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 G12: Wind Turbines: Wind Farms I |
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Chair: Marc Calaf, University of Utah Room: 200 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G12.00001: Wind Turbine Box - energy fluxes around a characteristic wind turbine Marc Calaf, Gerard Cortina, Varun Sharma This research project presents a new tool, so called ``Wind Turbine Box'', that allows for the direct comparison between the flow around a single wind turbine and the flow around a characteristic wind turbine immersed within a large wind farm. The Wind Turbine Box consists of a limited control volume defined around each wind turbine that is timely co-aligned with each corresponding turbine's yaw-angle. Hence it is possible to extract flow statistics around each wind turbine, regardless of whether the turbine is fully isolated or it is plunged within a large wind farm. The Wind Turbine Box tool has been used to compute the energy fluxes around a characteristic wind turbine of a large wind farm to better understand the wake replenishment processes throughout a complete diurnal cycle. The effective loading of the wind farm has been gradually increased, ranging from quasi-isolated wind turbines to a highly packed wind farm. For this purpose, several Large Eddy Simulations have been run, forced with a constant geostrophic wind and a time varying surface temperature extracted from a selected period of the CASES-99 field experiment. Results illustrate the differences in the flow dynamics as it evolves around a characteristic wind turbine within a large wind farm and its asymptotic transition to the fully developed wind turbine array boundary layer. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G12.00002: Mean kinetic energy budget of wakes within an array of model wind turbines and porous discs Ra\'{u}l Bayo\'{a}n Cal, Elizabeth Camp Wind turbines are often modeled as porous actuator discs within computational studies. In this wind tunnel study, stereo particle image velocimetry (SPIV) is used to characterize the wakes within a 4$\times$3 model wind turbine array and an analogous array of porous disks. The SPIV measurements are performed upstream between $-2.9\leq x/D \leq -0.3$ and downstream between $0.7\leq x/D \leq 5.6$ of the center turbine in the fourth row. To provide context, the similarities and differences in the flow fields as well as the mean and turbulent stresses are found. The primary analysis revolves around the mean kinetic energy budget in the wakes for both cases, model turbines and discs, obtained by the computation of mean kinetic energy, production of turbulence and flux of kinetic energy as these are equivalent to a measure of extracted power. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G12.00003: Proper orthogonal decomposition of wakes within a model wind turbine array and a matched array of porous discs Elizabeth Camp, Ra\'{u}l Bayo\'{a}n Cal Porous actuator discs are commonly used in computational simulations to represent wind turbines. Wind tunnel data of a 4$\times$3 model wind turbine array and an array porous discs is obtained via stereo particle image velocimetry. Snapshot Proper Orthogonal Decomposition (POD) is used to compare the characteristics of the wake of the center turbine model in the fourth row with those of a matched porous disk in the same position. In considering the near- and far-wake, an examination of the energy content of the modes, nature of the modes themselves as well as the rate of reconstruction of low dimensional representations of flow quantities is attained. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G12.00004: Effects of Turbine Spacing in Very Large Wind Farms S{\O}ren Juhl Andersen, Jens N{\O}rk{\AE}r S{\O}rensen, Robert Flemming Mikkelsen The Dynamic Wake Meandering model(DWM) by Larsen et al.(2007) is considered state of the art for modelling the wake behind a wind turbine. DWM assumes a quasi-steady wake deficit transported as a passive tracer by large atmospheric scales. The approach is also applied to wake interaction within wind farms, although certain aspects of the complex wake interaction are not captured, see Churchfield et al.(2014). Recent studies have shown how turbines introduce low frequencies in the wake, which could describe some of the shortcomings. Chamorro et al.(2015) identified three regions of different lengths scales. Iungo et al.(2013) related low frequencies to the hub vortex instability. Okulov et al.(2014) found Strouhal numbers in the far wake stemming from the rotating helical vortex core. Simulations by Andersen et al.(2013) found low frequencies to be inherent in the flow inside an infinite wind farm. LES simulations of large wind farms are performed with full aero-elastic Actuator Lines. The simulations investigate the inherent dynamics inside wind farms in the absence of atmospheric turbulence compared to cases with atmospheric turbulence. Resulting low frequency structures are inherent in wind farms for certain turbine spacings and affect both power production and loads. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G12.00005: LES studies of wind farms including wide turbine spacings and comparisons with the CWBL engineering model Richard Stevens, Dennice Gayme, Johan Meyers, Charles Meneveau We present results from large eddy simulations (LES) of wind farms consisting of tens to hundreds of turbines with respective streamwise and spanwise spacings approaching 35 and 12 turbine diameters. Even in staggered farms where the distance between consecutive turbines in the flow direction is more than 50 turbine diameters, we observe visible wake effects. In aligned farms, the performance of the turbines in the fully developed regime, where the power output as function of the downstream position becomes constant, is shown to primarily depend on the streamwise distance between consecutive turbine rows. However, for other layouts the power production in the fully developed regime mainly depends on the geometrical mean turbine spacing (inverse turbine density). These findings agree very well with predictions from our recently developed coupled wake boundary layer (CWBL) model, which introduces a two way coupling between the wake (Jensen) and top-down model approaches (Stevens et al. JRSE 7, 023115, 2015). To further validate the CWBL model we apply it to the problem of determining the optimal wind turbine thrust coefficient for power maximization over the entire farm. The CWBL model predictions agree very well with recent LES results (Goit \& Meyers, JFM 768, 5-50, 2015). [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G12.00006: Shifted periodic boundary conditions for large-eddy simulation of wind farms Wim Munters, Charles Meneveau, Johan Meyers In wall-bounded turbulent flow simulations, periodic boundary conditions combined with insufficiently long domains lead to persistent spanwise locking of large-scale turbulent structures. In the context of wind-farm large-eddy simulations, this effect induces artificial spanwise inhomogeneities in the time-averaged local wind conditions as seen by the wind turbines, leading to spurious differences in power prediction between otherwise equivalent columns of wind turbines in a wind farm (a column is defined here as a set of turbines parallel to the mean flow direction). We propose a shifted periodic boundary condition that eliminates this effect without the need for excessive streamwise domain lengths. Instead of straightforwardly reintroducing the velocity from the outlet plane back at the inlet, as in classic periodic boundary conditions, this plane is first shifted in the spanwise direction by a predefined and constant distance. The method is tested based on a set of direct numerical simulations of a turbulent channel flow, and large-eddy simulations of a high Reynolds number rough-wall half-channel flow. Finally, we apply the method in a precursor simulation, generating inlet conditions for a spatially developing wind-farm boundary layer. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G12.00007: Effects of subgrid-scale modeling on wind turbines flows Umberto Ciri, Maria Vittoria Salvetti, Stefano Leonardi The increased demand for wind energy had led to a continuous increase in the size of wind turbines and, consequently, of wind farms. A potential drawback of such large clusters lies in the decrease in the efficiency due to the wake interference. Large-Eddy Simulations (LES) coupled with blade models have shown the capability of resolving the unsteady nature of wind turbine wakes. In LES, subgrid-scale (SGS) models are needed to introduce the effect of the turbulence small scales not resolved by the computational grid. Many LES of wind farms employ the classic Smagorinsky model, despite it suffers from some major drawbacks, e.g. (i) the presence of an input tuning parameter and (ii) the wrong behaviour near solid walls. In the present work an analysis of the effects of various SGS models is carried out for LES in which the turbine tower and nacelle are directly simulated with the Immersed Boundaries method. Particular attention is dedicated to the region of separated flow behind the tower where the impact of the SGS models is expected to be important. We focus herein on non-dynamic eddy-viscosity models, which have proven to have a correct behaviour near solid walls. A priori and a posteriori tests are performed for a configuration reproducing an experiment conducted at NTNU. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G12.00008: Structure Function Scaling Exponent and Intermittency in the Wake of a Wind Turbine Array Aleksandr Aseyev, Naseem Ali, Raul Cal Hot-wire measurements obtained in a $3 \times 3$ wind turbine array boundary layer are utilized to analyze high order structure functions, intermittency effects as well as the probability density functions of velocity increments at different scales within the energy cascade. The intermittency exponent is found to be greater in the far wake region in comparison to the near wake. At hub height, the intermittency exponent is found to be null. ESS scaling exponents of the second, fourth, and fifth order structure functions remain relatively constant as a function of height in the far-wake whereas in the near-wake these highly affected by the passage of the rotor thus showing a dependence on physical location. When comparing with proposed models, these generally over predict the structure functions in the far wake region. The pdf distributions in the far wake region display wider tails compared to the near wake region, and constant skewness hypothesis based on the local isotropy is verified in the wake. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G12.00009: Inverse structure functions in the canonical wind turbine array boundary layer Bianca Viggiano, Moira Gion, Naseem Ali, Murat Tutkun, Ra\'ul Bayo\'an Cal Insight into the statistical behavior of the flow past an array of wind turbines is useful in determining how to improve power extraction from the overall available energy. Considering a wind tunnel experiment, hot-wire anemometer velocity signals are obtained at the centerline of a 3 x 3 canonical wind turbine array boundary layer. Two downstream locations are considered referring to the near- and far-wake, and 21 vertical points were acquired per profile. Velocity increments are used to quantify the ordinary and inverse structure functions at both locations and their relationship between the scaling exponents is noted. It is of interest to discern if there is evidence of an inverted scaling. The inverse structure functions will also be discussed from the standpoint of the proximity to the array. Observations will also address if inverted scaling exponents follow a power law behavior and furthermore, extended self-similarity of the second moment is used to obtain the scaling exponent of other moments. Inverse structure functions of moments one through eight are tested via probability density functions and the behavior of the negative moment is investigated as well. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G12.00010: Wind Turbine Experiments at Full Dynamic Similarity Mark Miller, Janik Kiefer, Carsten Westergaard, Marcus Hultmark Performing experiments with scaled-down wind turbines has traditionally been difficult due to the matching requirements of the two driving non-dimensional parameters, the Tip Speed Ratio (TSR) and the Reynolds number. Typically, full-size turbines must be used to provide the baseline cases for engineering models and computer simulations where flow similarity is required. We present a new approach to investigating wind turbine aerodynamics at full dynamic similarity by employing a high-pressure wind tunnel at Princeton University known as the High Reynolds number Test Facility (or HRTF). This facility allows for Reynolds numbers of up to 3 million (based on chord and velocity at the tip) while still matching the TSR, on a geometrically similar, small-scale model. The background development of this project is briefly presented including the design and manufacture of a model turbine. Following this the power, thrust and wake data are discussed, in particular the scaling dependence on the Reynolds number. [Preview Abstract] |
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