21st Congress of International Council of the Aeronautical Sciences, Melbourne, Australia, 13-18 September, 1998
Paper ICAS-98-3.1.3

NONLINEAR EVOLUTION OF A THREE-DIMENSIONAL WAVETRAIN IN A FLAT PLATE BOUNDARY LAYER

De Medeiros M.A.F.
University of Minas Gerais, Brazil

Keywords: three-dimensional wavetrain, flat plate boundary layer

This paper presents results of an experimental study of the transition in boundary layers. The experiments were conducted in a low-turbulence wind tunnel. The process was triggered by a three-dimensional Tollmien¬Schlichting wavetrain excited by a harmonic pdint source in the plate. Hot-wire anemometry was used to measure the signal and investgate the nonlinear regime of these waves. It was observed that the three-dimensional wavetrain behaved very differently from two-dimensional ones. In particular, it did not involve the growth of subharmonics or higher harmonics. The first nonlinear signal to appear was a mean flow distortion. This had a spanwise structure consisting of regions of positive and negative mean distortion distributed like longitudinal streaks, which became more complex as the nonlinearity developed. The structure in the direction perpendicular to the wall has also been studied. Initially the distortion profiles resembled Klebanoff modes, but further downstream it also became more complex. Elsewhere studies have revealed the existence of streak-structures in turbulent flows. It is conjectured that the current experiments may provide a link between early wave-like instabilities and some coherent structures of turbulent boundary layers.

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