Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G13: Vortex Dynamics and Vortex Flows V |
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Chair: Amy Lang, University of Alabama Room: 301 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G13.00001: Unsteadiness of Flow Structure on Low Swept Delta Wing Mohammadreza Zharfa, Ilhan Ozturk, Mehmet Metin Yavuz The flow structure of low-sweep 35 degree delta wing has been analyzed experimentally using flow visualization and flow measurement techniques. Laser illuminated smoke visualization, Laser Doppler Anemometry (LDA), and surface pressure measurements are performed to understand the steady and unsteady behavior of the flow regimes. Reynolds number varying from 10,000 to 100,000 and attack angles varying from 3 to 10 are tested. For the corresponding Reynolds Numbers and attack angles, prestall and poststall regimes are identified. The amplitude and frequency of the pressure and velocity fluctuations at different locations are compared with the regions of reattachment, vortex breakdown, and stall. Using statistics and spectral analysis, the unsteadiness of flow is studied in detail. Both the lift performance of the wing and the regions possibly exposed to surface buffeting are tried to be interpreted. Using the results of the study, an effective active flow control strategy to delay stall and to reduce surface buffeting is going to be determined next. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G13.00002: Flow Structure over Moderate Swept Delta Wing: \textit{Effects of Reynolds Number and Attack Angle} Ilhan Ozturk, Mohammadreza Zharfa, Mehmet Metin Yavuz Recent investigations have revealed the appearance of a distinctive type of leading edge vortex, dual vortex structure, over simple delta wing planforms having moderate sweep angles. Flow over a moderate swept 45-degree wing has been investigated using laser illuminated smoke visualization, Laser Doppler Anemometry (LDA), and surface pressure measurements. The effects of Reynolds number and attack angles on dual vortex structure, vortex breakdown, and poststall regime are reported. The footprint of flow regimes on the surface of the planform is captured by the pressure measurements, and the lift performance of the wing is tried to be extracted. The relation between surface pressure fluctuations and near surface velocity fluctuations is investigated. The reattachment region of the separated shear layer on the surface, vortex breakdown, and stall regime are studied with considering the aforementioned relation, which will enlighten some of the aspects of the buffeting on the wing planform. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G13.00003: Vorticity Transport in the Leading Edge Vortex of a Plunging Airfoil Azar Eslam Panah, James Akkala, James Buchholz The development of the leading edge vortex on a plunging flat plate airfoil is investigated using time-resolved particle image velocimetry and time-resolved surface pressure measurements. Interaction of the leading-edge vortex with the surface of the plate results in the creation of a secondary vortex similar to that found in many other flows such as those over delta wings. Temporal and spatial variations in the surface vorticity flux are computed from the time-varying pressure distributions on the surface of the plate. The net circulation resulting from this boundary flux is smaller in magnitude than that from the leading edge shear layer, but of the same order of magnitude. Entrainment of secondary vorticity into the leading-edge vortex results in cross-cancelation within the leading-edge vortex, weakening the leading edge vortex. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G13.00004: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 8:52AM - 9:05AM |
G13.00005: Three Dimensional Vortex Wake Structure of Flapping Wings in Hovering Flight Bo Cheng, Yun Liu, Xinyan Deng Flapping wings create complex vortex structures in the wake, as the vortices of one wing stroke shed periodically and travel downwards with the induced flow. However, the detailed three-dimensional vorticity distribution and evolution in the far wake are scarcely understood experimentally. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically-scaled flapping wing was investigated experimentally, using volumetric three component velocimetry. Summarily, the overall result of the wing action is to create a coherent vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex (LEV). The shed TESL rolls up into a root vortex (RV); together with the TV in the wake, they contracts radially but stretch tangentially in the wake. Concurrently, the downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of TVs and RVs. Overall, a closed vortex ring structure is not observed in the current study, because there is no well-established starting and stopping vortex structures that smoothly connect to TV and RV. Finally, evaluation of the vorticity transport equation shows that both TV and RV, while convected downwards, undergo vortex stretching, a three-dimensional phenomenon in rotating flows. It also confirms that the vorticity evolution is dominated by convection with secondary tilting/stretch effects, while the magnitude of vorticity dissipation is negligible. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G13.00006: Volumetric visualization of the near and far field wake in flapping wings Yun Liu, Bo Cheng, Xinyan Deng The flapping wings of flying animals create complex vortex wake structure, understanding its spatial and temporal distribution is fundamental to animal flight theory. In this study, we applied the volumetric 3-component velocimetry to capture both the near- and far-field flow generated by a pair of mechanical flapping wings. For the first time, the complete three-dimensional wake structure and its evolution throughout a wing stroke were quantified and presented. The general vortex wake structure maintains a quite consistent form: vortex rings in the near-field and two shear layers in the far-field. In specific, vortex rings shed periodically from the wings and are linked to each other in successive strokes. In the far-field, the shed vortex rings evolve into two parallel shear layers with dominant vorticity convected from tip and root vortices. The shear layers are nearly stationary in space compared to the periodic vortex rings shed in the near field. In addition, downwash passes through the centers of the vortex rings and extends downward between the two shear layers. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G13.00007: Influence of wing tip morphology on vortex dynamics of flapping flight Swathi Krishna, Karen Mulleners The mechanism of flapping wing flight provides insects with extraordinary flight capabilities. The uniquely shaped wing tips give insects an edge in flight performance and the interaction between the leading edge vortices and wing tip vortices enhance their propelling efficiencies and manoeuvrability. These are qualities that are sought after in current-day Micro Air Vehicles. A detailed understanding of the vortex dynamics of flapping flight and the influence of the wing tip planform is imperative for technical application. An experimental study is conducted to investigate the effects of different wing tip planforms on the formation, evolution and interaction of vortical structures. We thereby focus on the interaction between the coherent structures evolving from the leading edge and the wing tip during pitching and flapping motions.The spatial and temporal evolution of the three-dimensional flow structures are determined using Scanning (Stereo) Particle Image Velocimetry and an in-depth coherent structure analysis. By comparing the vortex dynamics, the aerodynamic performance of various wing tip planforms are evaluated. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G13.00008: Vortex shedding in flow past an airfoil using boundary layer approximation Xinjun Guo, Shreyas Mandre We present an extension of the Kutta condition using matched asymptotic expansion applied to the Navier-Stokes equation. The goal is to study the influence of unsteady fluid dynamical effects like leading edge vortex, unsteady boundary layer separation, etc. in flow around a solid body. In order to describe accurately the location and strength of vortex shedding, we solve the simplified Navier-Stokes equations in the form of boundary layer approximation in the thin inner region close to the solid body. In the outer region far from the structure, the vortex methods are applied, which significantly reduces the computational cost compared to CFD in the whole domain. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G13.00009: Performance of Piezoelectric Energy Harvesters in Isotropic Turbulence Amir Danesh-Yazdi, Oleg Goushcha, Niell Elvin, Yiannis Andreopoulos A piezoelectric harvester beam is used to extract energy from the turbulence in a surrounding fluid. An experimental investigation is carried out in a large scale wind tunnel in which turbulence-generating grids of varying dimensions are used to excite a flexible cantilever beam with a piezoelectric patch. The beam is instrumented with a strain gauge and the strain and voltage generated by the piezoelectric material are recorded as a function of time at various distances from the grid. We observe that the presence of the beam breaks the flow isotropy in the near field but the pressure forcing content retains some of the isotropy features. The pertinent parameters that influence the voltage output and performance of the beam are identified as (1) the dimensionless distance of the beam from the grid with respect to the grid size and (2) the dimensionless length of the beam. The voltage output is also shown to obey an exponential decay law with respect to the dimensionless distance parameter. A theoretical solution to the voltage output and tip displacement is also suggested and the results are compared with experimental data. [Preview Abstract] |
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