Inferring Propeller Inflow and Radiation from Near-Field Response, Part 2: Empirical Application

Data analysis techniques were previously developed for rotating machinery that predicted the far-field radiation, inferred inflow characteristics, and defined the near-field/far-field acoustic Green's function based on measurements of the pressure near field (Minniti, R. J., Blake, W. K., and Mueller, T. J., Inferring Propeller Inflow and Radiation from Near-Field Response, Part 1: Analytic Development, AIAA Journal, Vol. 39, No. 6, 2000, pp. 1030-1036). The techniques are applied to a free-running propeller in subsonic flow. As a first case, the propeller ingesting large-scale, mean-flow distortions as would be present downstream of stators or inlet guide vanes was considered. This simplified case allowed qualitative analysis in the time domain and complimenting quantitative analysis in the frequency domain. In addition, the case acted as a calibrating configuration to map the frequency response of the individual blades to the incoming flow by varying the number of distortions present and the rotational speed of the propeller. Based on the results of the first case, the analysis was extended to the propeller ingesting grid-generated turbulence. Because of the complex nature of the flow, all analysis was completed in the frequency domain. By the use of the techniques in Inferring Propeller Inflow and Radiation from Near-Field Response, Part 1: Analytic Development, an estimate of the blade summation gain was used to complete the direct solution of the aeroacoustic problem and predict the acoustic far field from a measurement of the ingested flow. Additionally, the inflow character was inferred from the near-field measurements.