Quantifying microbubble clustering in turbulent flow from single-point measurements

Single-point hot-wire measurements in the bulk of a turbulent channel have been performed in order to detect and quantify the phenomenon of preferential bubble accumulation. We show that statistical analysis of the bubble-probe colliding-times series can give a robust method for investigation of clustering in the bulk regions of a turbulent flow where, due to the opacity of the flow, no imaging technique can be employed. We demonstrate that micro-bubbles (radius R_0 ~ 0.1 mm) in a developed turbulent flow, where the Kolmogorov length-scale is, eta ~ R_0, display preferential concentration in small scale structures with a typical statistical signature ranging from the dissipative range, O(eta), up to the low inertial range, O(100 eta). A comparison with Eulerian-Lagrangian numeri- cal simulations is also presented to further support our proposed way to characterize clustering from temporal time series at a fixed position.