This paper provides comparisons between experimental data and numerical results for impulsively started flows in a three-dimensional rectangular lid-driven cavity of aspect ratio 1:1:2 at Reynolds number 1000. The initial evolution of this flow is studied up to the dimensionless time t = 12 and is found both experimentally and numerically to exhibit high sensitivity to geometrical perturbations. Three different flow developments generated by very small changes in the boundary geometry are found in the experiments and are reproduced by the numerics. This indicates that even at moderate Reynolds numbers the predictability of three-dimensional incompressible viscous flows in bounded regions requires controlling the shape of the boundary and the values of the boundary conditions more carefully than needed in two dimensions.

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International audienceThe kinematics of hydrodynamic turbulent flows developed in vertical slot fishways (VSF) was studied in detail in flow patterns not yet published to date for the purposes of modifying existing devices and to allow for the passage of all fishes, particularly the smaller species. A transparent device based on the typical prototype dimensions of VSF in France was constructed for the experiment. The velocity measurements were carried out by Particle Image Velocimetry (PIV). These measurements were used to determine the various kinematics parameters characterizing the flow. From the dimensions and slope of the fishway, two flow topologies highlighting the swirling pattern were proposed. The method of Proper Orthogonal Decomposition (POD) was used to undertake unsteady and energetic analyses to characterize the main phases of flow evolution that fish passing through the passage may encounter

The paper reports on the experimental results of a vertical slot fishway (VSF) study. Particle image velocimetry and Acoustic Doppler Velocimetry were used to estimate the effects of a supplementary cylinder, appropriately positioned inside existing VSFs, on the hydraulic parameters and the instantaneous flow patterns. Special attention was paid to the analysis of the spatial flow distribution and to the instantaneous flow dynamics initiated by the addition of a cylinder. Modifications of velocity amplitudes, flow fluctuations and vorticity values were quantified for two dimensions of VSF. The contribution of a supplementary cylinder has been quantified in order to "smooth" the effects of turbulence and to facilitate the passage of the largest number of fish irrespective of their size or swimming capacity. The results provided insights on the control of the fishway turbulence, which might help engineers to develop effective systems for the passage of fish with low swimming capabilities.

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In standard particle image velocimetry, the spatial resolution of the velocity field depends on the final window size of the cross-correlation. For small and micro experimental configurations, the PIV measurement is often limited by the optical accesses, the depth of focus and the number of particles in the interrogation window. With the aim of obtaining a resolution of one vector per pixel, a single pixel resolution correlation (SPRC) algorithm is described and applied to the study of a Poiseuille flow that is steady on average between two close walls, one of which has grooves normal to the streamwise direction. The influences of the number of real images and the application of different thresholds to the correlation are discussed. From 10 000 double frames, the velocity field is calculated by the SPRC and compared to the mean velocity field calculated by standard cross-correlation.

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