Effect of multifrequency forcing on the near-field development of a jet

Abstract

Flow visualization and hot-wire measurements are used to study the effect of acoustically forcing a jet with low-level disturbances containing more than one frequency component. Specifically for this study, forcings with combined frequencies at a ratio of 1 to 2 (1-2) and a ratio of 1 to 3 (1-3) are examined. For the 1-2 forcing, if the initial growth of the longer wave is sufficiently strong compared with that of the shorter wave, three manners of vortex merging can be observed: 1) pairing between two vortices of similar size, 2) pairing between two vortices of different size, and 3) vortex tearing. Both the initial phase angle and the Strouhal number play a key role in determining the manner of vortex merging. When forced at the condition where the first mode is close to the initially most amplified frequency, no vortex tearing can be observed, and vortex pairing occurs mostly between vortices of similar size. The phase angle affects mainly the pairing location under this situation. For the 1-3 forcing, three merging modes are identified. The initial phase angle has a major effect in determining the merging mode. Regardless of the mode of forcing, the specific total energy tends toward the same asymptotic value.