Image analysis applied to the study of mixing in a stably stratified shear layer

The development and breaking of Kelvin-Helmholtz waves is one of the primary causes of mixing in many geophysical and engineering flows with layers of fluids having different densities and horizontal velocities. Although this phenomenon was extensively studied in the field, a complete description can be experimentally obtained only by the use of image analysis techniques that are applicable only in laboratory experiments. The particular nature of the flow, especially before the development of the waves when the flow is parallel but in opposite direction, makes the application of the classical image velocimetry techniques non-trivial. With this in mind, a stably stratified shear flow was reproduced in the laboratory by means of a tilting tank. The velocity and density fields were measured simultaneously with multipoint time-resolved techniques during the formation and development of the Kelvin-Helmholtz waves. A novel particle tracking procedure is proposed that includes the stretching of the acquired images in the direction orthogonal to the main motion. Tests on synthetic images show a meaningful improvement in the effectiveness of particle tracking when using the proposed technique. Laser-Induced Fluorescence (LIF) data have been acquired by a second camera, equipped with a band-pass filter in order to measure only the fluoresced light. Particle Tracking Velocimetry (PTV) and LIF data have been referenced to the same frame by a registration procedure based on an affine transformation. In the range of the parameters investigated during the experiments, the evolution of the interface thickness and sharpness scales with the advective time scale. The analysis of the space-time evolution of the longitudinal statistics gives a comprehensive picture of the development and breaking of the waves