Fine vortex structure and flow transition to the geostrophic regime in rotating Rayleigh-Bénard convection

We present spatially resolved measurements of the columnar vortex structures in rotating Rayleigh-Benard convection. The scaled radial profiles of the azimuthal velocity $u_{\varphi }\left(r\right)$ and vertical vorticity $\omega \left(r\right)$ of the vortices are analyzed and compared with the predictions of the asymptotic theory. The results reveal that the asymptotic theory predicts accurately $u_{\varphi }\left(r\right)$ and $\omega \left(r\right)$ in the geostrophic convection regime. In the weak rotation regime, it is found that the experimental data deviate from the theoretical predictions. Our measurements of the mean velocity, vorticity of the vortices, and the strength of the vortex shield structure all indicate a flow transition from weakly rotating convection to geostrophic convection. Results of the parameter values for the transition are supportive of the scaling relationship obtained from previous heat-transfer measurements.