Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session M10: Convection and Buoyancy Driven Flows: Confined Flows |
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Sponsoring Units: DFD GPC Chair: Elizabeth Camp, Portland State University Room: B118-119 |
Tuesday, November 22, 2016 8:00AM - 8:13AM |
M10.00001: Stratified shear flow in an inclined duct: near-instantaneous 3D velocity and density measurements Jamie Partridge, Adrien Lefauve, Stuart Dalziel, Paul Linden We present results from a new experimental setup to study the exchange flow in an inclined square duct between two reservoirs containing fluids of different densities. This system can exhibit stratified shear wave motions, and has a distinct parameter threshold above which turbulence is triggered and progressively fills a larger fraction of the duct. To probe these intrinsically 3D flows, we introduce a new setup in which a traversing laser sheet allows us to obtain near-instantaneous 3D velocity and density fields. Three components of velocity are measured on successive 2D planes using stereo particle image velocimetry (PIV) with density information obtained simultaneously using laser induced fluorescence (LIF). [Preview Abstract] |
Tuesday, November 22, 2016 8:13AM - 8:26AM |
M10.00002: Stratified shear flow in an inclined duct: coherent structures and mixing Adrien Lefauve, Jamie Partridge, Stuart Dalziel, Paul Linden We present laboratory experiments on the exchange flow in an inclined square duct connecting two reservoirs at different densities. This system generates and maintains a stratified shear flow, which can be laminar, wavy or turbulent depending on the density difference and inclination angle. It is believed that the mean dissipation is set by the angle, and that high buoyancy Reynolds numbers (i.e. turbulent intensity) can be maintained, making this system suited for the study of continuously forced stratified turbulence. The talk will focus on the analysis of time-resolved, near-instantaneous 3D velocity and density data obtained by stereo particle image velocimetry (PIV) and laser induced fluorescence (LIF). This data allow for the visualisation of 3D coherent structures as well as turbulent mixing properties, which are key in understanding the dynamics of stratified turbulence. [Preview Abstract] |
Tuesday, November 22, 2016 8:26AM - 8:39AM |
M10.00003: Harmonic Forcing on the Stratified Square Lid Driven Cavity Jason Yalim, Bruno Welfert, Juan Lopez, Stephanie Taylor Stratified fluids that are driven at an interface, such as oceans or seas, can be periodically driven by wind. As a canonical flow, the square lid driven cavity with a harmonic forcing and a linear temperature gradient serves as a idealized model. Resonances of the harmonic forcing with the internal modes of the system aide energy transfer from the surface to the bulk, leading to interesting dynamics. Using a numerical spectral collocation method, the internal waves of the system are investigated, including their possible interaction and annihilation. [Preview Abstract] |
Tuesday, November 22, 2016 8:39AM - 8:52AM |
M10.00004: Numerical investigation of the onset of centrifugal buoyancy in a rotating cavity Diogo B. Pitz, Olaf Marxen, John Chew Buoyancy-induced flows in a differentially heated rotating annulus present a multitude of dynamics when control parameters such as rotation rate, temperature difference and Prandtl number are varied. Whilst most of the work in this area has been motivated by applications involving geophysics, the problem of buoyancy-induced convection in rotating systems is also relevant in industrial applications such as the flow between rotating disks of turbomachinery internal air systems, in which buoyancy plays a major role and poses a challenge to accurately predict temperature distributions and heat transfer rates. In such applications the rotational speeds involved are very large, so that the centrifugal accelerations induced are much higher than gravity. In this work we perform direct numerical simulations and linear stability analysis of flow induced by centrifugal buoyancy in a sealed rotating annulus of finite gap with flat end-walls, using a canonical setup representative of an internal air system rotating cavity. The analysis focuses on the behaviour of small-amplitude disturbances added to the base flow, and how those affect the onset of Rossby waves and, ultimately, the transition to a fully turbulent state where convection columns no longer have a well-defined structure. [Preview Abstract] |
Tuesday, November 22, 2016 8:52AM - 9:05AM |
M10.00005: The oscillation modes of large-scale circulation in turbulent Rayleigh-Bénard convection in a cubic container Dandan Ji, Kunlun Bai, Eric Brown We present measurement of the large-scale circulation (LSC) of turbulent Rayleigh- Bénard convection of a cubic cell. We measured the LSC orientation angle $\theta_0$ and off-center displacement angle $\alpha$. We found the LSC oscillates around one corner. The oscillation frequency matches the turnover time and the natural frequency based on the geometry of the cell predicted by the stochastic model presented by Brown and Ahlers (Phys. Fluids, 2008), however the model with advection which was used to predict oscillations in cylinders predicts the system is over-damped. The structure of the LSC breaks the symmetry of the cube. [Preview Abstract] |
Tuesday, November 22, 2016 9:05AM - 9:18AM |
M10.00006: Statistical convergence and the effect of large-scale motions on turbulent Rayleigh-Bénard convection in a cylindrical domain with 6.3 aspect ratio Philip Sakievich, Yulia Peet, Ronald Adrian At high Rayleigh numbers in moderate aspect-ratio cylindrical domains turbulent Rayleigh-Bénard convection (RBC) exhibits coherent large-scale motions with patterns like some of those found in laminar flow. In this work we show how the patterns of the largest scales in turbulent RBC affect the bias and convergence of the flow statistics at aspect-ratio 6.3 (diameter/ height). Large scale motions influence two of the finite-time statistical mean’s inherent properties: 1) the orientation of the patterns changes so slowly that it may appear almost fixed during a finite averaging time interval, thereby imbedding a preferred azimuthal direction in the sampled data; 2) they also have at least two states associated with the occurrence of up and down motions near the center of the convection cell. We will present a novel technique for triggering additional states of RBC in DNS simulations that are targeted for improving the statistical convergence of the flow. This technique gently perturbs the flow so that the new variations of the large scale patterns can be sampled. [Preview Abstract] |
Tuesday, November 22, 2016 9:18AM - 9:31AM |
M10.00007: Aspect-ratio dependence of the large-scale circulation in Rayleigh-B\'{e}nard convection with weak rotation Guenter Ahlers, Ping Wei We report measurements for slowly rotating turbulent thermal convection in cylindrical samples with aspect ratios $\Gamma=1.0$ and $2.0$ for a Prandtl number $Pr = 12.3$. The results are for the large-scale circulation (LSC) strength $\delta$, Fourier-energy E$_{tot}$, and relative flow strength S, as well as for two Reynolds numbers $Re_{ret}$ and $Re_{sl}$, for the Nusselt number $Nu$, and for the vertical temperature gradient $\partial \Theta/\partial z$ at the sample center. They cover the Rayleigh-number range $3\times10^{10} \leq Ra \leq 4\times 10^{11}$ and the inverse Rossby-number range $0 \leq 1/Ro \leq 1/Ro_{c}$. $Nu$, E$_{tot}$, S, and $\partial \Theta/\partial z$ showed sharp transitions at $1/Ro_c$. The LSC underwent retrograde rotation with period $\tau_{ret}$ and showed sloshing oscillations with period $\tau_{sl} << \tau_{ret}$. At constant $Ra$ and $1/Ro$ $\delta$ grew and decayed with a period equal to $\tau_{ret}$. We found that $Re_{ret} \equiv 4 L^2 / \tau_{ret} \nu \propto Ra^{0.65}$ ($\propto Ra^{0.50}$) for $\Gamma = 1.0$ ($\Gamma = 2.0$) ($\nu$ is the kinematic viscosity and $L$ the sample height) and $Re_{sl} \equiv 4 L^2 / \tau_{sl} \nu \propto Ra^{0.50}$ ($\propto Ra^{0.42}$) for $\Gamma = 1.0$ ($\Gamma = 2.0$). [Preview Abstract] |
Tuesday, November 22, 2016 9:31AM - 9:44AM |
M10.00008: Aspect-ratio dependence of small-scale temperature properties in turbulent Rayleigh-B\'{e}nard convection ping wei, Guenter Ahlers We report measurements of the variance $\sigma^2$, skewness $S$, and kurtosis $K$ of temperature fluctuations in turbulent Rayleigh-B\'{e}nard convection of a fluid with Prandtl number $Pr=12.3$ in cylindrical samples with aspect ratios $\Gamma=D/L$ (D is the diameter and L the height) of $0.50, 1.00$ and $2.00$ in the Rayleigh-number range $6\times10^{9} \leq Ra \leq 2\times 10^{12}$. The measurements were primarily for the radial positions $\xi=1.00$ (along the sample center line) and $\xi=0.063$ (near the side wall) at several vertical locations $z/L$. For all $\Gamma$ we found that $\sigma^2$ could be fitted by $\sigma^2 \sim (z/L)^{-\zeta}$ with $\zeta \simeq 0.7$ near the side wall and $\zeta \simeq 1.0$ along the sample center line ($\xi=1.00$). At the sample center and for $\Gamma=1$, the temperature probability distribution was very close to a Laplace distribution, with $K$ close to 6 independent of Ra. However, for $\Gamma=0.5$ the distribution was intermediate between Gaussian and Laplace, with $K$ close to 4 and also independent of Ra. For $\Gamma=2$ the distribution was close to Gaussian near the peak but had exponential tails, yielding $K$ values that decreased from about 6 to about 4 as Ra increased. [Preview Abstract] |
Tuesday, November 22, 2016 9:44AM - 9:57AM |
M10.00009: Experimental container shape dependence and heat transport scaling of Rayleigh-B\'{e}nard convection of high-Prandtl-number fluids Stephen Johnston, Enrico Fonda, Katepalli R. Sreenivasan, Devesh Ranjan Both experiments and simulations on Rayleigh-B\’{e}nard convection with fluids of Prandtl numbers 5 and below have shown that the container shape influences the flow structure. Here, we investigate similar dependences of convection of fluids with Prandtl numbers of up to 10$^4$. The convection cells have aspect ratio of order unity, and we use cubic and cylindrical shapes. Visual analysis using a noninvasive photochromic dye technique indicates the distinct large-scale flow patterns in both square and cylindrical test cells. The stability of these flow patterns is explored. Also presented are results on the Nusselt-Rayleigh scaling for moderate Rayleigh numbers. References: Z. A. Daya and R. E. Ecke, Phys. Rev. Lett. 87, 184501 (2001) N Foroozani, JJ Niemela, V Armenio, KR Sreenivasan, Phys. Rev. E 90, 063003 (2014) [Preview Abstract] |
Tuesday, November 22, 2016 9:57AM - 10:10AM |
M10.00010: Different motion modes of a mobile plate on top of a thermally convecting fluid Yadan Mao, Jin-Qiang Zhong, Jun Zhang Numerical simulations are conducted to model the dynamics of a mobile, insulating plate floating on top of a Rayleigh-Benard convecting fluid with infinite Prandtl number in a two dimensional rectangular domain, which is roughly analogues to the geological model of continent drift over mantle. We focus on the effect of plate size on the dynamic feedback between the plate and the underlying convection. Four different modes of coupling are revealed as plate size varies. Among them, two transient stable modes are identified:1. a very small plate tends to linger for long time over a cold downwelling bordering two counter-rotating convection cells; 2. a relatively small plate sometimes lingers over an upwelling plume bordering two convection cells with cold downwellings on the edges of the plate. A relatively large plate rides on a moving convection cell and oscillates periodically between the two ends walls. A very large plate executes only small excursions in response to the competition between the two neighbouring cells underneath and no longer touches the end walls. These modes are well related to different continent motions since the breakup of the Pangaea supercontinent. [Preview Abstract] |
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