Distribution of Energy Spectra, Reynolds Stresses, Turbulence Production, and Dissipation in a Tidally Driven Bottom Boundary Layer

Abstract

Seven sets of 2D particle image velocimetry data obtained in the bottom boundary layer of the coastal ocean along the South Carolina and Georgia coast [at the South Atlantic Bight Synoptic Offshore Observational Network (SABSOON) site] are examined, covering the accelerating and decelerating phases of a single tidal cycle at several heights above the seabed. Additional datasets from a previous deployment are also included in the analysis. The mean velocity profiles are logarithmic, and the vertical distribution of Reynolds stresses normalized by the square of the free stream velocity collapse well for data obtained at the same elevation but at different phases of the tidal cycle. The magnitudes of ?u?u??, ?w?w??, and ??u?w?? decrease with height above bottom in the 25?160-cm elevation range and are consistent with the magnitudes and trends observed in laboratory turbulent boundary layers. If a constant stress layer exists, it is located below 25-cm elevation. Two methods for estimating dissipation rate are compared. The first, a direct estimate, is based on the measured in-plane instantaneous velocity gradients. The second method is based on fitting the resolved part of the dissipation spectrum to the universal dissipation spectrum available in Gargett et al. Being undervalued, the direct estimates are a factor of 2?2.5 smaller than the spectrum-based estimates. Taylor microscale Reynolds numbers for the present analysis range from 24 to 665. Anisotropy is present at all resolved scales. At the transition between inertial and dissipation range the longitudinal spectra exhibit a flatter than ?5/3 slope and form spectral bumps. Second-order statistics of the velocity gradients show a tendency toward isotropy with increasing Reynolds number. Dissipation exceeds production at all measurement heights, but the difference varies with elevation. Close to the bottom, the production is 40%?70% of the dissipation, but it decreases to 10%?30% for elevations greater than 80 cm.