Draft:R. I. Sujith

Sujith Raman Pillai Indusekharan Nair (popularly known as R. I. Sujith) is an Institute Professor and the D. Srinivasan Chair Professor at the Department of Aerospace Engineering at the Indian Institute of Technology Madras. In 2023, he was inducted as International Member of the United States National Academy of Engineering. He was selected as a Fellow of the Combustion Institute in 2022, and was awarded the Distinguished Fellowship of the International Institute of Acoustics and Vibration in 2021. He received the J. C. Bose Fellowship in 2019 and the Swarnajayanti Fellowship in 2005 from the Department of Science and Technology of the Government of India. He is a Fellow of the Indian Academy of Sciences, an Associate Fellow of the American Institute of Aeronautics and Astronautics , a Fellow of the Indian National Academy of Engineering , and an Honorary Fellow of the Indian Society of Systems for Science and Engineering (ISSE). He has been awarded Hans Fischer Senior Fellowship from the Institute for Advanced Study, Technical University of Munich, Germany in 2010. He was awarded Alexander von Humboldt Fellowship in 2001.

Early Life
Sujith was born on May 11th, 1967, in Thiruvananthapuram, Kerala, India. He completed his B. Tech. in Aerospace Engineering from the Indian Institute of Technology Madras, where he received the HAL prize for securing First Rank in his Department. He earned his Ph.D. in Aerospace Engineering on "Behavior of Droplets in Axial Acoustic Fields" working with the legendary Ben T. Zinn at Georgia Institute of Technology, from 1990-1994 and graduating as the “Top Graduate Student from the College of Engineering”. Subsequently, he worked as a Postdoctoral Fellow in the same department from 1994-1995, where he worked in the area of combustion in microgravity and Laser Doppler Velocimetry. He then joined his alma mater, the Indian Institute of Technology Madras, as a Lecturer in the Department of Aerospace Engineering. He has held visiting positions at the Georgia Institute of Technology, DLR Institute für Stromungsforschung Göttingen, Technical University of Munich, and Potsdam Institute of Climate Impact Research.

Current research
Sujith’s work is characterized by beautiful experiments, deep theoretical insight, and a constant connection to industrial applications. This rare combination has led to unexpected findings that transformed the subject of thermoacoustic instability. Some of his key discoveries are described below.

Intermittency route to oscillatory instability
Traditionally thermoacoustic instability is modelled as a transition from a fixed point to a limit cycle. Instead, Sujith suggested that the low-amplitude aperiodic pressure fluctuations during the stable operation of the combustor (known as combustion noise in the community) are deterministic and have chaotic behaviour. The transition from combustion noise to periodic oscillations happens via intermittency - a state composed of bursts of large-amplitude periodic oscillations, amidst regions of low-amplitude aperiodic fluctuations. Sujith established that this intermittency route is a common feature of oscillatory instabilities in turbulent flow.

Oscillatory instability as loss of multifractality
Sujith showed that combustion noise displays scale invariance and has a multifractal signature that disappears at the onset of thermoacoustic instability. Multifractality of combustion noise reflects the complex nature of the dynamics involved in combustion systems that arises due to the nonlinear interaction between combustion, turbulent flow and duct acoustics. Further, Sujith provided a multifractal description for lean blowout in combustors with turbulent flow, thus introducing a unified framework within which thermoacoustic instability and blowout can be described. He identified universal scaling laws that characterize the transitions to oscillatory instabilities in widely different systems that are controlled by aeroacoustic, aeroelastic and turbulent flame processes. He showed that the onset of oscillatory instabilities in turbulent systems is analogous to Bose–Einstein condensation transition observed for bosons, by defining phase space cycles as particles and the periodic orbits as energy levels.

Complex Networks
He introduced complex networks in the study of thermoacoustics. He showed that the networks obtained from the time series of acoustic pressure fluctuations during the occurrence of combustion noise have a scale-free structure. This scale-free nature disappears, and order emerges in the network topology during the transition from combustion noise to combustion instability. Sujith used complex networks to formalize the identification of the pattern during the transition from combustion noise to thermoacoustic instability, as a structural change in the topology of the network. He was also the first to identify the presence of hubs in complex reacting combustor flow fields that offer optimal locations for implementing control strategies using single layer and multilayer networks.

These paradigm shifts that Sujith introduced enabled him to obtain precursor measures for detecting transitions, such as oscillatory instabilities or blowouts in turbulent systems. Sujith’s precursor technology has been demonstrated successfully in the GE Power & Water engine tests, and liquid rocket engines of ISRO. He has also worked with Siemens and Rolls Royce. Sujith’s approaches are applicable not just in thermoacoustics, but in any system where periodic oscillations emerge out of turbulent flow, such as aeroelastic and aeroacoustic systems.

Chimera
He demonstrated that the stable operation of the combustor is characterized by desynchronized aperiodic oscillations of the acoustic field and the heat release rate, which, with an increase in the control parameter, transitions to synchronized periodic oscillations. In between these states exist intermittent phase synchronized oscillations, wherein the two oscillators are synchronized during the periodic epochs and desynchronized during the aperiodic epochs of their oscillations. Sujith’s spatiotemporal analysis showed that regions of synchronous and asynchronous motion co-exist during the state of intermittency in the reaction zone. Such a state is known as chimera state. He is now intensifying the research on spatiotemporal analysis to understand the processes that occur as we approach impending instability. Sujith is translating this understanding of spatiotemporal dynamics to technologies for smart control strategies.

R-tipping and Amplitude Death
Sujith demonstrated R-tipping in thermoacoustic systems, deciphered its mechanism, and constructed a model. His group is the first to show such a phenomenon experimentally in physics. Thus, in recent years, Sujith’s group has engineered a number of paradigm shifts that have transformed the subject of thermoacoustics. He established the perspective of dynamical systems theory and complex systems theory to thermoacoustics. He provided the first experimental evidence of the coexistence of amplitude death and phase flip bifurcation in physical systems using experiments involving candle oscillators and Rijke tubes, studied the synchronization route to weak chimera via quenching, clustering, and chimaera states in a system coupled candle-flame oscillators and investigated the susceptibility of minimal networks to a change in topology and number of oscillators.

Interdisciplinarity
Sujith's research is based on in-depth transdisciplinary fundamental research that transcends the boundaries between engineering, mathematics and physics and has successfully been translated to technology development, providing practical solutions to difficult engineering problems. Transcending the traditional boundaries between engineering, physics and mathematics, he reformulated thermoacoustic instabilities as “order emerging from chaos through self-organization”, and has advocated this perspective through a monograph and review papers; this reformulation has attracted a large number of physicists, complex system theorists, and fluid mechanics researchers to this problem.

Sujith’s review artciles in Annual Review of Fluid Mechanics, The European Physical Journal Special Topics, Physics of Fluids, and Chaos summarize his rapid strides in recent years. He was the editor-in-chief of the International Journal of Spray and Combustion Dynamics from 2009 to 2016. He is currently a member of the editorial advisory board member of Chaos: An Interdisciplinary Journal of Nonlinear Science.

Students

 * 1)  Ramanarayanan Balachandran 
 * 2)  Koushik Balasubramanian 
 * 3)  Sathesh Mariappan 
 * 4)  Priya Subramanian 
 * 5)  Gireeshkumaran Thampi 
 * 6)  Vishnu R. Unni 
 * 7)  Vineeth Nair 
 * 8)  Lipika Kabiraj 
 * 9)  Aditya Saurabh 
 * 10)  Samadhan A. Pawar 
 * 11)  Amitesh Roy 
 * 12)  Induja Pavithran 
 * 13)  Praveen Kasthuri 

Honors and awards

 * 1) 2023 - Fellow of the United States National Academy of Engineering
 * 2) 2022 - Fellow of the Combustion Institute
 * 3) 2021 - Distinguished Fellowship of the International Institute of Acoustics and Vibration
 * 4) 2019 - J. C. Bose Fellowship from the Department of Science & Technology, Government of India
 * 5) 2018 - Institute Chair Professor at IIT Madras. (Chair named as D. Srinivasan Chair in 2020)
 * 6) 2017 - August Wilhelm Scheer Visiting Professorship at TU Munich
 * 7) 2017 - Fellow of the Indian Academy of Sciences
 * 8) 2010 - Hans Fischer Senior Fellowship from Institute for Advanced Study, Technical University of Munich, Germany
 * 9) 2010 - Associate Fellow of the American Institute of Aeronautics and Astronautics
 * 10) 2008 - Fellow of the Indian National Academy of Engineering
 * 11) 2005 - Swarnajayanti Fellowship from the Department of Science & Technology, Government of India
 * 12) 2002 - Young Engineer Award, Indian National Academy of Engineering
 * 13) 2000 - Alexander von Humboldt Fellowship

Publications

 * Tipping in complex systems under fast variations of parameters (2023)
 * Rijke tube: A nonlinear oscillator (2022)
 * Dynamical systems and complex systems theory to study unsteady combustion (2021)
 * Thermoacoustic Instability: A Complex Systems Perspective (2021)
 * Complex system approach to investigate and mitigate thermoacoustic instability in turbulent combustors (2020)
 * Experimental Evidence of Amplitude Death and Phase-Flip Bifurcation between In-Phase and Anti-Phase Synchronization (2018)
 * Sensitivity and Nonlinearity of Thermoacoustic Oscillations (2018)
 * Non-normality and nonlinearity in thermoacoustic instabilities (2016)
 * Combustion noise is scale-free: transition from scale-free to order at the onset of thermoacoustic instability (2015)
 * Intermittency route to thermoacoustic instability in turbulent combustors (2014)
 * Non-normality and nonlinearity in combustion–acoustic interaction in diffusion flames (2008)