Bubble clustering in a horizontal turbulent channel flow investigated by bubble-tracking velocimetry

We investigated bubble clustering in a horizontal turbulent channel flow experimentally. Accurate image processing was used to detect individual bubble centroids with a 100% success rate. The successful detections realize bubble tracking velocimetry for Lagrangian mapping of bubble-bubble interaction both visually and quantitatively. The bubble-bubble interaction becomes explicit as the channel Reynolds number (Re) increases in the range 5,000 < Re < 11,000, incorporating streamwise attraction, lateral repulsion, and tandem circulation within a space narrower than 3 bubble diameters. We attribute this to the turbulent boundary layer where wall-sliding bubbles display streamwise velocity differences due to size. The resulting clustering can be attributed to poly-dispersed bubbles. This idea is verified by experimental data classified by bubble size. The present findings offer an explanation for the initiation of void waves in a bubbly two-phase boundary layer at a scale larger than that of the original coherent structures involved in the turbulence.