Regenerative Orr mechanism yielding large non-modal perturbation energy growth in a viscosity stratified plane shear flow

Transiently growing non-modal perturbations can play a crucial role in the transition of plane shear flows in modally stable regimes. In terms of the extent of transient amplification, three-dimensional perturbations are typically more prominent due to the lift-up effect. In contrast, two-dimensional (2D) spanwise-independent perturbations are often considered less important as they typically undergo modest levels of transient growth and are short-lived. The Orr mechanism is key to the amplification of energy for 2D perturbations. In this work, we discuss 2D non-modal perturbations of three-layer viscosity stratified flows with the mean shear rates of the outer layers being equal. Strikingly, a novel regenerative Orr mechanism is found that allows for significant amount of energy amplification despite the 2D nature of the perturbations. Moreover, these perturbations survive for considerably long times. The perturbation structure shows symmetry about the middle layer and evolves such that the Orr mechanism can repeatedly occur in a regenerative manner resulting in the perturbation energy evolving in a markedly non-monotonic fashion. When these same perturbations are introduced in a uniform plane shear flow, the corresponding non-modal transient amplifications are shown to be much smaller.