Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session M33: Convection and Buoyancy Driven Flows: Turbulent ConvectionConvection
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Chair: Stephan Weiss, Max Planck Institute for Dynamics and Self-Organization Room: 106 |
Tuesday, November 21, 2017 8:00AM - 8:13AM |
M33.00001: Heat Flux measurements under strongly Non-Oberbeck Boussinesq conditions Stephan Weiss, Valentina Valori, Lucia Wesenberg, Eberhard Bodenschatz We use sulfur-hexafluoride (SF$_6$) in a Rayleigh-B\'enard convection cell to investigate how deviations from the Oberbeck-Boussinesq approximations influence the heat flux. The pressure (P) and temperature (T) of the working fluid are above their critical values, where fluid properties are very sensitive to changes in T and P. We make measurements at various (T,P) that are chosen, such that the Rayleigh- and Prandtl numbers are kept constant, while the variations of the fluid properties across the cell ($X_{NOB}=X_t/X_b-1$, with $X=\varrho, \kappa, \nu, \alpha, \lambda$) change. In this way, we can directly measure how changes in $X_{NOB}$ affect the heat flux. Our preliminary results suggest the existence of different regimes, where the heat flux is either increased or decreased for increasing $X_{NOB}$. These regimes are separated by lines of maximal and minimal $X$ in the $T$-$P$ parameter space. Our results also contribute to a better understanding of RBC under OB conditions, since one can tune to some extend the conditions at the boundary layers and see the effect of such changes onto the heat transport. [Preview Abstract] |
Tuesday, November 21, 2017 8:13AM - 8:26AM |
M33.00002: Heat transport and temperature profiles in turbulent convection of fluids heated locally from below ping wei, Guenter Ahlers We measured mean-temperature profiles in turbulent convection of fluids heated locally from below in a cylindrical sample with aspect ratio $\Gamma \equiv D/L = 1.00$ (D= 24.1 cm is the diameter and L the height). Heating was near the bottom-plate center over a circular area of diameter $D_h/D = 0.053$, while cooling was over the entire top plate. Rayleigh- and Prandtl-number ranges were $6\times10^{9} \leq Ra \leq 2\times 10^{12}$ and $5 \le Pr \le 12.3$. The measurements were at radial distances $r$ from the vertical center line with $\xi \equiv 1 - 2r/D = 1.00$ (along the center line) and $\xi=0.135$ (near the side wall) and at several vertical locations $z/L$. The center temperature was well below the mean temperature. Near the side wall the mean temperature varied logarithmically with $z/L$ near the bottom and $1-z/L$ near the top plate. Along the center line such a log layer was found near the bottom plate, but not near the top plate. [Preview Abstract] |
Tuesday, November 21, 2017 8:26AM - 8:39AM |
M33.00003: Oscillatory radiatively-forced internal convection Stefan Llewellyn Smith Internal convection, in which stably stratified fluid is destabilized by internal heating, shows interesting differences from the canonical situation of Rayleigh-Benard convection with forcing at the boundaries. We consider the case when the thermal forcing is the result of radiative heating, yielding an exponential profile in the vertical, rather than a uniformly distributed source of buoyancy, and when the forcing is oscillatory in time. These two effects do not appear to have been treated together previously. We examine the linear instability problem considering steady, harmonic and more general periodic forcings. We also discuss nonlinear effects. The underlying problem is relevant to Springtime heating in the Great Lakes, in which case heating destabilizes the water column because the temperature is in the anomalous regime when water becomes denser with heating. [Preview Abstract] |
Tuesday, November 21, 2017 8:39AM - 8:52AM |
M33.00004: Controlling heat transport and flow structures in thermal turbulence using ratchet surfaces Chao Sun, Hechuan Jiang, Xiaojue Zhu, Varghese Mathai, Roberto Verzicco, Detlef Lohse In this combined experimental and numerical study on thermally driven turbulence in a rectangular cell, the global heat transport and the coherent flow structures are controlled with an asymmetric ratchet-like roughness on the top and bottom plates. We show that, by means of symmetry breaking due to the presence of the ratchet structures on the conducting plates, the orientation of the Large Scale Circulation Roll (LSCR) can be locked to a preferred direction even when the cell is perfectly leveled out. By introducing a small tilt to the system, we show that the LSCR orientation can be tuned and controlled. The two different orientations of LSCR give two quite different heat transport efficiencies, indicating that heat transport is sensitive to the LSCR direction over the asymmetric roughness structure. Through analysis of the dynamics of thermal plume emissions and the orientation of the LSCR over the asymmetric structure, we provide a physical explanation for these findings. [Preview Abstract] |
Tuesday, November 21, 2017 8:52AM - 9:05AM |
M33.00005: Energy conservation in incompressible convection Tayler Quist, Evan H. Anders, Benjamin P. Brown, Keaton J. Burns, Daniel Lecoanet, Jeffrey S. Oishi, Geoffrey M. Vasil In classic Rayleigh-B\'enard convection, energy is not conserved. Here we study a set of incompressible equations that do conserve energy when thermal diffusion is present. Using the Dedalus pseudospectral framework, we study heat transport by convection in simulations of incompressible but energy-conserving equations. We compare heat transport properties to classic Rayleigh-B\'enard convection. [Preview Abstract] |
Tuesday, November 21, 2017 9:05AM - 9:18AM |
M33.00006: An experimental study on the near-source region of lazy turbulent plumes Francesco Ciriello, Gary R. Hunt The near-source region of a `lazy' turbulent buoyant plume issuing from a circular source is examined for source Richardson numbers in the range of $10^1$ to $10^7$. New data is acquired for the radial contraction and streamwise variation of volume flux through an experimental programme of dye visualisations and particle image velocimetry. This data reveals the limited applicability of traditional entrainment laws used in integral modelling approaches for the description of the near-source region for these source Richardson numbers. A revised entrainment function is proposed, based on which we introduce a classification of plume behaviour whereby the degree of `laziness' may be expressed in terms of the excess dilution that occurs compared to a `pure' constant Richardson number plume. The increased entrainment measured in lazy plumes is attributed to Rayleigh-Taylor instabilities developing along the contraction of the plume which promote the additional engulfment of ambient fluid into the plume. [Preview Abstract] |
Tuesday, November 21, 2017 9:18AM - 9:31AM |
M33.00007: Laminar boundary layers in Navier-slip Rayleigh-Bénard convection Jared Whitehead We investigate the structure of the boundary layers for turbulent Rayleigh-Bénard convection with Navier-slip velocity and fixed flux temperature boundary conditions. Careful, rigorous analysis shows that the Prandtl boundary layer equations are effectively linear in the inviscid and/or zero diffusive limit. This demonstrates that no matter how strongly forced the system is, for these boundary conditions the boundary layers will always remain laminar. Implications of these results on the non-appearance of the ultimate regime of turbulent convection for these boundary conditions is discussed. [Preview Abstract] |
Tuesday, November 21, 2017 9:31AM - 9:44AM |
M33.00008: Adoint-based shape optimization of heat transfer surface in turbulent flows with DNS-based eddy viscosity {\&} diffusivity Yukinori Kametani, Yosuke Hasegawa A new adjoint-based shape optimization algorithm for turbulent heat transfer problem is proposed. In this algorithm, direct numerical simulation (DNS) of relevant velocity and thermal fields is first conducted. Based on the statistics obtained from DNS, the spatial distribution of the eddy viscosity and the eddy diffusivity are determined so as to reproduce the local productions of turbulent kinetic energy and temperature fluctuation, respectively. Then the Reynolds-averaged Navier-Stokes (RANS) equations are constructed with the eddy viscosity and the eddy diffusivity obtained from DNS, and their adjoint equations are derived to achieve shape optimization. For validation, the present algorithm is applied to a wavy fin between two parallel walls under the conditions of a constant mean pressure gradient and uniform heating in fluid. A cost functional is consist of a bulk velocity and a bulk temperature for drag reduction and heat transfer enhancement, respectively. First, it is confirmed that the RANS simulation with the current DNS-based eddy viscosity and diffusivity can reproduce the mean velocity and temperature fields. It is found that the shape optimization is successfully achieved. In the obtained optimal shape, heat transfer is enhanced by the increase of heat transfer area near the reattachment and circulation region of the separated flow behind the wave crest. Meanwhile, pressure drag is reduced by holes near the channel walls which reduce the cross-sectional area in the streamwise direction. [Preview Abstract] |
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