An acoustic localization method is applied in a reverberant environment to locate the sources of discrete sounds having unknown timing and waveform. In particular, the localization method is applied to study low event rate cavitation in a vortical flow in a water-tunnel test-section with characteristic cross section dimension of 0.3 m. The primary frequency and bandwidth of the acoustic pulses from the small isolated cavitation bubbles are 10 kHz and 200 kHz respectively, and the measured pulse duration is ∼15–20 micro-seconds. The localization method involves using an array of receiving hydrophones to record the cavitation sound pulses. These hydrophone recordings, which include direct-path signal, reflected path signal, and noise, are time windowed and cross-correlated to obtain direct-path arrival-time differences. These arrival time differences are used in conjunction with a simple ray-based acoustic model to estimate the source location in three dimensions via a robust Monte-Carlo routine. The ratio of the primary-frequency wavelength to the water-tunnel cross-section dimension is ∼1/2. Consequently the time-windowing is tight; only 1 to 1.5 center-frequency cycles at the beginning of a signal pulse are readily useful for localization purposes. The remainder of the signal is contaminated by reflections and is not used in the present effort. To check and validate the results of the acoustic method, two-camera high-speed video data was taken synchronously with the acoustic data for 53 cavitation events. The acoustic localization scheme provided an unambiguous location estimate for all 53 cavitation bubbles. The average distance between the optical and acoustic measurement of the bubble location was 18.4 mm, or ∼1/8 of the wavelength of the primary signal frequency.

This paper presents the preliminary results from a friction drag reduction experiment conducted at high Reynolds numbers. The experiments were conducted in two phases at the U. S. Navy’s William B. Morgan Large Cavitation Channel (LCC) in Memphis, TN on a flat, hydraulically smooth plate (12.9 meters long by 3.0 meters wide by 0.18 meters thick) at flow speeds ranging from 0.5 to 20 m/s. The employed drag reduction technique involved the injection of microbubbles into the boundary from a line source over a range of injection flow rates from 100 to 800 scfm. These results include mean velocity data and spatially averaged shear stress data acquired in the single-phase flow, as well as spatially averaged shear stress data, bubble images, and void fraction measurements made in the bubble-injected flow.