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 G10: Convection and Buoyancy Driven Flows: Turbulent Convection |
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Sponsoring Units: DFD GPC Chair: Joerg Schumacher, Technische Universitat Ilmenau Room: B118-119 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G10.00001: High-amplitude dissipation event in a turbulent convection cell Janet Scheel, Joerg Schumacher A high-amplitude dissipation event in the bulk of a closed three-dimensional turbulent convection cell is found to be correlated with a strong reduction of the large-scale circulation flow in the system that happens at the same time as a plume emission event from the bottom plate. The reduction in the large-scale circulation allows for a nearly frontal collision of down- and upwelling plumes and the generation of a high-amplitude thermal dissipation layer in the bulk. This collision is locally connected to a subsequent high-amplitude energy dissipation event in the form of a strong shear layer. Our analysis illustrates the impact of transitions in the large-scale structures on extreme events at the smallest scales of the turbulence. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G10.00002: Local patches of turbulent boundary layer behaviour in classical-state vertical natural convection Chong Shen Ng, Andrew Ooi, Detlef Lohse, Daniel Chung We present evidence of local patches in vertical natural convection that are reminiscent of Prandtl--von K{\'a}rm{\'a}n turbulent boundary layers, for Rayleigh numbers $10^5$--$10^9$ and Prandtl number 0.709. These local patches exist in the classical state, where boundary layers exhibit a laminar-like Prandtl--Blasius--Polhausen scaling at the global level, and are distinguished by regions dominated by high shear and low buoyancy flux. Within these patches, the locally averaged mean temperature profiles appear to obey a log-law with the universal constants of Yaglom (1979). We find that the local Nusselt number versus Rayleigh number scaling relation agrees with the logarithmically corrected power-law scaling predicted in the ultimate state of thermal convection, with an exponent consistent with Rayleigh--B{\'e}nard convection and Taylor--Couette flows. The local patches grow in size with increasing Rayleigh number, suggesting that the transition from the classical state to the ultimate state is characterised by increasingly larger patches of the turbulent boundary layers. [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G10.00003: Turbulent convection driven by internal radiative heating of melt ponds on sea ice Andrew Wells, Tom Langton, David Rees Jones, Woosok Moon The melting of Arctic sea ice is strongly influenced by heat transfer through melt ponds which form on the ice surface. Melt ponds are internally heated by the absorption of incoming radiation and cooled by surface heat fluxes, resulting in vigorous buoyancy-driven convection in the pond interior. Motivated by this setting, we conduct two-dimensional direct-numerical simulations of the turbulent convective flow of a Boussinesq fluid between two horizontal boundaries, with internal heating predicted from a two-stream radiation model. A linearised thermal boundary condition describes heat exchange with the overlying atmosphere, whilst the lower boundary is isothermal. Vertically asymmetric convective flow modifies the upper surface temperature, and hence controls the partitioning of the incoming heat flux between emission at the upper and lower boundaries. We determine how the downward heat flux into the ice varies with a Rayleigh number based on the internal heating rate, the flux ratio of background surface cooling compared to internal heating, and a Biot number characterising the sensitivity of surface fluxes to surface temperature. Thus we elucidate the physical controls on heat transfer through Arctic melt ponds which determine the fate of sea ice in the summer. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G10.00004: Effects of Exit Variability on Near-Field Statistics for Turbulent Buoyant Jets Nicholas Wimer, Caelan Lapointe, Torrey Hayden, Jason Christopher, Gregory Rieker, Peter Hamlington Many engineering systems involve the use of high-temperature jets to heat nearby objects or surfaces. In such instances, proximity to the jet exit means that specific properties of the exit velocity and temperature can be of substantial importance in determining conditions at the heated object or surface. Moreover, compared to non-heated jets, the flow field complexity of high-temperature jets is subtantially increased due to the presence of buoyant forcing. In this talk, we examine the effects of variability in exit velocity and temperature on near-field flow statistics of a high-temperature turbulent buoyant jet. The analysis is based on large eddy simulations (LES) of turbulent buoyant jets for a variety of velocity and temperature exit distributions, including uniform, pseudo-random, and Gaussian distributions with different means and standard deviations. The resulting near-field turbulent statistics are compared to properties of the exit distributions, with a specific focus on predicting spatial and temporal spectral exponents for velocity and temperature in the near-field. The importance of these results for the prediction and understanding of engineering applications involving high-temperature jets is outlined. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G10.00005: The Turbulent Diffusivity of Convective Overshoot Daniel Lecoanet, Josiah Schwab, Eliot Quataert, Lars Bildsten, Frank Timmes, Keaton Burns, Geoffrey Vasil, Jeffrey Oishi, Benjamin Brown There are many natural systems with convectively unstable fluid adjacent to stably stratified fluid; including the Earth's atmosphere, most stars, and perhaps even the Earth's liquid core. The convective motions penetrating into the stable region can enhance mixing, leading to changes in transport within the stable region. This work describes convective overshoot simulations. To study the extra mixing due to overshoot, we evolve a passive tracer field. The horizontal average of the passive tracer quickly approaches a self-similar state. The self-similar state is the solution to a diffusion equation with a spatially dependent turbulent diffusivity. We find the extra mixing due to convection can be accurately modeled as a turbulent diffusivity, and discuss implications of this turbulent diffusivity for the astrophysical problem of mixing in convectively bounded carbon flames. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G10.00006: Sustained shear flows in Rayleigh-Bénard convection Tayler Quist, Evan Anders, Benjamin Brown, Jeffrey Oishi Zonal shear flows play important roles in both the solar and geo dynamos. In two dimensional simulations, and at relatively narrow aspect ratios, Rayleigh-Bénard convection naturally achieves zonal shear flows. These zonal flows are driven by the convection and modify it, significantly altering the heat transport and convective structures. Here we study shear flows in two and three-dimensional simulations of Rayleigh-Bénard convection using the Dedalus pseudospectral framework. At small aspect ratios and at Prandtl number 1, a large horizontal shear naturally occurs. At larger aspect ratios, we find that shearing is naturally prevented unless manually induced; there is a bistability between states dominated by “flywheel” modes and states dominated by large scale shear. We explore these states and the possibilities of sustained large scale shear in 3-D simulations. [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G10.00007: Non-adiabatic Rayleigh-Taylor instability Jesse Canfield, Nicholas Denissen, Jon Reisner Onset of Rayleigh-Taylor instability (RTI) in a non-adiabatic environment is investigated with the multi-physics numerical model, FLAG. This work was inspired by laboratory experiments of non-adiabatic RTI, where a glass vessel with a layer of tetrahyrdofuran (THF) below a layer of toluene was placed inside a microwave. THF, a polar solvent, readily absorbs electromagnetic energy from microwaves. Toluene, a non-polar solvent, is nearly transparent to microwave heating. The presence of a heat source in the THF layer produced convection and a time-dependent Atwood number ($A_{t}$). The system, initially in stable hydrostatic equilibrium $A_{t} < 0$, was set into motion by microwave induced, volumetric heating of the THF. The point when $A_{t} > 0$, indicates that the system is RTI unstable. The observed dominant mode at the onset of RTI was the horizontal length scale of the vessel. This scale is contrary to classical RTI, where the modes start small and increases in scale with time. It is shown that the dominant RTI mode observed in the experiments was determined by the THF length scale prior to RTI. The dominant length scale transitions from the THF to the toluene via the updrafts and downdrafts in the convective cells. This happens when $A_{t}$ passes from negative to positive. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G10.00008: Scale-to-scale energy and enstrophy transport in two-dimensional Rayleigh-Taylor turbulence Quan Zhou We apply a recently developed filtering approach, i.e. filter-space technique (FST), to study the scale-to-scale transport of kinetic energy, thermal energy, and enstrophy in two-dimensional (2D) Rayleigh-Taylor (RT) turbulence. Although the scaling laws of the energy cascades in 2D RT system follow the Bolgiano-Obukhov (BO59) scenario due to buoyancy forces, the kinetic energy is still found to be, on average, dynamically transferred to large scales by an inverse cascade, while both the mean thermal energy and the mean enstrophy move toward small scales by forward cascades. In particular, there is a reasonably extended range over which the transfer rate of thermal energy is scale-independent and equals the corresponding thermal dissipation rate at different times. This range functions similar to the inertial range for the kinetic energy in the homogeneous and isotropic turbulence. Our results further show that at small scales the fluctuations of the three instantaneous local fluxes are highly asymmetrically distributed and there is a strong correlation between any two fluxes. These small-scale features are signatures of the mixing and dissipation of fluids with steep temperature gradients at the fluid interfaces. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G10.00009: Investigation of turbulent Prandtl number subject to local acceleration and deceleration Eunbeom Jung, Wook Lee, Seongwon Kang, Gianluca Iaccarino The main objective of the present study is to analyze the turbulent Prandtl number ($\Pr_{t} )$ varying over space in a wall-bounded turbulent flow under local acceleration and deceleration. The $\Pr_{t} $ shows the opposite trends for the conditions of acceleration and deceleration. In order to explain these phenomena, the convection velocity from the space-time correlation is investigated. It is shown that small-scale motions experience larger acceleration and deceleration compared to large-scale ones. Also, a discrepancy between the momentum and heat transfer at small scales results in the spatially varying $\Pr_{t} $. The budgets of the turbulent kinetic energy and temperature variance show a hint for the variation of $\Pr_{t} $. The results from DNS and RANS with a constant $\Pr _{t} $ are compared and show that RANS prediction can be improved by using a modeled $\Pr_{t} $. From the turbulent statistics, a few flow variables showing higher correlations with $\Pr_{t} $ are identified. Based on this, simple phenomenological models are devised and the corresponding simulations show a more accurate prediction of the heat transfer rate. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G10.00010: Heat and momentum transfer for magnetoconvection in a vertical external magnetic field Till Z\"urner, Wenjun Liu, Dmitry Krasnov, J\"org Schumacher The scaling theory of Grossmann and Lohse (J. Fluid Mech. \textbf{407}, 27 (2000)) for the turbulent heat and momentum transfer is extended to the magnetoconvection case in the presence of a (strong) vertical magnetic field. The comparison with existing laboratory experiments and direct numerical simulations in the quasistatic limit allows to restrict the parameter space to very low Prandtl and magnetic Prandtl numbers and thus to reduce the number of unknown parameters in the model. Also included is the Chandrasekhar limit for which the outer magnetic induction field $\mathbf{B}$ is large enough such that convective motion is suppressed and heat is transported by diffusion. Our theory identifies four distinct regimes of magnetoconvection which are distinguished by the strength of the outer magnetic field and the level of turbulence in the flow, respectively. [Preview Abstract] |
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