Yuta Okuno

Characterization of degeneration process in combustion instability based on dynamical systems theory.

We present a detailed study on the characterization of the degeneration process in combustion instability based on dynamical systems theory. We deal with combustion instability in a lean premixed-type gas-turbine model combustor, one of the fundamentally and practically important combustion systems. The dynamic behavior of combustion instability in close proximity to lean blowout is dominated by a stochastic process and transits to periodic oscillations created by thermoacoustic combustion oscillations via chaos with increasing equivalence ratio [Chaos 21, 013124 (2011); Chaos 22, 043128 (2012)]. Thermoacoustic combustion oscillations degenerate with a further increase in the equivalence ratio, and the dynamic behavior leads to chaotic fluctuations via quasiperiodic oscillations. The concept of dynamical systems theory presented here allows us to clarify the nonlinear characteristics hidden in complex combustion dynamics.

Dynamics of self-excited thermoacoustic instability in a combustion system: Pseudo-periodic and high-dimensional nature

We have examined the dynamics of self-excited thermoacoustic instability in a fundamentally and practically important gas-turbine model combustion system on the basis of complex network approaches. We have incorporated sophisticated complex networks consisting of cycle networks and phase space networks, neither of which has been considered in the areas of combustion physics and science. Pseudo-periodicity and high-dimensionality exist in the dynamics of thermoacoustic instability, including the possible presence of a clear power-law distribution and small-world-like nature.

Low-dimensional dynamical system for Rayleigh-Bénard convection subjected to magnetic field

We have numerically investigated the dynamical behavior of Rayleigh-Benard (RB) convection in an incompressible conducting fluid subjected to a magnetic field by solving a low-dimensional dynamical system. Its dynamical properties are quantified by nonlinear time series analysis based on chaos theory. The stretching and folding in the phase space for the chaos region (normalized Rayleigh number r = 28) and the intermittent chaos region (r = 166.1) of RB convection at a high magnetic Prandtl number of Pm = 10 become complex with increasing applied magnetic field, and the degeneration of chaos is induced by the limit of the strong magnetic field owing to the overwhelming Lorentz force compared with the buoyancy. The results obtained in this study show the importance of the magnetic Prandtl number to the dynamical behavior of RB convection subjected to a magnetic field.