Phase analysis of lock-exchange gravity currents

Abstract

An image analysis technique is used to evaluate time-space varying density distribution of unsteady density currents produced by full-depth lock-release of saline water. Density currents with four initial densities were generated in a 3.0 m long, 0.20 m wide and 0.30 m deep flume, with horizontal smooth bed, and recorded with a 25 Hz CCD video camera. Using dye concentration as a tracer, a calibration procedure was applied for each pixel in the image relating an amount of dye uniformly distributed in the channel and grey scale values in the corresponding images. Subsequently, the quantity of dye is converted into salt concentration allowing the estimation of the 2D instantaneous current density distribution. Temporal evolution of the current front position and front velocity are analysed and related to different phases of the current development. Different Froude numbers, accounting for the initial and local reduced gravity and established with different length scales, are the nondimensional form of the front velocity and help defining the ruling quantities on each phase.