Low-dimensional techniques for sound source identification in high speed jets

The most energetic modes of the turbulent mixing layer from an axisymmetric jet at Mach 0.85 are examined using multi-component forms of the joint POD-Fourier decomposition techniques. Measurements of the velocity field (r, theta cross-plane) are performed using a stereo PIV system traversed along the sound-source regions of the, flow ( zD = 3 to 8, Deltaz = 0.25D). The results indicated a dominance in the m = 5 Fourier-azimuthal mode at z = 3D, with a shift to the m = 3 Fourier-azimuthal mode at z = 8D. A grid sensitivity indicates that the inclusion of additional components in the decomposition are not shown to shift the energy amongst modes, but rather change the relative energy in each mode, thus supporting previous investigations by Citriniti & George [97], Jung et al. [103], and Glauser & George [99] who used the scalar decomposition (streamwise component) in the axisymmetric mixing layer (incompressible) and Ukeiley et al . [58] who used a vector form (streamwise and radial component) in the compressible Mach 0.30 & 0.60 axisymmetric jet. By employing a cross-spectral-based modified form of Bonnet et al.'s [54] Complementary Techniques, a dynamical estimate of the sound-source events are realized. This low-dimensional first-order estimate used the pressure signatures from the jet's near-lip region as the unconditional events because of their high bandwidth capabilities, (sampled at 30kHz) in order to overcome low sampling speeds with conventional PIV systems. More importantly, the application of this technique is shown to greatly reduce, if not eliminate, the intrusiveness on the acoustic characteristics of the sound-source events.; The near-field pressure region was investigated using a variety of dynamic pressure transducers situated at several spatial locations outside of the turbulent shear-layer region near the jet's lip. An LDV system was traversed along several positions in the potential core and turbulent regions of the jet's flow and was sampled simultaneously with the pressure transducers. It was discovered that the ability to capture pressure signatures capable of correlating with the jet's velocity field (via LDV) was sensitive to radial, streamwise and azimuthal locations. Peak correlations were found as high as 25% when the LDV's measurement volume was situated at zD ≈ 4. The convective speeds of the large-scale events ranged between 0.75Ucl in the potential core at Mach 0.6 and 0.85, and 0.65Ucl along the center of the mixing layer at Mach 0.60. No acoustic feedback mechanisms were found between the turbulent events and the near field pressure.; Future investigations will use this low-dimensional model to evaluate the modal evolution of the Lighthill source terms in order to estimate the far-field noise, and will be compared to a simultaneous survey (already performed) of the acoustic far-field regions. From this model, we expect to determine the key signature events at the jet lip that are responsible, in an evolutionary sense, for the more energetic sources of noise. Eventual applications of these exciting findings will extract the necessary information for controlling these signature events in order to reduce their radiated noise.