Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G24: Rayleigh-Benard Convection IV: Turbulence |
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Chair: Guenter Ahlers, University of California, Santa Barbara Room: 327 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G24.00001: ABSTRACT WITHDRAWN |
Monday, November 21, 2011 8:13AM - 8:26AM |
G24.00002: Wind velocities and their connection to boundary layer characteristics in turbulent Rayleigh-B\'{e}nard convection Sebastian Wagner, Olga Shishkina, Claus Wagner Highly resolved direct numerical simulations have been performed for turbulent Rayleigh-B\'{e}nard convection in a cylindrical container with aspect ratio unity and Prandtl number Pr=0.786. The Rayleigh numbers in the simulations are up to $10^9$. The calculated fields are reduced to the plane of the large scale circulation and analyzed with respect to the viscous and thermal boundary layer thickness and corresponding quantities in a statistical manner, i.e. probability density functions and their characteristics have been calculated a posteriori from instantaneous flow fields. Different methods to determine the boundary layer thickness are suggested and compared. The results are analyzed and compared with the Prandtl-Blasius theory of laminar boundary layers as well as the Grossmann-Lohse theory of states in Rayleigh-B\'{e}nard convection. Thereby fundamental relations between wind velocities and the generated wall shear stress are determined. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G24.00003: Local fluctuations and fluxes in turbulent Rayleigh-B\'{e}nard convection Rajaram Lakkaraju, Richard J.A.M. Stevens, Roberto Verzicco, Siegfried Grossmann, Chao Sun, Detlef Lohse We numerically investigate the local velocity and temperature fluctuations and the local heat flux in a cylindrical Rayleigh-Benard cell for Rayleigh numbers from Ra = 2 $\times $10$^{6 }$ to 2$\times $10$^{9}$ at a fixed Prandtl number Pr = 5.2. The numerical measurements at different points in the cell reveal that the heat flux at the plume dominated edge is larger and that it always has a power law relationship with \textit{Ra}, i.e. $j \quad \sim $ \textit{A Ra}$^{\gamma }$ . The scaling exponent linearly depends on the radial distance from the center, with = 0.44 at the center to = 0.27 at the edge, similarly has found by Shang, Tong and Xia, PRL 2008 and predicted by Grossmann and Lohse, Phys. Fluids 2004. These values imply a transition to bulk dominated heat flux beyond Ra = 10$^{15}$. The velocity and temperature fluctuations also have a power law dependence on \textit{Ra} and the corresponding scaling exponents again dependent on the radial position. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G24.00004: Studies of the boundary layer structure in turbulent Rayleigh-B\'{e}nard convection Joerg Schumacher, Nan Shi, Mohammad Emran The structure of the laminar boundary layer in turbulent Rayleigh-B\'{e}nard convection is studied by three-dimensional direct numerical simulations. We consider convection in a cylindrical cell at an aspect ratio of one for Rayleigh numbers of $3\times 10^9$ and $3\times 10^{10}$ and at a Prandtl number of $Pr=0.7$. The laminar boundary layers of the velocity and temperature fields are found to deviate from the classical prediction of the Prandtl-Blasius-Pohlhausen theory, even when a dynamical rescaling of the data with an instantaneously defined thickness scale is performed in a plane that is aligned with the instantaneous orientation of the large-scale wind. We show on the basis of the existing numerical data that none of the assumptions that enter this classical solution are satisfied. Three-dimensional structures are present and important, the large-scale wind changes direction and amplitude and the pressure gradients are found to fluctuate significantly. The local boundary layer structure is compared with perturbative expansions of the boundary layer equations of forced and natural convection. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G24.00005: Transport Velocities in the Convection Zone of the Sun Are Anomalously Weak Katepalli Sreenivasan, Shravan Hanasoge, Thomas Duvall, Jr. Understanding the dynamo action in the Sun, the process by which its magnetic fields are created and sustained, remains a major challenge. Our current understanding of interior convective turbulence, thought to play a critical role in regulating the dynamo, is derived solely from phenomenology and simulation, neither of which addresses the appropriate parameter regime. Here, we seismically constrain transport velocity magnitudes as a function of spherical-harmonic degree $\ell$ and depth in the convection zone to be 20 - 100 times lower than predictions from the current simulation methodologies and mixing length models. These bounds are obtained by isolating waves that propagate to specific depths and searching for statistical evidence for deviations in their travel times due to advection by convective flows. Turbulence on scales $\ell \le 60$ persists in a strongly rotationally-dominated regime of {\it geostrophy} since the Rossby numbers at $r/R_\odot = 0.96$, where $R_\odot$ is the solar radius, are less than $\sim 10^{-2}$. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G24.00006: On Unsteady Natural Convection Between Spherical Shells Yuri Feldman, Tim Colonius Natural convection between two concentric spheres is investigated with three-dimensional numerical simulations. Buoyancy is achieved by preserving a temperature difference between the internal hotter and the external colder boundaries of the spherical shell. The numerical simulations were performed for the two basic configurations characterized by external to internal radius ratios of 1.2 and 1.5. Slightly supercritical laminar regimes characterized by the Rayleigh numbers of order Ra $\sim $ $O$(10$^{4}$-10$^{5})$ were simulated by utilizing a Direct Numerical Simulation (DNS) approach while a Large Eddy Simulation (LES) was used for investigation of turbulent regimes for Ra $\sim O$(10$^{8}$-10$^{9})$. We discuss the topological characteristics of the both laminar and turbulent flows. One of the possible scenarios of steady-unsteady transition is proposed as well. Implications of the results for the design of a double-walled Montgolfiere aerobot for the exploration of Titan's atmosphere are discussed. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G24.00007: Turbulent Rayleigh-B\'enard convection of a nematic liquid crystal Stephan Weiss, Guenter Ahlers While studying Rayleigh-B\'enard convection (RBC) of Newtonian fluids is a challenging and interesting topic on its own, we want to expand this field by including anisotropic fluids such as liquid crystals. In these liquids certain fluid properties, such as for example the heat conductivity $\lambda$ and the diamagnetic susceptibility $\chi$, depend on the average orientation of the molecules (the director). We present results from experiments where the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) was used as a working fluid in turbulent RBC. Since $\chi$ is anisotropic, one can alter the director orientation, and thus $\lambda$, by applying a magnetic field $H $. Our measurements show that the effective heat conductivity $ \lambda_{eff}=q/\Delta T$ of the turbulent sample increases quadratically with $H$ ($\Delta T$ is the temperature difference between top and bottom plate and $q$ the corresponding heat flux). We suggest a simple model that considers the interplay between the director-aligning magnetic field and the vigorous fluid motion that randomize the director orientations inside the thermal boundary layers at the top and bottom plates. [Preview Abstract] |
Monday, November 21, 2011 9:31AM - 9:44AM |
G24.00008: Enhanced and Reduced Heat Transport in Turbulent Thermal Convection with Polymer Additives Ping Wei, Rui Ni, Ke-Qing Xia We present an experimental study of heat transport in turbulent Rayleigh-B\'{e}nard (RB) convection with polymer additives in convection cells both with smooth and rough surfaces on the top and bottom solid conductive plates. For the cell with smooth plates, a reduction of the measured Nusselt number (Nu) was observed. Furthermore, the amount of Nu- reduction increases with increasing polymer concentration ($c$), reaching $\sim 12\%$ for $c = 120$, ppm and an apparent saturation of the Nu-reduction thereafter. For the cell with rough plates, however, an enhancement of Nu of was observed when the polymer concentration is greater than $120$. [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G24.00009: Connecting flow structures and heat flux in turbulent Rayleigh-B\'enard convection Erwin van der Poel, Richard Stevens, Detlef Lohse The aspect ratio ($\Gamma$) dependence of the heat transfer (Nusselt number $Nu$ in dimensionless form) in turbulent (two- dimensional) Rayleigh-B\'enard convection is numerically studied in the regime $0.4 \le \Gamma \le 1.25$ for Rayleigh numbers $10^7 \le Ra \le Ra^{9}$ and Prandtl numbers $Pr =0.7 $ (gas) and $4.3$ (water). $Nu (\Gamma )$ shows a very rich structure with sudden jumps and sharp transitions. We connect these structures to the way the flow organizes itself in the sample and explain why the aspect ratio dependence of $Nu$ is more pronounced for small $Pr$. Even for fixed $\Gamma$ different turbulent states (with different resulting $Nu$) can exist, between which the flow can or cannot switch. In the latter case the heat transfer thus depends on the initial conditions. [Preview Abstract] |
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