Microbubble clustering in turbulent flow

Single-point hot-wire measurements in the bulk of a turbulent channel have been performed in order to detect and quantify the phenomenon of bubble preferential 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 ($R_0 \simeq100\ \mu m$) in a developed turbulent flow, where the Kolmogorov length-scale is $\eta \simeq R_0$, display preferential concentration in small scale structures. In particular, it is found that the clustering process is enhanced by increasing the turbulence intensity. A comparison with Eulerian-Lagrangian numerical simulations has also been performed and arising similarities and differences will be discussed.