Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session E11: Rotating Flows I: Rotating Convection |
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Chair: Antonio Rubio, University of Colorado Room: 26A |
Sunday, November 18, 2012 4:45PM - 4:58PM |
E11.00001: Generation of a large-scale barotropic circulation in rotating convection Antonio Rubio, Keith Julien, Jeffrey Weiss We recently reported on the existence of a slow-growing large scale barotropic mode in DNS of rotating Rayleigh-Benard convection using the non-hydrostatic balanced geostrophic equations (NHBGE) (Julien et al 2012). Such large scale modes had been previously observed as an inverse cascade in stable layer quasi-geostophic dynamics or via instability mechanisms of thermal Rossby waves occuring in presence of sloping endwalls (i.e quasi-geostrophic beta-convection). In this talk we report on the early time history of this large scale mode and discuss the generating physical mechanism as a ``symmetry-breaking'' forcing function of the barotropic vorticity equation. Impacts of the large scale barotropic mode on the smaller scale baroclinic components of the flow are detailed with a specific emphasis on the changing nature of the heat transport as the barotropic mode evolves. [Preview Abstract] |
Sunday, November 18, 2012 4:58PM - 5:11PM |
E11.00002: Acceleration PDFs of particles in rotating turbulent convection Herman Clercx, Prasad Perlekar, Valentina Lavezzo, Federico Toschi Particle dispersion in buoyancy-driven rotating turbulent flows has direct relevance for many industrial and environmental applications. We have used a Lattice Boltzmann Method coupled with Lagrangian particle tracking algorithm to investigate the behaviour of passive and inertial particles released in turbulent rotating Rayleigh-B\'{e}nard (RB) convection. The flow domain is horizontally periodic and vertically confined. Both the gravity and the rotation vector are oriented in the vertical direction. Here we present the results of the acceleration PDFs of particles in both non-rotating and strongly rotating RB convection. It is found that the bulk acceleration PDF in non-rotating RB turbulence is like in homogeneous isotropic turbulence whereas rotation introduces anisotropy similar to acceleration PDFs obtained from experiments in (isothermal) forced rotating turbulence [1]. These results and those obtained for inertial particles will be discussed.\\[4pt] [1] L. Del Castello and H.J.H. Clercx, Phys. Rev. Lett. \textbf{107}, 214502 (2011). [Preview Abstract] |
Sunday, November 18, 2012 5:11PM - 5:24PM |
E11.00003: Low rossby number heat transport in rotating Rayleigh-Benard convection Keith Julien, Antonio Rubio, Geoffrey Vasil, Edgar Knobloch Recent laboratory experiments of turbulent rotating Rayleigh-B\'enard convection, performed entirely within the regime of strong rotational constraint, have revealed a sharp transition in the scaling of the heat transport as a function of the thermal forcing. This is embodied by the nondimensional Nusselt-Rayleigh scaling law, $Nu\propto Ra^\alpha$, where a steep scaling regime $(\alpha>1)$ gives way to a comparatively shallower regime $(\alpha <1/2)$ typical of non-rotating turbulent convection. A crossover between the thermal and viscous boundary layers has been proposed as the root-cause of this remarkable result, yet a similar transition is found in the presence of stress-free boundary conditions where viscous layer boundary layers are absent. Unfortunately, the dynamics within the thermal boundary layer remain poorly understood due to resolution challenges at low Rossby number. Utilizing numerical simulations of the asymptotically exact nonhydrostatic balanced geostrophic equations we present an alternative explanation, not reliant on the form of the mechanical boundary conditions, but based on loss of geostrophic balance within the thermal boundary layers as a result of vigorous vortical motions. We show that the bottleneck for heat transport is the turbulent interior. [Preview Abstract] |
Sunday, November 18, 2012 5:24PM - 5:37PM |
E11.00004: Heat Transport and Local Temperature Measurements of Geostrophic Rotating Thermal Convection Robert Ecke, Scott Backhaus, Sridhar Balasubramanian Rotating Rayleigh-Benard convection is an idealized model of geophysical convective motions where buoyancy and rotation compete. The parameters governing such flows are the Rayleigh number $Ra$ proportional to $\Delta T$ across the cell height $h$, the Taylor number $Ta$ proportional to $\Omega^2$ where $\Omega$ is the angular rotation rate, and the Prandtl number $Pr$. In the turbulent state, experiments have demonstrated that normalized heat transport $Nu$ for the rotating state at small $Ro = \sqrt{Ra/(Pr Ta)}$ scales in the same manner as the non-rotating heat transport with a small enhancement of heat transport that depends on $Ra$ and $Pr$. We explore global heat transport and local temperature measured at multiple vertical positions along the cell center line of a square convection cell with aspect ratio $\Gamma=L/h\approx 4$ where $L$ is a lateral side and $h=12.1$ cm is the cell height. We focus on the Ra range $5 \times 10^6 < Ra < 5 \times 10^8$ for $5 \times 10^8 < Ta < 5 \times 10^{10}$ from onset up to the crossover to turbulent scaling where $Nu \sim Ra^{0.29}$. We report on the scaling of $Nu$ with $Ra$ at constant $Ta$ in that range and infer local convective structure from vertical spatial correlation of temperature fluctuations. [Preview Abstract] |
Sunday, November 18, 2012 5:37PM - 5:50PM |
E11.00005: Flow structure in turbulent rotating Rayleigh--B\'enard convection Rudie Kunnen, Yoann Corre, Herman Clercx Turbulent Rayleigh-B\'enard convection is usually studied in an upright cylinder. The addition of axial rotation has profound effects on the flow structuring. The well-known large-scale circulation (LSC) of the non-rotating case is still found at low rotation rates but is replaced by an irregular array of vertically aligned vortical plumes at higher rotation rates. We report PIV measurements of turbulent rotating convection in a cylindrical cell of diameter-to-height aspect ratio $\Gamma=1/2$ at Rayleigh number $Ra=4.5\times 10^9$ and at many rotation rates covering both the LSC and the vortical-plume regime. We focus on: (i) the azimuthal precession of the LSC, (ii) collective motions of the vortical plumes, and (iii) the sidewall boundary layers. With these results we can clarify remarkable differences between the $\Gamma=1$ and $\Gamma=1/2$ cases reported recently in the literature. [Preview Abstract] |
Sunday, November 18, 2012 5:50PM - 6:03PM |
E11.00006: DNS of turbulent co- and counterrotating Taylor Couette flow up to Re=30,000 Bruno Eckhardt, Hannes Brauckmann We study global and local torque fluctuations in turbulent Taylor-Couette flows for shear Reynolds numbers $Re_S$ up to $3\times10^4$ at various mean rotations for radius ratios $\eta=0.71$ and $0.5$ and $\Gamma=2$. Convergence of simulations is tested using three criteria of which the agreement of dissipation values estimated from the torque and from the volume dissipation rate turns out to be most demanding. The typical spatial distribution of the different convective and viscous contributions to the local current are identified and PDF's of local current fluctuations calculated. The results agree with experimental observations after an additional spatial average to account for finite resolution. Simulations realising the same shear $Re_S\ge2\times10^4$ show a maximum in torque for moderate counter-rotation. For lower values $Re_S\le4\times10^3$ the torque features a maximum for a stationary outer cylinder. For stronger counter rotation the flow develop intermittently fluctuating boundary layers near the outer cylinder. We demonstrate the phenomenon in direct numerical simulations and propose a theoretical model for the critical value in the rotation ratio that agrees well with the observations. [Preview Abstract] |
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