Berkeley Fluids Seminar

University of California, Berkeley

Bring your lunch and enjoy learning about fluids!

Monday, December 4, 2017

12:00-13:00, 3110 Etcheverry Hall

Dr. Karthik Kashinath (Lawrence Berkeley National Laboratory)

Open-loop control of periodic and aperiodic thermoacoustic oscillations:
numerical simulations, experiments and low-order modelling

Abstract: Synchronization is a universal concept in nonlinear science but has received little attention in thermoacoustics. We investigate the influence of harmonic acoustic forcing on three different types of self-excited thermoacoustic oscillations: periodic, quasiperiodic and chaotic. We show that open-loop application of harmonic acoustic forcing is an effective strategy for controlling large-amplitude thermoacoustic oscillations. We find that (i) above a critical forcing amplitude, the system locks into the acoustic forcing by oscillating only at the forcing frequency ff, leaving no sign of the natural self-excited mode; (ii) the critical forcing amplitude required for lock-in decreases as ff approaches the natural frequencies, giving rise to characteristic V-shaped lock-in boundaries around the natural modes; (iii) for a wide range of forcing frequencies, the system’s oscillation amplitude can be reduced to less than 50% of that of the unforced system, and in some cases even up to 90%; (iv) amplitude reduction can be weakened as forcing amplitude is increased above lock-in boundary. Findings from numerical simulations are corroborated by experimental measurements. We then model the system as coupled van der Pol oscillators. We find that the response amplitude of the flame and lock-in boundary around the natural modes are qualitatively similar. These findings show that (i) the application of harmonic acoustic forcing can be an effective strategy for controlling periodic and aperiodic thermoacoustic oscillations; and (ii) this complex forced self-excited system can be modelled reasonably well as a simple forced selfexcited oscillator, which could be helpful to delve deeper into the dynamics of such systems and to develop new control strategies.

Bio: Karthik Kashinath received his Bachelors from the Indian Institute of Technology, Madras in 2007, Masters from Stanford University in 2009 and PhD from the University of Cambridge, U. K. in 2013. His background is in mechanical and aerospace engineering and applied physics. He has worked on various projects spanning a wide range of disciplines from supersonic air-breathing engines to battery technologies to acoustics to complex chaotic systems and turbulence. Since 2013 he has been a part of Lawrence Berkeley National Laboratory working as a climate and atmospheric scientist. His current research interests lie in extreme weather and climate events and novel big data analytics and pattern discovery methods for large complex systems such as Earth’s climate. He was the winner of the Leonardo da Vinci prize in 2013 for the best PhD thesis in Europe in flow, turbulence and combustion, and winner of the Osborne Reynolds prize in 2012 in the U.K. for outstanding research in fluid dynamics. He has also won best paper awards from the Combustion Institute of the U.K. and the American Society of Mechanical Engineers. When he is not in front of the computer or in the lab, he runs up mountains, swims in lakes and cooks exotic global cuisines.

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