Experimental study of the scalar concentration field in turbulent flows.

摘要

An experimental investigation has been carried out in a relatively simple turbulent flow in order to directly measure for the first time the Expected Mass Fraction (EMF) of the state of a contaminant concentration field within a contaminant cloud. Particle image velocimetry (PIV) and Planar laser induced fluorescence (PLIF) were used to measure simultaneous velocity and concentration fields, respectively. The EMF is a relatively simple measure of the state of a contaminant cloud. It has been shown that the EMF is approximately self-similar when concentrations are normalized by the centreline mean concentration. It has been shown that a reasonable approximation of the EMF moments is possible by using the centreline absolute moments. The results are compared with the theoretical and experimental results established for a line source of scalar in grid turbulence.;Key words: Turbulent diffusion, Scalar, Contaminant plume, Dispersion, Particle image velocimetry (PIV), Planar laser induced fluorescence (PLIF), self-similarity, Expected mass fraction (EMF), Probability density function (PDF), Moment, Batchelor scale.;The two closure approximations in the evolution of the moments of the probability density function of a scalar concentration are validated experimentally using simultaneous measurements of velocity and concentration fields. The effect of molecular diffusivity is brought into the convective closure approximation by introducing a representative 'local concentration scale', which appears to be a robust improvement in the approximation and can be estimated directly from the centreline moments. The concept of fractal scaling is used in dealing with under-resolved dissipation measurements by using an extrapolation scheme. This leads to two distinct self-similar regions within the Batchelor scale and the Integral scale, separated by the Kolmogorov scale, in the measured constant of the dissipative closure approximation.