1. e breakdown. The A-shaped ' spanwise corruga€P- ons of streaklines. which correspond to the peak-valley structure of amplitude variation, are a result of weak 3-0 displacements of fluid particles across the critical Jayer precede the appearance of KIebanoff's
2. Different types of three-dimensional transition phenomena recently observed (see References 2 4-10) are characterized by Sta ered patterns of pgaks and valleys (see Fig. 2+and by their occurrence at very low amplitudes of the fundament a l T-S wave. This pattern also evolves rapidly into transition. These experiments showed that the subharmonic of the fundamental wave (a necessary feature of the sta ered pattern) was excited i n the boundary l a h p r o d u c e d either resonant wave Interaction predicted by Craik the (Fig 2) called the C t e or the secondary instabill;* of Herbert +i, 3) called the H*-Spectral broadening to' turbulence w i G re -excited subharmonics has been observed i n acoustics. convection. and free shear layers and was not observed i n boundary layers until the results of Kachanov et al. * They initiated the interest in subharmonics and prompted the simultaneous verification of C-type resonance 2'5'6, Subharmonics have also been confirmed for channel flows 13. This work i s reviewed by Saric and Thomas 'and Levchenko and the background for this paper can be found there. The recent theoretical treatment i s given by Herbert 14.
3. Uhen the source of the 3-0 subharmonic i s the background disturbance environment. then two such subharmonics (different i n phase by 180° ) can be phase locked with the fundamental 6. Since the background disturbances are random to some extent, phase synchronization randomly alternates between these two waves and thus accounts for the spectral broadening of the subharmonic. This contributes to a definite unsteadiness i n the signals being measured. Moreover. a long fetch (distance from the vibrating ribbon) i s required to phase-lock and entrain the Subharmonic from the background so that i t s amplitude i s of the order of the fundamental. It has been our experience three-dimensionality of long-retch waves that the
4. In order to achieve a steady occurrence of the subharmonic resonance, a signal of one-half the frequency of the fundamental i s introduced on the vibrating ribbon. This i s done using the same technique as Sarjc and Reynolds 15. Thus, two 2-0 waves at F and F are introduced into the boundary layer. The symbol * will always denote the subharmonic at F/2. The relative amplitude and phase are accurately controlled by separately computing the signals on a OEC-MINC-11/23 computer and D/A converting. The appropriate 3-0 subharmonic then evolves from the 2-0 subharmonic. Under certain conditions. it i s possible t o obtain spatiallysteady resonance. The results are described below.