Velocity measurements in rotating Rayleigh-B\'enard convection and the Boundary Zonal Flow

Rotating turbulent thermal convection is of great importance in various astro- and geophysical systems, where the buoyancy driven flow strongly influenced by Coriolis forces due to rotation of the celestial bodies. It has been studied for decades in the so-called Rayleigh-B\'{e}nard setup, where a horizontal fluid layer is heated at the bottom and cooled at the top and rotated around the vertical axis. We investigate the horizontal velocity field using 2D-particle image velocimetry (PIV) in a cylindrical cell ($H=196\,$mm high) with aspect ratio $\Gamma=D/H=1$. We use water and various water-glycerol mixtures as working fluid resulting in a Prandtl number (Pr) in the range $6\leq Pr\leq70$ and Rayleigh numbers $10^8 < Ra < 2\times 10^9$. With our rotating table we reach $Ek$ as low as $10^{-5}$. We are mainly interested in studying the recently discovered {\em Boundary Zonal Flow} (BZF, see Zhang et al., Phys.Rev.Lett. 2020). The BZF is observed in a region close to the lateral sidewall with a cyclonic flow, i.e, a positive mean azimuthal velocity that is separated from and anticyclonic bulk, with negative mean azimuthal velocity. We measure the size of the BZF as a function of $Ek$ and $Ra$, and compare the results with DNS (Zhang and Shishkina, 2020).