| The present research examines the relationship between the large-scale structure dynamics of a controlled jet and the far-field sound. This was achieved by exploring the flowfield and the far field of an axisymmetric Mach 0.9 jet with a Reynolds number based on jet diameter of approximately 7.6 x 105. The jet is controlled by eight localized arc filament plasma actuators (LAFPA), which operate over a frequency range that spans the jet column instability, the initial shear layer instability, and higher. Varying the phase between the eight actuators allows excitation of azimuthal modes (m) 0, 1, 2, and 3.; The far field of the jet is probed with a microphone array positioned at 30° with respect to the downstream jet axis. The array is used to estimate the origin of high amplitude sound waves in space and time and also to provide metrics including sound pressure level (SPL) and overall sound pressure level (OASPL). The lower forcing Strouhal numbers (StDF's) increase the OASPL and move noise source distributions upstream while higher StDF 's slightly decrease the OASPL and have noise source distributions similar to the baseline jet. The cross-stream distribution of noise events changes from being localized along the jet centerline when the OASPL is amplified to being rather diffused when the OASPL is attenuated.; The flowfield is investigated using flow visualization and particle image velocimetry (PIV). A Reynolds decomposition of the PIV results showed the importance of the streamwise velocity fluctuations for the symmetric azimuthal modes (m = 0 and 2), and the cross-stream velocity fluctuations for the asymmetric azimuthal modes (m = 1 and 3). A proper orthogonal decomposition (POD) of the PIV data was performed to extract information about how forcing affects the large-scale flow features. The first POD mode based on v' shows the existence has large-scale flow features from forcing over a large range of StDF 's (0.09 to 1.08), while the first POD mode based on u' only has large-scale flow features from forcing around the preferred St DF of 0.36.; The first two POD modes were used to conditionally average the PIV data. When forcing at StD's other than the preferred mode, the conditional-averaged images show large-scale flow features from forcing that grow, saturate, and decay closer to the nozzle exit. When forcing near the jet preferred St DF, the large-scale flow features are very robust in the region of noise generation. The growth, saturation, and decay of the largescale flow features from forcing correlate to the far-field sound and noise source distribution. When exciting a symmetric azimuthal mode, m = 0, near the preferred St DF, the streamwise conditional-averaged velocity grows quickly and saturates over a relatively long spatial range. When exciting an asymmetric azimuthal mode, m = 1, near the preferred StDF, the cross-stream conditional-averaged velocity grows slowly, saturates, and then decays relatively quickly. The noise source distribution occurred in the decay region for both m = 0 and m = 1, and the distribution changed in accordance with changes in the decay rate. The quicker decay of m = 1 was accompanied by a narrower noise source distribution, while m = 0 decayed slower and the noise source distribution was spread over a longer streamwise span. A model was used to relate the centerline growth, saturation, and decay of the large-scale flow features to the far-field sound. The model was slightly changed based on the azimuthal mode, and the changes lead to differences in the far-field radiation. |
