The Emergence and Identification of Large-Scale Coherent Structures in Free Convective Flows of the Rayleigh-Bénard Type

Sergei Smirnov,Sergey Bogdanov,Alexander Smirnovsky

doi:10.3390/fluids6120431

Abstract

The revealing of the turbulence archetypes is one of the fundamental problems in the study of turbulence, which is important not only from the fundamental point of view but also for practical applications, e.g., in geophysics of ocean and lakes. The paper is devoted to the study of the emergence of coherent structures and the identification of their turbulent archetypes, typical for the free convective flows of the Rayleigh-Bénard type. Using Direct Numerical Simulation, we perform a numerical study of two refined convective flows: convection in a cylinder heated from below and internally heated convection in a layer. The main purpose of the study is identifying coherent structures (CS), investigating its main features and properties, and determining the turbulence archetypes using the anisotropy invariant map (AIM). We show that, in both configurations considered, CS takes place. In a cylinder, CS is a single large-scale vortex that can rotate azimuthally in non-titled container, but is almost “fixed” in the case of slightly tilted cylinder; in a layer, CS is a quasi-2D vortex, which can arise, exist for some time, disrupt, and then re-emerge again in the orthogonal direction. Nevertheless, the turbulence archetypes represented by the AIM are quite similar for both cases, and there are the distinct CS fingerprints on AIM.

Highlights

Accepted: 18 November 2021Usually the studies of turbulent transfer processes in natural geophysical flows are complicated due to the variety of energy forcing
We show the results of our numerical simulation of the Rayleigh-Bénard convection in a cylinder, both tilted and non-tiled, and perform the anisotropy invariant map (AIM) analysis of the large-scale coherent structure
Temporal changes in the dimensionless vertical velocity component at the point located at the distance of 0.17D from the sidewall are shown in Figure 4c

Summary

Introduction

Accepted: 18 November 2021Usually the studies of turbulent transfer processes in natural geophysical flows are complicated due to the variety of energy forcing. The structure of turbulence generated by mean velocity gradient (wind stress), wave-shear interaction (like in the case of Langmuir circulation), or gravitational instability (during night cooling or under-ice inhomogeneous heating of the water column) depends crucially on the type of the forcing. In this regard one can even distinguish between the different “archetypes” of turbulence [1]. The thorough description of the archetypes is of great practical importance, considering the variety of water bodies’ responses to external forcing of different nature. To achieve the proper forecasting of system response, the splitting criteria for deriving the contribution of each type of forcing is necessary

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Discussion

Conclusion