Mean and turbulence dynamics

What is it about?

Unsteady pressure gradients in turbulent flows not only influence the mean, but also affect the higher statistical moments of turbulence. In these flows, it is important to understand if and when turbulence is in quasi-equilibrium with the mean in order to better capture the dynamics and develop effective closure models. Therefore, this study aims to elucidate how turbulence decays or develops relative to a time-varying mean flow, and how the turbulent kinetic energy (TKE) production, transport and dissipation respond to changes in the imposed pressure forcing. The focus is on the neutral unsteady Ekman boundary layer, where pressure-gradient, Coriolis and turbulent friction forces interact, and the analyses are based on a suite of large-eddy simulations with unsteady pressure forcing. The analyses elucidate the physical mechanisms that trigger these dynamics, and investigate the implications on turbulence closure models and similarity laws.

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Why is it important?

The large majority of these previous studies, did not systematically vary the forcing time scale relative to the inertial and turbulent time scales, and, more importantly, were conducted under the assumption of a very large Rossby number (Ro = inertial force/Coriolis force), i.e. neglecting the Coriolis force. In this paper, we bridge this gap.