Directional change of tracer trajectories in rotating Rayleigh-Bénard convection

The angle of directional change of tracer trajectories in rotating Rayleigh-Benard convection is studied as a function of the time increment tau between two instants of time along the trajectories, both experimentally and with direct numerical simulations. 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 probability density functions (PDFs) of the angle of directional change theta(t, tau) show similar behavior as found in homogeneous isotropic turbulence, up to the timescale of the large-scale coherent flow structures. The scaling of the averaged (over particles and time) angle of directional change Theta(tau) = with tau shows a transition from the ballistic regime Theta(tau) similar to tau(c) with c = 1] for tau less than or similar to tau(n), with tau(n) the Kolmogorov timescale, to a scaling with smaller exponent c for tau(n) less than or similar to tau less than or similar to T-L, with T-L the Lagrangian integral timescale. This scaling exponent is approximately constant in the weakly rotating regime (Rossby number Ro greater than or similar to 2.5) and is decreasing for increasing rotation rates when Ro less than or similar to 2.5. We show that this trend in the scaling exponent is related with the large-scale coherent structures in the flow; the large-scale circulation for Ro greater than or similar to 2.5 and vertically aligned vortices emerging from the boundary layers (BLs) near the top and bottom plates and penetrating into the bulk for Ro less than or similar to 2.5. In the viscous BLs, the PDFs of theta(t, tau) and scaling properties of Theta (tau) are in general different from those measured in the bulk and depend on the type of boundary layer, in particular whether the BL is of Prandtl-Blasius type (Ro greater than or similar to 2.5) or of Ekman type (Ro less than or similar to 2.5). When it is of Ekman type, a stronger dynamic coupling exists between the BL and the bulk of the flow, resulting in similar scaling exponents in BL and bulk.