David R. Dowling

Phase conjugation in underwater acoustics

Phase‐conjugate mirrors are used in optics to compensate for aberrations caused by inhomogeneities in the propagation medium and by imperfections in optical components. In acoustics, analogous behavior can be achieved by a time‐reversed retransmission of signals received by an array. Compensation for multipath propagation and array imperfections is automatic and does not require knowledge of the detailed properties of either the medium or the array. The behavior of acoustic phase‐conjugate arrays is illustrated in several examples, some highly idealized and some more realistic. The effects of aperture size and inhomogeneities in the propagation medium are treated for both the near‐field and far‐field regions. It is concluded that phase‐conjugate arrays offer an attractive approach to some long‐standing problems in underwater acoustics.

Underwater acoustic communication by passive-phase conjugation: theory and experimental results

A new method for coherent underwater acoustic communication called passive phase conjugation is evaluated. The method is so named because of conceptual similarities to active phase conjugation methods that have been demonstrated in the ocean. In contrast to active techniques, however, the array in passive phase conjugation needs only receive. The procedure begins with a source transmitting a single probe pulse. After waiting for the multipathed arrivals to clear, the source then transmits the data stream. At each element in the distant receiving array, the received probe is cross-correlated with the received data stream. This cross-correlation is done in parallel at each array element and the results are summed across the array to achieve the final communication signal suitable for demodulation. As the ocean changes, it becomes necessary to break up the data stream and insert new probe pulses. Results from an experiment conducted in Puget Sound near Seattle are reported. Measurements were made at multiple ranges and water depths in range-dependent environments.

Similarity of the concentration field of gas-phase turbulent jets

This work is an experimental investigation of the turbulent concentration field formed when the nozzle gas from a round, momentum-driven, free turbulent jet mixes with gas entrained from a quiescent reservoir. The measurements, which were made with a non-intrusive laser-Rayleigh scattering diagnostic at Reynolds numbers of 5000, 16000, and 40000, cover the axial range from 20 to 90 jet exit diameters and resolve the full range of temporal and spatial concentration scales. Reynolds-number-independent and Reynolds-number-dependent similarities are investigated. The mean and r.m.s. values of the concentration are found to be consistent with jet similarity laws. Concentration fluctuation power spectra are found to be self-similar along rays emanating from the virtual origin of the jet. The probability density function for the concentration is also found to be self-similar along rays. Near the centreline of the jet, the scaled probability density function of jet fluid concentration is found to be nearly independent of the Reynolds number.

A Summary of the Physical Properties of Cirrus Clouds

Abstract A review of existing literature is made to determine typical values for the physical properties cirrus clouds. The properties examined (with typical values and measured ranges) are cloud-center altitude (9 km, 4 to 20 km), cloud thickness (1.5 km, 0.1 to 8 km), crystal number density (30 L−1, 10−4 to 10−4 L−1), condensed water content (0.025 g m −3, 10−4 to 1.2 g m −3), and crystal size (250 μm, 1 to 8000 μm). A typical crystal size distribution is also reported.

Bubble friction drag reduction in a high-Reynolds-number flat-plate turbulent boundary layer

Turbulent boundary layer skin friction in liquid flows may be reduced when bubbles are present near the surface on which the boundary layer forms. Prior experimental studies of this phenomenon reached downstream-distance-based Reynolds numbers (

Acoustic pulse compression using passive phase‐conjugate processing

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Narrow‐band performance of phase‐conjugate arrays in dynamic random media

) of several million, but potential applications may occur at

Bubble-induced skin-friction drag reduction and the abrupt transition to air-layer drag reduction

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Using cross correlations of turbulent flow-induced ambient vibrations to estimate the structural impulse response. Application to structural health monitoring

orders of magnitude higher. This paper presents results for

Phase-conjugate array focusing in a moving medium

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