Instabilities in laminar separation bubbles

What is it about?

Wall-bounded flows, in their transition from a laminar state to turbulence, pass through a set of particular stages characterized by different physical processes. Among wallbounded flows, separated flows have a special place because their dynamics can either be noise amplifiers or oscillators. For several years Marxen and co-workers have been studying the evolution of two- and three-dimensional perturbations in the laminar part of a laminar separation bubble. In Marxen et al. (J. Fluid Mech., vol. 728, 2013, p. 58) they study vortex formation and its evolution in laminar–turbulent transition in a forced separation bubble. By the combined use of numerical and experimental methods, different mechanisms of secondary instabilities have been highlighted: elliptic instability of vortex cores and hyperbolic instability responsible for three-dimensionality in the braid region. This work shows, for the first time in laminar separation bubbles, the first nonlinear stages of transition to turbulence of such a flow. However, since this type of flow is very sensitive to various environmental stresses, several scenarios for transition to turbulence remain to be explored.

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