Lagrangian dynamics of large inertial balls in turbulent Rayleigh-Bénard convection

In this work, we report an experimental study of the Lagrangian dynamics of large inertial balls in turbulent Rayleigh-Bénard convection. The measurements were conducted in a quasi-two-dimensional rectangular cell with the Rayleigh number range of 8.8 x 10⁹ - 8.8 x 10¹⁰. The inertial balls used have a diameter of 1.7 cm, comparable to the integral length scale of the turbulent flow. It is found that there exists a well-defined characteristic time in many Lagrangian quantities (e.g., the displacement, velocity, and acceleration) of the inertial ball, the magnitude of which is consistent with the turn-over time of the large-scale mean flow in the system. In particular, the mean square displacement of a single ball follows a ballistic-typed diffusion within one turn-over time and exhibits an oscillation after reaching the plateau caused by spatial confinement. It is further found that the probability density distributions of Lagrangian velocity and acceleration have Gaussian and stretched exponential shapes, respectively, which are similar to previous findings obtained by tracer particles. However, while the mean flow intensity characterized by these inertial balls is in good agreement with the result obtained by particle image velocimetry, the corresponding rms value is much smaller. These results could be useful for the applications of ocean buoy technology.