Advances in Decomposition Techniques for Turbulent Jet Analysis
Date & Time: Thursday, 23.05.2024, at 14:00.
Speaker: Dr. Akhil Nekkanti
Topic: “Advances in Decomposition Techniques for Turbulent Jet Analysis”
Abstract: Turbulent flows are high-dimensional systems characterized by instabilities, nonlinearity, and randomness. Within the chaotic motions, these flows exhibit structures that are coherent in space and time. Further insights into coherent structures can aid our understanding of turbulence and help develop reduced-order models for flow control. My work involves developing data-driven techniques for identifying the dynamically relevant structures and using these techniques to investigate the flow physics of turbulent jets. The first part of my talk will encompass our contributions to spectral modal decomposition techniques. Spectral proper orthogonal decomposition (SPOD) is the frequencydomain variant of principal component analysis, and it computes modes that are coherent in space and time. In recent years, SPOD has emerged as a major tool for analyzing turbulent flows. We extend SPOD for low-rank reconstruction and frequencytime analysis. A convolution-based strategy is proposed for frequency-time analysis that characterizes the intermittency of spatially coherent flow structures. Next, bispectral mode decomposition (BMD), a technique that extracts flow structures associated with nonlinear triadic interactions by optimizing the third-order statistics, is presented. These techniques are applied to high-fidelity numerical simulations in the second part of the talk. Large-eddy simulations of two unforced and four forced jets at a Reynolds number of 50,000 were performed and validated against companion experiments. In a transitional jet, the emphasis of our analysis is on characterizing vortex pairing and the associated energy transfer. The SPOD-based spectral energy budget reveals that the energy flows from the fundamental to its subharmonic, resulting in the growth of the subharmonic frequency. Finally, forced turbulent jets are investigated using BMD to analyze the triads created due to the sum and difference interactions between the fundamental forcing and its harmonics.
About the speaker: Dr. Akhil Nekkanti is a postdoctoral scholar in the Department of Mechanical and Civil Engineering at California Institute of Technology, USA. He received his Ph.D. in Department of Mechanical and Aerospace Engineering at University of California San Diego. His research interests are on turbulent flows, modal decomposition techniques, and computational fluid dynamics. His work involves performing high-fidelity numerical simulations of turbulent and cavitating flows and developing reduced-order models to investigate the underlying physics of turbulent flows, primarily in jets.