Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session HU: Convection and Buoyancy Driven Flows III |
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Chair: Aline Cotel, University of Michigan Room: Hilton Chicago Marquette |
Monday, November 21, 2005 1:20PM - 1:33PM |
HU.00001: Natural ventilation of interconnected boxes Morris R. Flynn, Colm P. Caulfield We examine the natural ventilation flow which occurs when a source of buoyancy is confined within a forced room with three vents: one low level exterior vent; and high and low level vents to an unforced room, which in turn has a high level exterior vent. This generalizes toward more realistic building planforms the classic single room flow considered by Linden et al. (1990). The steady state flow in the forced room is very similar to the single room case, with a well-mixed buoyant layer whose relative depth is determined purely by the vent geometry. However, it is essential to consider the system's time history to identify even the steady state properties of the flow in the unforced room. The development of a vertically stratified buoyant layer in the unforced room is inevitable; its depth depends in a non-trivial way on the cross-sectional areas of not only all the vents, but also the two rooms. We compare the predictions of a hierarchy of numerical models with the results of analogue laboratory experiments, demonstrating the critical role played by the developing vertical stratification in the unforced room. [Preview Abstract] |
Monday, November 21, 2005 1:33PM - 1:46PM |
HU.00002: Ventilation of two rooms interconnected thorough a bottom opening. Ramon Tovar, Paul F. Linden, Luis P. Thomas Spaces are often cooled by the addition and extraction of cold air at the ceiling. As a model of this processes, we study experimentally the transient ventilation of two interconnected rooms with a buoyancy source located in the forced room and an exit vent in the unforced room. The two rooms connect through an opening located at the bottom of the interior wall. Initially the average density inside the model increases linearly with time. As the forced room ventilates through the unforced one, mixed fluid enters the unforced room causing a two-layer displacement flow. After the dense layer in the unforced room reaches the exit vent, the average density increases exponentially towards the source density. Thus, on the long term, as the buoyancy difference in the forced room becomes weaker, a dominant jet-like flow develops and the stratification decreases in both rooms. Two flow regimes develop through the communication opening depending on its vertical extent: a unidirectional flow develops for small openings, while an exchange flow develops for openings with larger vertical height. We analyze these flow regimes in terms of the neutral pressure level and discuss its implications for optimizing the cooling of the two rooms. R. Tovar acknowledges the support of UC-MEXUS-CONACYT, grant 6925. [Preview Abstract] |
Monday, November 21, 2005 1:46PM - 1:59PM |
HU.00003: Thermal plume dispersion around a ``cargo container'' in crossflow Wayne Smith, Gary Settles A passive scalar is released in a thermal plume emanating from a vent in a rectangular solid on a ground plane in crossflow. The scalar concentration accumulates in the recirculatory wake of this complex turbulent separated flowfield. The flowfield is simulated by a numerical solution of the RANS equations and by wind tunnel experiments. The practical application of this scenario lies in the detection of trace chemicals in vented plumes from sea cargo containers sitting in the sun. The transport is driven by the temperature difference between the air inside and outside the container. In this way contraband chemical traces are carried into the ambient air where standoff optical sensors may be employed to detect them. The measured and computed plume trajectories are compared and correlations are developed to describe the plume behavior based on the overall flow parameters. One result of this research is guidance on where to aim standoff optical detectors in order to detect chemical traces emanating from cargo containers. [Preview Abstract] |
Monday, November 21, 2005 1:59PM - 2:12PM |
HU.00004: Self-Preserving Velocity Properties of Steady Round Buoyant Turbulent Plumes in Uniform Crossflows Francisco J. Diez, Luis P. Bernal The velocity properties of steady round buoyant turbulent plumes have been studied, motivated by applications to the dispersion of heat and potentially harmful substances from steady exhaust flows. The experiments involved salt water sources injected into uniform crossflows in a water channel. Mean and fluctuating velocity fields were measured over cross sections of the flow using Particle-Image-Velocimetry (PIV). Matching the index of refraction of the source and ambient fluids was required in order to avoid image distortion and scattering the laser beam away from the buoyant flow. The self-preserving structure properties of the flow are correlated successfully based on the scaling analysis of Fischer et al. (1979). The resulting self-preserving structure consists of two counter-rotating vortices having their axes nearly aligned with the crossflow direction, which move away from the source in the streamwise (vertical) direction due to the action of buoyancy. This alignment is a strong function of the source/crossflow velocity ratio, u$_{o}$/v$_{\infty }$. Finally, the counter-rotating vortex system is responsible for substantial increases in the rate of mixing of the source fluid with the ambient fluid compared to axisymmetric round buoyant turbulent plumes in still environments. [Preview Abstract] |
Monday, November 21, 2005 2:12PM - 2:25PM |
HU.00005: Laboratory investigation of a single mantle plume Aline Cotel, Carolina Lithgow-Bertelloni Although many have studied the chemistry and dynamics of mantle plumes, fundamental questions remain. These can be grouped into two general issues: a) Plume structure and dynamical interaction with the surrounding mantle, b) The degree of entrainment and mixing in mantle plumes of chemically distinct material from the deep mantle. We address these fundamental questions by performing detailed fluid dynamical experiments to determine the structure, velocity, and degree of entrainment in mantle plumes. Heat is used as the driving convective mechanism to form a single mantle plume. The experiments are conducted in a Plexiglas tank (outer dimensions of 0.3$\times $0.3$\times $0.3 m$^{3})$. A short copper cylinder, 0.03 m tall with a diameter of 0.025 m, is connected to a heater and is attached to the center of the tank bottom. By varying voltage settings we can simulate varying heat fluxes in the deep mantle. Two main techniques are employed in our experiments: shadowgraph and Particle Image Velocimetry (PIV). Penetration height and plume head size are related to the varying buoyancy flux. In addition, velocity and vorticity fields determined using Particle Image Velocimetry provide insight into the plume structure and the nature of the entrainment process. [Preview Abstract] |
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