Transient growth on time-dependent strongly-stratified flows

Abstract eng:
We study the competition between the zigzag instability and Lilly’s mechanism of vertical decorrelation through independentlyevolving horizontal layers by computing the linear optimal perturbations to different types of vertically-invariant base flows with strong stratification. This allows us to find, as a function of vertical wavelength, the potential of linear perturbations to extract energy from each base flow. We observe that the zigzag instability is more relevant than Lilly’s mechanism only when the base flow is composed of distinct vortices, while the zigzag scaling laws remain always valid except for viscous effects. Additionaly, the optimal perturbations reveal other, more efficient mechanisms of transient energy extraction. Performing toroidal-poloidal decomposition of the optimal perturbations we observe that, as the vertical wavenumber increases, the optimal perturbations are increasingly associated to energy extraction from the vertical vorticity into vertical velocity and density perturbations.

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