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 H24: Aerodynamics III |
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Chair: Jana Anirban, Pittsburgh Supercomputing Center Room: 319 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H24.00001: An Experimental Investigation on the Interferences among Multiple Turbines in Onshore and Offshore Wind Farms Wei Tian, Ahmet Ozbay, Hui Hu We report an experimental study to investigate the wake interferences among multiple wind turbines on onshore and offshore wind farms. The experimental studies are conducted in a large-scale Aerodynamic/Atmospheric Boundary Layer (AABL) Wind Tunnel with an array of scaled three-blade Horizontal Axial Wind Turbine (HAWT) models placed in atmospheric boundary layer winds with different mean and turbulence characteristics to simulate the situations in onshore and offshore wind farms. In addition to measuring dynamic wind loads (both forces and moments) and the power outputs of the scaled turbine models, a Particle Image Velocity (PIV) system is used to conduct detailed flow field measurements to quantify the turbulent wake vortex flows and the wake interferences among the wind turbines sited over onshore and offshore wind farms with non-homogenous surface winds. The detailed flow field measurements are correlated with the dynamic wind loads and power output measurements to elucidate underlying physics in order to gain further insight into the characteristics of the dynamic wind loads and wake interferences among multiple wind turbines for higher total power yield and better durability of the wind turbines. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H24.00002: Energetic Turbulence Structures in the Wake of Model Wind Turbines Jian Sheng, Faraz Mehdi, Leonardo P. Chamorro Wind turbine wakes contain complex and energetic flow structures. Characterizing the near-wake field is critical to assess flow-structure interactions and evaluate asymmetric loadings that trigger premature structural failure. Although the turbulence flow structure in the far-wake region is important in the wind farm design, an integrated characterization of the entire wake flow would provide clearer mechanistic view on other phenomena such wake meandering and unsteady interactions with the blades of downwind turbines. High-speed Particle Image Velocimetry (PIV) is carried out over a model wind turbine in a neutrally stratified boundary layer flow. The measurements are made at consecutive locations ranging from three rotor diameters upstream to twelve rotor diameters downstream of the unit. Vortical structures within the wake including tip, root and hub vortices are identified and followed as they advect downstream. The evolution of these dominant near-wake flow structures are quantified and provide us a better understanding of interactions between turbine wake and boundary layer. The spatial distribution of the mean and fluctuating velocity, as well as energy spectrum and turbulent kinetic budget are also discussed. [Preview Abstract] |
Monday, November 25, 2013 10:56AM - 11:09AM |
H24.00003: Computational analysis of a tip vortex structure shed from a bio-inspired blade Sebastian Gomez, Lindsay N. Gilkey, Bryan E. Kaiser, Svetlana V. Poroseva Understanding and predicting a tip vortex structure and its dynamics is of significant importance for all branches of aerodynamics. A particular focus of our research is the rotorcraft performance which is substantially influenced by a tip vortex. A tip vortex also is a major source of energy losses and acoustic noise. In the present study, an impact of a blade shape on a tip vortex structure is analyzed. Simulations are conducted of flows around a rectangular blade and a bio-inspired blade of the same area. An insect wing is chosen as a blade prototype. Indeed, insects developed physical characteristics that reduce energy consumption while permitting sustained and controlled flight at low level of noise. Analysis has been done to determine what insect poses flight characteristics closest to the small rotorcraft design goals. Commercial CFD software STAR-CCM$+$ is used for conducting computations on structured and unstructured grids and for data post-processing. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H24.00004: Analysis of Wake Profiles for Free Leading Edge Membranes in Low Reynolds Number Flow Andrew Wrist, Zheng Zhang, James P. Hubner MAVs (micro air vehicles) are similar in size and flight velocity to nature's evolved flyers such as bats. Bats have flexible membrane wings that provide them with aerodynamic advantages, effectively reducing energy necessary to maintain flight. This study was inspired by the free LE (leading edge) and TE (trailing edge) combinations found on certain bat species. In previous research, silicone substitutes for these membranes have been tested on rigid frames, and it was found in certain cases that their lift-to-drag ratios outperform those of flat plates. In this study, wake profiles for different LE/TE combinations were analyzed, as increasing wake depth and width are related to increasing drag. Silicone membranes with an aspect ratio of one were constructed and tested at various angles of attack, pretensions, and fixed/free LE/TE configurations in a low speed wind tunnel at 10 m/s (Re $=$ 50,000). The wake of each membrane configuration was measured using a hotwire probe. The results indicate that membrane airfoils with free LEs produced a greater momentum deficit due to increased losses on the leeward side of the membrane. Further characteristics and trends are discussed in the presentation. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H24.00005: Aerodynamic Design of Wing based on Humpback Whale Flipper Saif Akram, Faisal Baig The tubercles provide a bio-inspired design that has commercial viability for wing-like structures. Wind tunnel tests at low speeds of model humpback flippers with leading-edge tubercles have demonstrated improvements tubercles make, such as a staggering 32{\%} reduction in drag, 8{\%} improvement in lift, and a 40{\%} increase in angle of attack over smooth flippers before stalling. The tubercles on the leading edge act as a passive-flow control device that improves the performance and maneuverability of the flipper. Possible fluid-dynamic mechanisms for improved performance include delay of stall through generation of a vortex and modification of the boundary layer, and increase in effective span by reduction of both spanwise flow and strength of the tip vortex. In the present work, numerical investigation of a 3D wing with scalloped leading edge inspired by the humpback whale flipper is carried out at high subsonic speeds with variation in angle of attack from 0 to 25 degrees. The effect of using different turbulence models is also investigated in order to attain a better understanding of mechanism(s) responsible for improved aerodynamic performance. This new understanding of humpback whale flipper aerodynamics has strong implications for wing design. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H24.00006: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 11:48AM - 12:01PM |
H24.00007: Flexible body with drag independent of the flow velocity Thomas Barois, Emmanuel de Langre The drag of a rigid object is expected to increase with flow velocity. For wide ranges of velocities commonly found, the drag increases as the square of the relative velocity of the fluid. This strong dependence of the load with velocity accounts for specific survival strategies adopted by passive living systems such as plants in wind or algae in marine environments: through elastic reconfiguration, the drag on plants is reduced when compared to a rigid configuration and the velocity exponent for the drag is typically found between 1 and 1.5. In this work, a flexible body conceptual model is presented that exhibits a drag force that is almost independent of the free stream velocity. This surprising result is shown to be remarkably robust as it is experimentally observed for a range of geometries. This study opens the way for the design of devices subjected to a drag that is independent of the flow velocity. This possibility constitutes a key point in various fields involving flexible structures that are towed or subjected to wind. [Preview Abstract] |
Monday, November 25, 2013 12:01PM - 12:14PM |
H24.00008: Large-eddy simulations of the Lillgrund wind farm Karl Nilsson, Simon-Philippe Breton, Stefan Ivanell, Dan Henningson Large-eddy simulations using the EllipSys3D Navier-Stokes solver developed at DTU/Ris{\o} combined with an actuator disc (ACD) method for rotor modeling are performed to compute the power production of the turbines in the Lillgrund wind farm. The ACD method models the rotor with body forces determined from drag and lift coefficients which are tabulated as functions of the angle of attack. As the boundary layer over the blades is not resolved, this approach greatly reduces the computational costs compared to simulations involving the modeling of the full blade geometry. The simulations are performed both with a recently implemented power controller, which forces the turbines to adapt their rotational speed to the conditions they are operating in, and without any controller, where all turbines are given a fixed rotational speed. The atmospheric conditions are modeled using pre-generated turbulence and a prescribed boundary layer. Only a marginal difference is found between the results from the simulations with and without the controller. The simulation results show a very good agreement with measured production from the real farm. Therefore, it can be concluded that the simulation method realistically predicts the power production of the Lillgrund wind farm. [Preview Abstract] |
Monday, November 25, 2013 12:14PM - 12:27PM |
H24.00009: Experimental study on influence of pitch motion on the wake of a floating wind turbine model Stanislav Rockel, Raul Bayoan Cal, Joachim Peinke, Michael Hoelling Wind energy has become a major contributor to energy from renewable sources and is still demanded to increase its portion to the overall energy supply. Offshore wind energy was found to have the highest potential to fulfill these demands, due to better and steadier wind conditions found on seas. Offshore wind turbines which have been installed lately use monopiles as foundations and are feasible in shallow water up to a depth of 50m. Such shallow areas are rare and often exploited, so floating support structures for offshore wind turbines in deep water are possible solutions. The additional degrees of freedom of a floating support structure will influence the aerodynamics at the rotor and its wake. Wind tunnel experiments were performed using a classical fixed turbine model and a streamwise oscillating turbine in free pitch motion. For both cases the turbines were operated under same inflow conditions and wakes up to 7 rotor diameters were measured using 2D-3C stereographic particle image velocimetry (SPIV). The obtained data was statistically analyzed and a direct comparison of the wake of the fixed and oscillating turbine was performed. Our results show that inclinations and oscillations of the turbine have a strong impact on the structure of the wake and its development. [Preview Abstract] |
Monday, November 25, 2013 12:27PM - 12:40PM |
H24.00010: ABSTRACT WITHDRAWN |
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