ADV Laboratory Evaluation and Field Deployment in the Benthic Boundary Layer by WHOI

As part of the Coastal Mixing and Optics Accelerated Research Initiative by the Office of Naval Research, Woods Hole Oceanographic Institute deployed three SonTek/YSI Acoustic Doppler Velocimeters (ADV) south of Martha's Vineyard on a tripod at 70-m depth to measure velocities just 35 cm above the seabed.

Prior to deployment, an extensive laboratory investigation was undertaken to evaluate the performance of the ADV*. Simultaneous flow measurements were obtained using the ADV and a Laser Doppler Velocimeter (LDV) for a range of flow conditions. The measured mean flows agreed within one percent. The difference was attributed to uncertainties in vertically aligning the two sensors. direct estimates of Reynolds stress were underestimated by only one percent by the ADV.

Special characteristics of the flow were used to examine the effect of sample volume size in turbulence spectra. It was found that at relatively low-boundary Reynolds numbers, the effect of viscous dissipation was more important than the effect of the sample volume size. Spatial filtering is a function of the Kolmogorov microscale (i.e., less attenuation at less turbulent flows) and is greater in the horizontal component than in the vertical. This, combined with the significantly lower noise levels, make the vertical component the best candidate for studies of the structure of turbulence.

Figure 1. Time series of horizontal velocities (top),
root mean squared velocity (center),
and shear velocity (bottom)
within the benthic boundary layer.

ADV time-series are shown in Figure 1 from the first field deployment in 1996 beginning on August 19, 1996 until Hurricane Edouard struck the east coast on September 1, 1996. Data consisted of velocity time-series sampled at 25 Hz for 9.5 minutes each hour per instrument. The diurnal tide dominates the mean horizontal velocities, while the relative phase difference indicates rotary flow at this depth. The variance of the horizontal flow was initially almost constant, but increased in response to the approach of Hurricane Edouard. The increase of the mean shear velocity due to the presence of the wave boundary layer can be seen just before the hurricane arrives.

• ADV

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*Voulgaris, G. and Trowbridge, J. July 1998 (In Press). Evaluation of the Acoustic Doppler Velocimeter (ADV) for Turbulence Measurements. Journal of Atmospheric and Oceanic Technology. For more information, contact Dr. George Voulgaris (gvoulgaris@whoi.edu) or SonTek.