Direct measurements of Reynolds stress with an acoustic Doppler velocimeter

Ocean boundary layer dynamics is dominated by high-frequency, intermittent phenomena that can only be described in detail using turbulent statistics. Unfortunately, direct measurements of bulk turbulent parameters (kinetic energy, dissipation rates, Reynolds stress, etc.) have proven elusive because precise estimates require current meters that can measure all three velocity components, have a good 3D response, are reasonably accurate, and have a sampling rate that is high enough to capture the scales of interest. As a candidate sensor for ocean deployment, the SonTek ADV was tested in a flume located at the University of South Florida. In the test, data were collected simultaneously with a 2D LDV and the 3D ADV in a collocated sampling volume. Comparison of the Reynolds stress over a velocity range of 5-80 cm/s shows a very good correspondence at high flows and a slight bias in the ADV data at low flows (<10 cm/s). An array of seven ADVs was deployed as part of Duck '94, a large-scale field project taking place in the fall of 1994 off Duck, North Carolina. The sensors were mounted on a vertical mounting frame and deployed at a depth of approximately 4 m. The authors discuss methods for extracting the turbulent properties of the joint wave-current field using an ADV. The data examples are taken from a storm event at Duck with 2.2 m significant wave height on November 7, 1994.