Influence of transverse acoustic modal structure on the forced response of a swirling nozzle flow

This study describes continued investigations of the response of a swirling flow to transverse acoustic excitation. This work is motivated by transverse combustion instabilities in annular gas turbine engine architectures. This instability provides a spatially varying acoustic velocity disturbance field around the annulus, so that different nozzles encounter different acoustic disturbance fields. In this study, we simulate this effect by looking at a standing wave acoustic field where the nozzle is located at either a velocity anti-node, referred to as out-of-phase forcing, and a velocity node, referred to as in-phase forcing. The out-of-phase forcing condition provides an asymmetric forcing field about the center plane of the flow and excites an asymmetric response in the flow field; the in-phase forcing provides a symmetric forcing condition and results in symmetric flow response near the nozzzle. The symmetric versus asymmetric flow response was measured in two ways. First, in the r-x plane where axial and radial components of velocity are measured using high-speed particle image velocimetry (PIV), a helical and ring vortex rollup of the shear layers is evident in the asymmetric and symmetric forcing condition, respectively. Additionally, the swirling motion of the jet is measured in the r-θ plane at two downstream locations and a spatial decomposition is used to calculate the strength of azimuthal modes in radial velocity fluctuations. At the forcing frequency, the m=0 mode is strongly excited at the nozzle exit with symmetric forcing, while asymmetric forcing results in a strong peak in the m=1 mode, or the first helical mode. The results in this plane of view are congruent with those in the r-x plane. Further downstream, however, the mode strengths change as the vorticity is religned and natural asymmetries of the swirling jet set in. Finally, the low frequency self-excited motion of the vortex core was measured and characterized in the unforced flow. It is composed of an m=-1 and m=-2 mode, and the physical interpretation of these mode numbers is highlighted. High amplitude acoustic forcing decreases the amplitude of oscillation of this structure in both the in-phase and out-ofphase forcing, but to varying degrees.