Multipaths in acoustic tomography of aero-propulsion flows

Acoustic tomography is of great interest for aero-propulsion applications due to its potential for low-cost, robust, non-intrusive measurements of fluid velocities and temperatures. Recent work has shown the promise of this technique even in high subsonic flows. However, this prior work does not consider the possibility of multiple ray paths existing between the same acoustic source–receiver pair. By measuring the flow properties of a TFE731 turbofan engine with a Pitot rake probe, then using the measured properties to numerically calculate ray paths, we show that acoustic multipaths can occur in realistic environments. Using a pneumatic horn and several microphones located near the exhaust nozzle of this engine, we detect these multipaths, validating the predictions from ray theory. Furthermore, when these multipaths are unaccounted for, a fundamental ambiguity results in the tomographic reconstruction—multiple different flow profiles can produce identical first-arrival travel times. The later-arriving multipaths must be accounted for in order to produce a unique reconstruction. As a first step toward multipath-capable acoustic tomography of aero-propulsion flows, we adapt the Herglotz–Wiechert inverse (a technique from seismology) to account for high-speed fluid flows. This generalized technique can potentially be used to accurately reconstruct these flow profiles in the presence of multipaths.