Acoustic Doppler Current Profiler (ADCP) Measurements of Breaking Waves

Detailed flow measurements of the turbulent multiphase flow associated with wave breaking present a unique instrumentation challenge. Measurement systems must be capable of high sampling rates, large dynamic ranges, and be capable of making measurements in water, air and optically opaque regions. An experiment was performed at the Naval Surface Warfare Center, Carderock Division, in October 2007 to measure various characteristics of the breaking wave generated from a submerged ship transom. The primary objective of this work was to obtain full-scale qualitative and quantitative flow field data of a large breaking transom wave over a range of conditions, specifically transom drafts and Froude numbers. Several types of measurements were made on the transom stern wave during this experiment, however, this paper will focus on the Nortek Acoustic Wave and Current (AWAC) profiler measurements of the stern wake. The AWAC has a center acoustic beam in addition to the three angled beams typically found on an acoustic Doppler current profiler. The trends of the acoustic return from the AWAC and the trends of the bubble density and location on the water surface compare well, and it is anticipated that this return can be related to void fraction and bubble measurements in the future. This type of non-intrusive measurement could be very useful in the evaluation of breaking waves.