A lagrangian study of particle dispersion in the unstable boundary layer

Abstract

Most of the experimental techniques used to investigate the atmospheric turbulence give an Eulerian description of the velocity field. Nevertheless, pollutant dispersion phenomena are naturally described in the Lagrangian approach as the pollutant acts as tag of the fluid particles. The turbulence in the unstable boundary layer (UBL) of the atmosphere was simulated by means of a laboratory model and investigated from the Lagrangian point of view by use of a measuring technique (the particle tracking velocimetry) based on image analysis. The Lagrangian correlations of the horizontal and vertical velocity components were evaluated. From these, the Lagrangian integral time scales were obtained and compared with the Eulerian ones. The comparison showed that it is not possible to define a single value of the Lagrangian and Eulerian scale ratio for the whole UBL. Moreover, the data set was conditionally sampled in order to describe selectively the behaviour of upward- and downward-moving particles. Finally, assuming the horizontal homogeneity, steadiness, and frozen turbulence, the time histories of the particle locations were used to predict pollutant concentration fields, downwind continuous sources placed at any height.