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EXPERIMENTAL INVESTIGATION OF STREAKY STRUCTURES IN A RELAMINARIZING BOUNDARY LAYER

Alessandro Talamelli
II Facoltà di Ingegneria, Università di Bologna, Via Fontanelle 40, Forl`ı, 47100, Italy

N. Fornaciari
Department of Aerospace Engineering, Universita di Pisa Via G. Caruso 56100 Pisa, Italy

K. J. A. Westin
Department of Mechanics, KTH S-100 44 Stockholm, Sweden

P. Henrik Alfredsson
Liné Flow Centre, KTH Mechanics, Osquars Backe 18, SE-100 44 Stockholm, Sweden

Résumé

An experimental investigation of a boundary layer subjected to a relaminarization process is performed. The boundary layer, artificially made turbulent, develops on a plate mounted inside a two dimensional convergent channel. The effects of an high level of free stream turbulence are also investigated. Results show that the mean velocity profiles of the boundary layer develop towards the self similar laminar solution for both levels of free stream turbulence tested.
Two-point correlation measurements performed by means of two miniaturized single-wire hot-wire probes with variable relative spanwise separation, show the presence of elongated longitudinal structures. The dimension of these streaks decreases in the downstream direction. Conversely, if the size is normalised by the displacement thickness of the boundary layer, the relative streak dimension shows a significant increase. Also, when scaled by the viscous length scale l* the streak dimensions increase from about 60l* in the turbulent boundary layer to 150−180l* in the relaminarizing region. Finally, to process only the laminar part of the velocity signals a filtering procedure based on a threshold value of the second derivative of the time signals has been applied showing a slight increase in the detected dimensions of the structures.