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 variance 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 scalar variance advection ≃ dissipation is valid, similarly to the decay of turbulent kinetic energy in the wake of a grid. The simultaneous determination of both scalar variance and kinetic energy dissipation rates permit an experimental determination of the scalar to velocity time scale ratio. Finally, an analysis of the self similarity of the fluctuating concentration field is also provided.