Frequency Locking and Vortex Dynamics of an Acoustically Excited Bluff Body Stabilized Flame

Abstract

This paper presents measurements of the forced response of a bluff body stabilized flame. The nonreacting, unforced flow exhibits intrinsic oscillations associated with an unstable, global wake mode. This same global mode persists in the reacting flow at low density ratios, but disappears at high flame density ratios where the flow is dominated by the convectively unstable shear layers. The flow responds quite differently to forcing in these two situations, exhibiting a roughly linear input-output character in the convectively unstable regime, but exhibiting nonlinear behavior, such as frequency locking, during global mode oscillations. In this work, a reacting bluff body wake is subjected to harmonic, longitudinal, acoustic forcing, providing a symmetric disturbance. This experiment is conducted at several density ratios as well as different spacing between the forcing frequency and global mode frequency. As the spacing between these frequencies is narrowed, the wake response is drawn away from the global mode frequency and approaches (and eventually locks-into) the forcing frequency. This observation seems to be linked to the spatial distribution of vorticity fluctuations, as well as the symmetry of the vortex shedding. For example, for large spacing between the forcing and global mode frequencies, there is significant response at the forcing frequency in the shear layers, but little response is observed along the flow centerline (which itself exhibits strong oscillations at the global mode frequency). This seems to be due to the symmetry of vortical structures that are shed at the forcing frequency. For small spacing between these frequencies, however, the vortices shed symmetrically at the forcing frequency, but quickly stagger into an asymmetric configuration as they convect downstream. For such cases, we observe a strong response of the wake at the forcing frequency. The axial distance required for such staggering to occur is a function of the spacing between the forcing frequency and natural frequency, the flame density ratio, and the intensity of the forcing.