Low frequency oscillations in turbulent Rayleigh-Benard convection: laboratory experiments

Abstract

This is a report on observations of low frequency oscillations in turbulent Rayleigh-Benard convection, in the Rayleigh number (R) range 107–108, with Prandtl number (Pr) equal to 7. It has been known that for convecting layers with large aspect ratio A, a steady large scale flow sets in at R = 2 × 106. Tilted transient plumes embedded in this flow, and maintaining it through Reynolds stresses, drift in one direction along the bottom of the layer, and in the opposite direction along the top. At a fixed point near the bottom or top boundary, there is a variability associated with the passage of these plumes. We call this the high frequency variability. A new kind of organization is observed for 107 < R < 108; clusters of transient tilted plumes travel in a horizontal direction as coherent units. These clusters are separated from each other by quiescent zones with almost no plumes. Now at a fixed point near the bottom boundary, there is a low frequency variability associated with the passage of clusters, as well as the high frequency variability from the passage of plumes within the cluster. Quantitative information on this low frequency oscillation derived from space-time portraits and from temperature time series is presented.Heat flux measurements show a Nusselt number (N)-Rayleigh number relationship for 106 < R < 108 which is hysteretic for A = 12, but when the convecting layer is partitioned into 144 cells each with A = 1, hysteresis is not present and the N-R relationship then agrees with earlier results for single cells with A = 1.