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A new method for the calculation of vector acoustic intensity from pressure microphone measurements has been applied to the aeroacoustic source characterization of an unheated, Mach 1.8 laboratory-scale jet. Because of the ability to unwrap the phase of the transfer functions between microphone pairs in the measurement of a radiating, broadband source, physically meaningful near-field intensity vectors are calculated up to the maximum analysis frequency of 32 kHz. The new intensity method is used to obtain a detailed description of the sound energy flow near the jet. The resulting intensity vectors have been used with a raytracing technique to identify the dominant source region over a broad range of frequencies. Additional aeroacoustics analyses provide insight into the frequency-dependent characteristics of jet noise radiation, including the nature of the hydrodynamic field and the transition between the principal lobe and sideline radiation. Nomenclature d = spacing between two microphones = nozzle exit diameter , = transfer function between microphones a and b I = time-averaged vector intensity p = acoustic pressure = acoustic pressure magnitude = acoustic pressure phase , ( ) = quadspectrum between microphones a and b 1 Associate Professor, Dept. of Physics and Astronomy, N283 ESC, AIAA Senior Member.

Compressible flows over cavities with a series value of length-to-depth ratio (L/D) were investigated experimentally, with the objective being to elucidate the mechanism of the transition of types of cavity flows as their L/D increases or decreases. For open-type cavity flows, the freedom of backflow inside the cavity is found to be crucial in smoothing out adverse pressure gradient. The spreading of the shear layer and its gradual approach toward the cavity floor as L/D increases tend to suppress the freedom of backflow, causing the cavity flow to change from the open-type to the transitional-type. For closed-type cavity flows, the finite thickness of the upstream boundary layer leads to the presence of three kinds of characteristic lengths that correspond to the recirculation region, the deflection of the main flow and the recovery or the abrupt rise of the pressure, respectively. The compression fans at the foot of the impingement and the exit shock waves will approach each other well in advance of the possible merge of the vertices of the conventionally defined separation and recompression wakes. As the L/D of a closed cavity decreases, the reattached boundary layer on the mid-portion of the cavity floor will have less developing distance, thus it will be more susceptible to adverse pressure gradient and prone to separation. For the present two-dimensional cavity models, the critical values of L/D were found to be about 10 and 14 for the transition of cavity flows from the open-to the transitional-type and from the transitionalto the closed-type, respectively. The sum of the pressure lengths at the front and rear wakes agrees remarkably well with the second critical L/D.

Acoustic and flow fields of a Mj = 1.8 correctly-expanded jet impinging on a 45 ○ inclined flat plate are investigated experimentally. In the experiments, microphone measurement, Schlieren visualization, and wall static pressure measurement on the inclined plate surface are conducted for different nozzle-plate distances. When the nozzle-plate distance is 5D, two types of acoustic waves, whose characteristics are different in propagation directions, spectra, and source locations, are observed. One propagates in the 30 ○ direction from the inclined plate surface, and the other propagates in the 75 ○ direction. When the nozzleplate distance is varied, the change of the intensities of those two types of acoustic waves is observed, and the correlation between the acoustic waves and the shock structures of the flow field is discussed.

For an adequate understanding of the broadband acoustic phenomena generated by a rocket exhaust jet impinging on a flame deflector, this study experimentally clarifies the factors that cause the difference in the broadband acoustic field of a supersonic ideally expanded jet impinging on an inclined flat plate for various nozzle-plate distances. According to previous studies, there are two possible factors: the Mach waves, which are radiated from the free-jet region and reflected by the plate, and the acoustic waves generated in the impingement region. To distinguish the effects of these factors, this study compares the following three results: the overall sound pressure level distribution, images extracted from the schlieren visualization movies using acoustic-triggered conditional sampling, and tracing lines of the acoustic intensity vectors of the Mach waves. The results reveal that the nozzle-plate distance affects the fraction of the Mach waves that are generated in the free-jet region and reflected by the plate, resulting in a higher overall sound pressure level in the upstream direction for larger nozzle-plate distances. It is concluded that the location of the plate relative to the source region of the Mach waves significantly affects the acoustic phenomena, owing to the variation in the nozzle-plate distances.

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