Internal Velocity Measurements in a Rising Bubble

We measure the internal velocity field of a rising bubble using a direct optical technique augmented by physics-based interpolation. While the external flow outside rising bubbles has been studied over a wide range of Reynolds and Eotvos numbers, the flow inside bubbles has not been measured directly due to the challenge of imaging through a distorted interface. The lack of direct measurements means bubbly flow analyses must depend on bubble models based on highly simplified geometries or on costly and unvalidated computations. To make direct velocity measurements inside a rising bubble, we combine optical flow-based velocimetry with laser-induced fluorescence and a novel shape-reconstruction method to measure the velocity field away from the bubble interface. We then augment these measurements with an efficient numerical model for low Reynolds number flows to interpolate the velocity field very near the interface. Using this technique, we quantify the velocity field due to single and double toroidal vortices inside bubbles with classic dimple or skirt geometries.