Three-Dimensional Swirl Flow Velocity-Field Reconstruction Using a Neural Network With Radial Basis Functions

International audienceFor many studies, knowledge of continuous evolution of hydrodynamic characteristics is useful but generally measurement techniques provide only discrete information. In the case of complex flows, usual numerical interpolating methods appear to be not adapted, as for the free decaying swirling flow presented in this study. The three-dimensional motion involved induces a spatial dependent velocity-field. Thus, the interpolating method has to be three-dimensional and to take into account possible flow nonlinearity, making common methods unsuitable. A different interpolation method is thus proposed, based on a neural network algorithm with Radial Basis Functions

Numerical investigation of bend and torus flows—Part II : Flow simulation in torus reactor

International audienceFlow in a torus reactor with straight parts fitted with a marine impeller is investigated. The laser Doppler anemometry (LDA) is first employed to achieve experimental measurements of mean velocity profiles. Next, a numerical resolution of the steady-state flow is performed using a multiple reference frames (MRF) approach to represent the particular flow induced by the marine impeller in the geometry. A comparison of predictions using different turbulence models to LDA measurements is made, and a k–ω model is assessed.The numerical tool is used to investigate in more details the particular ow induced in the torus geometry. Evolution of the axial and rotating motions when moving away from the impeller is especially investigated, showing the complex hydrodynamical interaction between the main rotating swirl motion involved downstream the impeller, and bend curvature e ects. Two di erent ow conditions can be considered in the torus geometry, with a main swirling motion close to the impeller, which freely decays and vanishes when Dean vortices appear in bends. Simulations for two rotation velocities of the impeller and comparison with the study with simple bends (ÿrst part) reveal pertinence of the swirl number Sn to describe the change of ow conditions along the reactor axis. When this parameter decreases below a threshold value around 0.2 in a bend entry, centrifugal e ects due to bend curvature are more important than the swirl motion, and Dean vortices appear in bend outlet. One main consequence is the axial distance of the swirl motion persistence, which is found to be smaller for the higher impeller rotation velocity, due to the dual e ect of the marine impeller that generates simultaneously both axial and rotating motions

Residence time distribution in twisted pipe flows: Helically coiled system and chaotic system

This study compares residence time distributions in a helically coiled tube and a spatially chaotic system. The chaotic system consists of an array of bends, the plane of curvature of each one makes a 90 degrees angle with that of its neighbors. Chaotic trajectories are obtained by the switch in the symmetry plane of the Dean vortices which appear in the bends. Mixing in the two configurations is compared by modeling the residence time distributions, experimentally determined by means of a two-measurement-point technique, using the axial dispersion plug-flow model. For Reynolds numbers greater than 2500, axial dispersion in the chaotic system is more than 20% less than in a helically coiled tube having the same number of bends. The decrease in axial dispersion is due to the generation of chaotic trajectories, which also contribute to an increase in transverse dispersion. Thus, the chaotic curved pipes system appears very promising in producing good mixing, especially in a laminar flow regime

Hydrodynamics study in a plane ultrafiltration module using an electrochemical method and particle image velocimetry visualization

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Experimental and numerical characterisation of mixing in a steady spatially chaotic flow by means of residence time distribution measurements

This work describes an experimental study and a numerical simulation of residence time distributions (RTD) in a spatially chaotic three-dimensional flow. The experimental system is made up of a succession of bends in which centrifugal force generates a pair of streamwise Dean roll-cells. Fluid particle trajectories become chaotic through geometrical perturbation obtained by rotating the curvature plane of each bend +/-90 degrees with respect to the neighbouring ones. Different numbers of bends, ranging from 3 to 33, were tested. RTD is experimentally obtained by using a two-measurement-point conductimetric method, the concentration of the injected tracer being determined both at the inlet and at the outlet of the chaotic mixer. The experimental RTD is modelled by a plug flow with axial dispersion volume exchanging mass with a stagnant zone. RTD experiments were conducted for Reynolds numbers between 30 and 13,000. Peclet number based on the diameter of the pipe Pe(D) = (W) over bar D/D-ax) increases with Reynolds number, whatever the number of bends in the system. This reduction in axial dispersion is due to the secondary Dean flow and the chaotic trajectories. Globally, the flowing fraction increases with Reynolds number, whatever the number of bends, to reach a maximum value between 90 and 100%. For Reynolds numbers between 50 and 200, the flowing fraction increases with the number of bends. The stagnant zone models fluid particles located close to the tube wall. The pathlines become progressively chaotic in small zones in the cross section and then spread across the flow as the number of bends is increased, allowing more trapped particles to move towards the tube centre. In order to characterise more completely the efficiency of the device, a criterion is proposed that takes into account both the mixing characteristics and the pressure drop. The RTD for low Reynolds numbers has also been obtained numerically using a flow model based on Dean\u27s asymptotic perturbation solutions of the mean flow in a curved pipe. At the end of each bend, the velocity field is rotated by +/-90 degrees before entering the next bend. The RTD is calculated by following the trajectories of 250,000 \u27numerical\u27 particles along the device. Numerical results are in good agreement with experiments in the same Reynolds number range. (C) 2000 Elsevier Science Ltd. All rights reserved