Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D33: Environmental Flows I 
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Chair: Roberto Verzicco, University of Rome Tor Vergata Room: Georgia World Congress Center B405 
Sunday, November 18, 2018 2:30PM  2:43PM 
D33.00001: Flapping line fountains Francesco Ciriello, Gary R Hunt, Antoine LR Debugne Results of an experimental study are presented on a turbulent line fountain formed when a dense saline solution is ejected steadily upwards from a slender rectangular slot into a lighter aqueous environment. A combination of flow visualisations and measurements reveal the complex threedimensional structure of these fountains. The flow is interrogated by simultaneously viewing it from above and from the side. These views reveal that the fountain flaps about the slot in distinct patterns. Insights into the mechanism causing the flapping were acquired by examining the internal structure of the fountain along vertical sections cutting across the slot. Measurements of rise height and lateral excursion of flapping events, alongside their associated spatial and temporal statistics, are discussed to show that a single driving mechanism underlies the motion. 
Sunday, November 18, 2018 2:43PM  2:56PM 
D33.00002: Swirling flow induced by jets and plumes Wilfried Coenen, Prabakaran Rajamanickam, Adam Weiss, Antonio L Sanchez The farfield interactions of the slow flow induced by the entrainment of jets and plumes with the surrounding geometry may induce swirling motion. When the initial size of the jet or plume a is much smaller than the characteristic radial distance r_{∞} at which the swirl is generated, the evolution of the flow in the presence of bounding walls leads to a selfsimilar description with weak swirling motion, valid at intermediate radial distances a ≪ r ≪ r_{∞}. It is found that the selfsimilar solution for the circulation Γ is of the second kind, with the exponent λ in the radial decay rate Γ ∝ (r/r_{∞})^{λ} obtained as an eigenvalue. The resulting velocity distributions can find application in mathematical formulations of jet and plume problems involving interactions with ambient swirl, including dust devils and fire whirls. 
Sunday, November 18, 2018 2:56PM  3:09PM 
D33.00003: Solutions for the flow induced by a turbulent line plume in the nearnozzle region James Richardson, Gary R. Hunt In experimental studies of turbulent dense saline plumes, plumes typically emerge from a nozzle submerged in a visualization tank of freshwater. In a simple case, the exterior of a nozzle used to create a line plume can be characterized as a long rectangular prism of width 2S immersed to a depth D beneath the free surface. A long thin slot centered on the nozzle base provides the plume source. The plume and its induced flow are expected to be influenced by the boundaries near the source: the nozzle base and sides, and the free surface. We studied the effect of this external nozzle geometry on the flow induced by a turbulent line plume due to entrainment. Using a potential flow model and techniques from complex analysis, we solve Laplace's equation to find the stream function of the induced flow for any ratio D/S. The solutions show that the induced flow is sensitive to the nozzle geometry and that the streamline inclination can significantly depart from the expected horizontal inflow. Some implications for experimental design and measurements of plume entrainment are discussed. 
Sunday, November 18, 2018 3:09PM  3:22PM 
D33.00004: Enhanced entrainment into turbulent plumes driven by suspended sediment Craig McConnochie, Claudia Cenedese, Jim McElwaine We present results from laboratory experiments and direct numerical simulations that examine entrainment into particleladen turbulent plumes. Specifically, we consider a regime with dilute concentrations of small and dense particles carried by a positively buoyant plume. Within this regime, the settling velocity of the particles is smaller than and in the opposite direction to the plume velocity at all heights. Despite the settling velocity being much smaller than the plume velocity, the presence of suspended particles is seen to enhance the entrainment of ambient fluid into the plume by approximately 30%. The increased entrainment coefficient is attributed to inertial clustering of the particles leading to regions of the plume being denser than the ambient fluid. We hypothesize that these dense regions can then be convectively unstable leading to an increased entrainment coefficient. 
Sunday, November 18, 2018 3:22PM  3:35PM 
D33.00005: Enhanced heat flux in vertical natural convection by dropletinduced turbulence Chong Shen Ng, Vamsi Spandan, Detlef Lohse, Roberto Verzicco Bubbles are typically used in engineering applications to promote mixing or to enhance heat transport. Using direct numerical simulations with immersed boundaries, we show that light rising droplets enhance the volumeaveraged heat flux in vertical natural convection for Rayleigh numbers within one decade in separation and Prandtl number of 7. Specifically, we find that the heat flux can be manipulated by changing the volume fraction occupied by the droplets and the ratio of droplet density to fluid density. The underlying mechanism can be linked to the strength of dropletinduced turbulence, which can be quantified by the ratio of droplet buoyancy to the thermal buoyancy of vertical natural convection.

Sunday, November 18, 2018 3:35PM  3:48PM 
D33.00006: The fluid dynamics of deepsea mining Thomas Peacock In the next decade, with pressing societal for mineral resources, it is anticipated that deepsea mining activities will commence. Associated with such activities are numerous challenging fluid dynamics problems. A notable example is the behavior of sedimentladen plumes generated by collector vehicles operating on the sea floor at depths around 40005000m. Here we present an overview of the fluid dynamics associated with deepsea mining activities and results of our studies incorporating classical and numerical modeling, and the results of recent field experiments. 
Sunday, November 18, 2018 3:48PM  4:01PM 
D33.00007: Buoyancy effects on crossventilation Megan S Davies Wykes, Elkhansaa Chahour, Nouhaila Fahdi, Paul F Linden Winddriven crossventilation occurs when there are openings on opposite walls of a room, at equal height. This talk will present the results of laboratory experiments using a water flume and a crossventilated model room at 1/10th scale. These experiments examine the effect of an initial indooroutdoor temperature difference on the ventilation rate and the removal of contaminants from a crossventilated room. We present simple mathematical models that explain the observed ventilation rates. 
Sunday, November 18, 2018 4:01PM  4:14PM 
D33.00008: Entrainment in DNS of Turbulent Thermals Daniel Lecoanet, Nadir Jeevanjee Thermals are the result of a finite release of buoyant fluid. Thermals have been studied extensively in the laboratory, and more recently with large eddy simulations (LES). Here we present the first direct numerical simulations (DNS) of thermals; we study both laminar thermals (Re=630) and turbulent thermals (Re=6300) with Pr=1. Our goal is to quantitatively test the entrainment hypothesis of, e.g., Morton, Taylor, & Turner (1956). We use a thermal tracking algorithm based off techniques proposed in atmospheric science, and calculate the entrainment rate as the rate of change of the thermal volume. In line with previous work, we find that the entrainment rate is inversely proportional to the radius of the thermal, d log V/dz = e/r, where e is the entrainment efficiency. The entrainment efficiency for both laminar thermals (e=0.36) and turbulent thermals (e=0.47) is significantly lower than the reported values in laboratory experiments (e~0.75). We suggest this may be due to the use of salt as a source of density perturbations in laboratory experiments, which have Sc~700. 
Sunday, November 18, 2018 4:14PM  4:27PM 
D33.00009: On the Effective Buoyancy of Turbulent Thermals Nadir Jeevanjee, Nathaniel Tarshish, Daniel Lecoanet An isolated, buoyant fluid parcel (or thermal) does not accelerate with its Archimedean buoyancy, because of environmental inertia (i.e. virtual mass effects). This reduces its acceleration to an `effective buoyancy’. Here we utilize a novel correspondence with magnetostatics to derive the first complete, exact analytical expressions for effective buoyancy, for the case of ellipsoidal thermals. Such a geometry is exhibited by real, turbulent thermals in the laboratory (e.g. Scorer 1957) as well as turbulent thermals that we simulate with direct numerical simulation. The average effective buoyancy of these turbulent thermals is accurately captured by our analytical expressions. 
Sunday, November 18, 2018 4:27PM  4:40PM 
D33.00010: The Initial Development of a Thermal and the Application to Downdraughts Emily Kruger, Jamie Partridge, Gabriel Rooney, Paul F Linden Downward moving cold air within thunderstorms, known as downdraughts, can be used to determine the severity of a storm. Therefore an understanding of them is useful for weather forecasting. Typically in weather forecasting these downdraughts are modelled using the theory of a plume from from Morton, Taylor and Turner (1956), which inherently assumes that the plume is long and thin. Downdraughts are generally wider than they are high and hence deviate from the Morton, Taylor and Turner theory. 
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