Dynamics and Optics of Bubble Tracking Velocimetry for Airflow Measurement

Availability of particle tracking velocimetry (PTV) that is applied to movie images of soap bubbles in airflow is investigated experimentally. This study is positioned as a basic research for measuring environmental flow with a large spatial scale, such as flows around building, trees, and near-ground airflow boundary layer. Instead of solid fine particles, soap bubbles are used as the airflow tracer for reducing environmental impact. Typical bubble size provided by the present bubble generator is controlled around 20mm, at which the bubbles obviously have relative velocity to the airflow. We report three topics on the bubble tracking velocimetry (BTV) in this paper: 1) optics of bubble-imaging for quantitative visualization, and application to 3-D visualization using color illumination, 2) theoretical estimation of bubble’s relative velocity to airflow based on an equation of bubble’s translational motion, 3) comparison of velocity vector field obtained by the BTV with that from cross-correlation PIV applied for smoke image. For the latter two topics, airflow around a NACA airfoil is chosen as the target of BTV measurement since it causes significant slip motion of soap bubbles from the airflow that accompanies shear rate, convective acceleration, pressure gradient, and separation.