AE Seminar

Shock-Wave/Boundary-Layer Interactions and Transition: 
Critical Mechanisms of Unsteadiness in High-Speed Flows

ft. 

Unnikrishnan Sasidharan Nair

Assistant Professor | Department of Mechanical Engineering | Florida State University

Tuesday, March 4
11am - 12pm
Guggenheim 442

About the Seminar: 
Interaction of high-speed air vehicles with their fluid-environment inherently results in flow unsteadiness, which profoundly impacts drag and heat loads on the vehicle. A fundamental understanding of underlying mechanisms that drive this dynamic behavior is imperative for designing efficient, safe, and reliable aircraft. In this seminar, we explore two critical mechanisms that are often associated with unsteady flowfields in high-speed aviation: shock-wave/boundary-layer interactions (SBLIs), and boundary layer transition. In the first scenario, we study supersonic SBLIs typically encountered on fuselages, to understand its three-dimensional topology and unsteady nature, utilizing high-fidelity simulations. Spectral and modal analyses reveal primary, secondary, and tertiary vortical structures that drive flow dynamics in the interaction region, as well as in the downstream wake. Similarities to and deviations from canonical two-dimensional interactions will also be highlighted, including relevant scaling parameters and Reynolds number dependence. The second phenomenon is hypersonic boundary layer transition over a flight-relevant cone-cylinder-flare geometry. Using global stability methods, we evaluate the multi-modal instability mechanisms that drive transition, and its sensitivity to atmospheric noise levels. Dominant instabilities include Mack's first and second modes within the boundary layer, absolute instabilities within the separation bubble over the flare, and convective shear layer instabilities over the edge of the bubble. While transition is induced by absolute instabilities under quiet conditions, a noisier atmosphere results in inter-modal interactions between the first mode and streak instabilities, leading to intermittent turbulent spots that collapse the bubble, accelerate transition, and intensify aero-thermal loading. 

About the Speaker: 
Unnikrishnan (Unni) Sasidharan Nair is an Assistant Professor in the Department of Mechanical Engineering, Florida State University. His research focuses on developing high-fidelity simulation frameworks augmented through insightful modal and physical analysis techniques, to study compressible flows pertinent to aero-propulsion systems, aeroacoustics, and high-speed transition. Current areas of research include 3D SBLIs, hypersonic transitional and turbulent flows, supersonic inlet performance, jet noise reduction, wake dynamics of high-AoA nose cones, laminar separation bubbles over lifting surfaces, and nonequilibrium aerothermodynamics. He obtained his Doctoral and Post-Doctoral training at The Ohio State University.