Acoustic Phased Array Imaging of Turbulence
Robert P. Dougherty
President, OptiNav, Inc.
Aeroacoustic source location is important in commercial aeronautics for aircraft noise reduction and also for locating and tracking aircraft wake vortices. In both cases, a phased array of acoustic sensors can be used as an imager to visualize the acoustic source distribution. This process can be viewed as an inverse propagation technique, but beamforming algorithms that are effective in practice are both more powerful and more limited than mathematical descriptions of inverse problems. At one level, standard delay and sum beamforming, used with an appropriate degree of art in array design, “diagonal deletion” and deconvolution can reveal the apparent source locations and strengths in a wide range of applications including highly reverberant wind tunnels, static engine tests, and dusty plains near airports. At a deeper level, the details of the source distribution interact with the array data acquisition and processing assumptions in complex ways that challenge aeroacoustic theoreticians and experimentalists to understand what can be learned about turbulent noise sources from the array technique. Examples of acoustic beamforming methods and results in several applications will be given, and the relationship between aeroacoustic theory and beamforming practice will be illustrated.