LAGRANGIAN STATISTICS AND TRANSILIENT MATRIX MEASUREMENTS BY PTV IN ACONVECTIVE BOUNDARY-LAYER

The most common velocity measurement techniques, based on image analys is, calculate the velocity by cross-correlation of a portion of the di gitized images, and give a Eulerian description of the investigated fi eld. Particle tracking velocimetry (PTV), based on the recognition of trajectories of seeding particles, only furnishes a Eulerian descripti on provided the trajectories are shorter than the characteristic scale of the phenomenon. If particles are tracked for a longer time, a Lagr angian description is obtained. Consequently, in order to successfully evaluate Lagrangian statistics, a long series of single-exposed image s has to be acquired. PTV has been used to examine the pollutant dispe rsion in a laboratory simulation of the convective boundary layer of t he atmosphere. The convective layer has been simulated by a water tank heated from below, where the atmospheric thermal stratification has b een reproduced. Though the phenomenon was observed to be steady in the Eulerian reference frame, the same did not occur in the Lagrangian re ference frame. From the analysis of particle motion, it is possible to determine the characteristic time scale of the turbulence and to desc ribe the different behaviour of hot updraughts and cold downdraughts. The pollutant dispersion is described in detail by the transilient mat rix representing the probability of transition of a particle from one level to another of the convective layer. From the information given b y this matrix, it is possible, in principle, to estimate the concentra tion fields, due to a variety of concentrated and distributed pollutan t sources.