Numerical Investigation of Fundamental Mechanisms in Hypersonic Transition to Turbulence

All dates for this event occur in the past.

Scott Lab E525
201 W. 19th Avenue Columbus, OH 43210
Columbus, OH 43210
United States

Abstract:
Laminar-to-turbulent transition estimation is one of the key challenges in designing hypersonic vehicles, primarily due to uncertainties in the disturbance environment and the enormous parametric space involved. A comprehensive understanding of the underlying physical processes is lacking, hindering the development of efficient control strategies. In particular, the role of leading-edge bluntness during various stages of transition remains unresolved, which is of practical relevance. This thesis documents a series of five complementary numerical studies aimed at understanding the laminar-to-turbulent transition process on hypersonic flat plates with varying leading edge bluntness. Direct numerical simulations are used to accurately resolve the spatio-temporal scales of the flows. These are complemented with physics and data-driven techniques to gain insight into the flow fields. The first study investigates the role of leading-edge bluntness in the receptivity of broadband freestream disturbances and their linear amplification mechanisms. At small nose radii, the disturbances trigger Mack modes, whose growth rate reduces with increasing bluntness. After a critical radius, waves of a different class with predominant support in the entropy layer are amplified. With increasing bluntness, the amplification rates of these disturbances increase. In the second study, the laminar-to-turbulent transition on a sharp flat plate is examined with stochastic freestream forcing. Second-mode waves are most amplified, followed by their fundamental resonance and finally the onset of turbulence. During various stages of the transition, acoustic and vortical dissipation dominate the wall heating, and near-wall streaks enhance the skin-friction. The third study scrutinizes the characteristics of the late stages of transition by triggering an isolated bypassed turbulent spot on a hypersonic flat plate, and examines its evolution with the momentum potential theory. Mack modes and their fundamental resonance remain an integral part of the spot evolution. The roller-like structures close to the wall exhibit predominantly acoustic and entropic characteristics, while the freestream radiation emitted from the turbulence core is mainly acoustic in nature. The features of the entropy-layer disturbances on blunted flat plates are examined in the fourth study. Spanwise oblique perturbations amplifying in the entropy layer interact to generate streamwise streaks in the boundary layer. These entropy-layer disturbances act as the streak secondary instabilities and transfer perturbation energy into the boundary-layer through triadic interactions. The fifth study developed a reduced order model for the freestream disturbances from a supersonic channel tunnel. This model informs the receptivity on a flat plate, where broadband disturbances penetrate the boundary layer and generate first-mode waves. This is followed by the formation of streamwise streaks and intermittent turbulent spots, emulating natural transition in wind tunnels.

Zoom Link (or alternative) - if available
https://osu.zoom.us/j/96361199002?pwd=K3V3azRNZzFtVXpaNFh5eWRpQnUyZz09

Committee Members
Professor Datta Gaitonde
Professor Jack McNamara
Professor Lian Duan
 

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