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arXiv · 2510.13336

Stable stratification enhances transient growth in streaky shear flows

Will Oxley·Rich Kerswell
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Abstract

Recent work has found that the well-known `lift-up' mechanism is not important for, and may even inhibit, the transient growth possible on streaky wall-bounded shear flows which is believed an important process in the near-wall cycle for turbulent flows. Moreover, artificially removing the wall-normal velocity has been found to unleash 3 orders of magnitude more perturbation energy growth in an unbounded streaky flow model. Motivated by this, we examine the effect of introducing stable stratification which naturally suppresses wall-normal velocities (the `vertical' shear case) and find it permits the hugely enhanced linear energy growth predicted by simply removing the wall-normal velocity. Alternatively, imposing stable stratification such that the spanwise velocities are suppressed (`horizontal shear') not surprisingly inhibits transient growth by weakening the active `push over' mechanism. A formula for the critical stratification strength to completely suppress the preferred growth mechanism is determined which proves a useful predictor for what is seen in the full numerical solutions of the model. Implications for a stratified near-wall cycle are briefly discussed.

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Will Oxley, Rich Kerswell. 2025-10-15. Stable stratification enhances transient growth in streaky shear flows. https://arxiv.org/abs/2510.13336

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