Experimental study of K-regime of breakdown in straight and swept wing boundary layers

~Received 16 October 2000; accepted 28 February 2001!In this Brief Communication, the nonlinear evolution of periodical disturbances generated by anexternal sound field in a swept wing boundary layer is presented. All experimental results arecompared with corresponding data for a straight wing configuration. The Tollmien–Schlichtinginstability has been studied, and it was found that the disturbance flow field remained highlydeterministic and periodic in both time and space until the latest stages of the transition. Afrequency-wave number Fourier analysis shows that the disturbance spectra comprise only thefundamental wave and its higher harmonics. The K-type breakdown scenario was observed, and thenonsymmetry of flow patterns in the swept wing boundary layer was found to be due to the presenceof the cross flow. © 2001 American Institute of Physics. @DOI: 10.1063/1.1366667#It is well known that the surface friction is the mainsource of drag on aircraft wings, road vehicles, and otherstreamlined bodies. Since the skin friction of turbulentboundary layers is significantly greater than that of laminarboundary flows, it is most important to have an insight intohow the laminar–turbulent transition occurs in differentthree-dimensional ~3D! boundary layers. This knowledgewould enable prediction and, in a future perspective, alsocontrol of all stages of the transition.Whereas the stability of two-dimensional~2D! boundarylayers has been extensively studied theoretically, experimen-tally, and numerically, much less effort has been devoted tothe stability of 3D boundary layer flows owing to complexityof phenomena underlying the breakdown of laminar to tur-bulent flow.