Prime modes of fluid circulation in large-aspect-ratio turbulent Rayleigh-Bénard convection.

Based on a detailed experimental investigation in an aspect-ratio-4 rectangular cell in the range 3.7 x 10(7)<or=Ra<or=3.7 x 10(9), we present evidence of possible scenarios of the long-term dynamics of large-scale circulations (LSC) in bounded large-aspect-ratio turbulent Rayleigh-Bénard convection. Karhunen-Loève analysis of instantaneous velocity fields from long-time particle image velocimetry measurements suggests two different states, but both appear to be produced by a single large-scale flow structure. The measurements provide a three-dimensional picture of LSC in the Gamma=4 cell. A modified scaling relation between the LSC peak frequency omega(p) and Ra is proposed.