Non-intrusive density measurements in gravity currents interacting with an obstacle

Gravity currents are flows produced by a density difference between two fluids. Non-intrusive density measurements, based on image analysis, are employed to measure the instantaneous width averaged density fields of lock-release gravity currents. The developed technique uses dye concentration as a tracer. For each pixel of the acquired images, a calibration procedure is applied to relate the concentration of dye, uniformly distributed in the experimental tank, and the greyscale. Two calibration techniques with nine-image and two-image calibration curves, respectively, are employed to evaluate the instantaneous density fields of a gravity current with and without a bottom obstacle. The two different calibration curves affect the measurement results, i.e. instantaneous density fields, front propagation, current area and mixing. The image analysis using a nine-image calibration curve is found to give better results compared to the technique with a two-image calibration curve. The analysis performed shows how the image analysis based on a nine-image calibration curve is a more suitable technique for the study of the gravity currents dynamics and the associated mixing, which are strongly affected by the presence of a bottom obstacle.