Directional change of tracer trajectories in rotating Rayleigh-Bénard convection

In Lagrangian measurements of turbulence the complexity is reflected in the directional changes of tracer trajectories over many active timescales. We study angular statistics of tracer trajectories in rotating Rayleigh-Bénard convection both experimentally and numerically. Our aim is to explore the geometrical characterization of flow structures in turbulent convection in a wide range of timescales and how it is affected by background rotation. We find that the angle of directional change θ(τ) as a function of the time gap τ is distributed similarly as in homogeneous isotropic turbulence. The ensemble averaged angle Θ(τ) = 〈|θ(τ)|〉 displays a transition from a ballistic scaling Θ(τ) ∼ τ for τ < τ_η (the Kolmogorov timescale), to an inertial-range scaling Θ(τ) ∼ τ^c with smaller exponent c < 1 for τ_η < τ < T_L (the Lagrangian integral timescale). We show that the value of c is related with the dominant flow structures: the large-scale circulation for slow rotation and vertically aligned vortices emerging from the boundary layers for rapid rotation.