Associate Professor of Mechanical and Aerospace Engineering
Mechanical, Materials, and Aerospace Engineering Department, Illinois Institute of Technology, Chicago, Illinois (email)
At high Reynolds numbers, fluid particles within a boundary layer experience a momentum deficit relative to the external mainstream flow and are very susceptible to unsteady separation in regions of adverse external pressure gradient. There are many applications involving high-Reynolds-number wall-bounded flows in which a small-scale unsteady separation is a turning point in the evolution of the overall flow field and plays a central role in the dynamics of many of these applications. Essentially any unsteady, high-Reynolds-number flow in which a boundary layer is subject to an adverse streamwise pressure gradient can lead to unsteady separation involving formation of a recirculation region and an intense eruption of near-wall vorticity. Applications of unsteady separation include formation and detachment of the dynamic-stall vortex on pitching airfoils and helicopter blades, the turbulence generation mechanism near solid surfaces, the flow upstream of surface-mounted obstacles, and the flow through various portions of turbine and compressor passages and branching pipes. In dynamic stall at high Reynolds numbers, for example, it is an unsteady separation event that initiates the formation of the dynamic-stall vortex near the leading edge of the airfoil and a subsequent unsteady eruption that causes the dynamic-stall vortex to detach from the upper surface of the airfoil, leading to full stall. Our research seeks to increase our fundamental understanding of unsteady separation and leverage that knowledge into the development of appropriate strategies for controlling these events in applications.
Much of our effort in increasing our physical understanding of unsteady separation has focused on the viscous-inviscid interaction process that occurs during unsteady separation. The role of viscous-inviscid interaction has been investigated from two points of view, high-Reynolds number asymptotic theory and computational solutions of the full Navier-Stokes equations. Through this complementary approach, it has been found that interaction occurs somewhat differently in three Reynolds-number regimes. In addition to providing a clearer understanding of the interaction process, calculations of the full Navier-Stokes equations for a vortex-induced flow have also been calculated through unsteady separation to include vortex detachment.
In addition to the separation phenomena itself, high-Reynolds-number flows are also susceptible to instabilities. Two important types of instabilities have been considered. The first, which occurs very early in the separation process, is an inviscid Rayleigh instability that occurs soon after development of the recirculation region and requires very high Reynolds numbers. The second, which occurs much later in the separation process, does not require such high Reynolds numbers and occurs as a shear layer detaches from the remainder of the boundary layer.
This clearer understanding of the physical processes that take place in unsteady separation is essential in order to facilitate the control of unsteady eruptive events in practical applications. Potential control mechanisms that have been considered for unsteady separation include a heated surface and a moving wall. In both cases unsteady separation can be accelerated, delayed or suppressed through the action of the control mechanisms.
Funding: Army Research Office
Related Presentation: June 2009 Seminar