| Summary: | Introduction and background. Particles are dispersed in a turbulent fluid flow in many natural and industrial situations¿typical examples in the atmosphere include pollutant dispersion, cloud formation; in the oceans¿entrained air bubbles, plankton distribution; in industry¿sprays, combustion in engines, etc. Researchers aim to model and understand these systems to enable predictions, for example the ash release due to the recent Iceland volcano eruption (Eyjafjallajökull 2010), which caused major air-travel disruption around northern Europe. A fundamental understanding of the physics is essential and major research efforts are directed towards gaining information on the statistical properties (for example the velocity and acceleration) of such particles suspended in turbulent flows. Here we study, for the first time, the accelerations of particles lighter than the surrounding fluid (air bubbles in water) and focus on the effects of finite particle-size and role of gravity. We use a unique two-phase turbulence facility (Twente Water Tunnel) with a traversing particle-tracking setup to conduct state-of-the-art experiments (see the figure). The experimental results are compared to cutting-edge numerical simulations. Main results. The acceleration variances and intermittency indicate that both the effects of finite-size and gravity are important. We find that gravity has a surprising correction on the vertical component acceleration. The finite-sized bubbles do not respond to the smallest-scale fluctuations, and as a consequence, we find a decrease in the intermittency of the probability distribution function (PDF) of the bubbles, compared to tracers. The experiments and numerical simulations indicate a complex interplay between gravity and inertia, and this deserves further study. Wider implications. We provide a solid experimental validation of the widely used equations for modeling particles in turbulence (Maxey and Riley 1983 Phys. Fluids 26 883). Such complementary experimental and numerical efforts greatly improve our fundamental understanding of the physics of particles in turbulence.
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