Spatio-temporal dynamics of turbulent separation bubbles

TL;DR

This study investigates the complex spatio-temporal behavior of turbulent separation bubbles over a flat plate using direct numerical simulations, revealing distinct high- and low-frequency modes and their underlying mechanisms.

Contribution

It provides new insights into the dynamics of separation bubbles, especially the low-frequency breathing mode and the potential Görtler instability influence.

Findings

Identification of high-frequency vortex shedding mode.

Observation of a low-frequency breathing mode.

Evidence suggesting Görtler instability as a driving mechanism.

Abstract

The spatio-temporal dynamics of separation bubbles induced to form in a fully-developed turbulent boundary layer (with Reynolds number based on momentum thickness of the boundary layer of 490) over a flat plate are studied via direct numerical simulations. Two different separation bubbles are examined: one induced by a suction-blowing velocity profile on the top boundary and the other, by a suction-only velocity profile. The latter condition allows reattachment to occur without an externally imposed favourable pressure gradient and leads to a separation bubble more representative of those occurring over airfoils and in diffusers. The suction-only separation bubble exhibits a range of clearly distinguishable modes including a high-frequency mode and a low-frequency "breathing" mode that has been observed in some previous experiments. The high-frequency mode is well characterized by classical frequency scalings for a plane mixing layer and is associated with the formation and shedding of spanwise oriented vortex rollers. The topology associated with the low-frequency motion is revealed by applying dynamic mode decomposition to the data from the simulations and is shown to be dominated by highly elongated structures in the streamwise direction. The possibility of G\"{o}rtler instability induced by the streamwise curvature on the upstream end of the separation bubble as the underlying mechanism for these structures and the associated low frequency is explored.