Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session M10: Convection and BuoyancyDriven Flows: Rotation 
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Chair: Stephan Weiss, University of Michigan  Ann Arbor Room: 110 
Tuesday, November 24, 2015 8:00AM  8:13AM 
M10.00001: Effect of weak rotation on the largescale circulation in turbulent convection with a Prandtl number $Pr = 12.3$ Ping Wei, Guenter Ahlers We report measurements of largescale circulation properties for highRayleighnumber convection in a rotating cylindrical sample with aspect ratio $\Gamma = D/L = 1.00$ (D is the diameter and L the height). The Prandtl number was $Pr = 12.3$. The measurements covered the Rayleighnumber range $2\times 10^{10} \leq Ra \leq 4\times10^{11}$ and the inverse Rossbynumber range $0 \leq 1/Ro \leq 1/Ro_c = 0.28$ where the LSC was present. The azimuthal orientation $\theta_0$ of the LSC circulation plane remained fixed in the frame of the rotating sample for $Ra < Ra_{0} \simeq 5\times10^{10}$. The sloshing motion of the LSC showed oscillations with a short time period $\tau^{pl}$ of several tens of seconds. The temperature amplitude $<\delta>$ of the LSC increased as $1/Ro$ approached $1/Ro_c$, and decreased rapidly beyond it. For $Ra > Ra_{0}$, the circulation plane underwent retrograde rotation and hence caused timeperiodic temperature oscillations near the side wall with a large period $\tau_{ac}$ of hundreds of seconds. Remarkably, $\tau_{ac}$ persisted without a discontinuity even for $1/Ro > 1/Ro_c$ where the LSC ceased to exist, indicating that vortex structures in that regime undergo the same retrograde rotation as the LSC. [Preview Abstract] 
Tuesday, November 24, 2015 8:13AM  8:26AM 
M10.00002: Rayleigh and Prandtlnumber dependence of the largescale flowstructure in weaklyrotating turbulent thermal convection Stephan Weiss, Ping Wei, Guenter Ahlers Turbulent thermal convection under rotation shows a remarkable variety of different flow states. The Nusselt number (Nu) at slow rotation rates (expressed as the dimensionless inverse Rossby number 1/Ro), for example, is not a monotonic function of 1/Ro. Different 1/Roranges can be observed with different slopes $\partial Nu/\partial (1/Ro)$. Some of these ranges are connected by sharp transitions where $\partial Nu/\partial(1/Ro)$ changes discontinuously. We investigate different regimes in cylindrical samples of aspect ratio $\Gamma=1$ by measuring temperatures at the sidewall of the sample for various Prandtl numbers in the range $3 < Pr < 35$ and Rayleigh numbers in the range of $10^8 < Ra < 4\times 10^{11}$. From these measurements we deduce changes of the flow structure. We learn about the stability and dynamics of the largescale circulation (LSC), as well as about its breakdown and the onset of vortex formation close to the top and bottom plate. We shall examine correlations between these measurements and changes in the heat transport. [Preview Abstract] 
Tuesday, November 24, 2015 8:26AM  8:39AM 
M10.00003: Tuning transitions in rotating RayleighB\'enard convection Pranav Joshi, Rudie Kunnen, Herman Clercx Turbulent rotating RayleighB\'enard convection, depending on the system parameters, exhibits multiple flow states and transitions between them. The present experimental study aims to control the transitions between the flow regimes, and hence the system heat transfer characteristics, by introducing particles in the flow. We inject nearneutrally buoyant silver coated hollow ceramic spheres ($\sim$100 micron diameter) and measure the system response, i.e. the Nusselt number, at different particle concentrations and rotation rates. Both for rotating and nonrotating cases, most of the particles settle on the top and bottom plates in a few hours following injection. This rapid settling may be a result of ``trapping'' of particles in the laminar boundary layers at the horizontal walls. These particle layers on the heattransfer surfaces reduce their effective conductivity, and consequently, lower the heat transfer rate. We calculate the effective system parameters by estimating, and accounting for, the temperature drop across the particle layers. Preliminary analysis suggests that the thermal resistance of the particle layers may affect the flow structure and delay the transition to the ``geostrophic'' regime. [Preview Abstract] 
Tuesday, November 24, 2015 8:39AM  8:52AM 
M10.00004: Combined effects of a magnetic field and a helical force on the onset of a rotating RayleighB\'{e}nard convection with freefree boundaries Jean Bio Chabi Orou, Gis\`ele Pomal\'egni We investigate the combined effects of rotation , magnetic field and helical force on the onset of stationary and oscillatory convection in a horizontal electrically conducting fluid layer heated from below with freefree boundary conditions. For this investigation the linear stability analysis studied by Chandrasekhar (1961) is used. We obtain the condition for the formation of a single large scale structure. In (Pomal\'{e}gni et al, 2014) it was shown the existence of a critical value S$_{\mathrm{cr}}$ of the intensity of the helical force for which the apparition of two cells at marginal stability for the oscillatory convection is obtained. Then, we have shown here how the increasing of parameter Ta influences this critical value of the helical force intensity. [Preview Abstract] 
Tuesday, November 24, 2015 8:52AM  9:05AM 
M10.00005: Energetic dynamics of a rotating horizontal convection model with wind forcing Varvara Zemskova, Brian White, Alberto Scotti We present a new test case for rotating horizontal convection, where the flow is driven by differential buoyancy forcing along a horizontal surface. This simple model is used to understand and quantify the influence of surface heating and cooling and wind stress on the Meridional Overturning Circulation. The domain is a rectangular basin with surface cooling at both ends (the poles) and surface warming in the middle (equatorial) region. To model the effect of the Antarctic Circumpolar Current, reentrant channel is placed near the Southern pole. Freeslip boundary conditions are imposed in the closed box, while zonally periodic boundary conditions are enforced in the channel. The problem is solved numerically using a 3D DNS model based on a finitevolume AMR solver for the Boussinesq NavierStokes equations with rotation. The relative contributions of surface buoyancy and wind forcing and the energetic balance are analyzed at a Rayleigh number of 10$^{8}$ and a relatively high aspect ratio of [5, 10, 1] in zonal, meridional and vertical directions, respectively. The overall dynamics, including largescale overturning, baroclinic eddying, and turbulent mixing are investigated using the local Available Potential Energy framework introduced in [Scotti and White, J. Fluid Mech., 2014]. [Preview Abstract] 

M10.00006: ABSTRACT WITHDRAWN 
Tuesday, November 24, 2015 9:18AM  9:31AM 
M10.00007: Transitions in turbulent rotating convection Hadi Rajaei, Kim Alards, Rudie Kunnen, Federico Toschi, Herman Clercx This study aims to explore the flow transition from one state to the other in rotating RayleighB\`{e}nard convection using Lagrangian acceleration statistics. 3D particle tracking velocimetry (3DPTV) is employed in a waterfilled cylindrical tank of equal height and diameter.~ The measurements are performed at the center and close to the top plate at a Rayleigh number Ra $=$ 1.28e9 and Prandtl number Pr $=$ 6.7 for different rotation rates. In parallel, direct numerical simulation (DNS) has been performed to provide detailed information on the boundary layers. We report the acceleration pdfs for different rotation rates and show how the transition from weakly to strongly rotating RayleighB\`{e}nard affects the acceleration pdfs in the bulk and boundary layers. We observe that the shapes of the acceleration PDFs as well as the isotropy in the cell center are largely unaffected while crossing the transition point. However, acceleration pdfs at the top show a clear change at the transition point. Using acceleration pdfs and DNS data, we show that the transition between turbulent states is actually a boundary layer transition between PrandtlBlasius type (typical of nonrotating convection) and Ekman type. [Preview Abstract] 
Tuesday, November 24, 2015 9:31AM  9:44AM 
M10.00008: Lagrangian analysis of rotating RayleighB\'{e}nard turbulence Kim Alards, Hadi Rajaei, Rudie Kunnen, Federico Toschi, Herman Clercx Transitions between turbulent states can occur in RayleighB\'{e}nard convection, for example, due to rotation, which is known to change the flow structure and the heat transport. In this study we want to characterize these different states of turbulence using Lagrangian statistics of tracer particles.~ RayleighB\'{e}nard convection is modeled using DNS and tracer particles that perfectly follow the flow are included. The fluid velocity and the temperature at the particle position are calculated using a linear interpolation scheme. Lagrangian statistics of 1e6 particles are measured in the form of velocity, acceleration and temperature pdfs for different rotation rates. The influence of rotation on the flow structure and heat transport is analyzed. Statistics obtained in the cell center and near the top and bottom plate are compared in order to investigate the influence of the boundary layers on RB convection. On top of that the results are compared with experiments, in which neutrally buoyant particles are tracked in a rotating cylindrical RB setup. A good agreement between experiments and numerics is found.~ [Preview Abstract] 
Tuesday, November 24, 2015 9:44AM  9:57AM 
M10.00009: Retrograde rotation of the largescale flow in turbulent rotating RayleighBenard convection with high Rossby number JinQiang Zhong, HuiMin Li, XueYing Wang We present measurements of the azimuthal orientation $\theta(t)$ of the largescale circulation (LSC) for turbulent RayleighB\'enard convection in the presence of week rotations ${\Omega}$. Linear retrograde rotations of the LSC circulating plane are observed over the entire Rossbynumber range (1${\le}Ro{\le}$300) studied. When the $Ro$ increases, the ratio of the retrograde rotation rate, $\gamma={\langle}\dot{\theta}{\rangle}/{\Omega}$ remains nearly a constant $0.12$ in the range of (1${\le}Ro{\le}$80) and starts to increases when $Ro>80$. When $Ro{\simeq}300$, $\gamma$ approaches a value of $0.36$ close to the prediction from previous theoretical models. In a background of linear rotations, erratic changes in $\theta(t)$ accompanied by decreasing in the LSC amplitude $\delta$ are observed. These small$\delta$ events give rise to the increasing $\gamma$ with very high Ro numbers (80${\le}Ro{\le}$300). In this range, the diffusivity of $\theta$ is proportional to $\delta^{2}$. Moreover, the occurrence frequency of the small$\delta$ events, and their average duration are independent on $Ro$. We propose a model to include additional viscous damping for the LSC azimuthal motion due to turbulent viscosity and provide theoretical interpretations of the experimental results. [Preview Abstract] 
Tuesday, November 24, 2015 9:57AM  10:10AM 
M10.00010: The G\"ottingen rotating turbulent RayleighB\'enard convection facility Eberhard Bodenschatz, Dennis van Gils, Xiaozhou He, Guenter Ahlers This presentation will focus on the newly commissioned rotating RBC facility at the Max Planck Institute for Dynamics and SelfOrganization (MPIDS). The MPIDS has a pressure vessel, called the Uboot of G\"ottingen, which can house different RBC cells. By pressurizing the Uboot with sulfur hexafluoride, nitrogen, or helium up to 19 bars one can obtain Rayleigh numbers spanning $10^9 < Ra < 10^{15}$, at nearly constant Prandtl numbers. Recently, a rotating table was constructed that can operate outside as well as in the Uboot, on top of which the current RBC cell of aspect ratio 0.50 can be installed. The accessible parameter space is $0.02 < Ro^{1} < 20$ for the inverse Rossby number and $10^{8} < Ek < 10^{3}$ for the Ekman number. At strong rotation (small Ek) but still turbulently convective (large Ra) one enters the geostrophic turbulent regime. Recent experiments involve measuring in and near this regime of which preliminary results will be shown and discussed. [Preview Abstract] 
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