Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R19: Convection and Buoyancy-Driven Flows: Heat Transfer and Boundary Layers |
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Chair: Romain Volk, Ens de Lyon Room: 2006 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R19.00001: Melting dynamics of large ice balls in a turbulent flow Romain Volk, Nathanael Machicoane We study the melting dynamics of large ice balls in a turbulent flow at very high Reynolds number. Using an optical shadowgraphy setup, we record the time evolution of particle sizes from which we deduce the heat transfert coefficient as a function of the particle Reynolds number $\textit{Re}_{D}$. We study three cases: fixed ice balls melting in a region of strong turbulence with zero mean flow, fixed ice balls melting under the action of a strong mean flow with lower fluctuations, and ice balls freely advected in the whole flow. For the fixed particles cases, heat transfer is observed to be much stronger than in laminar flows, the Nusselt number behaving as a power law of the Reynolds number\string: $\textit{Nu} \propto\textit{Re}_{D}^{0.8}$. For freely advected ice balls, the turbulent transfer is further enhanced and the Nusselt number is proportional to the Reynolds number $\textit{Nu} \propto\textit{Re}_{D}$. The surface heat flux is then independent of the particles size, leading to an ultimate regime of heat transfer reached when the thermal boundary layer is fully turbulent. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R19.00002: Convection-driven shear and decreased heat flux David Goluskin, Hans Johnston, Glenn R. Flierl, Edward A. Spiegel When Rayleigh-B\'enard convection is simulated in a horizontally periodic domain, there may spontaneously arise horizontal mean flow that is vertically sheared. We simulate such ``shearing convection'' in conditions that are especially conducive to its formation---a 2D domain with free-slip top and bottom boundaries---for Prandtl numbers between 1 and 10 and Rayleigh numbers ($Ra$) up to $10^{10}$. At sufficiently large $Ra$, the horizontal mean flow is very strong in our simulations, accounting for over 99\% of the fluid's total kinetic energy. Vertical heat transport, quantified by the Nusselt number, is greatly depressed by this mean flow. As $Ra$ is raised, the Nusselt number grows much more slowly than in ordinary (non-shearing) Rayleigh-B\'enard convection and can even decrease. Shearing convection can assume two very different forms, depending mainly on the Prandtl number; the convective heat transport can be either significant at all times or significant only during bursts. We mention an analogy with tokamak plasmas and suggest ways to produce shearing convection in the laboratory. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R19.00003: Numerical investigation of heat transfer enhancement of natural convection in a square cavity filled with a Water-CuO nanofluid Jose Nunez Gonzalez, Alberto Beltran We presents a numerical study of natural convection heat transfer in a square cavity filled a water-CuO nanofluid. The governing equations for natural convection are solved numerically with a Chebyshev pseudo spectral method using and projection method as a decoupling strategy. The Nusselt number is determined as a function of Rayleigh number and the solid volume fraction. The high conductivity of the CuO nanoparticles modifies the overall thermal conductivity of the fluid, even with a decrement of the Nusselt number the effective thermal conductivity increase therefore higher heat transfer rate is obtained with the numerical model. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R19.00004: Enhanced heat transport in partitioned thermal convection Yun Bao, J. Chen, Bo-Fang Liu, Zhen-Su She, Jun Zhang, Quan Zhou Enhancing heat transport across a fluid layer is of fundamental interest as well as of great technological importance. For decades, Rayleigh-B\'{e}nard convection, i.e. the motion of a fluid layer that is heated from below and cooled from above, has been a paradigm for the study of convective heat transport, and how to improve its overall heat-transfer efficiency is still an open question. Here we report an experimental and numerical study that reveals a novel mechanism that leads to dramatically enhanced heat transport. When vertical partitions are inserted into a convection cell with thin gaps left open between partition walls and the cooling/heating plates, it is found that the convective flow becomes much more coherent and self-organized, leading to a dramatically enhanced heat transport of up to 2.3 times that without any partitions. We expect that this surprising effect will lead to broad applications. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R19.00005: Boiling Rayleigh-Benard flow Daniela Narezo Guzman, Yanbo Xie, Songyue Chen, Guenter Ahlers, Chao Sun, Detlef Lohse We report on heat transport due to boiling of Novec7000 (1-methoxyheptafluo-ropropane) at the bottom plate of a turbulent Rayleigh-B\'enard sample filled with liquid (except for small vapor bubbles when boiling took place). The top surfaces of the bottom plates were silicon wafers etched with a triangular lattice of 30 $\mu$m diameter and 100 $\mu$m deep cavities. The lattice spacing was different for each wafer (100 $\mu$m and 1 mm). The plate diameter and sample height both were 10 cm. Only a central bottom-plate area of 2.5 cm diameter was heated. When the cavities were activated (deactivated) by assuring that they were filled by vapor (liquid), then they nucleated (did not nucleate) bubble formation for bottom-plate temperatures $T_b$ larger than the boiling point. Results of the heat transport as a function of $T_b$ with a fixed applied temperature difference $\Delta T = T_b - T_t = 20$K and $\Delta T = 15$K ($T_t$ is the top plate temperature) will be reported. The effective conductivity of the 2-phase flow was enhanced relative to the supersaturated single-phase system by up to 40\%. Direct visualization of the boiling surface showed that larger (smaller) spacing lead to weak (strong) interaction between neighboring sites, which was determining for bubble departure size. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R19.00006: Optimum heat transport by coherent Rayleigh-B\'enard convection Fabian Waleffe, Anakewit Boonkasame, Leslie Smith A classic marginal boundary layer argument suggests that the heat transport $Nu$ scales like $Ra^{1/3}$ while upper bound theories give that $Nu$ is at most $\sim Ra^{1/2}$, where the Rayleigh number $Ra$ is the main control parameter for Rayleigh-B\'enard convection. Turbulent data have shown various scalings between $Nu\sim Ra^{2/7}$ and $Ra^{1/3}$, depending on domain aspect ratio and various corrections. Here, we investigate \emph{coherent} solutions of the Boussineq equations for the Rayleigh-B\'enard problem with no-slip boundary conditions and Prandtl number 7. The primary solution that bifurcates from the conduction state at $Ra \approx 1708$ has been calculated up to $Ra\approx 4.\, 10^6$ and shows $Nu \sim Ra^{1/4}$ with a delicate spiral structure. A related solution that maximizes $Nu$, at least locally, has been calculated up to $Ra=10^9$ and it scales as $Nu -1 \sim 0.12\, Ra^{0.31}$ for $10^7 < Ra < 10^9$, quite similar to turbulent data. This is a simple yet multi-scale coherent solution whose horizontal wavelength is $\sim Ra^{-0.22}$ in that range. It is unstable to larger scale perturbations and in particular to mean flows, yet it appears to be relevant as a backbone for turbulent solutions, in particular, setting the scale and strength of elemental plumes. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R19.00007: Turbulence Structure near the Hot Plate in Turbulent Natural Convection Vipin Koothur, Baburaj Puthenveettil We obtain the velocity field in a plane parallel to the hot plate in turbulent natural convection for $10^6\leq Ra_w \leq 10^9$ using Stereo PIV. The plane of measurement is inside the velocity boundary layer, estimated from natural convection boundary layer equations as well as inside the velocity boundary layer due to the large scale flow. We extract the line plumes from these velocity field and study the nature of boundary layer and the velocity statistics of these line plumes. We study the turbulent statistics from these velocity fields and show that at higher wavenumber the energy spectrum shows a Bolgiano-Obukhov $k^{-11/5}$ scaling at all $Ra_w$ considered. At lower wavenumbers, the energy spectrum scales approximately as $k^{-1.35}$ for $10^6\leq Ra_w \leq 10^8$ and $k^{-1}$ at $Ra_w =10^9$. The crossover lengthscales obeys the same power law dependence as the mean plume spacing on the near wall lengthscale, $Z_w$.\footnote{Puthenveettil et al, \textbf{J. Fluid Mech.} 685: 335-364.} [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R19.00008: Local boundary layer scales in turbulent Rayleigh-Benard convection Janet Scheel, Joerg Schumacher A method is presented for computing fully local boundary layer scales for the velocity and temperature fields obtained from simulations of three-dimensional turbulent Rayleigh-Benard convection. These local boundary layer scales reflect the strong spatial inhomogeneities of both boundary layers due to the large-scale, but complex and intermittent, circulation that builds up in closed convection cells. The statistics of the local boundary layer scales are discussed as well as the scaling of mean boundary layer thicknesses, the resulting shear Reynolds number and the friction coefficient with respect to Rayleigh number. Additionally, an analysis of the recently suggested dissipation layer thickness scales versus Rayleigh number is conducted. All investigations are based on highly accurate spectral element simulations which reproduce gradients and their fluctuations reliably. The study is done for a Prandtl number of 0.7 and for Rayleigh numbers which extend over nearly five orders of magnitude, $3\times 10^5\le Ra \le 10^{10}$ in cells of aspect ratio of one. One study of aspect ratio equal to three is also performed in the case of $Ra=10^8$. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R19.00009: Influence of roughness on boundary layer structure and large scale circulation in turbulent thermal convection Olivier Liot, Quentin Ehlinger, Thibaut Coudarchet, Julien Salort, Eleonore Rusaouen, Bernard Castaing, Francesca Chilla With sufficient forcing, measured by the Rayleigh number, Rayleigh-B\'enard convection becomes turbulent. Influence of controlled roughness on the heated bottom plate has been studied from a thermal point of view [1]: a regime transition has been observed corresponding to an increase of the heat flux compared to a smooth plate. With a parallelepipedic convection cell where mean flow can be considered as bi-dimensional and where controlled square-studs roughness have been added, we performed PIV measurements to visualize both large scale circulation and boundary layer close to roughness. We work at Rayleigh number from $10^{9}$ to $10^{10}$, either side of the transition. We show that the boundary layer is thinner above the rougness than between row of square-studs, and a dramatic change of flow structure is observed between rougness. It is in good agreement with previous temperature measurements [2] and brings an explanation to the heat flux increase. Moreover, for large scale circulation, turbulence structure changes: velocity r.m.s. is higher than in the smooth case and presents a large dissymetry.\\[4pt] [1] Tisserand et al., \emph{Physics Of Fluids}, \textbf{23}, 015105 (2011)\\[0pt] [2] Salort et al., \emph{Physics Of Fluids}, \textbf{26}, 015112 (2014) [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R19.00010: Turbulent convection against rough walls: Manipulating a broken symmetry Srikanth Toppaladoddi, Sauro Succi, John Wettlaufer We present results from well resolved numerical simulations of turbulent convection in a cell with rough walls in two dimensions. In order to examine hypotheses regarding the interaction between the boundary layers and the interior of the flow, we study classical Rayleigh-Benard convection with the top plate having a sinusoidal roughness distribution. The amplitude of the roughness is such that it is always larger than the thermal boundary layer thickness for all Rayleigh numbers ($Ra$) considered here. The lattice Boltzmann method is used to model the Navier-Stokes and heat transport equations within the Boussinesq approximation. We observe a scaling law, for the Nusselt number ($Nu$), $Nu \sim Ra^{1/3}$ over three decades in $Ra$, from $Ra = 10^6$ to $Ra = 10^9$, at a Prandtl number of $1$. The scaling law obtained is in good agreement with recent experiments. We discuss the effects of the additional top-down broken symmetry on the mean temperature in the core-flow region, the plumes generated at the rough surface, and the two-point temperature correlation function at different heights. It is found that the correlation length scales as the wavelength of the roughness distribution. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R19.00011: Viscous boundary layers in turbulent Rayleigh-B\'enard convection at low Prandtl number Ronald du Puits, Christian Resagk, Willert Christian We present time-resolved Particle Image Velocimetry measurements of the flow adjacent to the horizontal plates in turbulent Rayleigh-B\'enard convection (RBC) for the Rayleigh number $Ra=1.4\times 10^{10}$ and the Prandtl number $Pr=0.7$. The measurements have been undertaken in a large-scale RB experiment 7.15 m in diameter and 6.30 m in height which is called the ``Barrel of Ilmenau.'' They give detailed insight into the near-wall flow field in turbulent RB convection and provide experimental data to evaluate various competing theories on the heat transport. We characterize the flow field by analyzing typical quantities like the mean velocity profile and its fluctuations, the spatial and temporal evolution of the vorticity inside the boundary as well as the wall shear stress and its correlation with the outer flow. We will also show that the convective boundary layer becomes turbulent locally and temporarily although its shear Reynolds number $Re_s=U_{\infty}\delta /\nu \approx 265$ ($U_{\infty}$ - outer velocity, $\delta$ - boundary layer thickness, $\nu$ - kinematic viscosity) is considerably smaller than the value 420 underlying existing phenomenological theories. [Preview Abstract] |
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