PIV measurements in the bottom boundary layer of the coastal ocean

Turbulence measurements were recently performed in the bottom boundary layer of the coastal ocean. A submersible PIV system was deployed at two locations, the first in 12m deep water near to LEO-15, about 5km off the New Jersey coast and the second in 10m deep water just inside the mouth of Delaware Bay. The present system consists of two 12-bit, 2K×2K digital cameras, operating simultaneously. The 51×51cm sample area of each camera is aligned with the current. The light source is a pair of flash-lamp pumped dye lasers located at the surface. The beams are transmitted through optical fibers to submerged probes that produced the light sheets. The custom made cameras have a hardware based 'image shifter' which offsets the two exposures within a single recorded frame. The cameras, probes and auxiliary sensors are mounted on an adjustable seabed platform which allowed measurements to be made at any elevation, from close to the bed up to 1.7m into the water column. The equipment was submerged continuously for up to 13hr s, and images were recorded at 0.5 and SHz.In some measurements the two light sheets were parallel to each other and in others they were perpendicular. The data includes vertical mean velocity profiles and time evolution of the mean velocity. A log layer only forms when the mean velocity is substantially higher than the surface wave induced motion. Using the covariance of the difference between velocity components, it is possible to estimate the Reynolds stress without being affected by unsteady phenomena related to surface waves. The PIV measurements are complimented by data collected by other instruments, which allow the fine scale structure of the boundary layer to be related to the large scale mean characteristics of the entire water column.