Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L33: Turbulent Convection II 
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Chair: Charles Doering, University of Michigan Room: Georgia World Congress Center B405 
Monday, November 19, 2018 4:05PM  4:18PM 
L33.00001: Absence of evidence for transition to the 'ultimate' regime of RayleighBénard convection Charles R. Doering, John S. Wettlaufer, Srikanth Toppaladoddi There are several distinct predictions for the asymptotic behavior of heat transport Nu as the Rayleigh number Ra_{ }→_{ }∞ in thermal turbulence described by the fundamental model of RayleighBénard convection. One is Nu = O(Ra^{1/3}) and another is the socalled 'ultimate' scaling Nu = O(Ra^{1/2}) possibly modified by logarithmic corrections to Ra^{1/2}/(log Ra)^{3/2} or Ra^{1/2}/(log Ra)^{2}. Recently reported experimental and direct numerical simulation measurements of Nu as a function of Ra have been cited as evidence of transition to the 'ultimate' state. In this talk, without questioning the veracity of the NuRa data reported therein, we show that the data do not support the claims. 
Monday, November 19, 2018 4:18PM  4:31PM 
L33.00002: Investigation of Transition to Turbulence in Compressible Natural Convection Flow along a Vertical Heated Plate using Direct Numerical Simulation Deboprasad Talukdar, ChungGang Li, Makoto Tsubokura The report focuses on the flow dynamics involved in transitional phenomena generated under the sole action of buoyancy forces for a spatially developing compressible turbulent natural convection flow for air along a vertical flat surface heated with uniform heat flux using direct numerical simulation. No external perturbations are imposed on the flow field to generate transition to turbulence. Mean velocity and temperature profile distribution across boundary layer along with growth of corresponding boundary layer are measured. Velocity transition immediately follows thermal transition. Velocity and temperature fluctuations across the boundary layer are measured. Predominant frequencies of the disturbances are determined through FFT analysis. Beginning of transition happens around 157 W/m and end of transition at 226 W/m. Visualization of instantaneous flow field performed shows a very natural transition to turbulent. 
Monday, November 19, 2018 4:31PM  4:44PM 
L33.00003: Flow topology transition via global bifurcation in turbulent RayleighBenard convection YiChao Xie, GuangYu Ding, KeQing Xia We present experimental observation of a spontaneous symmetry breaking induced transition in turbulent RayleighBenard convection. The experiment was carried out in a vertical annulus convection cell with the aspect ratio Γ=H/(2R_{o})=1.87 and the radius ratio γ=R_{i}/R_{o}=0.88. Here H=100.0 mm is the height of the cell, R_{i}=84.5 mm and R_{o}=96.0 mm are the radius of the inner and outer sidewalls, respectively. Simultaneous measurements of the heat transport, i.e., the Nusselt number, and the topology of the LSF were performed. It is found that the heat transport shows a sharp transition with the increase of the Rayleigh number Ra in the range of 6.0×10^{7}≤ Ra ≤ 1.3×10^{9} and at fixed Prandtl number Pr=5.4. Analysis of the LSF structure suggests that the transition is resulted from a global bifurcation of its pattern from a highsymmetry quadruple structure (QS) to a lowsymmetry dipole structure (DS). In the transition zone spanned about half a decade in Ra, the system switches stochastically between the two metastable QS and DS states [1].[1]Xie, Ding and Xia, Phys. Rev. Lett. 120, 214501 (2018) 
Monday, November 19, 2018 4:44PM  4:57PM 
L33.00004: Turbulent superstructures in RayleighBénard convection detected by deep learning Enrico Fonda, Ambrish Pandey, Joerg Schumacher, Katepalli Raju Sreenivasan Turbulent convection in nature comprises vortices and plumes on many time and length scales that tend to assemble to gradually evolving largescale patterns which are termed turbulent superstructures. These patterns appear in the form of ridgelike contours of hot upwelling and cold downwelling fluid. To detect them by machine learning techniques, we use a fully convolutional deep neural network that generates precise image segmentation from relatively small training sets. The neural network reduces the turbulent superstructures in an extended threedimensional RayleighBénard convection layer to a planar network that connects defect points of the superstructure patterns. The dynamics of the network is manifest by the creation and annihilation of defects which determine the network topology. We estimate the fraction of heat transported across the network for varying Rayleigh numbers but fixed Prandtl number, and find the fraction to be significant. 
Monday, November 19, 2018 4:57PM  5:10PM 
L33.00005: Turbulent thermal convection over fractallike multiscale roughness Xiaojue Zhu, Richard Stevens, Roberto Verzicco, Detlef Lohse In turbulent RayleighBénard convection, roughness is often used as a means to enhance heat transport. Yet the community is still debating on whether the Nusselt vs Rayleigh number scaling exponent (Nu∼Ra^{β}) increases or remains unchanged. In our previous study we have shown that for periodic roughness of the same size, the scaling exponent β can reach up to 1/2 for one decade in Ra, and then saturates back to a value close to the smooth wall case. In contrast, we now investigate RayleighBénard convection with surface roughness of three length scales. We find that the range of Ra in which the 1/2 exponent manifests can be extended substantially. The reason is that with multiscale roughness, the state where a thin thermal boundary layer is uniformly distributed along the rough surfaces is very much delayed and can only be achieved at very high Ra.

Monday, November 19, 2018 5:10PM  5:23PM 
L33.00006: Transition of the Flow Reversal in Turbulent Thermal Convection Xin Chen, Shidi Huang, Keqing Xia, Hengdong Xi We present an experimental study on the flow reversals in quasi2D turbulent RayleighBenard convection. It is found the reversal rate decreases with Rayleigh number (Ra) and there is a transitional Ra, which separates the slow decease and fast decease regime of the reversal rate. Our velocity measurements show that for low Ra the interior of the main roll of the flow has already breaks into two small rolls although they are still enclosed by the envelope of the main roll, which is the reason for the frequent reversals for low Ra. In addition we decomposed the instantaneous flow field into Fourier modes and defined the stability of the singleroll mode, found that the stability of the singleroll mode experiences a slow increase followed by a fast increase. We believe this transition of the singleroll stability is responsible for the transition of the reversal rate. We also studied the Pr dependence of the transition and successfully revealed why reversal happens only in a restricted parameter ranges. 
Monday, November 19, 2018 5:23PM  5:36PM 
L33.00007: Mechanism of largescale flow reversals in turbulent thermal convection Yin Wang, PikYin Lai, Penger Tong It is commonly believed that heat flux passing through a closed thermal convection system is balanced so that the convection system can remain at a steady state. Here we report a new kind of convective instability for turbulent thermal convection, in which the convective flow stays over a long steady ``quiet period" having a minute amount of heat accumulation in the convection cell, followed by a short and intermittent ``active period" with a massive eruption of thermal plumes to release the accumulated heat. The rare massive eruption of thermal plumes disrupts the existing largescale circulation across the cell and resets its rotational direction. A careful analysis reveals that the distribution of the plume eruption amplitude follows the generalized extreme value statistics with an upper bound, which changes with the fluid properties of the convecting medium. The experimental findings have important implications to many closed convection systems of geophysical scale, in which massive eruptions and sudden changes in largescale flow pattern are often observed. 
Monday, November 19, 2018 5:36PM  5:49PM 
L33.00008: Effects of boundary asymmetry in turbulent RayleighBenard convection KeQing Xia, YiChao Xie, BuYingChao Cheng We present study of the effects of boundary asymmetry in turbulent RayleighBenard convection. This asymmetry was introduced by using roughness elements with different roughness aspect ratios λ on the top and the bottom plates, i.e., λ_{t}=1.0 (top plate) and λ_{b}=4.0 (bottom plate). The experiment was carried out in a cylindrical cell with an aspect ratio 1 and a diameter of 19.2 cm. The height of the roughness elements was 8 mm. The Rayleigh number Ra was in the range of 3.5×10^{8}≤Ra≤ 8.2×10^{9} and the Prandtl number was Pr=4.34. It is found that the scaling exponents of the Nusselt number and the Reynolds number with Ra in the asymmetric cell is larger than those obtained in a symmetric cell with λ_{t}=λ_{b}=1.0 and smaller than those obtained in a symmetric cell with λ_{t}=λ_{b}=4.0. Measurement of the temperature profile along the cell's centerline suggests the boundary asymmetry not only changes the transport properties, but also leads to deviation of the mean temperature T_{bulk} from the averaged temperature of the top and bottom plate T_{m}. In addition, this deviation is Radependent. The change of T_{bulk }might be due to the different plume dynamics at the asymmetric boundaries with increasing Ra. 
Monday, November 19, 2018 5:49PM  6:02PM 
L33.00009: Radiatively heated thermal convection: bypassing the boundary layers to achieve the « ultimate » scaling regime. Simon Lepot, Sébastien Aumaître, Basile Gallet The heat flux transported by thermal convection has important implications for geophysical, astrophysical and industrial flows: one seeks a powerlaw relation between the convective heat flux and the internal temperature gradients. Decades of investigations of the RayleighBénard (RB) setup indicate that the heat transport is strongly restricted by boundary layers near the hot and cold solid plates. This prevents the observation of the “ultimate” scalingregime of thermal convection, where bulk turbulence controls the convective heat flux independently of molecular viscosity and thermal diffusivity. In contrast with the RB setup, many geophysical and astrophysical convective flows are driven by radiation: absorption of incoming light by a body of fluid induces local heating. We have developed a laboratory experiment that reproduces such radiative heating: heat is input radiatively, directly inside the bulk turbulent flow and away from the boundary layers. We will provide experimental evidence that this naturally leads to the ultimate regime of thermal convection. These experimental results are confirmed quantitatively by Direct Numerical Simulations. 
Monday, November 19, 2018 6:02PM  6:15PM 
L33.00010: Experimental investigation of heat transport in inhomogeneous bubbly flow OnYu Dung, Biljana Gvozdić, Elise Alméras, Dennis P.M. Van Gils, Sander Huisman, Chao Sun, Detlef Lohse In this study the heat transport in inhomogeneous bubbly flow is experimentally investigated. The experiments were performed in a rectangular bubble column heated from one side wall and cooled from the other, with millimetric bubbles introduced through one half of the injection section (close to the hot wall or the cold wall). We characterise the global heat transport while varying two parameters: the gas volume fraction α =0.4%−5.1%, and the Rayleigh number Ra_{H} =4×10^9 − 2.2×10^10. Different flow regimes occur with increasing gas flow rates. In the generated inhomogeneous bubbly flow there are three main contributions to the mixing: (i) transport by the buoyancy driven recirculation, (ii) bubble induced turbulence and (iii) shearinduced turbulence (SIT). The strength of these contributions and their interplay depends on α which is reflected in the measured heat transport enhancement. Comparing with the case of homogeneous bubbly flow, for α<4%, inhomogeneous bubbly injection results in better heat transport due to induced largescale circulation. In contrast, for α>4%, when the contribution of SIT becomes stronger, but so does the competition between all three contributions, the homogeneous injection is more efficient. 
Monday, November 19, 2018 6:15PM  6:28PM 
L33.00011: Abstract Withdrawn

Monday, November 19, 2018 6:28PM  6:41PM 
L33.00012: Unusual aspects of solar convection Katepalli Raju Sreenivasan, Joerg Schumacher This talk will be about turbulent convection in the Sun, which is the dominant mode of heat transport outwards of 70% of its radius. Given the dimensions of the Sun (radius = 7 x 10^{8} m), all decent estimates of flow parameters such as the Rayleigh and Reynolds numbers suggest that the convective flow is highly turbulent. But the medium is severely stratified (many orders of magnitude variation in density), the Prandtl numbers are very low (10^{}^{6} or lower), the deviation from adiabaticity is miniscule, etc. It is hard to assess the effects of all these unusual features with certainty but an attempt will be made. After discussing the standard solar model which gives us an idea of the Sun’s basic unperturbed state, we discuss some features of its extraordinarily complex and rich structure in the convection zone. 
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