Detailed measurements of wall pressure fluctuations have been made in the intermittent (laminar-turbulent) region of a flat plate boundary layer. Digital sampling and processing techniques were used. The properties of these pressure fluctuations were found to be similar to the previous measurements made in the fully turbulent region. The measurements were repeated with a single two-dimensional surface roughness on the plate. The only changes in the results were a decrease in the transition Reynolds number from 2 × 106 to 1.6 × 106, and an increase in the decay rate of the longitudinal cross-spectral density magnitude by a factor of about 1.5. Emmons’ (1951) analytical model was applied for two cases: (1) a constant source density downstream of transition and, (2) a line source distribution at transition. Both predicted burst rates as functions of intermittency appreciably higher than measured values. Wall pressure spectra scaled on dynamic head showed a strong dependency on intermittency. This dependency was largely resolved, at least for intermittencies greater than 64 percent, by scaling on turbulent mean shear stress at the wall.

Mid-frequency active, surface ship sonar data from a recent shallow-water sea trial have been processed using synthetic aperture sonar (SAS) algorithms. The target of opportunity closed head-on directly toward the surface ship at a relative speed of 13 knots through an acoustic environment that was characterized by a downward refracting sound speed profile in 100-m-depth water over a sand/mud bottom during calm seas. This situation represents an endfire (90-degree squint angle) scenario for which the resolution of any line array (synthetic or real) is considerably degraded relative to the broadside case. The test was not designed for SAS application so the aperture is significantly undersampled. The processing approach utilizes signal adaptive, ping-to-ping correlation autofocusing to correct for the effects of platform and target motion, and for ocean instabilities. The processing is tuned to the specific environment by setting the cross-correlation window size based on the two-way group arrival structure of the channel. The aliasing that results from the undersampled aperture is reduced to acceptable levels through the appropriate use of cross-range matched field processing. Images formed by processing the data from a forward-look synthetic aperture measuring 2200 wavelengths with a mean target distance of 9.5 km are presented.

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An autonomous surface vehicle known as a wave glider, instrumented with a low-power towed hydrophone array and embedded digital signal processor, is demonstrated as a viable low-noise system for the passive acoustic monitoring of marine mammals. Other key design elements include high spatial resolution beamforming on a 32-channel towed hydrophone array, deep array deployment depth, vertical motion isolation, and bandwidth-efficient real-time acoustic data transmission. Using at-sea data collected during a simultaneous deployment of three wave glider-based acoustic detection systems near Stellwagen Bank National Marine Sanctuary in September 2019, the capability of a low-frequency towed hydrophone array to spatially reject noise and to resolve baleen whale vocalizations from anthropogenic acoustic clutter is demonstrated. In particular, mean measured array gain of 15.3 dB at the aperture design frequency results in a post-beamformer signal-to-noise ratio that significantly exceeds that of a single hydrophone. Further, it is shown that with overlapping detections on multiple collaborating systems, precise localization of vocalizing individuals is achievable at long ranges. Last, model predictions showing a 4× detection range, or 16× area coverage, advantage of a 32-channel towed array over a single hydrophone against the North Atlantic right whale upcall are presented for the continental shelf environment south of Martha's Vineyard.

An experiment is conducted in which plate vibration is used to successfully cancel a sound-excited Tollmien–Schlichting wave. With sound excitation, transition to turbulence in the boundary layer occurs at a streamwise Reynolds number (Rx) of 1.2×106. The addition of plate vibration delays the transition to Rx=1.8×106. In the laminar region of the boundary layer (at Rx=0.97×106) the addition of plate vibration reduces the level of streamwise velocity fluctuations from more than 100 times to only two times the particle velocity of the sound excitation.