Emergence of substructures inside the large-scale circulation induces transition in flow reversals in turbulent thermal convection

Abstract

We present an experimental study of the reversal of the large-scale circulation (LSC) in quasi-two-dimensional turbulent Rayleigh–Bénard convection. It is found that there exists a transition in the Rayleigh number ($Ra$) dependence of the reversal rate $f$ with two distinct scalings: for $Ra$ less than a transitional value $Ra_{t}$, the non-dimensionalized reversal rate $ft_{E}\sim Ra^{-1.09}$; however, for higher $Ra$ the scaling changes to $ft_{E}\sim Ra^{-3.06}$, where $t_{E}$ is the turnover time of the LSC. Flow visualization shows that this regime transition originates from a change in flow topology from a single-roll state to a new, less stable, abnormal single-roll state with substructures inside the single roll. The emergence of the substructures inside the LSC lowers the energy barrier for the flow reversals to occur and leads to a slower decay of $f$ with $Ra$. Detailed analysis reveals that, although it is the corner rolls that trigger the reversal event, the probability for the occurrence of reversals mainly depends on the stability of the LSC. This is supported by a model we proposed to predict the critical condition for the transition, which agrees well with the experimental results.

Thermal boundary layer structure in low-Prandtl-number turbulent convection

“…2018 b ; Chen et al. 2019). We exhibit the flow structures corresponding to a few dominant modes in our flow in figure 4( a – d ).…”

“…We estimate the strengths of various flow modes by computing their kinetic energy content by projecting the velocity field on a sine–cosine basis (Wagner & Shishkina 2013; Chen et al. 2019) as follows: Here , i.e. they are positive integers.…”

From zonal flow to convection rolls in Rayleigh–Bénard convection with free-slip plates

“…For Ra = 10 8 and 3 × 10 8 , N z z = 256 is used only for the large Γ cases (for example, Ra = 3 × 10 8 , Γ = 32 and 64) where very long simulations are performed, in order to test whether the zonal flow state can stably exist. study heat transport (Chong et al 2018) and flow reversals (Chandra & Verma 2011;Wang et al 2018bWang et al , 2019aChen et al 2019). An overview of the 2-D simulations and the grid resolutions used are given in table 1.…”

Reversals of coherent structures in turbulent mixed convection

“…More recent studies reduced the degrees of freedom for the LSC's orientation by using a rectangular (quasi) two-dimensional sample (Sugiyama et al 2010;Podvin & Sergent 2015;Castillo-Castellanos, Sergent & Rossi 2016;Podvin & Sergent 2017;Castillo-Castellanos et al 2019;Chen et al 2019). These studies include both numerical and experimental investigations aimed at gaining an insight into the mechanisms of the reversal events.…”

“…This overview on the different LSC reconfiguration processes reveals that their driving mechanism can be different for different boundary conditions. Another example for this is the frequency of the occurrence of reconfigurations, which shows different dependencies on the Rayleigh number in the above-mentioned studies: increasing Rayleigh numbers are associated with increasing (Araujo, Grossmann & Lohse 2005), decreasing (Ni, Huang & Xia 2015;Wang et al 2018;Chen et al 2019Chen et al , 2020, non-monotonic (Brown & Ahlers 2006) or independent (Xi & Xia 2007) behaviour of the occurrence frequencies.…”