Resemblance of K- and N-regimes of boundary-layer transition at late stages

This paper is devoted to an experimental study of late nonlinear stages of laminar-turbulent transition in a 2D flow close to the Blasius boundary-lay er The measurements were conducted at controlled disturbance conditions wit h excitation of a 2D large-amplitude fundamental instability wave with freq uency f(1) and/or a pair of low-amplitude oblique subharmonic instability w aves with frequency f(1)/2 and values of the spanwise wavenumber +/-beta(1/ 2). In the case with a phase shift between the fundamental mode and the pai r of the subharmonics favourable for the subharmonic resonance the transiti on process was found to be characterised by a rapid resonance growth of the 3D subharmonic modes. This was followed by a formation of the h-structures within each subharmonic period in time, positioned in a staggered order in space that is typical of the N-regime (or the subharmonic regime) of the b oundary layer breakdown. However, at late stages of the disturbance develop ment the local behaviour of the perturbations in the vicinity of the Lambda -structures turned out to be very similar to that typically observed in the K-regime of breakdown. This could be seen in the formation of an intensive Lambda-shaped 3D high-shear layer and the coherent structures associated w ith spikes in the time-traces of the hot-wire signal. Sets of consecutive s pikes were found to be generated in the vicinity of the tip of each Lambda- structure. The arrays of spikes had also the staggered order with the strea mwise and spanwise spacing characteristic of the subharmonic wave (as in th e N-regime) but their local properties were found to be qualitatively the s ame as those typical for the K-regime. Despite the significantly different nature of the initial stages of these two scenarios of transition, describe d usually in terms of weakly nonlinear interactions of the instability wave s, the late stages of these two types of breakdown (described usually in te rms of vortices attributed to the coherent structures) have approximately t he same physical nature. (C) 2000 Editions scientifiques et medicales Elsev ier SAS.