Some experimental investigations on the concentration variance and its dissipation rate in a grid generated turbulent flow

This paper is devoted to an experimental investigation of concentration var iance diffusion in a grid generated turbulent flow. Combined laser-induced fluorescence applied to concentration measurement and two-dimensional laser -Doppler velocimetry are implemented in order to measure simultaneously and instantaneously the molecular concentration of the passive tracer and two components of the carrier flow velocity. The different terms of the scalar variance transport equation can be measured directly in order to deduce the scalar fluctuations dissipation rate. It is shown that the approximation s calar variance advection similar or equal to dissipation is valid, similarl y to the decay of turbulent kinetic energy in the wake of a grid. The simul taneous determination of both scalar variance and kinetic energy dissipatio n rates permit an experimental determination of the scalar to velocity time scale ratio. Finally, an analysis of the self similarity of the fluctuatin g concentration field is also provided. (C) 2000 Elsevier Science Ltd. All rights reserved.