Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session E10: Instability: Jets, Wakes and Shear Layers III: Wakes II |
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Chair: Sherwin Maslowe, McGill University Room: 25C |
Sunday, November 18, 2012 4:45PM - 4:58PM |
E10.00001: Hub vortex helical instability as the origin of wake meandering in the lee of a model wind-turbine Francesco Viola, Giacomo Valerio Iungo, Simone Camarri, Fernando Porte-Agel, Francois Gallaire Wind tunnel measurements were performed for the wake produced by a three-bladed wind turbine immersed in uniform flow. These tests show the presence of a vorticity structure in the near wake region mainly oriented along the streamwise direction, which is denoted as hub vortex. The hub vortex is characterized by oscillations with frequencies lower than the one connected to the rotational velocity of the rotor, which are ascribed to wake meandering by previous works. This phenomenon consists in transversal oscillations of the wind turbine wake, which are excited by the shedding of vorticity structures from the rotor disc acting as a bluff body. In this work temporal and spatial linear stability analyses of a wind turbine wake are performed on a base flow obtained through time-averaged wind tunnel velocity measurements. This study shows that the low frequency spectral component detected experimentally is the result of a convective instability of the hub vortex, which is characterized by a counter-winding single-helix structure. Simultaneous hot-wire measurements confirm the presence of a helicoidal unstable mode of the hub vortex with a streamwise wavenumber roughly equal to the one predicted from the linear instability analysis. [Preview Abstract] |
Sunday, November 18, 2012 4:58PM - 5:11PM |
E10.00002: Inviscid Instability of a Trailing Vortex Sherwin Maslowe, Jan Feys A similarity solution for an aircraft trailing vortex valid far downstream of its wingtip was found by Batchelor (1964). Its linear stability has been the focus of many papers following the pioneering work of Lessen et al. (1974). In the parallel flow version of Batchelor's solution, the azimuthal and axial velocities can be written $W = e^{- r^2}$ and $V = q (1 - e^{- r^2})/r$. The swirl component $V (r)$ is stabilizing and for $q > 2.31$ the vortex is stable. It was pointed out, however, by Spalart (1998) that closer to the aircraft a family of profiles found by Moore \& Saffman (1973) provide a more accurate description of the axial flow. Comparing with the recent experiments of Lee \& Pereira (2010), we find that the latter profiles better describe the axial flow deficit in the vortex core. We present results for the Moore \& Saffman profiles showing them to be considerably more unstable than the Batchelor vortex. [Preview Abstract] |
Sunday, November 18, 2012 5:11PM - 5:24PM |
E10.00003: Reflectional symmetry breaking of the separated flow over 3D bluff bodies Mathieu Grandemange, Marc Gohlke, Olivier Cadot The first experimental observation of a permanent reflectional symmetry breaking (RSB) is reported for a laminar three-dimensional wake. Based on flow visualizations, a first bifurcation from the trivial steady symmetric state to a steady RSB state is characterized at Re = 340. The RSB state becomes unsteady after a second bifurcation at Re = 410. It is found that this RSB regime is persistent at large Reynolds numbers and responsible for a bi-stable turbulent wake. [Preview Abstract] |
Sunday, November 18, 2012 5:24PM - 5:37PM |
E10.00004: ABSTRACT WITHDRAWN |
Sunday, November 18, 2012 5:37PM - 5:50PM |
E10.00005: Oscillation, bifurcation and growth of modal instability in bluff-body wakes: a new understanding Amalendu Sau Past experimental findings suggest during nucleation of a vortex over a bluff body the near-wake instability initiates a wavy trail even at \textit{Re}=30, and wavelength of this oscillation decreases with increasing \textit{Re} while amplitude increases. We discovered that, such a wavy oscillation has a strong spanwise counterpart which gets fast augmented with \textit{Re}, and enforces growth of a new class of bifurcations along the K\'{a}rm\'{a}n vortex cores. Notably, the detected pressure oscillation along a vortex core reaches a threshold value at the onset of shedding process and initiates growth of a Hopf bifurcation in spanwise coordinate. With \textit{Re} the pressure oscillation gains momentum; enforcing occurrence of multiple local pressure maxima and bifurcations along a vortex coreline. Our detailed simulations with square cylinders of different aspect ratios and \textit{Re} up to 240 unfold development of two physically distinct stages of spanwise wake undulations and bifurcations. While growth of uniform length-scaled bifurcations and their spatio-temporal swapping initiate formation of ``Mode A'' instability structures, a transition scenario for the ``Mode B'' is prompted with the eruption of variable length-scaled spanwise bifurcations. [Preview Abstract] |
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