Non-intrusive Density Measurements in a Gravity Current Over a Porous Bed

Gravity currents are buoyancy driven flows in which the density gradient causes the development of the flow predominantly in the horizontal direction. In this work, full depth lock-release gravity currents, propagating over a porous bed, are investigated by laboratory experiments. All experiments were recorded by a high-speed camera and an image analysis technique was applied for the evaluation of the instantaneous density fields of the surface gravity current and of the ground flow. The different steps of the non-intrusive density measurements, i.e. the gray scale calibration, the double calibration, the calibration curves and the image subtraction, are described and discussed. In particular, due to the large optical difference between the images captured for gravity currents and ground flow, a double calibration was adopted to analyze the two different flows. Preliminary results of the non-intrusive density measurements are shown. In the upper measurement domain, the instantaneous density fields show a gravity current with a well-defined head loosing mass as it propagates downstream. In the bottom measurement domain, a second gravity current with an approximately triangular shape propagates slowly in the same direction of the surface current. In addition, infiltration, due to the presence of a top layer of salty water, develops a fingering instability.