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 A18: Vortex Dynamics: Energy Harvesting and Atmospheric Flows |
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Chair: George Triantafyllou, National Technical University of Athens Room: 206 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A18.00001: Sensitivity of two-dimensional flow past transversely oscillating cylinder to streamwise cylinder oscillations George Triantafyllou, Sofia Peppa The sensitivity of flow past transversely oscillating cylinder to streamwise harmonic perturbations is studied. The value of the Reynolds number is equal to 150. We start with a transversely oscillating cylinder and then impose a small streamwise (in-line) perturbation with a frequency equal to twice the transverse oscillation frequency. The cylinder is thus following an eight-shaped trajectory, which can be traversed in a counter-clockwise or clockwise direction. For the counter-clockwise mode of motion, we find that, for low to moderate values of the streamwise amplitude, the power transfer from the fluid to the structure increases with the amplitude of oscillation in the streamwise direction, even though the magnitude of the fluctuations of the forces is decreased. The increase of the power transfer becomes more important at higher values of the transverse amplitude of oscillation. For the clockwise mode of motion we observe the reverse trend, the hydrodynamic forces increase with the amplitude of oscillation and the power transfer decreases. It is shown that the variation of the power transfer in both types of motion is due entirely to the variation of the phase between the transverse oscillation of the cylinder and the vortex formation process. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A18.00002: Computations of flow in an anchored Solar Vortex Dahhea Min, Paul F. Fischer, Arne J. Pearlstein In regions with high solar insolation, there is a potential to extract mechanical energy from the gravitationally unstable ground-heated air layer, using the substantial axial and azimuthal momentum of an anchored buoyancy-induced columnar vortex to drive a vertical-axis turbine. The seasonal and diurnal availability (which extends well into the late afternoon and even past sunset, due to utilization of the thermal capacity of the ground to heat the air, rather than direct use of photons) is well-matched to air-conditioning loads in the southwestern US. Critical issues in the design of such systems are the geometry of the enclosure that serves to anchor the dust devil-like vortex and prevent it from being blown away by ambient wind, as well as the geometry of the stationary vanes used both to enhance entrainment of ground-heated air into the vortex from a collection area much larger than that of the enclosure, and to utilize any ambient wind to enhance the vortex. Here, we report computations (using the spectral-element code Nek5000) of heated and unheated flows in several geometries of interest. The results are discussed in the context of field experiments. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A18.00003: Buoyancy-Induced, Columnar Vortices Mark Simpson, Ari Glezer Free buoyancy-induced, columnar vortices (dust devils) that are driven by thermal instabilities of ground-heated, stratified air in areas with sufficient insolation convert the potential energy of low-grade heat in the surface air layer into a vortex flow with significant kinetic energy. A variant of the naturally-occurring vortex is deliberately triggered and anchored within an azimuthal array of vertical, stator-like flow vanes that form an open-top enclosure and impart tangential momentum to the radially entrained air. This flow may be exploited for power generation by coupling the vortex to a vertical-axis turbine. The fundamental mechanisms associated with the formation, evolution, and dynamics of an anchored, buoyancy-driven columnar vortex within such a facility are investigated experimentally using a heated ground plane. Specific emphasis is placed on the manipulation of the vortex formation and structure and the dependence of the vorticity production and sustainment mechanisms on the thermal resources and characteristic scales of the anchoring flow vanes using stereo-PIV. It is shown that manipulation of the formation and advection of vorticity concentrations within the enclosure can be exploited for increasing the available kinetic energy. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A18.00004: Numerical Investigation of Synthetic Buoyancy-Induced Columnar Vortices Nicholas Malaya, Roy Stogner, Robert Moser Much of the solar energy incident on the Earth’s surface is absorbed into the ground, which in turn heats the air layer above the surface. This buoyant air layer contains considerable gravitational potential energy. The energy can drive the formation of columnar vortices (“Dust-Devils”) which arise naturally in the atmosphere. These “Dust-Devils” occur over a wide range of scales in many different locations across the Earth, as well as on Mars. A new energy harvesting approach makes use of this ubiquitous process by creating and anchoring the vortices artificially and extracting energy from them. In this talk we explore the characteristics of these vorticies through numerical simulation. Computational models of the turning vane system used to generate the vortex have been developed. We will discuss the formulation of these models and their validation against available experimental measurements. We will also describe the use of these simulations to optimize the turning vane configuration to maximize the power extraction, as well as serving as a vehicle to probe the dynamics of the underlying physical processes. Finally, this talk will conclude with comparisons between the synthetic vorticies and the naturally occurring phenomena. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A18.00005: NonBoussinesq effects on vorticity and kinetic energy production S. Ravichandran, Harish Dixit, Rama Govindarajan The Boussinesq approximation, commonly employed in weakly compressible or incompressible flows, neglects changes in inertia due to changes in the density. However, the nonBoussinesq terms can lead to a kind of centrifugal instability for small but sharp density variations, and therefore cannot be neglected under such circumstances (see, e.g., \textsc{DIXIT {\&} GOVINDARAJAN, JFM , 2010, 415)}. Here, we study the evolution of a light-cored Gaussian vortex and find that the nonBoussinesq terms can lead to significant changes in how vortices evolve. The problem is governed by three nondimensional numbers---Reynolds number (i.e. viscosity), Atwood number, and a ratio of gravitational and centrifugal Froude numbers. We find that the production of kinetic energy and vorticity in a light-cored Gaussian vortex are affected significantly by the nonBoussinesq terms, and varies non-monotonically with the parameters of the problem. In general, these nonBoussinesq effects depend both on the strength of gravity and on the Reynolds number associated with the initial vortex. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A18.00006: Flow hydrodynamics and contaminant transport in the flow past a lateral square cavity Cristian Escauriaza, Juan Ignacio Polanco, Olivia August, Diogo Bolster Turbulent flows past lateral cavities play an important role in the transport of contaminants in rivers and streams. Cavities are surface storage zones, where large-scale unsteady coherent structures are the leading mechanisms that produce longer residence times and control the fate of contaminants in the river. In this work we study the recirculating flow and mass transport in a lateral square cavity, by performing numerical simulations with a hybrid URANS/LES turbulence model (DES-LR). We focus on the dynamics of the coherent structures and their impacts on the transport and storage of a passive scalar. In addition, we use the numerical results to develop new 1D models that improve the description of the evolution of the averaged concentration inside the cavity. By transferring the information to larger spatial scales, we provide new insights on the mechanisms of contaminant transport and analyze the overall effects of surface storage zones in open channel flows. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A18.00007: Propulsive effects of vortex coupling between parallel pulsed jets Athanasios Athanassiadis, Douglas Hart For vehicles that use pulsed jet propulsion, nature provides inspiration for different ways to improve propulsive performance. Communities of marine invertebrates called salps improve the efficiency of cruising locomotion by aggregating into large multi-animal chains. In this process, the cylindrical animals physically connect to each other side-by-side to form an array of individual pulsed jets whose synchronous pulsing propels the entire chain forward. Some benefits of this chaining behavior can be described using existing models of pulsed jet propulsion for steady, cruising conditions. However, during unsteady conditions such as impulsive maneuvering at low speeds, it remains unclear how interactions between neighboring jets will affect the chain's propulsive performance. Using bench-top experiments, we investigate the unsteady interactions between two parallel pulsed jets. Under some conditions, the pulsed jets form vortex rings that coalesce before vortex formation is complete, coupling the hydrodynamics of the independent jets. We measure how different degrees of vortex coupling alter the energy and momentum transfer in the two-jet system. Finally, we explore the energy and momentum scalings that would guide the design of a vehicle using multi-jet maneuvering techniques. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A18.00008: Vortex dynamics in high-speed rarefied cavity flows Vishnu Venugopal, Girimaji Sharath S Space-access vehicles are frequently exposed to high non-equilibrium conditions, particularly during an atmospheric re-entry. Any cavity on the surface of such vehicles including suction chambers and impact damages can drastically alter the aerodynamic behavior around the vehicle. For instance, even if the freestream is rarefied, the flow within the cavity could be close to continuum due to the entrapment and accumulation of many molecules. This can significantly change the dynamics of vortex patterns, that are commonly present in a cavity flow, eventually affecting the surface properties along cavity walls. So, it is important to characterize the vortex dynamics in a cavity flow as a function of the degree of rarefaction (Knudsen number), cavity size and flow speed. Direct numerical simulations are performed for lid driven cavity flows using a Unified Gas Kinetic Scheme. A parametric study is performed to quantify physically possible vortex configurations for a given combination of Knudsen number, cavity aspect ratio and lid velocity. The underlying physical mechanisms involved in the production of different vortex structures are highlighted. Finally, an effort is made to develop a reference diagram that clearly classify the regions of physically possible vortex configurations. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A18.00009: Vortex dynamics and flapping patterns of the inverted flag with a bluff body Hyeonseong Kim, Junyoung Kim, Daegyoum Kim Flow-induced vibration of flexible structures for energy harvesting has drawn attention recently. The inverted flag whose trailing edge is clamped and leading edge is free to move was known to self-excite in a uniform flow of both water and air. In this study, we investigated the effect of an upstream bluff body, a flat plate, on the dynamics of the downstream inverted flag. Periodic vortical structures created by an upstream bluff body make the dynamics of the inverted flag quite different from those of the inverted flag without the bluff body. We examined the motion of the inverted flag by varying the relative displacement of the inverted flag from the bluff body and their relative size. Our results show that the inverted flag can flap with higher frequency and larger amplitude with the upstream bluff body. We also compared the dynamics of the inverted flag with those of the general flag with the upstream bluff body. In order to better understand the dynamics of the flag, the analysis of the flow patterns using particle image velocimetry was also conducted. [Preview Abstract] |
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