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 A12: Vortex I |
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Chair: Arindam Banerjee, Lehigh University Room: 26B |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A12.00001: Accuracy of current actuator-line modeling methods in predicting blade loads and wakes of wind turbines Pankaj Jha, Matthew Churchfield, Patrick Moriarty, Sven Schmitz Actuator-line modeling has become a prominent method for computing individual wind turbine wakes and their interaction within large wind turbine arrays. The advantage of actuator-line modeling is rooted in discretizing wind turbine blades as compact lines of body forces within an outer RANS- or LES-type solver. This eliminates the need for prohibitively expensive fully-resolved and detailed blade flow simulations. However, the close linkage between sectional blade forces and their reactive momentum deficit distribution just behind the rotor and its evolution and recovery further downstream in the wake has not been addressed in much detail in the literature. We have observed that current actuator-line modeling overpredicts blade tip loads by as much as twenty percent in comparison to blade-element momentum analyses and available data. The effect of the observed discrepancies in blade loading on the wake recovery and blade loading of a downstream wind turbine is unknown. This work addresses current efforts in quantifying the uncertainty and improving the predictive capability of actuator-line modeling. A detailed study of the combined effects of local grid refinement, body force projection width, and actuator element distribution is presented. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A12.00002: Vertical Axis Wind Turbine flows using a Vortex Particle-Mesh method: from near to very far wakes Stephane Backaert, Philippe Chatelain, Gregoire Winckelmans, Stefan Kern, Thierry Maeder, Dominic von Terzi, Wim van Rees, Petros Koumoutsakos A Vortex Particle-Mesh (VPM) method with immersed lifting lines has been developed and validated. The vorticity-velocity formulation of the NS equations is treated in a hybrid way: particles handle advection while the mesh is used to evaluate the differential operators and for the fast Poisson solvers (here a Fourier-based solver which simultaneously allows for unbounded directions and inlet/outlet boundaries). Both discretizations communicate through high order interpolation. The immersed lifting lines handle the creation of vorticity from the blade elements and its early development. LES of Vertical Axis Wind Turbine (VAWT) flows are performed, with a relatively fine resolution (128 and 160 grid points per blade) and for computational domains extending up to $6\,D$ and $14\,D$ downstream of the rotor. The wake complex development is captured in details, from the blades to the near wake coherent vortices, to the transitional ones, to the fully developed turbulent far wake. Mean flow statistics in planes (horizontal, vertical and cross) are also presented. A case with a realistic turbulent wind inflow is also considered. The physics are more complex than for HAWT flows. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A12.00003: Three dimensional visualization of the interaction between energetic coherent motions and tip vortices in the wake of an axial-flow marine turbine Daniel Troolin, Leonardo Chamorro, Seung-Jae Lee, Roger Arndt, Fotis Sotiropoulos Tip vortices of axial-flow turbines play a key role in modulating the mean and turbulent characteristics of the wake. Understanding their evolution and the mechanisms that trigger instability is crucial to improve bulk power extracted in a wind farm. In this study, we study the interaction between the tip vortices generated by a miniature axial-flow turbine and strong coherent motions present in the flow. The turbine was placed in a water flume at the St. Anthony Falls Laboratory at the University of Minnesota under subcritical conditions. A circular cylinder was placed upstream of the turbine to induce the energetic coherent structures in the flow. Three-dimensional three-component (3D3C) velocity measurements were made in the flow downstream of the miniature turbine. The focus was placed on visualizing the complex interaction between the von Karman vortices shed by the cylinders and the turbine tip vortices. New insights on the tip vortex dynamics and three dimensional characteristics of the wake flow will be discussed. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A12.00004: Flow Behavior Around Coupled, Rotating Turbines in Steady Flow Matthew Fu, John Dabiri Counter-rotating vertical axis turbines (VATs) have been shown to yield increased power density in wind farms as compared to typical horizontal axis wind turbine (HAWT) farms. However, the governing physical mechanisms remain poorly understood. Scale model experiments in a free-surface water tunnel were conducted to characterize the effect of parameters such as turbine separation, tip speed ratio, and flow speed on the downstream flow field and the resulting vortex shedding from VATs. The flow field was visualized using particle image velocimetry (PIV) and planar laser induced fluorescence. The results are compared and contrasted with recent studies of counter-rotating circular cylinders to determine if suppression of vortex shedding plays a similarly important role in dictating the overall wake dynamics. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A12.00005: Breaking the Symmetry with Flexible Blades Julia Cosse, Daegyoum Kim, Morteza Gharib Savonious turbines take advantage of shapes that experience higher drag when moving with the wind and lower drag when moving against the wind. Generally curved blades (e.g. semi circles) have supplied this characteristic drag differential, which allows the turbine to spin. While flexibility is often associated with drag reduction via reconfiguration, it is less well known that this same mechanism can be used for drag enhancement. Inspired by these unique properties we designed a turbine and placed it in a wind tunnel to further investigate the potential of flexible materials. The model was built such that it can be equipped with blades made out of different materials and the blade pitch angle can be chosen arbitrarily. As expected, the turbine didn't rotate when rigid blades were fixed to the turbine because both sides of the turbine experienced identical drag forces. However, when flexible blades were used, the wind reconfigured the shape of the blades such that there was a drag differential across the turbine which resulted in rotation. The characteristics of the flexible blades will be described in detail along with an analysis of their performance. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A12.00006: Optimum design of Hydrokinetic turbine based on Fluid structure interaction analysis Nitin Kolekar, Arindam Banerjee Hydrokinetic turbines, unlike conventional hydraulic turbines are zero head energy conversion devices, which utilize the kinetic energy of flowing water for power generation. Though the basic operation is similar to wind turbines, due to denser working media, these turbines are subjected to higher loads and stresses. The present work aims at hydrodynamic design and coupled fluid structure interaction (FSI) analysis for a horizontal axis hydrokinetic turbine. Blade element momentum (BEM) theory is utilized to analyze fluid forces and torque developed on turbine blades. The results of BEM are compared with a detailed three-dimensional computational fluid dynamics (CFD) analysis. The CFD domain is coupled with the structural domain using arbitrary Lagrangian-Eulerian scheme to find stresses in turbine components. A parametric study will be carried out to understand the effect of various parameters like blade pitch angle, flow velocity and RPM on the stresses developed on blades for different blade materials (aluminum and steel). Based on the one-way FSI analysis, the flow conditions and turbine design parameters will be optimized to achieve maximum possible efficiency. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A12.00007: The Influence of Spanwise Flow on Leading-Edge Vortex Growth Jaime Wong, Jochen Kriegseis, David Rival It has been postulated that a spanwise component velocity through the core of a leading-edge vortex (LEV) can limit its growth and allow the LEV to remain attached to the wing. In the case of a delta wing, spanwise velocity is produced by wing sweep. However, in the case of flapping-wing flight, centripetal and Coriolis accelerations produce spanwise velocities which vary periodically. In order to understand the effect of various spanwise velocity profiles on LEV growth a simple analytical model for vortex growth has been developed. This model is based on the transport of vorticity-containing mass into the LEV through the leading-edge shear layer. By first neglecting spanwise effects, the model has been verified against a nominally two-dimensional plunging profile using Particle Image Velocimetry (PIV). With the addition of a spanwise transport of vorticity-containing mass, swept and flapping spanwise velocity profiles have been modeled and compared with three-dimensional, three-component velocity data collected using Particle Tracking Velocimetry. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A12.00008: Global Vorticity Shedding on Rectangular and Streamlined Foil Geometries Stephanie Steele, Jason Dahl, Gabriel Weymouth, Michael Triantafyllou We explore several aspects of the fluid phenomenon we call global vorticity shedding. Global vorticity shedding occurs when an object in a fluid with circulation suddenly vanishes, shedding the entirety of the boundary layer vorticity into the wake at once. Global vorticity shedding is in distinct contrast with traditional massive separation shedding, in which vorticity is shed from a body from only a few separation points into the fluid. In our experiments, we approximate the disappearance of a towed foil by rapidly retracting the foil in the span-wise direction. We show that for a square-tipped vanishing foil at an angle of attack, the globally shed boundary layer vorticity forms into primary vortices, which evolve and eventually amalgamate with secondary vortices to leave two lasting vortices in the wake. The secondary vortices are a result of three-dimensionality in the flow. We further explore streamlined foil geometries to achieve a simpler and less three-dimensional wake. Vortex formation times are small, with vortices fully formed nearly instantaneously in the flow, making the application of global vorticity shedding promising for a force transducer to impart large and fast maneuvering forces on an underwater vehicle. [Preview Abstract] |
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