UC Irvine

The near acoustic field of a subscale ducted fan was experimentally measured for the purposeof understanding the physics of sound generation and refining low-order models for aft-emitted tonal fan noise. To properly resolve the detailed features of the pressure field, measurements were obtained using a phased microphone array comprising of fixed and scanning sensors. The traversing of a sensor through a spatially-varying acoustic field introduces signal non-stationarity which must be addressed by dividing the complete signal into quasistationary blocks and applying a frequency-dependent window during spectral estimation. The harmonic content of the pressure along the scan line was obtained from the signals of the axially scanning sensors using the Vold-Kalman (VK) filter and partial fields decomposition (PFD). Accurate construction of the axial pressure distribution from the microphone measurements required advanced interpolation and filtering techniques based on the the decomposition of the pressure into its unwrapped phase and amplitude. The resulting axial pressure field has sufficient spatial resolution to perform space-time correlations and calculate the convective Mach number distribution. The azimuthal distribution of the tonal pressure field was obtained from the fixed sensors using the VK filter and can be combined with the axial pressure distribution to generate a cylindrical radiation surface. The radiation surface can then be implemented into propagation and scattering tools, such as the boundary element method (BEM), to efficiently predict acoustic emission in isolated and installed configurations. The near-field measurements are compared to the predictions of a model for aft tonal emission where the source features are partially informed by the internal duct acoustics. Reasonable agreement is found in the axial pressure distributions and space-time correlations. The near-field measurements can provide guidance for refinement of the model.