Revisiting the oscillation of large-scale circulation in turbulent Rayleigh–Bénard convection: a perspective from linear-amplification mechanism

We investigate the oscillation of the large-scale circulation (LSC) in turbulent Rayleigh–Bénard convection by combining laboratory experiments and numerical simulations. The experiments are conducted mainly in a laterally confined rectangular cell, and the flow fields are measured by particle image velocimetry in the vertical mid-plane. It is found that the velocity field exhibits not only the commonly observed oscillation frequencies (fτ ₒ=1,2) but also higher-order harmonic frequencies (fτ ₒ> 2) , where τ ₒ is the turnover time of the LSC. Spectral proper orthogonal decomposition (SPOD) reveals that the coherent structures underlying these frequencies display an approximate azimuthal periodicity when viewed in polar coordinates. These structures can be interpreted as travelling waves, consistent with the advected-oscillation picture proposed by Brown & Ahlers (2009 J. Fluid Mech. , vol. 638, pp. 383–400). A data-driven resolvent analysis, used here as an effective input–output model inferred from the experimental data, yields response modes and gain peaks that are consistent with the SPOD results, and support the interpretation that the dominant oscillations are selected through linear amplification about the mean circulation. This perspective is further supported by direct numerical simulation in a rectangular cell and by velocity measurements in a cylindrical cell. These findings provide an additional viewpoint for understanding the oscillation mechanism of the LSC in turbulent Rayleigh–Bénard convection.