Turbulent Flow Hydrodynamics and Sediment Transport: Laboratory Research with LDA and PIV

Abstract

Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) are indirect techniques to measure flow velocities. Both require small particles in the moving fluid, both measure specific statistics of the velocity of such particles, and both are among the least intrusive techniques to measure flow velocities. LDA techniques have been used in the scope of river hydraulics since the early 1980s, while PIV techniques have been growingly used in the last 15 years. The main objective of the present chapter is to show applications of LDA and PIV techniques to fluvial hydraulics, in the scope of research work conducted at Instituto Superior Tecnico, Technical University of Lisbon, Portugal. The work is divided in three parts. The first is dedicated to the presentation of basic concepts of LDA and PIV measurements. The second part addresses the characterization of turbulent open-channel flows over flat rough mobile beds and over flat rough fixed beds with LDA. The near-bed turbulent flow structure and the bursting cycle in mobile and immobile beds are discussed. The third part is aimed at describing the flow within the scour hole of a wall-mounted vertical cylinder on a mobile bed, using PIV. The structure of the flow, namely the downflow and the vortex system inside the hole, is presented and discussed, and the main scour mechanism is identified. In both cases, the relevance of LDA and PIV techniques for the understanding of sediment-flow interaction is highlighted.