Visualization and characterization of gravity currents over rough beds by means of PIV measurements

Abstract

Particle Image Velocimetry (PIV) is herein used to investigate the influence of the bed roughness in the development of unsteady gravity currents, namely in what concerns the instantaneous two-dimensional velocity and vorticity fields. Density currents are performed by lock-release of a fixed volume of saline water into a fresh water tank. The experiments were conducted in a 3.0 m long Perspex flume of horizontal bed and rectangular cross section of 0.20 x 0.30 m2. Four experiments were performed by varying the bed roughness maintaining the initial density of the saline water in the lock, the lock length and the water depth. The analysis of instantaneous velocity and vorticity maps obtained in the vertical centreline plane of the channel show that bed roughness reduces the overall streamwise velocity within the current and confers a vertically homogenized velocity distribution, therefore reducing the vorticity magnitude.