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 R31: Wind Energy III |
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Chair: John Dabiri, California Institute of Technology Room: 33B |
Tuesday, November 20, 2012 1:00PM - 1:13PM |
R31.00001: Thermal stratification effects on a 4x3 wind turbine array boundary layer Elizabeth Camp, Murat Tutkun, Raul Bayoan Cal Efforts have intensified to investigate the effects of thermal stratification on wind turbine performance from a fluid mechanics the perspective. Recently, it has been shown that power production is highly dependent on the temperature variation of the atmospheric turbulent boundary layer. Hence, flow within a 4x3 turbine array is studied under neutral, unstable and stable thermal stratification regimes. The flow upstream of the model wind turbine array was modified using an active grid, strakes, roughness elements, and thermally controlled floor panels. In this wind tunnel experiment, the mean velocities, Reynolds stresses, and trends in C$_{p}$ curves were measured along the centerline of the array using Particle Image Velocimetry in conjunction with torque sensors. The measurements consequently yield the flow development as well as the differences produced by the thermal effects within the array. [Preview Abstract] |
Tuesday, November 20, 2012 1:13PM - 1:26PM |
R31.00002: Statistical analysis of kinetic energy entrainment in a model wind turbine array boundary layer Raul Bayoan Cal, Nicholas Hamilton, Hyung-Suk Kang, Charles Meneveau For large wind farms, kinetic energy must be entrained from the flow above the wind turbines to replenish wakes and enable power extraction in the array. Various statistical features of turbulence causing vertical entrainment of mean-flow kinetic energy are studied using hot-wire velocimetry data taken in a model wind farm in a scaled wind tunnel experiment. Conditional statistics and spectral decompositions are employed to characterize the most relevant turbulent flow structures and determine their length-scales. Sweep and ejection events are shown to be the largest contributors to the vertical kinetic energy flux, although their relative contribution depends upon the location in the wake. Sweeps are shown to be dominant in the region above the wind turbine array. A spectral analysis of the data shows that large scales of the flow, about the size of the rotor diameter in length or larger, dominate the vertical entrainment. The flow is more incoherent below the array, causing decreased vertical fluxes there. The results show that improving the rate of vertical kinetic energy entrainment into wind turbine arrays is a standing challenge and would require modifying the large-scale structures of the flow. [Preview Abstract] |
Tuesday, November 20, 2012 1:26PM - 1:39PM |
R31.00003: Effects of downstream and cross-stream spacings in an array of aligned wind turbines Daniel Houck, Matt Melius, Raul Cal This study seeks to address the effects of downstream and cross-stream spacings in a finite array of aligned wind turbines. A series of wind tunnel experiments were performed in which a 3x4 array of scale wind turbines were arranged in four patterns of different downstream (DS) and cross-stream (CS) spacings. The four cases, using the rotor diameter as unity, were: 6DSx3CS, 6DSx1.5CS, 3DSx3CS, and 3DSx1.5CS. All cases were subjected to the same inflow conditions, which replicated an atmospheric boundary layer. Particle image velocimetry was used to measure the flow field at the entrances and exits of the first and third rows of the array and velocity field data was collected. Results indicate effects of the downstream and cross-stream spacings separately, but also effects that appear to correlate to turbine density and not a specific arrangement. The cases often pair up with similar trends, but do so differently in the first and third rows and even, at times, in the entrances and exits and at different heights. The effects of cross-stream spacing are more pronounced in the first row than the third, and vice versa for downstream spacing. [Preview Abstract] |
Tuesday, November 20, 2012 1:39PM - 1:52PM |
R31.00004: Flow development comparison in two-bladed and three-bladed model wind turbine arrays C. Dalton McKeon, Jonathan Sullivan, Elizabeth Camp, Matthew Melius, Dominic Delucia, Raul Bayoan Cal, Luciano Castillo Vertical entrainment of energy through turbulent structures is compared between two-bladed and three-bladed model wind turbine arrays. A wind tunnel study under neutrally stratified conditions has been performed to compare the differences in large-scale structures of energy fluxes in two 3 $\times$ 4 arrays. Both arrays have three turbines with 3D spacing in the spanwise direction and four turbines with 6D spacing in the streamwise direction. The rotor diameter for both is 12 cm. The same mean velocity at hub height is maintained for both arrays. The power coefficient for both models is matched, resulting in different tip speed ratios. Consequently, both arrays of turbines are extracting energy from the flow at the same rate, which results in the identification of differences in energy fluxes due to the distinct number of blades on the rotor. Velocity data is collected via stereoscopic PIV; planes are located along the centerline of the array and are parallel with the streamwise direction. Profiles of mean velocity, Reynolds stresses, energy flux, and energy dissipation are generated. These profiles are used to compare the mechanisms of energy exchange in the two-bladed and three-bladed arrays. [Preview Abstract] |
Tuesday, November 20, 2012 1:52PM - 2:05PM |
R31.00005: Markovian properties of wind turbine wakes within a 3x3 array Matthew Melius, Murat Tutkun, Ra\'ul Bayo\'an Cal Wind turbine arrays have proven to be significant sources of renewable energy. Accurate projections of energy production is difficult to achieve because the wake of a wind turbine is highly intermittent and turbulent. Seeking to further the understanding of the downstream propagation of wind turbine wakes, a stochastic analysis of experimentally obtained turbulent flow data behind a wind turbine was performed. A 3x3 wind turbine array was constructed in the test section of a recirculating wind tunnel where X-wire anemometers were used to collect point velocity statistics. In this work, mathematics of the theory of Markovian processes are applied to obtain a statistical description of longitudinal velocity increments inside the turbine wake using conditional probability density functions. Our results indicate an existence of Markovian properties at scales on the order of the Taylor microscale, $\lambda$, which has also been observed and documented in different turbulent flows. This leads to characterization of the multi-point description of the wind turbine wakes using the most recent states of the flow. [Preview Abstract] |
Tuesday, November 20, 2012 2:05PM - 2:18PM |
R31.00006: Low-order flow modeling of vertical-axis wind turbine arrays Daniel Araya, Anna Craig, John Dabiri We present a potential flow model of recent experimental measurements of a full-scale array of vertical-axis wind turbines. Potential flow elements are used to approximate the flow physics of the array. Average velocity measurements, taken over the course of several months from different locations within the array, are used to compute velocity residuals, which are minimized to find the best-fit model. In addition, we present an approach to extend the empirical model to larger turbine array sizes by deducing the relation of the local flow velocity to the potential flow element strengths. This low-order modeling approach has the advantage of being simple enough for rapid optimization of small turbine arrays, yet robust enough for also at least qualitatively predicting the performance of larger arrays. [Preview Abstract] |
Tuesday, November 20, 2012 2:18PM - 2:31PM |
R31.00007: Low dimensional model of energy reconstruction for inline and offset wind turbine arrays Nicholas Hamilton, Murat Tutkun, Raul Bayoan Cal Wind tunnel data was obtained via stereo-Particle Image Velocimetry for wind turbine models in rectangular and row-offset array configurations. Inflow and wakes were compared statistically and through proper orthogonal decomposition (POD). Spacing within the array configurations lead to varied wake recovery conditions visible in statistics and POD analyses. The number of snapshot POD modes required to reconstruct specified energy levels for the wakes demonstrates a strong dependence on the array configuration. Quantities including flux of kinetic energy and production rebuild with fewer POD modes than vorticity and dissipation as they rely on large-scale structures for shape andintensity. Energy content and organization in small-scale structures contribute to the delay of recovery of the flow to upstream inflow conditions. [Preview Abstract] |
Tuesday, November 20, 2012 2:31PM - 2:44PM |
R31.00008: Experimental study of flow around scaled wind turbine arrays Ramiro Chavez Alarcon, B.J. Balakumar, Fangjun Shu An array of scaled model wind turbines, designed using blade element momentum theory, were investigated in a low-speed wind tunnel under uniform laminar inflow conditions. To investigate the influence of neighbor wind turbines, a 2D particle image velocimetry (PIV) system was used to measure the flow field in the wake of the turbines. It is found that flow in the wake of the turbine is axisymmetric about the turbine axis, and is not dependent on the blades orientation. A detailed dataset containing wake structure variations including velocity deficit and Reynolds stresses was obtained and compared with previously obtained data corresponding to a single wind turbine. The power extracted by a single wind turbine under the influence of the array was obtained from the velocity deficit observed at the wake and compared with the power obtained from torque sensor measurements. The complete investigation is compared with previously obtained data corresponding to a single wind turbine. [Preview Abstract] |
Tuesday, November 20, 2012 2:44PM - 2:57PM |
R31.00009: Performance and wake measurements for a cross-flow axis lift-driven turbine Peter Bachant, Martin Wosnik A 2.67 m wide, 3.67 m deep tow tank was used to measure performance and wake flow from a cross-flow axis (CFA) lift-driven turbine. A custom turbine test bed was designed that allows precise control of turbine tip speed ratio via servomotor, as well as measurements of power and overall drag on the turbine. Blade forces are measured with high spatio-temporal resolution via piezoelectric film sensors. The flow-field in the near wake is measured via high frame rate PIV and acoustic Doppler velocimetry. The size of the turbine model and a newly renovated tow tank mechanism allowed the blade chord Reynolds number to be roughly an order of magnitude higher than in previous studies. The overall goal of this study is to accurately measure fluid-blade interactions to observe effects of design parameter changes to performance and wake structure; also to help validate (or invalidate) numerical models. The higher Reynolds numbers of these results, especially those under dynamic stall conditions, make them more applicable to full scale commercial installations. [Preview Abstract] |
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