COBEM-2017-0208 The Influence of the Reynolds Number on the Linear Instability of Laminar Separation Bubbles in the Absence of Environmental Disturbances

The understanding and identification of the physical instability mechanisms responsible for the appearance of unsteadiness and three-dimensionality on laminar separation bubbles can provide a fundamental insight into the aerodynamic characteristics of airfoils at stall conditions, and guide the development of strategies for the control of separated flows. Separation bubbles have been demonstrated to have an intrinsic instability mechanism, which results in the appearance of spanwise-periodic three-dimensional structures. The instability mechanism responsible for three-dimensionality was found to become active, in incompressible flow, under conditions in which wavelike, two-dimensional perturbations are only convectively unstable, and thus would require of continuous external excitation to dominate the physics. The aim of this work is to extend this analysis to compressible subsonic separation bubbles, as compressibility could alter the qualitative picture. Direct numerical simulations are carried out to obtain two-dimensional base flows, determining the conditions for the appearance of self-sustained two-dimensional oscillations. For those bubbles that remain steady in the two-dimensional simulations, a linear instability analysis based on two-dimensional eigenmodes is applied to the study of the stationary three-dimensional instability. The effect of Reynolds and Mach numbers on both kinds of instabilities and the conditions under which they become active are also documented.