper, we extend and test qualitatively and quantitatively some of the basic results of Part 1, in particular the initial longshore vortex transport for a wide range of geometries ranging from narrow rip current topographies to isolated breakwaters. Accounting for the presence of a shoreline is found to be necessary for the representation of longshore vortex transport. Results show vortex motion ranging from strongly offshore in the case of a narrow rip current, to strongly longshore in the case of an isolated breakwater. Even with the significant approximations inherent in the analytical predictions, numerical computations using time-domain Boussinesq-type equations and laboratory experiments confirm the trends. Part 3 examines the horizontal mixing features of wave-induced flows over isolated (single-breakwater configuration) or multiple (ripcurrent configuration) submerged structures.

Topographically controlled, breaking-wave-induced macrovortices. Part 2. Changing geometries

Turbulent mixing layers associated with streamwise uniform and non-uniform flows in compound channels (main channel with adjacent floodplains) are experimentally investigated. The experiments start with uniform flow conditions. The streamwise non-uniformity is then generated by imposing an imbalance in the upstream discharge distribution between main channel (MC) and floodplains (FPs), keeping the total discharge constant, which results in a transverse depth-averaged mean flow. This study firstly aims at assessing the effect of a transverse flow on the mixing layer and coherent structures that form at the MC/FP interfaces. A wide range of initial velocity ratio or dimensionless shear between MC and FP is tested. The study secondly aims at assessing the effect of this velocity ratio on the mixing layer, for a fixed vertical confinement of flow. The total discharge was then varied to quantify the confinement effect. The results show that, far from the inlet section, Reynolds-stresses increase with local velocity ratio for a fixed confinement, and decrease with confinement for a fixed velocity ratio. It is also shown that, irrespective of confinement, the existence of quasi-two-dimensional coherent structures is driven by velocity ratio and the direction and magnitude of transverse flow. These structures cannot develop if velocity ratio is lower than 0.3 and if a strong transverse flow towards the MC occurs. In the latter case, the transverse flow is the predominant contribution to momentum exchange (compared with turbulent mixing and secondary currents), convex mean velocity profiles are observed, preventing the formation of quasi-two-dimensional structures.

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Mixing layer and coherent structures in compound channel flows: Effects of transverse flow, velocity ratio, and vertical confinement

The authors wish to thank Fabien Thollet and Alexis Buffet for their technical support, and the anonymous referees for their valuable comments that have helped to improve the paper. This research work was funded by the French National Research Agency under grant N◦ANR-14-CE03-0010 (Flowres ANR project, https://flowres.irstea.fr/en/). Declaration of Interests. The authors report no conflict of interest.

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Compound open-channel flows : effects of transverse currents on the flow structure

We investigate, within the framework of the nonlinear shallow water equations (NSWE), the generation and evolution of large-scale eddies with vertical axis (macrovortices hereinafter) which are responsible for much of the horizontal mixing occurring at the boundaries between the main channel and the flood planes of a compound channel. We show that the mechanism of generation of vorticity is essentially inviscid and is analogous to that occurring at a curved shock. Numerical experiments performed by means of a recently developed shock-capturing model for the solution of the NSWE, and described in Brocchini et al. (2001), clarify some features of macrovortices generation and allow us to quantify the momentum transfer across the channel.

Macrovortices-induced horizontal mixing in compound channels

The series of papers on the flow dynamics due to wave-induced macrovortices is completed with a statistical analysis of the mixing of the shallow flows occurring around submerged structures used for coastal protection. This is investigated with specific focus on the role played by large-scale horizontal eddies shed in coastal areas by waves breaking corresponding to topographic features like submerged breakwaters. As in Part 2, conditions due to isolated or arrays of breakwaters are studied. Analysis of particle statistics is used to determine both the features of the induced quasi-two-dimensional flow and to derive general properties. In particular three distinct regimes are found to characterize the flow evolution. Asymptotic regimes for small and large times share in any of the features of typical 'ballistic' and 'Brownian' regimes. Focus is mainly placed on properties of the 'intermediate time' regime which are seen to depend on the chosen topographic configuration. In agreement with the deterministic results of Part 2, we find that, because of an intense longshore current, an isolated breakwater induces a larger dispersion than that due to an array of breakwaters, characterized by a rip current. Moreover, for the same topography, the diffusivity grows with the local wavelength. Comparison with field data suggests that results of scaled-down laboratory experiments reproduce well natural mixing conditions. A simple formulation of absolute diffusivity, to be used in practical applications related to environmental quality management, is, finally, proposed.

A. Piattella

Papers

Macrovortices-induced horizontal mixing in compound channels

Topographically controlled, breaking-wave-induced macrovortices. Part 3. The mixing features

La circolazione idrodinamica in presenza di barriere sommerse: un'analisi sperimentale

Nearshore macrovortices generated at a submerged breakwater: experimental investigation and statistical modeling

Macrovortices-induced mixing in coastal and riverine environments