Len Zedel

Performance of a single-beam pulse-to-pulse coherent Doppler profiler

We report on the testing of a single-beam 1.7-MHz coherent Doppler sonar. The system is PC-controlled, using a digital signal processor (DSP) to acquire and extract the velocity and backscatter amplitude data. Results from a series of tow-tank calibration tests demonstrate an accuracy in the order of 5 mm s/sup -1/ for data rates of 10 profiles/second over a 1-2 m range with 1.5-cm range bins. An expression for system accuracy is developed which allows generalization to other pulse-to-pulse coherent Doppler systems. We present data showing the systematic decorrelation of backscatter signals due to particle advection: increased decorrelation is seen in the transducer near-field. Example observations of velocity profiles in laboratory-generated waves are presented.

Organized structures in subsurface bubble clouds: Langmuir circulation in the open ocean

Observations of the movement and distribution of subsurface bubbles reveal the structure of coherent motions near the ocean surface. The measurements were obtained in the open ocean from a freely drifting, self-contained acoustical instrument equipped with side scan sonar, dual-frequency echo sounders, and ambient sound recording systems. The bubble clouds are organized into long, narrow plumes aligned with the wind, consistent with windrows caused by Langmuir circulations. They have widths of about 3 to 5 m and lengths up to 100 m, with multiple scales coexisting. Their mean separation depends upon wind speed: below 5 m s−1, mean spacings were about 5 m, increasing to 10 m when winds exceeded 10 m s−1 The maximum depth to which the plumes were observed to penetrate depended to some extent on wind speed, with the greatest penetrations of 12 m occurring at the highest wind speeds (13 m s−1). However, the time-averaged depth of observed bubble plume penetration bears only a weak dependence on wind speed. Whenever winds exceeded 5 m s−1, a mean penetration depth of 6 m was observed. Associated with these bubble plumes were maximum downward velocities of about 0.06 m s−1. These speeds were detected at 8 m depth in the center of the plumes; magnitudes decreased to zero near the surface and horizontally toward the boundaries of the plumes.

A coherent Doppler profiler for high-resolution particle velocimetry in the ocean: Laboratory measurements of turbulence and particle flux

Abstract A pulse-to-pulse coherent acoustic Doppler profiler has been developed for high-resolution particle velocimetry in the ocean, in particular for remote measurements of suspended sediment flux and turbulence in the nearshore and continental shelf bottom boundary layer. Acoustic backscatter estimates of suspended particle concentration and velocity are determined simultaneously from the phase and amplitude of the backscattered signal over an O(1 m)-long profile with subcentimeter resolution, at an ensemble-averaged rate of O(25 Hz). To characterize the performance of the profiler as a remote turbulent flux sensor, laboratory experiments were carried out in a particle-laden high–Reynolds number round jet. The results include comparisons between the acoustic Doppler and standard laboratory sensors, and to previous experimental and theoretical results for turbulent jets. The observed mean radial particle flux profile is found to be consistent with the computed mean flux profile for a turbulent jet. The...

Resolving Velocity Ambiguity in Multifrequency, Pulse-to-Pulse Coherent Doppler Sonar

Coherent Doppler sonar allows noninvasive velocity measurements and is suitable for both laboratory and field applications. The approach is particularly attractive in those environments where optical techniques are not suitable either because of power requirements or more critically, water turbidity. Notably, the technique has been employed successfully in oceanic and river boundary layer studies. However, the occurrence of range and velocity ambiguities limit the more general application of the technique. This paper introduces a method to overcome speed ambiguities by acquiring acoustic backscatter at two (or more) frequencies simultaneously with a broadband transmit pulse. The different frequencies have distinct velocity ambiguities allowing disambiguation of the velocity measurements. The approach is conceptually similar to the use of multiple transmit pulse rates but has the advantage that the data can be acquired simultaneously and so there is no loss in data rate. In addition, system geometry often restricts the allowed pulse repetition rate so that disambiguation using frequency is more flexible and more generally applicable. Theoretically, the effective ambiguity velocity of a dual-frequency system can be extended arbitrarily but phase noise in a practical system restricts the method to about a fivefold increase in ambiguity velocity.

Modeling Pulse-to-Pulse Coherent Doppler Sonar

Abstract Coherent Doppler sonar provides a powerful tool for probing boundary layer flows under field and laboratory conditions. However, velocity profiling applications of this technique are complicated by characteristic velocity and range ambiguities. Proper implementation for any application requires careful design so that performance can be accurately predicted. In this paper, a computer model capable of simulating coherent Doppler operation is presented. The point scatterer model operates in three dimensions and accommodates bistatic transducer geometries. Excellent agreement is demonstrated between model simulations and laboratory trials. Some simple model applications are presented. The model has been developed for profiling applications but is equally suited to modeling point measurement acoustic Doppler velocimeters.

A three-component bistatic coherent Doppler velocity profiler: error sensitivity and system accuracy

We report on a 1.7-MHz coherent Doppler sonar system designed for near-shore environments that provides three-component velocity profiles over an O(1-m-depth) range. The use of a bistatic geometry allows three independent components of velocity to be measured simultaneously in time and coincident in space. The system was calibrated in a tow-tank facility for horizontal velocities up to 2 m/s. For multiple pulse-pair ensemble-averaged velocity estimates generated at a rate of 30 profiles/second in 0.7 cm depth bins, the standard deviation in measured vertical velocity is about 1% of the total flow speed. For the horizontal components (based on bistatic measurements), the standard deviation of velocity estimates is about 5%. Absolute accuracies are also about 5% of the speed but a portion of this bias is caused by flow disturbance around the instrument package: this disturbance is modeled using potential flow past a cylinder.

Deep Ocean Wave Measurements Using a Vertically Oriented Sonar

Abstract A vertically oriented 200-kHz sonar is used to make estimates of (one-dimensional) surface wave spectra in the deep ocean. Two independent approaches are used to determine wave spectra from this system: a direct measurement of range to the surface and the vertical velocity estimate of the surface. Estimated wave spectra are compared with observations from a Datawell Waverider buoy positioned directly above the acoustic instrument. Comparisons are provided for relatively calm conditions and also during a period of mixed swell and wind waves forced by a 13 m s−1 wind. Wave spectra from the three estimates appear qualitatively similar and agree to within 10% in total wave energy density.

Turbulence measurements in a jet: Comparing the vectrino and vectrinoii

Results are reported from an experiment carried out with the newly developed Nortek VectrinoII and the standard Nortek Vectrino in a turbulent axisymmetric jet at a Reynolds number of 5000. The mechanical and acoustic characteristics of these instruments are identical. However, the electronics and signal processing scheme in the VectrinoII represent advancements over those in the original Vectrino (referred to as VectrinoI in this paper). In addition, the VectrinoII provides for profiling over a ca. 3 cm range interval thereby allowing direct measurement of the spatial structure of the flow. The two instruments deliver comparable performance as measured by mean velocity profiles, turbulent kinetic energy spectra, and the derived values of Reynolds stress and dissipation. The Vectrino measurements are compared to the mean and turbulent properties observed by [1] using hot-film and Laser Doppler anemometry. Here, there is good agreement in mean velocity and Reynolds stress measurements. Significant differences are seen in dissipations and velocity variance.

Observations of the space‐time structure of flow, turbulence, and stress over orbital‐scale ripples

The spatial and temporal structure of flow, turbulence, and stress over equilibrium orbital-scale sand ripples are investigated at turbulence-resolving scales with a wide-band coherent Doppler profiler (MFDop) and an oscillating tray apparatus. The oscillation period and horizontal excursion were 10 s and 0.5 m. A single trial was also executed at 0.6 m excursion. Ripple wavelength and amplitude were 25 and 2.2 cm. Ensemble-averaged velocity profiles were acquired with 3 mm vertical resolution at 42 Hz. The spatial pattern of flow as a function of oscillation phase was determined by combining the phase-averaged velocity measurements from trials with the MFDop at different positions relative to a particular ripple crest. The MFDop measurements are used to investigate the coevolution of the lee vortex, turbulent kinetic energy, Reynolds stress, and turbulence production as a function of phase. Shear stress is determined from the vertically integrated vorticity equation and the double-averaged momentum equations. Friction factors obtained from the two methods are comparable and range from 0.1 to 0.2.

Acoustic Doppler current profiler observations of herring movement

Observations were made of over-wintering (December 1997) and migrating (January 1998) Norwegian, spring-spawning herring (Clupea harengus) using a moored 307 kHz acoustic Doppler current profiler (ADCP). The location of herring in ADCP data is identified by regions of volume-backscatter strength greater than −60 dB re 1 m −1 . The presence of herring was verified using net trawls and 38 kHz, EK500 data. While the ADCP cannot make speed measurements of individual fish, the system does provide a measure of the swimming speed and direction of large herring schools. Herring were observed to move both horizontally and vertically: horizontal speeds were from 0 to 50 cm s −1 . Higher speeds were observed during daylight hours for both deployments with somewhat increased activity at both dawn and dusk. At night-time, over-wintering herring demonstrated no well-defined velocity.