Turbulence characteristics and dissipation estimates in the coastal ocean bottom boundary layer from PIV data

Turbulence characteristics in the coastal ocean bottom boundary layer are m easured using a submersible Particle Image Velocimetry (PIV) system with a sample area of 20 x 20 cm(2). Measurements are performed in the New York Bi ght at elevations ranging from 10 cm to about 1.4 m above the seafloor. Rec orded 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 instantan eous spatial velocity distributions within the sample area. The vertical di stribution of mean velocity indicates the presence of large-scale shear eve n 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 individua l vector maps. The data extend well beyond the peak in the dissipation spec trum and demonstrate that the turbulence is clearly anisotropic even in the dissipation range. The vector maps are also patched together to generate e xtended velocity distributions using the Taylor hypothesis. Spectra calcula ted from the extended data cover about three decades in wavenumber space. F or the overlapping range the extended spectra show small differences from t hose determined using the instantaneous distributions. Use of the Taylor hy pothesis causes "contamination'' of the extended spectra with surface waves . Nevetheless, the results still indicate that the turbulence is also aniso tropic at low wavenumbers (energy containing eddies). The vertical componen t of velocity fluctuations at energy containing scales is significantly dam ped 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 compar ed. Several of these methods are possible only with 2D data, such as that p rovided by PIV, including a "direct'' method, which is based on measured co mponents of the deformation tensor. Estimates based on assumptions of isotr opy are typically larger than those based on the direct method (using avail able velocity gradients and least number of assumptions), but the differenc es vary from 30% to 100%.