Statistical Characterization of Helium-Filled Soap Bubbles Tracing Fidelity for PIV

The present work follows a previous study on the aerodynamic characterization of helium-filled soap bubbles (HFSBs) for large-scale PIV measurements. HFSBs were found to yield, on average, a time response of about 10s. However, the response of each individual tracer remained to be ascertained, which is the topic of the present study. 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 micro-size 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. The results are discussed and related to the differences observed in the bubbling and jetting regimes for bubble production. Finally, the HFSBs relative density to the air is estimated using a modified Stokes drag law.AerodynamicsAerodynamics, Wind Energy & Propulsio