Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session D27: Focus Session: Wind Energy Fluid Dynamics II |
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Chair: John Dabiri, Caltech Room: Ballroom I-II |
Sunday, November 20, 2011 2:10PM - 2:23PM |
D27.00001: Competing mechanisms of momentum transport in large wind farms Johan Meyers, Charles Meneveau In very large wind farms in the atmospheric boundary layer, energy, and momentum are on average transported from layers above the farm downward towards the turbines (Calaf, Meneveau, Meyers, Phys. Fluids 2010). In the current work, we investigate in more detail the three-dimensional flows of mass, momentum and energy towards individual turbines, based on a suite of large-eddy simulations. We find that there are two competing mechanisms which bring momentum to the turbines, i.e. a sideways flux, and a top-down flux of momentum (sideways fluxes themselves are fed by a top-down flux in regions outside the turbine wake area). For large spanwise turbine spacings, sideways momentum fluxes are dominating; for small spanwise spacings, the top-down mechanism is dominant. Inspired by these observations, we propose a new integral model for wind-farm performance, in which competing fluxes of momentum are represented by closed analytical expressions obtained by integrating momentum equations over different regions in the ABL. [Preview Abstract] |
Sunday, November 20, 2011 2:23PM - 2:36PM |
D27.00002: Large-eddy simulation of atmospheric boundary layer flow through wind farms Fernando Porte-Agel, Yu-Ting Wu, Valerio Iungo, Hao Lu Simulating atmospheric boundary layer flow and its interactions with wind turbines is of great importance for optimizing the design (layout) and overall efficiency of wind farms. This presentation focuses on recent efforts to develop and validate a large-eddy simulation (LES) framework for wind energy applications. The subgrid-scale fluxes of momentum and heat are parameterized using tuning-free Lagrangian scale-dependent dynamic models (Stoll and Porte-Agel 2006). The turbine-induced forces are parameterized using two types of models: an actuator disk model that allows for non-uniform distribution of the forces and includes rotational effects (Wu and Porte-Agel 2011); and an actuator line model that distributes the forces on lines that follow the position of the blades. The LES code is validated against wind-tunnel measurements collected with hot-wire anemometry inside and above a large model wind farm. The characteristics of the wind farm wakes simulated with the proposed LES framework, and particularly the spatial distribution of the velocity deficit and turbulence intensity, are in good agreement with the measurements. Finally, in order to extend the LES validation to field conditions, results are presented from a field experiment aiming to characterize wind-turbine wakes using three scanning wind LiDARs. [Preview Abstract] |
Sunday, November 20, 2011 2:36PM - 2:49PM |
D27.00003: Power measurements on a stratified 3 $\times$ 3 wind turbine array Dominic DeLucia, Nicholas Hamilton, Ra\'{u}l Bayo\'{a}n Cal Extracting energy out of the wind is the ultimate goal of wind turbine technology and understanding its variation due to the local environment is of interest. Power measurements are carried out in a wind turbine array boundary layer. Measurements are done independently using a torque sensing device which employs strain gages to measure it; following the design of Kang \& Meneveau (2010).\footnote{H. S. Kang and C. Meneveau (2010), Meas. Sci. \& Tech. 21, 105206.} The power curves are obtained based on a three by three array submerged in an atmospheric turbulent boundary layer, which is generated through the use of roughness elements, an active grid and spires. The power measurements are acquired for the turbines in the array while modifying the local environment. This is done using a thermal floor where an unstable boundary layer is created and then compared with a neutral case. [Preview Abstract] |
Sunday, November 20, 2011 2:49PM - 3:02PM |
D27.00004: LES investigation of turbine spacing effects in wind farms Xiaolei Yang, Fotis Sotiropoulos We study turbine spacing and layout effects in large wind farms using large-eddy simulation (LES) with the actuator disk model to represent individual turbines. The actuator disk model is implemented in our second-order accurate immersed boundary finite-difference solver using the discrete delta functions to interpolate the velocities on the disk and distribute the body forces to the surrounding fluid points. For aligned wind farms, the effects of the streamwise and spanwise spacings on the extracted power and turbulence intensities in the vicinity of the turbines are systematically studied. A model for the effective roughness length is also developed based on the present numerical results. As part of our future work, the effect of turbine spacing in staggered wind farms will be investigated. [Preview Abstract] |
Sunday, November 20, 2011 3:02PM - 3:15PM |
D27.00005: Large Eddy Simulation study of scalar transport in fully developed wind-turbine array boundary layers Marc Calaf, Marc B. Parlange, Charles Meneveau Large wind farms are attaining scales at which two-way interactions with the atmospheric boundary layer must be taken into account. A recent study by Baidya et al. (PNAS 2010) has shown that wind farms increase scalar fluxes at the surface. Numerical simulations from Calaf et al. (Pof 2010) together with laboratory experiments from Cal et al. (JSRE 2010) showed that the friction velocity underneath the wind turbines is decreased. Conversely, above the turbine, friction velocity is increased. To shed light onto the relevant phenomena, a suite of Large Eddy Simulations of an infinite (fully developed) wind turbine array boundary layer, including passive scalar transport, is performed. Results clearly show an overall increase of scalar fluxes in the presence of wind turbines, of about 10-15\%. And this increase is not highly dependent on wind turbine loading or spacing. This resultant increase in the scalar fluxes can be explained through a balance between two competing effects. Further, following the approach of Calaf et al. (PoF 2010), a single-column model has been developed which confirms the observed trends. [Preview Abstract] |
Sunday, November 20, 2011 3:15PM - 3:28PM |
D27.00006: Turbine layout effects on the flow structure inside an above large wind farms Leonardo Chamorro, Roger Arndt, Fotis Sotiropoulos An understanding of the role of the wind farm layout on the vertical momentum transport above a wind farm is essential to improve energy production of the different turbines. We investigate the turbulent structure of the flow inside and above a large model wind farm (roughly in fully developed conditions). The large array of turbines consisted of several columns of turbines spaced three abreast in an aligned configuration. The length of the wind farm was over fifteen boundary layer thicknesses. Turbine spacing of 6, 8, 10 and 12 rotor diameter was considered for the analysis. Full characterization of the turbulent flow was obtained between two rows of turbines far inside the wind farms in a vertical plane parallel to the direction of the flow and two spanwise-vertical planes were also included. A cross-wire anemometer was used to obtain high resolution measurements of streamwise and vertical velocity components at various locations at 10 KHz for a sampling period of 120 s at each location. Turbulence statistics, scale-to-scale interaction and TKE budget terms are evaluated to determine the role of the turbine layout on the turbulent dynamics of the flow. [Preview Abstract] |
Sunday, November 20, 2011 3:28PM - 3:41PM |
D27.00007: Wind Resource Evaluation at the Caltech Field Laboratory for Optimized Wind Energy Quinn Mulligan, Matthias Kinzel, John Dabiri Wind resources are evaluated at the Caltech Field Laboratory in order to understand how an array of vertical-axis wind turbines extracts energy from the flow. A tower with sonic anemometers placed every meter over the turbine's rotor height is deployed in upwind and downwind positions relative to the array of turbines to obtain the three dimensional wind velocity vectors. Upwind of the array, far enough to be considered free stream, the measured velocity profile represents the turbulent boundary layer flow at the site. Downwind, the measured wind velocities are reduced significantly and display a smaller variance over the rotor height. The topmost sensor, located above the top of the rotor height, reports flow velocities close to the free stream quantities. Sweeps and ejections are both present in the downwind velocity profile. The talk will present the data from these field measurements, discuss the similarities and differences to canopy flows and draw conclusions about the interaction between the wind turbine array and the flow. [Preview Abstract] |
Sunday, November 20, 2011 3:41PM - 3:54PM |
D27.00008: Experimental Study of Aligned and Staggered Wind Farms in a Convective Boundary Layer Corey Markfort, Wei Zhang, Fernando Porte-Agel Wind farm-atmosphere interaction is complicated by turbine configuration and thermal effects on momentum and kinetic energy fluxes. Wind farms of finite length have been modeled as increased surface roughness or as a sparse canopy; however it is not clear which approach is more appropriate. Experiments were conducted in a thermally controlled boundary layer wind tunnel, using a custom x-wire/cold wire and surface heat flux sensors, to understand the effect of aligned versus staggered turbine configurations on momentum absorption and flow adjustment in a convective boundary layer (CBL). Results for experiments of a large farm show the span-wise averaged flow statistics exhibit similar turbulent transport properties to that of canopy flows. The wake adjusts within and grows over the farm more quickly for a staggered compared to an aligned farm. Using canopy flow scaling, we show that the flow equilibrates faster and the overall momentum absorption is higher in a staggered compared to an aligned farm. Wake recovery behind a single turbine is facilitated by buoyancy in a CBL (Zhang et al. under review). We find a similar effect in wind farms resulting in reduced effective roughness and momentum absorption. We also find a reduction of surface heat flux for both wind farms, but greater for the staggered farm. [Preview Abstract] |
Sunday, November 20, 2011 3:54PM - 4:07PM |
D27.00009: Experimental investigation of turbulence characteristic around a scaled wind turbine Ramiro Chavetz Alarcon, B.J. Balakumar, Fangjun Shu Experiments on a scaled model wind turbine, designed using blade element momentum theory, were performed under laminar inflow conditions with and without yaw. A detailed dataset containing wake structure variations under yawed inflow was obtained to provide useful validation data for certain classes of simulation codes. Phase locked PIV experiments performed at various blade orientations (phases) showed that the turbulence characteristics in the mid- to far-wake region is approximately axisymmetric. The power extracted by the model was obtained from the horizontal velocity deficit observed at the wake and compared with the power obtained from torque sensor measurements. Significant differences between these two measurements demonstrate the importance of losses due to viscous and turbulent dissipation. [Preview Abstract] |
Sunday, November 20, 2011 4:07PM - 4:20PM |
D27.00010: Boundary Layer Effects on Wind Turbine's Tip Vortices using PIV Measurements David Green, Leonardo Chamorro, Fotis Sotiropoulos, Roger Arndt, Jian Sheng Understanding the complex interactions between vortical flow structures of the Horizontal Axis Wind Turbine (HAWT) and the atmospheric boundary layer is crucial to optimize blade design and turbine spacing in a wind farm. Tip vortices shed by the blades often play a key role. This paper focuses on the boundary layer flow interacting with a single turbine and multiple turbines in an in-line configuration using Particle Image Velocimetry (PIV) technique. The model has three blades with a span of 6.4 cm and 1.5 cm chord length. The tip speed ratio is set at roughly 5. The models are roughly within one fourth of the boundary layer. PIV measurement is phase locked on the position as the blade is passing through the measurement plane. Flow fields are captured up to 10 diameters or 87 chord lengths downstream. The effects on the turbine-generated vortical structures in a turbulent boundary layer are analyzed. Comparison to tip vortices produced in a free-stream mean flow will also be presented. [Preview Abstract] |
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