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 H12: Wind Turbines: Vertical Axis |
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Chair: John Dabiri, Stanford University Room: 200 |
Monday, November 23, 2015 10:35AM - 10:48AM |
H12.00001: Optimization of wind farm performance using low-order models John Dabiri, Ian Brownstein A low order model that captures the dominant flow behaviors in a vertical-axis wind turbine (VAWT) array is used to maximize the power output of wind farms utilizing VAWTs. The leaky Rankine body model (LRB) was shown by Araya et al. (JRSE 2014) to predict the ranking of individual turbine performances in an array to within measurement uncertainty as compared to field data collected from full-scale VAWTs. Further, this model is able to predict array performance with significantly less computational expense than higher fidelity numerical simulations of the flow, making it ideal for use in optimization of wind farm performance. This presentation will explore the ability of the LRB model to rank the relative power output of different wind turbine array configurations as well as the ranking of individual array performance over a variety of wind directions, using various complex configurations tested in the field and simpler configurations tested in a wind tunnel. Results will be presented in which the model is used to determine array fitness in an evolutionary algorithm seeking to find optimal array configurations given a number of turbines, area of available land, and site wind direction profile. Comparison with field measurements will be presented. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H12.00002: Transition to bluff body dynamics in the wake of vertical axis turbines Daniel Araya, John Dabiri A unifying characteristic among bluff bodies is a similar wake structure independent of the shape of the body. We present experimental data to demonstrate that the wake of a vertical axis wind/water turbine (VAWT) shares similar features to that of a bluff body, namely a circular cylinder. For a fixed Reynolds number (Re $\approx $ 10$^{\mathrm{4}})$ and variable tip-speed ratio, 2D particle image velocimetry (PIV) is used to measure the velocity field in the wake of three different laboratory-scale turbines: a 2-bladed, 3-bladed, and 5-bladed VAWT, each with similar geometry. From the PIV measurements, the time-averaged and dynamic characteristics of the wake are evaluated. In all cases, we observe three distinct regions in the VAWT wake: (1) the near wake, where periodic blade shedding dominates; (2) a transition region, where blade vortices decay and growth of a shear layer instability occurs; (3) the far wake, where bluff body wake oscillations dominate. We further characterize this wake transition with regard to turbine solidity and examine its relation to the mean flow, an important metric for power production within a wind farm. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H12.00003: Patterns of 3D flow in a rotating cylinder array Anna Craig, John Dabiri, Jeffrey Koseff Experimental data are presented for large arrays of rotating, finite-height cylinders, which show that the three-dimensional flows are strongly dependent on the geometric and rotational configurations of the array. Two geometric configurations of the cylinders, each with two rotational configurations, were examined for a total of four arrays. 2D PIV was conducted in multiple intersecting horizontal and vertical sheets at a location far downstream of the leading edge of the array in order to build up a picture of the 3D developed flow patterns. It was found that the rotation of the cylinders drives the formation of streamwise and transverse flow patterns between cylinders. These horizontal flow patterns, by conservation of mass, drive vertical flows through the top of the array. As the array of rotating cylinders may provide insight into the flow kinematics of an array of vertical axis wind turbines, this planform flux is of particular interest as it would bring down into the array high kinetic energy fluid from above the array, thus increasing the energy resource available to turbines far downstream of the leading edge of the array. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H12.00004: In Situ Particle Tracking around kW Sized Wind Turbines Ian Brownstein, John Dabiri Laboratory studies of model wind turbines are typically unable to match both the Reynolds number (Re) and tip speed ratio (TSR) of full-scale wind turbines. In order to match both relevant parameters, a quantitative flow visualization method was developed to take in situ measurements of the flow around full-scale wind turbines. The apparatus constructed was able to seed an approximately 9mx9mx5m volume in the wake of the turbine using artificial snow. Quantitative results were obtained by tracking the evolution of the snow using particle tracking algorithms. As a step toward full 3D-PTV measurements, results will be presented in which a 2D measurement is taken with a single camera positioned at the base of the turbine looking upward. The resulting tracking is therefore integrated in the span-wise direction. This method is demonstrated through a comparative study of a five-bladed VAWT producing power in different wind conditions at the Field Laboratory for Optimized Wind Energy (FLOWE) in Lancaster, CA. Future work to expand this method to 3D-PTV is also discussed. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H12.00005: Computational Study of Savonius Wind Turbines with Stators Aaron Alexander, Arvind Santhanakrishnan The dynamics of a stator assembly that directs incoming wind into an internal cylindrical trapped flow that exits vertically has been previously studied using particle image velocimetry and computational fluid dynamics (CFD). The present study uses the commercial CFD package Star-CCM$+$ (CD-adapco) to investigate how a Savonius rotor is affected by the introduction of cylindrical flow trapped by a stator enclosure. The results are then compared with the flow field around an identical Savonius rotor without a stator assembly. The flow characteristics are investigated at Reynolds numbers on the order of one million to examine local flow effects around the rotor as well as downstream wake vorticity. Additionally, the minimum free stream wind velocity needed for rotor start-up and rotor output power will be compared with and without the use of a stator. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H12.00006: Morping blade design for vertical axis wind turbines David MacPhee, Asfaw Beyene Wind turbines operate at peak efficiency at a certain set of operational conditions. Away from these conditions, conversion efficiency drops significantly, requiring pitch and yaw control schemes to mitigate these losses. These efforts are an example of geometric variability, allowing for increased power production but with an unfortunate increase in investment cost to the energy conversion system. In Vertical-Axis Wind Turbines (VAWTs), the concept of pitch control is especially complicated due to a dependence of attack angle on armature azimuth. As a result, VAWT pitch control schemes, both active and passive, are as of yet unfeasible. This study investigates a low-cost, passive pitch control system, in which VAWT blades are constructed of a flexible material, allowing for continuous shape-morphing in response to local aerodynamic loading. This design is analyzed computationally using a finite-volume fluid-structure interaction routine and compared to a geometrically identical rigid rotor. The results indicate that the flexible blade increases conversion efficiency by reducing the severity of vortex shedding, allowing for greater average torque over a complete revolution. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H12.00007: The effects of Reynolds number, tip speed ratio, and solidity in VAWTs Colin Parker, Allen Schult, Megan C. Leftwich The wakes of several scale models of vertical axis wind turbines (VAWTs) are investigated in a wind tunnel using particle imaging velocimetry (PIV). The tip speed ratio, Reynolds number, and solidity (chord to diameter ratio) is varied to see effect each parameter. The solidity is changed by varying the chord length of a three blade turbine of constant diameter. The range of parameters (Reynolds number and tip-speed ratio) investigated, closely matches those of full size turbines. Time averaging behind the turbines shows the asymmetry in wake. A more complete picture of the wake is seen using phase averaging by syncing the imaging to the position of the turbine. These results show a cycle of structures developing on the blades and then being shed into the wake. Imaging is done at the midplane of the turbine from upstream of the turbine back into the wake. Additionally a vertical plane behind the center of the turbine is used to measure the horizontal components in the wake. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H12.00008: Wake Development of a Model Vertical Axis Wind Turbine Hawwa Kadum, Sasha Friedman, Elizabeth Camp, Rau'l Cal At the Portland State University wind tunnel facility, an experiment is conducted to observe the downstream development of the wake past a model vertical axis wind turbine (VAWT). The flow domain is composed of streamwise-spanwise planes at mid-height of the VAWT rotor and data is obtained via particle image velocimetry (PIV). The flow field is assessed by analyzing contours of mean velocities and the full Reynolds stress tensor. Furthermore, profiles of the aforementioned quantities and flow parameters are discussed in the context of downstream evolution/flow development. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H12.00009: Experimental Performance of a Novel Trochoidal Propeller Bernard Roesler, Brenden Epps In the quest for energy efficiency in marine transportation, a promising marine propulsor concept is the trochoidal propeller. We have designed and tested a novel trochoidal propeller using a sinusoidal blade pitch function. The main results presented are measurements of thrust and torque, as well as the calculated efficiency, for a range of advance coefficients. The experimental data show narrow 95\% confidence bounds, demonstrating high accuracy and repeatability in the experimental methods. We compare our sinusoidal-pitch trochoidal propeller with prior cross-flow propellers, as well as a representative screw propeller. While the efficiency of our propeller exceeds that of the cycloidal-pitch trochoidal propeller, it is slightly lower than the efficiencies of the other propellers considered. We also present a theoretical model that can be used to further explore and optimize such trochoidal propellers, leading to new avenues for improvements in marine propulsion systems. [Preview Abstract] |
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