Turbulence Characteristics and Dissipation Estimates in the Coastal Ocean Bottom Boundary Layer from PIV Data

Abstract

Turbulence characteristics in the coastal ocean bottom boundary layer are measured using a submersible Particle Image Velocimetry (PIV) system with a sample area of 20 ? 20 cm2. Measurements are performed in the New York Bight at elevations ranging from 10 cm to about 1.4 m above the seafloor. Recorded data for each elevation consists of 130 s of image pairs recorded at 1 Hz. After processing, the data at each elevation consist of 130 instantaneous spatial velocity distributions within the sample area. The vertical distribution of mean velocity indicates the presence of large-scale shear even at the highest measurement station. The flow also undergoes variations at timescales longer than the present data series. Spatial spectra of the energy and dissipation are calculated from individual vector maps. The data extend well beyond the peak in the dissipation spectrum and demonstrate that the turbulence is clearly anisotropic even in the dissipation range. The vector maps are also patched together to generate extended velocity distributions using the Taylor hypothesis. Spectra calculated from the extended data cover about three decades in wavenumber space. For the overlapping range the extended spectra show small differences from those determined using the instantaneous distributions. Use of the Taylor hypothesis causes ?contamination? of the extended spectra with surface waves. Nevetheless, the results still indicate that the turbulence is also anisotropic at low wavenumbers (energy containing eddies). The vertical component of velocity fluctuations at energy containing scales is significantly damped as the bottom is approached, while the horizontal component maintains a similar energy level at all elevations. Different methods of estimating the turbulent energy dissipation are compared. Several of these methods are possible only with 2D data, such as that provided by PIV, including a ?direct? method, which is based on measured components of the deformation tensor. Estimates based on assumptions of isotropy are typically larger than those based on the direct method (using available velocity gradients and least number of assumptions), but the differences vary from 30% to 100%.