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 A11: Convection and Buoyancy-Driven Flows: Plumes |
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Chair: Rachael Bonnebaigt, University of Cambridge Room: 111 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A11.00001: Plumes from vertical buoyancy sources detrain - where does it matter? Rachael Bonnebaigt, C. P. Caulfield, Paul Linden Buildings often have heated vertical surfaces, such as patches of wall heated by sunlight. How do these heated surfaces affect the temperature stratification in a room? Using analogue laboratory experiments and a theoretical model, we investigate the stratification in a sealed, insulated space containing a vertically distributed buoyancy source. In the experiments, the source drives a turbulent plume, as in the theoretical model. However, the plume then detrains (intrudes into the ambient) at intermediate heights. This detrainment is not accounted for in current theoretical models. We compare theoretical and experimental ambient density profiles to see whether detrainment is significant for various boundary conditions on the heated wall. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A11.00002: Predicting The Intrusion Layer From Deep Ocean Oil Spills Dayang Wang, Aaron Chow, E.Eric Adams Oil spills from deep ocean blowout events motivate our study of multiphase plumes in a water column. Key to understanding the long-term fate of these plumes is the ability to predict the depth and persistence of intrusion layers. While intrusion layers from multiphase plumes have been studied under stagnant conditions, their behavior in the presence of crossflow, especially in mild crossflow, remains poorly understood. The classical classification of plume behavior identifies two regimes: crossflow-dominant and stratification-dominantâ€”but it does not account for the interplay between the two effects, leaving the transition region unexplored. We conduct laboratory tank experiments to investigate the behavior of intrusion layers under the simultaneous action of crossflow and stratification. Our experiments use an inverted frame of reference, using glass beads with a range of sizes to simulate oil droplets. We find that crossflow creates enhanced mixing, which in turn leads to a shallower intrusion layer of the released fluid (correspondingly, a deeper layer in the case of a deep ocean blowout). We develop a mathematical formulation that extends previous models to account for crossflow effects, and use field observations to validate the analytical and experimental findings. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A11.00003: Critical modes due to Archimedean buoyancy and dielectrophoretic force in a dielectric liquid in cylindrical annulus Antoine Meyer, Harunori Yoshikawa, Olivier Crumeyrolle, Innocent Mutabazi An incompressible dielectric fluid is confined in a cylindrical annulus maintained at two different temperatures and an electric tension in Earth gravity. The coupling between the electric field and the thermal variation of the permittivity leads to a dilectrophoretic force that acts as a buoyancy force to induce convective flows. We have performed the linear stability analysis to determine the critical parameters and the nature of critical modes for different values of the control parameters. Four types of modes were found: For weak values of the electric tension, the critical modes are either hydrodynamic or thermal modes depending on the Prandtl number and for large values of electric tension lead to electric modes $\left[1\right]$. For its intermediate values, critical modes are columnal vortices, similar to those observed in simulations of the convection in a cylindrical annulus with a radial gravity $\left[2\right]$. \\[4pt] $\left[1\right]$ H.N. Yoshikawa et al., Phys.Fluids \textbf{25}, 024106(2013).\\[0pt] $\left[2\right]$ A. Alonso et al., Fluid. Dyn. Res. \textbf{24}, 133 (1999). [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A11.00004: Bifurcation and Stability Analyses in Horizontal Convection Pierre-Yves Passaggia, Alberto Scotti, Brian White Horizontal Convection is a flow driven by differential buoyancy forcing across a horizontal surface. It has been considered as a simple model to study the influence of heating and cooling at the ocean surface on the Meridional Overturning Circulation. The mechanisms responsible for transition to turbulence are presented using a bifurcation and global stability analyses of the two-dimensional baseflows. The forcing imposed at the surface creates a circulation characterized by a sinking plume near the pole and an upwelling at the equator. Increasing the magnitude of the forcing, the steady states are shown to undergo a sub-critical bifurcation, leading to a transition in the behaviour of the descending plume These steady states are shown to become unstable to both two and three-dimensional perturbations. The three-dimensional instability modes are characterized by counter-rotating vortices located in the plume and are associated with the Rayleigh-Taylor instability. The two-dimensional instability modes are associated with the vortex shedding of the plume, spreading into the abyss. Using the available potential energy analysis framework, we identify the instability mode responsible for the best mixing efficiency. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A11.00005: Sedimentation from Particle-Laden Plumes in Stratified Fluid Bruce Sutherland, Youn Sub Hong Laboratory experiments are performed in which a mixture of particles, water and a small amount of dye is continuously injected upwards from a localized source into a uniformly stratified ambient. The particle-fluid mixture initially rises as a forced plume (which in most cases is buoyant, though in some cases due to high particle concentration is negative-buoyant at the source), reaches a maximum height, collapses upon itself and then spreads as a radial intrusion. The particles are observed to rain out of the descending intrusion and settle upon the floor of the tank. Using light attenuation, the depth of the particle mound is measured after the experiment has run for a fixed amount of time. In most experiments the distribution of particles is found to be approximately axisymmetric about the source with a near Gaussian structure for height as a function of radius. The results are compared with a code that combines classical plume theory with an adaptation to stratified fluids of the theory of Carey, Sigurdsson and Sparks (JGR, 1988) for the spread and fall of particles from a particle-laden plume impacting a rigid ceiling. Re-entrainment of particles into the plume is also taken into account. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A11.00006: Computational study of the formation and evolution of a three-dimensional gravity current Andrew Ooi, Shuang Zhu, Nadim Zgheib, Balachandar Sivaramakrishnan Gravity currents occur when fluids of different density are brought together. They are relevant in many engineering applications such as the dispersion of hazardous gas cloud or the spillage heavy chemicals from marine vehicles. Thus far, most of the studies have assumed that the gravity current is two-dimensional (or ``planar'') as it travels down the slope, i.e. the gravity current is homogeneous in the spanwise direction. In this study, we utilise data from direct numerical simulation to investigate the evolution and formation of a fully three-dimensional gravity current propagating down a uniform slope. Previous theoretical studies have predicted that three-dimensional gravity current will evolve towards a ``self-similar'' circular wedge shape. Flow visualization from experiments showed that, contrary to the theoretical prediction, the gravity current takes on a shape that is more akin to a triangular wedge. Data from our direct numerical simulation agrees with the experimental observation. Furthermore, it has been found that the shape of this triangular wedge is relatively insensitive to the initial shape of the gravity current. The physical mechanisms leading to formation of this triangular shape and the entrainment properties of such a structure will be presented. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A11.00007: Trajectory of a plume in a power-law velocity profile Ali Tohidi, Nigel Kaye Highly buoyant plumes, bent-over by a cross flow, occur in many situations ranging from waste-water discharges into rivers up to wildfire plumes in the atmosphere. Highly buoyant plumes have a steeper initial trajectory and, therefore, rise to regions of higher velocity. Hence, their trajectory will be more greatly affected by vertical variations in horizontal velocity. It is shown that, for a power-law boundary layer, the volume and momentum fluxes scale on the square of the plume's path $(s^{2})$ compared to $s^{3/4}$ for a uniform velocity. The plume's trajectory is flatter with the plume angle scaling on $s^{-1}$ compared to $s^{-1/3}$ in the uniform case. However, experimental evidence in the literature indicates that, under certain conditions, the boundary layer velocity profile makes little difference to the plume trajectory and algebraic equations developed for plumes in a uniform cross flow are adequate. Source length scale analysis is used to establish criteria for when to include boundary layer velocity variations. Such variations are only important when either the momentum length scale or buoyancy length scale is considerably greater than the release height of the plume. This result is particularly crucial for modeling wildfire plumes. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A11.00008: Airborne Detection and Dynamic Modeling of Carbon Dioxide and Methane Plumes Jamey Jacob, Taylor Mitchell, Seabrook Whyte To facilitate safe storage of greenhouse gases such as CO$_2$ and CH$_4$, airborne monitoring is investigated. Conventional soil gas monitoring has difficulty in distinguishing gas flux signals from leakage with those associated with meteorologically driven changes. A low-cost, lightweight sensor system has been developed and implemented onboard a small unmanned aircraft that measures gas concentration and is combined with other atmospheric diagnostics, including thermodynamic data and velocity from hot-wire and multi-hole probes. To characterize the system behavior and verify its effectiveness, field tests have been conducted over controlled rangeland burns and over simulated leaks. In the former case, since fire produces carbon dioxide over a large area, this was an opportunity to test in an environment that while only vaguely similar to a carbon sequestration leak source, also exhibits interesting plume behavior. In the simulated field tests, compressed gas tanks are used to mimic leaks and generate gaseous plumes. Since the sensor response time is a function of vehicle airspeed, dynamic calibration models are required to determine accurate location of gas concentration in $(x,y,z,t)$. Results are compared with simulations using combined flight and atmospheric dynamic models. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A11.00009: Direct numerical simulation of temporal plumes for entrainment analysis Dominik Krug, Jimmy Philip, Daniel Chung, Ivan Marusic Temporally evolving jets have already proven to be a valuable tool in studying details of the entrainment process. The present work aims to extend this concept to buoyancy driven flows. In this spirit, we report the direct numerical simulation of pure turbulent plumes evolving in time on a triply periodic computational domain and present details on the choice of key simulation parameters. Further, we use the plume data to investigate entrainment in a buoyancy driven flow. To this end, we determine the entrainment coefficient via two independent approaches. The first approach employs an integral analysis while the second is based on a relation between the bulk entrained flux and the local propagation of the turbulent/non-turbulent interface (TNTI) defined by an isosurface of enstrophy. This allows us to quantify entrainment in terms of quantities related to the propagation of the TNTI relative to the fluid, namely the local entrainment velocity and an amplification factor due to the convoluted shape of the enstrophy isosurface. [Preview Abstract] |
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