Fluid-thermodynamic (FT) analysis of compressible flows
Speaker: Dr. S. Unnikrishnan
Ohio State University
Date & Time: 07 June 2019 @ 1100 hrs
Venue: AE Auditorium
Unsteadiness in fluid flows generally excite a wide range of spatio-temporal scales, the analysis of which can yield important insights into their physics. We adopt a generalized energy-based framework to identify crucial dynamics in compressible flows, by first extracting and then quantifying the evolution of Kovasznay-type physical modes in these systems. These modes constitute the hydrodynamic, acoustic and entropic components of fluctuations in a compressible fluid, and are referred to as fluid-thermodynamic (FT) modes. The approach is used to analyze two classes of flows (generated through validated simulations): turbulent jets and hypersonic boundary layers.
Turbulent jets: The primary issue addressed here is genesis and transmission of noise. The method accurately isolates the energetically trivial, but highly detrimental acoustic component, which is identified to possessa spatio-temporally coherent wavepacket structure in the turbulent plume, having scales significantly larger than local turbulent integral-scales. On the other hand, the hydrodynamic content exhibits a chaotic nature, expected of a turbulent flowfield. A phenomenological model is thendeveloped, which identifies vortex intrusions into the potential core as important intermittent events that activate the acoustic wavepacket, leading to coherent sound radiation. The strong low-rank behavior observed in the acoustic component helps in the development of robust and inexpensive sound propagation and modeling tools for turbulent jets.
Hypersonic boundary layers: Transition in hypersonic boundary layers is closely associated with the evolution of an instability wave called Mode S, which triggers the critical second-mode or Mack-mode. Although generally associated with an acoustic instability, no clear definition exists that identifies the FT character of the Mack-mode. We utilize linear stability theory augmented with the above energy-based analysis to quantify the variations in vortical, acoustic and entropic components in Mode eigenfunctions, as this wave experiences Mack mode amplification. In spite of Mode S being dominated by the vortical content, the crucial growth of Mack mode is accompanied by a local amplification of the acoustic eigenfunction and sensitization of thermal source mechanisms. Complementary direct simulations extract a rich set of dynamics within the amplified Mack mode, with its vortical content exhibiting counter-rotating recirculation cells, and the acoustic content remaining trapped between the wall and the critical layer. Current simulations explore 3D-breakdown scenarios and effects of highly cooled walls from the perspective of these instabilities.
About the Speaker: Unnikrishnan is a post-doctoral researcher in the Department of Mechanical and Aerospace engineering at The Ohio State University. He obtained his Ph. D. in aerospace engineering from OSU in 2016, with a thesis on developing techniques to identify noise source mechanisms in supersonic jets. Ongoing development of this work has resulted in computationally inexpensive far-field noise prediction tools and acoustic models for subsonic and supersonic jets. As a post-doctoral researcher, he has been working on an energy-based approach that identifies Kovasznay-type modes in hypersonic transitional boundary layers, to yield physical insights into the behavior of first and second modes. High-fidelity simulations are an integral part of these studies, and are often complemented by state-of-the-art analysis techniques, involving modal and statistical tools.
All are welcome.
Date(s) - 07/06/2019
11:00 am - 12:15 pm