Numerical and experimental hydrodynamic studies of a lagoon pilot

The determination of residence time is of major interest in terms of the ch aracterisation, design and modelling of most environmental engineering proc esses, where a proper and homogeneous fluid distribution is often essential , and especially in the domain of wastewater or waste material treatment. I n this paper, two different numerical methods for obtaining theoretical pre dictions of residence time distributions using the finite volume method apr used. The first one consists of solving a transport equation of the local mean age of the fluid, which is the average time that a fluid particle take s to reach any point of the domain from a supply inlet. The result obtained is a spatial distribution of the local mean are of tile fluid, which may h e displayed as isocontours in the space domain considered. The second one c onsists of injecting a virtual particle stream (i.e, a fluid particle havin g the same density as the surrounding fluid, and treated numerically as a t racer) and measuring the time elapsed between the injection and the termina tion or the trajectory using a Lagrangian reference frame. The result obtai ned is expressed as an exit time distribution and may be displayed as a his togram. Finally, a comparison with measurements of mean residence times of tracers in a lagoon is made in order to establish the relevance of this met hod in concrete form. The prediction enables the instantaneous determinatio n of the geometrical characteristics of the flow that contribute actually t o the residence time dispersion, its appropriate use prior to tile design o f water treatment and waste material treatment installations should contrib ute to prevent undesirable flow patterns such as short-circuiting and dead spaces. (C) 2001 Elsevier Science Ltd. All rights reserved.