Pulsatility delays the transition to sustained turbulence in quasi-two-dimensional shear flows

TL;DR

This study shows that pulsatility delays the transition to sustained turbulence in quasi-two-dimensional shear flows, with non-oscillatory forcing being more effective, though pulsatility reduces the initial disturbance needed for turbulence.

Contribution

It reveals that oscillatory base flows primarily cause decay rather than promote turbulence, highlighting the efficiency of non-oscillatory forcing in triggering sustained turbulence.

Findings

Oscillatory flows delay turbulence transition.

Transient growth is dominated by modal intracyclic growth.

Pulsatility reduces the initial disturbance amplitude needed for turbulence.

Abstract

This work investigates efficient routes to turbulence in quasi-two-dimensional shear flows. Two-dimensional disturbances require high Reynolds numbers to incite transition from a steady base flow, as transient growth is modest. With the addition of an oscillatory base flow component, this work shows that the transient growth experienced by two-dimensional initial perturbations is often well above that provided by the steady component. However, as has been shown for three-dimensional flows [Pier & Schmid J. Fluid Mech. 926, A11 (2021)], the transient growth is almost entirely composed of modal intracyclic growth, rather than a transient mechanism which takes advantage of non-normality. This lack of transient growth, relative to the severe decay induced by the favorable pressure gradient during the acceleration phase of the oscillatory base flow, only ever delays the transition to sustained turbulence. Thus, a non-oscillatory driving force remains the most efficient strategy for sustained turbulence in quasi-two-dimensional shear flows. The only benefit provided by pulsatility is that the amplitude of the initial condition required to trigger intermittent turbulence is orders of magnitude smaller.