Evaluation of time-space varying density distribution on gravity currents by image analysis technique

Density currents are flows driven by density differences within a fluid which can be due to temperature differences, dissolved substances or particles in suspension. The present work describes a procedure based on an image-analysis technique to experimentally evaluate the time and space varying density distribution on density currents. The experiments were conducted in the Laboratory of Hydraulics of University Roma Tre in a 3.0 m long, 0.20 m wide and 0.30 m deep transparent Perspex flume, with a horizontal smooth bed. Density currents were performed by lock exchange release of saline water and recorded with a 25 Hz CCD camera under controlled light conditions. A controlled quantity of dye is added to the saline water in the lock to provide flow visualization and to work as density tracer. A calibration procedure technique is applied to gray scale matrices, converted from the acquired video frames, by using an experimentally determined non-linear relationship between these and the quantity of dye in the water (Fig. 1). Subsequently, the quantity of dye is converted in salt concentration by assuming a linear relation between both quantities, dye and salt, allowing thus the estimation of the instantaneous current density 2D distribution. With the resulting instantaneous density maps (Fig. 2), temporal and spatial evolution of the current form and structure can be assessed and discussed as well as time varying depth averaged density, front position and front velocity.