Due to turbulence-associated excessive heating and drag, the issue of laminar-to-turbulent transition in high-speed boundary layers remains one of the primary difficulties that prevents achieving reliable long-range hypersonic flight. Thus, accurate and reliable prediction of transition to turbulence in high-speed flows is a key design requirement in order to ensure safety and efficiency. Regardless of how they occur (e.g. naturally in the atmosphere or artificially by the design components), small-amplitude fluctuations in pressure, temperature and/or velocity are the most influential players in the transition scenario during a high-altitude flight. A key factor to which the transition location and mechanism is extremely sensitive, is the spectral makeup of the instability waves in the pre-transitional regions. Since the environmental conditions are often uncertain during a hypersonic flight, the main challenge is how can we guarantee robust flow-design when we do not know the exact disturbance spectra? In order to address this challenge, for a given level of free-stream energy we seek the inflow disturbance that is nonlinearly most dangerous: This disturbance will be a collection of waves with optimally selected amplitudes and phases such that it causes the earliest possible breakdown to turbulence, and thus establishes a strict nonlinearly bound on transition Reynolds number. Any other free-stream disturbance with the same energy would cause transition farther downstream, and therefore, design strategies based on the nonlinearly most dangerous disturbance are robust in unknown environmental conditions. When designing high-speed air vehicles, the primary objective is to reduce drag and heat transfer. Therefore, once the nonlinearly most dangerous disturbance is identified, it is tempting to use flow-control strategies to suppress it. In current work, the viability of two common strategies is investigated: Breakdown to turbulence is delayed as much as possible by using (i) thermal texture and (ii) physical roughness, when the environmental disturbance of the simulations is set to correspond to the most dangerous condition from the standpoint of transition. An ensemble-variational optimization technique is developed to seek the objectives of current work.
About the Speaker
Reza Jahanbakhshi earned a B.Sc. and a M.Sc. in Mechanical Engineering from Sharif University of Technology in 2008 and 2010, respectively, during which time he conducted research in the field of cryogenics. After receiving his master’s degree, he moved to the United States in 2011 to pursue a doctoral degree in the Department of Mechanical and Aerospace Engineering at the State University of New York at Buffalo. During his time as a Ph.D. candidate, he worked on high-fidelity numerical simulations of reacting compressible turbulent flows. After receiving his Ph.D. in 2016, Dr. Jahanbakhshi was awarded a post-doctoral fellowship at Johns Hopkins University. The project, on which he has continued to work, is on the topic of laminar-to-turbulent transition in hypersonic boundary layers. Dr. Jahanbakhshi is currently an assistant professor in department of aerospace, physics, and space sciences at the Florida Institute of Technology.
