Coherence of temperature and velocity superstructures in turbulent Rayleigh-B\'enard flow

TL;DR

This study reveals that temperature and velocity superstructures in turbulent Rayleigh-Bénard convection are larger, highly correlated, and scale with Rayleigh number, with distinct spectral features and a self-similar velocity structure.

Contribution

It demonstrates that velocity and temperature superstructures are of similar size and highly correlated, challenging previous assumptions, and identifies their scaling and spectral characteristics across Rayleigh numbers.

Findings

Superstructures of temperature and velocity are nearly perfectly correlated.

Superstructure size increases monotonically with Rayleigh number.

Temperature fluctuation distribution is bimodal with a large-scale and a small-scale peak.

Abstract

We investigate the interplay between large-scale patterns, so-called superstructures, in the fluctuation fields of temperature $\theta$ and vertical velocity $w$ in turbulent Rayleigh-B\'{e}nard convection at large aspect ratios. Earlier studies suggested that velocity superstructures were smaller than their thermal counterparts in the center of the domain. However, a scale-by-scale analysis of the correlation between the two fields employing the linear coherence spectrum reveals that superstructures of the same size exist in both fields, which are almost perfectly correlated. The issue is further clarified by the observation that in contrast to the temperature, and unlike assumed previously, superstructures in the vertical velocity field do not result in a peak in the power spectrum of $w$. The origin of this difference is traced back to the production terms of the $\theta$- and $w$-variance. These results are confirmed for a range of Rayleigh numbers $Ra = 10^5$--$10^9$, the superstructure size is seen to increase monotonically with $Ra$. Furthermore, the scale distribution of particularly the temperature fluctuations is pronouncedly bimodal. In addition to the large-scale peak caused by the superstructures, there exists a strong small-scale peak. This `inner peak' is most intense at a distance of $\delta_\theta$ from the wall and associated with structures of size $\approx 10 \delta_\theta$, where $\delta_\theta$ is the thermal boundary layer thickness. Finally, based on the vertical coherence relative to a reference height of $\delta_\theta$, a self-similar structure is identified in the velocity field (vertical and horizontal components) but not in the temperature.