TY - GEN

T1 - Convective and absolute instabilities in reacting bluff body wakes

AU - Emerson, Benjamin

AU - Lundrigan, Julia

AU - O'Connor, Jacqueline

AU - Noble, David

AU - Lieuwen, Tim

PY - 2011

Y1 - 2011

N2 - This paper describes the variation of bluff body wake structure with flame density ratio. It is known that the bluff body flow structure at "high" and "low" flame density ratios is fundamentally different, being dominated by the convectively unstable shear layers or absolutely unstable Von Karman vortex street, respectively. This paper characterizes the aforementioned transition and shows that the bifurcation in flow behavior does not occur abruptly at some ρu/ρ b value. Rather, there exists a range of transitional density ratios at which the flow exists intermittently in both flow states, abruptly shifting back and forth between the two. The fraction of time that the flow spends in either state is a monotonic function of ρu/ρb. This behavior is to be contrasted with lower Reynolds number, laminar flow problems where the convective/absolute instability transition occurs at a well defined value of bifurcation parameter. With this distinction in mind, however, this paper also shows that local parallel stability analyses developed for laminar base wake flows can capture many of the observed flow dependencies. These results have important implications on the dynamics of high Reynolds number, vitiated flows, where typical parameter values fall into the highly intermittent flow regime characterized in this study. This suggests that such flows exhibit two co-existing dynamical states, intermittently jumping between the two.

AB - This paper describes the variation of bluff body wake structure with flame density ratio. It is known that the bluff body flow structure at "high" and "low" flame density ratios is fundamentally different, being dominated by the convectively unstable shear layers or absolutely unstable Von Karman vortex street, respectively. This paper characterizes the aforementioned transition and shows that the bifurcation in flow behavior does not occur abruptly at some ρu/ρ b value. Rather, there exists a range of transitional density ratios at which the flow exists intermittently in both flow states, abruptly shifting back and forth between the two. The fraction of time that the flow spends in either state is a monotonic function of ρu/ρb. This behavior is to be contrasted with lower Reynolds number, laminar flow problems where the convective/absolute instability transition occurs at a well defined value of bifurcation parameter. With this distinction in mind, however, this paper also shows that local parallel stability analyses developed for laminar base wake flows can capture many of the observed flow dependencies. These results have important implications on the dynamics of high Reynolds number, vitiated flows, where typical parameter values fall into the highly intermittent flow regime characterized in this study. This suggests that such flows exhibit two co-existing dynamical states, intermittently jumping between the two.

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U2 - 10.1115/GT2011-45330

DO - 10.1115/GT2011-45330

M3 - Conference contribution

AN - SCOPUS:84865529670

SN - 9780791854624

T3 - Proceedings of the ASME Turbo Expo

SP - 377

EP - 387

BT - ASME 2011 Turbo Expo

T2 - ASME 2011 Turbo Expo: Turbine Technical Conference and Exposition, GT2011

Y2 - 6 June 2011 through 10 June 2011

ER -