A submersible particle image velocimetry system for turbulence measurements in the bottom boundary layer

This paper introduces an oceanic particle image velocimetry (PTV) system th at has been under development at The Johns Hopkins University over the past three years. PIV maps two components of the instantaneous velocity distrib ution within a selected sample area by recording the motion of microscopic tracer particles illuminated by a pulsed laser sheet. The primary purpose o f this instrument, in its current configuration, is to directly measure the Reynolds stresses, velocity profile, vorticity, and turbulent spectra in t he bottom boundary laver of the coastal ocean. Due to the basic principles of PIV, the validity of these measurements is independent of any of the usu al assumptions about the nature of the flow, such as the existence of a log layer in the velocity profile, a constant stress layer, or an inertial sub range in the turbulence spectra. In addition, at scales up to those of a si ngle image, the analysis does not require the use of the Taylor hypothesis. The primary focus of this paper is a detailed description of the PIV system and its principles of operation. The light source is a surface-mounted, hi gh-power laser connected to a submerged probe using an optical fiber. Image s of particles are recorded on a high-resolution, digital camera that feeds the data to a surface-mounted acquisition and analysis system. All the sub merged components are mounted on an adjustable platform that enables data a cquisition at any desired orientation and at a broad range of elevations. T his system has been successfully tested in the NASA/Langley Research Center saltwater towing tank and off the dock at the Woods Hole Oceanographic Ins titution.