Laboratory measurements in the convective boundary layer by means of a PTV system

BADAS, MARIA GRAZIA;QUERZOLI, GIORGIO

2008-01-01

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Abstract

The aim of this study is to better characterize the Convective Boundary Layer of the atmosphere (CBL). The investigation has been performed in a water tank model by means of a PTV system, which allowed measuring the velocity field during the evolution of the phenomenon. Experimental data show a good agreement with other laboratory measures and field measurements, supporting the validity of the performed simulation and of the achieved results. The analysis has been focused on the spatial features of the CBL, which have seldom been studied due to the difficulties in measuring the whole velocity field in controlled and repeatable conditions. Spatial characterization reflects fundamental features of CBL, such as heterogeneity and anisotropies. The computation of spatial autocorrelations and the subsequent estimates of the integral scales have provided important information on the convective structures responsible for the mixing processes within the atmospheric boundary layer. Actually, spatial CBL characterization has also important implications in the development of appropriate Sub Grid Scale (SGS) parameterizations reproducing small scale physics in Large Eddy Simulation (LES) models. Indeed, most recent SGS schemes are based on directly computing the stress tensors, needed by LESs, from the small scales velocity field reproduced by means of scale invariant tools, which use, as input, the resolved velocity field. Therefore, studying the spatial scale invariant properties of CBL velocity fields has a meaningful practical significance. Specifically, different scaling regimes have been investigated on the velocity field at different non dimensional elevations. Experimental data display Generalized Extended Self Similarity (GESS) scaling for both horizontal and vertical velocity components. Moreover, GESS scaling exponents computed on vertical velocities are higher than horizontal ones and exhibit a distinct trend with the non dimensional elevation.