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The current work investigates the transition of a swirling jet between two flow states, namely, columnar-jet (CJ) and wall-jet (WJ) states, under the influence of external transverse acoustic forcing. Here, we have performed simultaneous time-resolved stereoscopic particle image velocimetry and dynamic pressure measurements to understand the structure and dynamics of these swirling jets under external forcing. It is observed that at low Reynolds number (Re), the swirl flow transitions from CJ to WJ state due to transverse forcing. And the flow returns to the CJ state when the acoustic is turned OFF. However, above a critical flow Re, the swirl flow does transition from CJ to WJ state when subjected to transverse forcing, but upon turning OFF the acoustics, the flow stays in the WJ state. Thus, the swirling flow demonstrates bistability in the flow states only above a critical flow Re. It is observed that upon forcing, there is an increase in the streamwise velocity fluctuations (u ′ x ) near the centerline and in the radial velocity fluctuations (u ′ r ) near the injector lip. This finding is also confirmed through spectral proper orthogonal decomposition analysis. In addition, it is observed that as the flow transitions from CJ to WJ state, the relative contribution of the convective term U r ¯ ∂ U r ¯ ∂ r toward the radial pressure gradient ( ∂ P ¯ ∂ r ) increases in comparison to the centrifugal force term ( U ¯ θ 2 r ). The study highlights the effect of acoustic-induced velocity fluctuations on the bistability of swirl flows over a range of flow Re. [ABSTRACT FROM AUTHOR]

We investigate the effect of transverse acoustic excitation on nonreacting swirling jets. The work is motivated by the azimuthal instabilities in annular gas turbine combustor which are one of the major challenges in aero-engines. We have designed and fabricated a multinozzle linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features a dual corotating radial swirler configuration. Two compression drivers placed on either side of the combustion chamber are used to generate acoustic fields in the direction transverse to the flow. Simultaneous two-dimensional (2D) particle image velocimetry (PIV) and high-frequency pressure measurements are conducted to measure the time-averaged velocity field and the chamber acoustics, respectively. It is observed that once the swirling jet is excited with a transverse acoustic forcing, it instantaneously transitions to a wall-jet state. In wall-jet state, the flow moves radially outwards and remains attached to the walls on either side of the nozzle, and is characterized by a recirculation zone with a strong negative axial velocity. In our experiments, we demonstrate that transverse acoustic excitation can lead to a bistable state in swirling flows. We investigated the acoustic response to low-amplitude forcing on the combustion chamber by performing a forced acoustic response analysis using comsol. It is observed that acoustic forcing leads to a peak response of the radial and azimuthal velocities at the flare of the swirler, which could induce an axisymmetric deflection of the jet shear layer pushing it into a wall-jet state. [ABSTRACT FROM AUTHOR]