Numerical-assisted design of a wind tunnel used in the estimation of volatilization from water, waste or soil.

Numerical simulations of the flow inside two wind tunnel geometries used for determining emission volatilization have been conducted. The flow pattern was examined in a plane close to the emitting surface. A reduced surface, characterizing the low velocity zones in this plane, has been defined. The proposed geometry modification, consisting of a convergent/divergent system, decreases the low velocity regions by a factor ranging between 2 and 3.5. Large vortices present in the conventional configuration are almost eliminated. The new velocity distribution, close to the emitting surface, is more uniform and should therefore generate a more efficient mass transfer to the flowing air.

Numerical and experimental hydrodynamic studies of a lagoon pilot.

The determination of residence time is of major interest in terms of the characterisation, design and modelling of most environmental engineering processes, where a proper and homogeneous fluid distribution is often essential, and especially in the domain of wastewater or waste material treatment. In this paper, two different numerical methods for obtaining theoretical predictions of residence time distributions using the finite volume method are 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 takes to reach any point of the domain from a supply inlet. The result obtained is a spatial distribution of the local mean age of the fluid, which may be displayed as isocontours in the space domain considered. The second one consists of injecting a virtual particle stream (i.e. a fluid particle having the same density as the surrounding fluid, and treated numerically as a tracer) and measuring the time elapsed between the injection and the termination of the trajectory using a Lagrangian reference frame. The result obtained is expressed as an exit time distribution and may be displayed as a histogram. Finally, a comparison with measurements of mean residence times of tracers in a lagoon is made in order to establish the relevance of this method in concrete form. The prediction enables the instantaneous determination of the geometrical characteristics of the flow that contribute actually to the residence time dispersion. Its appropriate use prior to the design of water treatment and waste material treatment installations should contribute to prevent undesirable flow patterns such as short-circuiting and dead spaces.