Velocity measurements in rotating Rayleigh-Bénard convection and the Boundary Zonal Flow*

We report on velocity measurements in rotating Rayleigh-Bénard convection (RBC). Experiments are conducted in a cylindrical plexiglass transparent cell

with diameter (D) and height (H) of 20 cm, resulting in the aspect ratio Γ=D/H=1. As working fluid, we use mixtures of water and glycerol, resulting in Prandtl numbers between 6.6≤Pr≤75, and Rayleigh numbers in the range 10⁸≤Ra≤3×10⁹. The cell rotates around its cylinder axis with rotation rates that correspond to Ekman numbers 1.5×10^-5≤Ek. We measure horizontal velocity components at a cross-section at half height using Particle Image Velocimetry. With our results we show the first direct measurements of the Boundary Zonal Flow - BZF, which develops near the sidewall and was recently discovered in numerical simulations (see Zhang et al., Phys.Rev.Lett.  2020) and localised temperature measurements (Wedi et al., JFM 2021). We analyse the thickness δ₀ of the near wall region as well as typical velocities therein as function of Pr, Ra, and Ek, and compare these results with numerical simulations.