A Submersible Particle Image Velocimetry System for Turbulence Measurements in the Bottom Boundary Layer

Abstract

This paper introduces an oceanic particle image velocimetry (PIV) system that has been under development at The Johns Hopkins University over the past three years. PIV maps two components of the instantaneous velocity distribution within a selected sample area by recording the motion of microscopic tracer particles illuminated by a pulsed laser sheet. The primary purpose of this instrument, in its current configuration, is to directly measure the Reynolds stresses, velocity profile, vorticity, and turbulent spectra in the bottom boundary layer of the coastal ocean. Due to the basic principles of PIV, the validity of these measurements is independent of any of the usual 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 subrange in the turbulence spectra. In addition, at scales up to those of a single 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, high-power laser connected to a submerged probe using an optical fiber. Images of particles are recorded on a high-resolution, digital camera that feeds the data to a surface-mounted acquisition and analysis system. All the submerged components are mounted on an adjustable platform that enables data acquisition at any desired orientation and at a broad range of elevations. This system has been successfully tested in the NASA/Langley Research Center saltwater towing tank and off the dock at the Woods Hole Oceanographic Institution.