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 M11: Convection and Buoyancy-Driven Flows: Numerical Studies |
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Chair: Rudie Kunnen, Eindhoven University of Technology Room: 111 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M11.00001: Transition to geostrophic convection: the role of boundary conditions Rudie Kunnen, Rodolfo Ostilla-M\'onico, Erwin van der Poel, Roberto Verzicco, Detlef Lohse The so-called geostrophic regime of rapidly rotating Rayleigh--B\'enard convection is dominated by rotation with strong enough thermal forcing to attain a turbulent flow. It is the appropriate regime for the description of the large-scale geophysical and astrophysical convective flows. Only very recently, numerical simulations and experiments have become able to enter into this regime with distinctly different scalings than the traditional rotation-affected regime, with many open questions remaining. We explore the transition to the geostrophic regime using direct numerical simulations of the Navier--Stokes and heat equations by varying the rotation rate (Ekman number~$Ek$) at two constant values of the thermal forcing (Rayleigh number~$Ra=1\times 10^{10}$ and~$5\times 10^{10}$) and constant Prandtl number~$Pr=1$. We focus on the differences between the application of no-slip or stress-free boundary conditions on the horizontal plates. We find the transition as changes in heat transfer, boundary-layer thickness, bulk/boundary-layer distribution of dissipation and bulk mean temperature gradient. The transition is gradual: many statistics reveal a change in scaling, but not sharp and not at exactly matching~$Ek$. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M11.00002: Buoyancy induced modification of the law-of-the-wall in an unstably stratified turbulent channel flow Federico Toschi, Andrea Scagliarini, Halldor Einarsson, Armann Gylfason We present results on the influence of buoyancy on the boundary layer dynamics and on mean quantities, like velocity profiles, in an unstably stratified turbulent channel flow. The study is based on direct numerical simulations where we investigated a broad range of friction Reynolds numbers and Rayleigh numbers. We primarily focused on the modification of the logarithmic law of the wall, due to buoyancy, and we provide a simple phenomenological model that is able to capture the observed deviations, in the log-law region, from the usual neutral case. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M11.00003: Compressible convection in geophysical fluids: comparison of anelastic, anelastic liquid and full numerical simulations Jezabel Curbelo, Thierry Alboussiere, Stephane Labrosse, Fabien Dubuffet, Yanick Ricard In this talk we describe the numerical method implemented to study convection in a fully compressible two-dimensional model, which may be reduced to the different simplifications such as the anelastic approximation and the anelastic liquid approximation. Various equations of state are considered, from the ideal gas equation to equations related to liquid or solid condensed matter. We are particularly interested in the total value and spatial distribution of viscous dissipation. We analyze the solutions obtained with each approximation in a wide range of dimensionless parameters and compare the domain of validity of each of them. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M11.00004: Eddy Sensitivity to Resolution and Viscosity in Density Driven Ocean Currents Shanon Reckinger, Mark Petersen, Scott Reckinger Density driven currents (in the ocean, known as oceanic overflows) impact global ocean circulation and affect intermediate and deep-water properties in numerous regions in the ocean. General circulation models currently rely on parameterizations for representing dense overflows due to resolution restrictions. These parameterizations rely on a detailed understanding of the mixing properties, which is enhanced by studying idealized overflows. This work looks at how numerical parameters like viscosity and resolution affect the eddying behavior of the dense plume inside an idealized domain. The simulations encompass a large numerical parameter study using MPAS-Ocean, which is the ocean component of an unstructured grid climate model framework called the Model for Prediction Across Scales (MPAS). Results show that eddies respond to changes in viscosity and resolution through a complicated and interrelated set of dynamical processes. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M11.00005: Quantitative saltwater modeling for validation of sub-grid scale LES turbulent mixing and transport models for fire Pietro Maisto, Andre Marshall, Michael Gollner A quantitative understanding of turbulent mixing and transport in buoyant flows is indispensable for accurate modeling of combustion, fire dynamics and smoke transport used in both fire safety design and investigation. This study describes the turbulent mixing behavior of scaled, unconfined plumes using a quantitative saltwater modeling technique. An analysis of density difference turbulent fluctuations, captured as the collected images scale down in resolution, allows for the determination of the largest dimension over which LES averaging should be performed. This is important as LES models must assume a distribution for sub-grid scale mixing, such as the ?-PDF distribution. We showed that there is a loss of fidelity in resolving the flow for a cell size above $0.54D^*$; where $D^*$ is a characteristic length scale for the plume. Such a point represents the threshold above which the fluctuations start to monotonically grow. Turbulence statistics were also analyzed in terms of span-wise intermittency and time and space correlation coefficients. An unexpected condition for the core of the plume, where a substantial amount of ambient fluid (fresh water) is found, and the mixing process under buoyant conditions were found depending on the resolution of measurements used. [Preview Abstract] |
(Author Not Attending)
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M11.00006: Effect of inlet conditions on the turbulent statistics in a buoyant jet Rajesh Kumar, Anupam Dewan Buoyant jets have been the subject of research due to their technological and environmental importance in many physical processes, such as, spread of smoke and toxic gases from fires, release of gases form volcanic eruptions and industrial stacks. The nature of the flow near the source is initially laminar which quickly changes into turbulent flow. We present large eddy simulation of a buoyant jet. In the present study a careful investigation has been done to study the influence of inlet conditions at the source on the turbulent statistics far from the source. It has been observed that the influence of the initial conditions on the second-order buoyancy terms extends further in the axial direction from the source than their influence on the time-averaged flow and second-order velocity statistics. We have studied the evolution of vortical structures in the buoyant jet. It has been shown that the generation of helical vortex rings in the vicinity of the source around a laminar core could be the reason for the larger influence of the inlet conditions on the second-order buoyancy terms as compared to the second-order velocity statistics. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M11.00007: The Role of Convective and Diffusive Mixing in Porous Media Shyam Sunder Gopalakrishnan, Jorge Carballido-Landeira, Anne De Wit, Bernard Knaepen The classical Rayleigh--Taylor (RT) instability that triggers convective and diffusive mixing when a denser fluid lies on top of a less dense one is characterised both numerically and experimentally in an ideal two-dimensional porous media. The universal nature of the flow dynamics starting with a stable diffusive regime, that is followed by a linearly unstable regime, and eventually to a nonlinear regime is presented. Though the fundamental behaviour has been studied extensively, the roles of convective and diffusive mixing on the flow features are not yet explored. It has been a long held view that diffusive mixing is significant only during the initial stages, and once the transition has occurred, the dynamics are governed by convection. We show that this is not the case, and both convection and diffusion play an important role even during the nonlinear regime, albeit at different regions of the flow with convection dominant locally at the tip of the fingers, and balanced by diffusion in the rest of the mixing zone. This also provides a quantitative measure for the evolution of the width of the fingers. The computational findings are well supported using our experimental observations, where an excellent agreement on the flow dynamics are obtained. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M11.00008: Adjoint-based shape optimization of fin geometry for enhanced solid/liquid phase-change process Kenichi Morimoto, Yuji Suzuki In recent years, the control of heat transfer processes, which play a critical role in various engineering devices/systems, has gained renewed attention. The present study aims to establish an adjoint-based shape optimization method for high-performance heat transfer processes involving phase-change phenomena. A possible example includes the application to the thermal management technique using phase-change material. Adjoint-based shape optimization scheme is useful to optimal shape design and optimal control of systems, for which the base function of the solution is unknown and the solution includes an infinite number of degrees of freedom. Here we formulate the shape-optimization scheme based on adjoint heat conduction analyses, focusing on the shape optimization of fin geometry. In the computation of the developed scheme, a meshless local Petrov-Galerkin (MLPG) method that is suited for dealing with complex boundary geometry is employed, and the enthalpy method is adopted for analyzing the motion of the phase-change interface. We examine in detail the effect of the initial geometry and the node distribution in the MLPG analysis upon the final solution of the shape optimization. Also, we present a new strategy for the computation using bubble mesh. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M11.00009: Simulation of a Supercritical Fluid Flow with Large Temperature Difference under the Assumption of Constant Pressure Satoko Komurasaki Eruption of geothermally heated water from the hydrothermal vent in deep oceans of depth over 2,000 meters is numerically simulated. The hydrostatic pressure of water is assumed to be over 200 atmospheres, and the temperature of heated water is occasionally more than $300^{\circ}$C. Under these conditions, a part of heated water can be in the supercritical state, and the physical properties can change significantly by the temperature. Particularly, thermal diffusivity at the critical temperature becomes so small, which prevents heat diffusion, and the temperature gradients can become high. Simulation of this kind of fluid flow can be carried out only by using a highly robust scheme. In this paper, a scheme for a highly-unsteady-flow computation is introduced, and a supercritical fluid flow with a large temperature difference is simulated at a constant pressure. In the computation, the compressible Navier-Stokes equations are solved using a method for the incompressible equations under constant pressure. The equations are approximated by the multidirectional finite difference method and KK scheme is used to stabilize the high-accuracy computation. [Preview Abstract] |
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