Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D19: Convection and Buoyancy-Driven Flows: General I |
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Chair: Max Koerner, Technische Universitaet Ilmenau Room: 2006 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D19.00001: Direct numerical simulations of mixed convection in a turbulent channel flow Samir Sid, Vincent Terrapon, Yves Dubief Wall-bounded turbulence has been extensively studied by the scientific community during the last decades. Much effort has been devoted to identify the role that coherent structures and energy exchanges play in turbulent channel flows. However, in many engineering applications, wall-bounded flows are subjected to additional physical phenomena. For instance, applying a temperature differential to the channel walls leads to a modified turbulent state which results from a balance between buoyancy, inertia and viscosity effects. Although, forced and natural convection have been widely studied separately, the coupling between both and its consequences on turbulence features are still not fully understood. In the present work, direct numerical simulations of a buoyant turbulent channel flow are reported for different values of the Reynolds and Richardson numbers. The energy exchanges between potential and kinetic energy and their impact on coherent structures are investigated. Macroscopic quantities (e.g.: Nusselt number) and statistics are compared with those obtained in forced convection flows. Finally, the influence of the ratio between inertia and buoyancy effects (i.e. Richardson number) is discussed. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D19.00002: Experimental Investigation of Large-Scale Flow Structures in Turbulent Mixed Convection Max Koerner, Christian Resagk, Andre Thess We report on experimental investigations of the temporal and spatial behavior of large-scale flow structures (LSC) in turbulent mixed convection. Using a reduced scale model room with a passenger cabin based geometry allows a global view on the LSCs, which are mainly responsible for thermal comfort and air quality within rooms. Moreover, the usage of pressurized working gases like air or sulfur hexafluoride (SF6) enables experimental investigations within broad ranges of the Reynolds number Re and Rayleigh number Ra. Thus, it is also possible to achieve realistic values of the dimensionless numbers allowing direct conclusions to be drawn about the LSCs in rooms similar to passenger cabins. The LSCs are determined by measurements of the 2D velocity field using a 2D2C particle image velocimetry system. In order to characterize three-dimensionally evolved flow structures, the measurement plane can be moved throughout the depth of the model room. We found very complex LSCs ranging from two-dimensional to three-dimensional structures and from one-roll systems over simple two-roll ones to chaotic behavior of the flow. The formation the LSCs has a strong dependency on the relation between Re and Ra and they often show distinct coherent oscillations. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D19.00003: A Study of Mixed Convection in a Heated Channel M.Z. Hossain, Jerzy M. Floryan Mixed convection in a channel subject to a spatially periodic heating along one of the walls has been studied. The pattern of the heating is characterized by the wave number $\alpha $ and its intensity is expressed in terms of the Rayleigh number Ra$_{p}$. The primary convection occurring in response to the applied heating has the form of counter-rotating rolls with the wave vector parallel to the wave vector of the heating. The resulting net heat flow between the walls increases proportionally to Ra$_{p}$ but the growth saturates when Ra$_{p} =$ 0(10$^{3})$. The most effective heating pattern corresponds to $\alpha \approx $1 as this leads to the most intense transverse motion. The primary convection is subject to transition to secondary states with the onset conditions depending on $\alpha $. Conditions leading to transition between different forms of secondary motions have been determined using the linear stability theory. Three patterns of secondary motion may occur at small Reynolds numbers Re, i.e., the longitudinal rolls, the transverse rolls and the oblique rolls, with the critical conditions varying significantly as a function of $\alpha $. Increase of $\alpha $ leads to the elimination of the longitudinal rolls and, eventually, elimination of the oblique rolls with the transverse rolls assuming the dominant role. For large $\alpha $ the transition is driven by the Rayleigh-B\'{e}nard mechanism while for $\alpha =$0(1) the spatial parametric resonance dominates. It is shown that the global flow characteristics are identical regardless of whether the heating is applied either at the lower or at the upper walls. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D19.00004: Simulations of surfactant laden drops settling in sharp stratifications David Martin, Francois Blanchette To model oil droplets in the oceans, we present simulations of surfactant-laden drops settling in stratifications. Our model uses a thin, axisymmetric interface, treated as a two dimensional front, and we track the local surfactant concentration. By altering the relative surface tension of the drop with the ambient, surfactants impact the flow around the drop as well as the settling speed of the drop. The ambient stratification also affects surface tension, giving rise to complex dynamics. Settling speeds are obtained in the presence of surfactant, and compared to the surfactant free case, and the effects of the stratification are quantified. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D19.00005: Effects of Droplet Size on Intrusion of Sub-Surface Oil Spills Eric Adams, Godine Chan, Dayang Wang We explore effects of droplet size on droplet intrusion and transport in sub-surface oil spills. Negatively buoyant glass beads released continuously to a stratified ambient simulate oil droplets in a rising multiphase plume, and distributions of settled beads are used to infer signatures of surfacing oil. Initial tests used quiescent conditions, while ongoing tests simulate currents by towing the source and a bottom sled. Without current, deposited beads have a Gaussian distribution, with variance increasing with decreasing particle size. Distributions agree with a model assuming first order particle loss from an intrusion layer of constant thickness, and empirically determined flow rate. With current, deposited beads display a parabolic distribution similar to that expected from a source in uniform flow; we are currently comparing observed distributions with similar analytical models. Because chemical dispersants have been used to reduce oil droplet size, our study provides one measure of their effectiveness. Results are applied to conditions from the `Deep Spill' field experiment, and the recent Deepwater Horizon oil spill, and are being used to provide ``inner boundary conditions'' for subsequent far field modeling of these events. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D19.00006: Experiments and models of particle slurries Matthew Molinare, Sarah Burnett, Andrew Li, Katherine Varela, Dirk Peschka, Jeffrey Wong, Andrea Bertozzi We present new experimental and theoretical results for the resuspension of bidisperse particle-laden flows on an inclined plane. In particular, we study the case of two negatively buoyant particle species of similar size and dissimilar densities in a viscous fluid of finite volume. Different regimes of particle separation are observed and studied by adjusting the angle of inclination, total particle concentration, and relative particle volume ratio. In addition to obtaining information about the height profile of shock formations, we measure the advancement and separation of particle and fluid front positions in mono- and bidisperse scenarios. These dynamics are the basis for a quantitative understanding of polydisperse cases, which can be readily applied to industry and catastrophe modeling. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D19.00007: Numerical simulation of convective sedimentation using a two-way coupled Eulerian-Lagrangian model Yi-Ju Chou, Yun-Chuan Shao, Yi-Chun Yeh Numerical simulations of convective sedimentation are conducted using an Eulerian-Lagrangian (EL) model. The present EL model is a two-way coupled system that further accounts for added mass and volumetric effects, which makes it more suitable for solid-liquid suspension problems. By comparing with different modeling strategies, the present modeling results reveal the significant deviation from the traditional single-phase modeling results when concentration becomes dense. Moreover, comparison with the point particle representation demonstrates the importance of the volumetric effect in dense suspension problems. Settling of particle clouds in homogeneous and stratified background flow fields is investigated. The results show different transport patterns associated with different conditions of density and particle-induced stratification. The influence of the background stratification on convection of particle clouds is further discussed. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D19.00008: Turbulent flow inside a solar concentrator receiver Manuel Ramirez, Eduardo Ramos A solar concentrator receiver is a heat exchanger designed to absorb a beam of radiant heat coming from a field of heliostats. Inside the device, a slow forced flow generated bye an external pressure gradient is present, together with a natural convective a turbulent flow produced by the large temperature gradients due to intense heating. We present a model of this device based on the numerical solution of the mass, momentum and energy conservation equations. We consider heating conditions that lead to turbulence convective flow. For this season, a large eddy simulation model is incorporated. The results are potentially useful for the design of solar concentrator receivers. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D19.00009: Numerical investigation of high temperature heat pipe incorporated in thermal energy storage systems Mahboobe Mahdavi, Songgang Qiu, Saeed Tiari In the present work, a new type of high temperature heat pipe is investigated which can be incorporated in the thermal energy storage for concentrated solar power systems. A detailed two dimensional axisymmetric numerical procedure is developed to analyze the steady state thermal-hydrodynamic characteristics of the heat pipe. The model accounts for conduction in the wall and wick regions as well as compressible flow in vapor chambers. The geometrical features, working fluid type, wick structure and operational temperature of the heat pipe are adjusted satisfying the heat transport limitations. The proposed numerical model agrees well with available experimental data. The effects of evaporator heat input and vapor core radius on the vapor velocity and pressure fields, vapor and wall temperature distributions as well as heat pipe thermal resistance are studied. The results revealed that the thermal resistance increases with the increase of heat input while decreases with the increase of radius, however, there exists a certain radius which further increase over would not affect the thermal resistance significantly. [Preview Abstract] |
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