The stability of streaks, generated by vortices in wall-bounded shearows, is studied analytically, numerically and experimentally. A novelanalytical approximation of the linear transient growth in Couette owallows investigating the secondary stability of spanwise periodic streaksusing Floquet theory. The optimal parameters for instability correspondto the strongest inection points, those having maximal shear, ratherthan initial conditions maximizing the energy growth. For the sym-metric transient growth the most dangerous secondary disturbances aresinuous, associated with spanwise inection points having a spanwisewavenumber of = 3:6 (as opposed to = 1:67 which maximizes energygrowth) and the varicose instabilities are associated with spanwise inec-tion points as well. For the antisymmetric transient growth both sinu-ous and varicose instabilities are observed, associated with spanwise andwall-normal inection points, respectively. The theoretical results areveried by obtaining transition in a direct numerical simulation (DNS)initiated by the corresponding analytical expressions. The rapid evolu-tion of the secondary disturbance on top of the slowly evolving transientgrowth enables us to use the multiple time scales method to follow theevolution of the secondary disturbance. The very good agreement be-tween the DNS and analytical expressions veries the theoretical predic-tions. Finally, the above results are discussed with respect to previoustransitional pipe and Poiseuille ow experiments.
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