Turbulent Wake of a Bridge Pier in a Tidal Current

The evolution of a large Reynolds number O(10 ⁶) wake was investigated using data from a ship-mounted acoustic Doppler current profiler (ADCP). The ADCP was a custom configured 600-kHz Teleydyne RD Instruments Workhorse augmented to have a fifth beam that was nominally vertical. The test was conducted in Narragansett Bay, RI, USA, using a large support pier of the Newport Bridge as the turbulent source in the bay´s tidal flow. The turbulence was manifested as a vigorously fluctuating wake in the lee of the bridge pier. A series of wake crossings between x = 10-100 D were made during the ebb of the tidal cycle, where x and D are, respectively, the downstream distance and bridge pier diameter. The data set serves as one of a handful where wake evolution has been documented for a large Reynolds number system, and is unique in that high-horizontal resolution vertical velocity profiles were taken with the vertical fifth beam of the ADCP. A range of scales was observed in the vertical velocity field within the wake, including small-scale turbulence O(2-10 m) and large vortex structures O(20-40 m). When cast in terms of nondimensional wake age ( N τ), where N and τ are, respectively, the buoyancy frequency and wake age, the decay of root mean square vertical velocity fluctuations was observed to be proportional to (N τ)^-0.82, faster than the decay of an unstratified 2-D plane wake.