Identification of coherent structures in the Convective Boundary Layer

BADAS, MARIA GRAZIA;QUERZOLI, GIORGIO

2009-01-01

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Abstract

According to several results presented in literature, Finite-Time Lyapunov Exponents (FTLE) methodology represents a powerful tool for the analysis and characterization of unsteady phenomena. Using a laboratory model, we investigated the adoption of the FTLE for the identification and characterization of coherent structures in the Convective Boundary Layer (CBL). The investigation was carried out in a convective tank, using distilled water as the working fluid. The simulated phenomenon was then analyzed by means of image analysis techniques on an illuminated vertical plane. Specifically, the 2D velocity fields were retrieved by means of RIV (Robust Image Velocimetry). Comparison with numerical and in situ measurements reported in literature confirmed the soundness of the obtained velocity fields. The inspection of FTLE spatial fields obtained by the time-resolved RIV measurements allowed the identification of CBL coherent structures. This result is not trivial due to the unsteady and fully turbulent nature of the analyzed phenomenon. Moreover, FTLE statistics were analyzed both for the whole CBL and as a function of the non-dimensional elevation. Results revealed important phenomenological features of coherent structures linked to the mixing processes in the CBL.