How gravity and size affect the acceleration statistics of bubbles in turbulence
Résumé
We report results from the rst systematic Lagrangian experimental investigation in the previously unexplored regime of very light (air bubbles in water) and large particles (D/eta >> 1) in turbulence. Using a traversing camera setup and particle tracking, we study the Lagrangian acceleration statistics of ~ 3 mm diameter (D) bubbles in a water tunnel with nearly homogeneous and isotropic turbulence generated by an active-grid. The Reynolds number (Re ) is varied from 145 to 230, resulting in size ratios, D/eta in the range of 7.3,12.5, where eta is the Kolmogorov length scale. The experiments reveal that gravity increases the acceleration variance and reduces the intermittency of the PDF in the vertical direction. Once the gravity o set is subtracted, the variances of both the horizontal and vertical acceleration components are about 5 +- 2 times larger than the one measured in the same flow for fluid tracers. Moreover, for these light particles, the experimental acceleration PDF shows a substantial reduction in intermittency at growing size ratios, in contrast to neutrally buoyant or heavy particles. All these results are closely matched by numerical simulations of fi nite-size bubbles with the Fax en corrections.