Condensation of Coherent Structures in Turbulent Flows

Chong, Kai Leong; Huang, Shi Di; Kaczorowski, Matthias; Xia, Ke‐Qing

doi:10.1103/physrevlett.115.264503

Cited by 73 publications

(87 citation statements)

References 21 publications

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“…Special attention is paid to the abnormal Ra dependence of flatness in the Γ = 0.1 cell, which is also reflected in its very different time series signals and the bimodal shaped PDF at Ra = 8.6 × 10 7 . Based on the recent finding by Chong et al (2015), these strange behaviours for Γ = 0.1 could be due to the fact that the LSC no longer exists in this case.…”

Section: Local Temperature Fluctuationsmentioning

confidence: 74%

Effects of geometric confinement in quasi-2-D turbulent Rayleigh–Bénard convection

Huang

Xia

2016

J. Fluid Mech.

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We report an experimental study of confinement effects in quasi-2-D turbulent Rayleigh-Bénard convection. The experiments were conducted in five rectangular cells with their height H and length L being the same and fixed, while the width W was different for each cell to produce lateral aspect ratios (Γ = W/H) of 0.6, 0.3, 0.2, 0.15 and 0.1. Direct flow field measurements reveal that the large-scale flow slows down as Γ decreases and there are more plumes travelling through the bulk region. Moreover, the reversal frequency of the large-scale flow is found to increase drastically in smaller Γ cells, by more than 1000-fold for the highest value of Rayleigh number reached in the experiment. The reversal frequency can be well described by a stochastic model developed by Ni et al. (J. Fluid Mech., vol. 778, 2015, R5) and the probability density functions (PDF) of the time interval between successive reversals are found to follow Poisson statistics as in the 3-D system. It is further observed that the bulk temperature fluctuation increases significantly and its PDF changes from exponential to Gaussian as Γ decreases. The influences of geometric confinement on the global heat transport are also investigated. The measured Nu-Ra relationship suggests that, as the lateral aspect ratio decreases, the relative weight of the boundary layer contribution in the global heat transport increases compared to that from the bulk. These results demonstrate that in the quasi-2-D geometry, geometric confinement has strong effects on both the global and local properties in turbulent convective flows, which are very different from the previous findings in 3-D and true 2-D systems.

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Section: Local Temperature Fluctuationsmentioning

confidence: 74%

Effects of geometric confinement in quasi-2-D turbulent Rayleigh–Bénard convection

Huang

Xia

2016

J. Fluid Mech.

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“…As the strengths of stabilization increase, the global transport behaviour undergoes a transition from a typical RB regime to the regime dominated by the stabilizing force. However, earlier studies in CRB [22,24], RRB [26,27] and DDC [30,35] separately revealed an intermediate regime with enhancement in scalar transport, and in CRB and DDC even the decoupling of scalar and mass transport was observed. Figures 1(a,b,c) show the normalized N u against 1/Γ for CRB, 1/Ro for RRB, and Λ for DDC, respectively.…”

mentioning

confidence: 69%

“…In CRB, it is the reciprocal of the width-to-height ratio 1/Γ that characterizes the relative strength of stabilizing [22][23][24]. In RRB, the stabilization is characterized by the ratio of Coriolis force to buoyancy which is the reciprocal Rossby number 1/Ro [25][26][27][28].…”

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confidence: 99%

Confined Rayleigh-Bénard, Rotating Rayleigh-Bénard, and Double Diffusive Convection: A Unifying View on Turbulent Transport Enhancement through Coherent Structure Manipulation

et al. 2017

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Many natural and engineering systems are simultaneously subjected to a driving force and a stabilizing force. The interplay between the two forces, especially for highly nonlinear systems such as fluid flow, often results in surprising features. Here we reveal such features in three different types of Rayleigh-Bénard (RB) convection, i.e. buoyancy-driven flow with the fluid density being affected by a scalar field. In the three cases different stabilizing forces are considered, namely (i) horizontal confinement, (ii) rotation around a vertical axis, and (iii) a second stabilizing scalar field. Despite the very different nature of the stabilizing forces and the corresponding equations of motion, at moderate strength we counterintuitively but consistently observe an enhancement in the flux, even though the flow motion is weaker than the original RB flow. The flux enhancement occurs in an intermediate regime in which the stabilizing force is strong enough to alter the flow structures in the bulk to a more organised morphology, yet not too strong to severely suppress the flow motions. Near the optimal transport enhancements all three systems exhibit a transition from a state in which the thermal boundary layer (BL) is nested inside the momentum BL to the one with the thermal BL being thicker than the momentum BL.

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“…In [22] by example of P r = 4.3 it was shown that the optimal aspect ratio Γ opt , which provides the maximal N u, exists for any Ra. The present results suggest that the optimal Γ opt also exists for any P r. This fact is very important for optimization of heat transport in practical applications.…”

Section: Numerical Methods and Simulation Parametersmentioning

confidence: 99%

Effect of Prandtl number on heat transport enhancement in Rayleigh-Bénard convection under geometrical confinement

et al. 2018

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We study, using direct numerical simulations, the effect of geometrical confinement on heat transport and flow structure in Rayleigh-Bénard convection in fluids with different Prandtl numbers.Our simulations span over two decades of Prandtl number P r, 0.1 ≤ P r ≤ 40, with the Rayleigh number Ra fixed at 10 8 . The width-to-height aspect ratio Γ spans between 0.025 and 0.25 while the length-to-height aspect ratio is fixed at one. We first find that for P r ≥ 0.5, geometrical confinement can lead to a significant enhancement in heat transport as characterized by the Nusselt number N u. For those cases, N u is maximal at a certain Γ = Γ opt . It is found that Γ opt exhibits a power-law relation with P r as Γ opt = 0.11P r −0.06 , and the maximal relative enhancement generally increases with P r over the explored parameter range. As opposed to the situation of P r ≥ 0.5, confinement-induced enhancement in N u is not realized for smaller values of P r, such as 0.1 and 0.2. The P r dependence of the heat transport enhancement can be understood in its relation to the coverage area of the thermal plumes over the thermal boundary layer (BL) where larger coverage is observed for larger P r due to a smaller thermal diffusivity. We further show that Γ opt is closely related to the crossing of thermal and momentum BLs, and find that N u declines sharply when the thickness ratio of the thermal and momentum BLs exceeds a certain value of about one. In addition, through examining the temporally averaged flow fields and 2D mode decomposition, it is found that for smaller P r the large-scale circulation is robust against the geometrical confinement of the convection cell.

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Condensation of Coherent Structures in Turbulent Flows

Cited by 73 publications

References 21 publications

Effects of geometric confinement in quasi-2-D turbulent Rayleigh–Bénard convection

Effects of geometric confinement in quasi-2-D turbulent Rayleigh–Bénard convection

Confined Rayleigh-Bénard, Rotating Rayleigh-Bénard, and Double Diffusive Convection: A Unifying View on Turbulent Transport Enhancement through Coherent Structure Manipulation

Effect of Prandtl number on heat transport enhancement in Rayleigh-Bénard convection under geometrical confinement

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