On the tracing fidelity of helium-filled soap bubbles for PIV experiments

Abstract

This thesis follows a previous study on the aerodynamic characterization of helium-filled soap bubbles (HFSBs) for large-scale PIV measurements and aims at characterizing statistically the tracing fidelity of HFSBs in PIV experiments, considering the statistical distribution of the bubbles diameter, slip velocity, relaxation time and density. High-speed visualizations identify two different operating regimes of the bubble generator. Two dedicated experiments are performed at a spatial resolution such to determine simultaneously the bubbles trajectory and their diameter. The velocity of the bubbles in the stagnation region ahead of a circular cylinder is evaluated by the PTV technique. The results are compared with micron-sized fog droplets taken as reference. The tracking error of individual trajectories is assessed by statistical analysis of the relative slip between the bubble and the airflow. The instantaneous particle relaxation time is retrieved from the ratio between slip velocity and local acceleration. Additional information on the bubble instantaneous properties is taken by inferring the diameter from the distance between the glare points. HFSBs were found to yield, on average, a time response of about 10 µs with a standard deviation that exceeds 30 µs when the nozzle is in a stable operating regime (bubbling). However, when the nozzle operates in a unstable operating regime (jetting), the standard deviation of the bubble diameter and relaxation time can be as high as 70 µm and 50 µs, respectively. The HFSBs relative density to air is estimated using a modified Stokes drag law. HFSBs as flow tracers in a laminar flat plate boundary layer feature a particle-free region close to the flat plate surface. The height of this region is a function of the particle diameter and can be partly explained by the conservation of mass principle of a streamtube far upstream that expands inside the boundary layer.