Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R26: Wind Turbines: Wake Analysis |
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Chair: Fotis Sotiropoulos, University of Minnesota Room: 2007 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R26.00001: Analysis of Tip Vortices Identified in the Instantaneous Wake of a Horizontal-Axis Model Wind Turbine Placed in a Turbulent Boundary Layer Akash Jain, Faraz Mehdi, Jian Sheng The near-wake field, a short region characterized by the physical specifications of a turbine, is of particular interest for flow-structure interactions responsible for asymmetric loadings, premature structural breakdown, noise generation etc. Helical tip vortices constitute a distinctive feature of this region and are dependent not only on the turbine geometry but also on the incoming flow profile. High-spatial resolution PIV measurements are made in the wake of a horizontal-axis model wind turbine embedded in a neutrally stratified turbulent boundary layer. The data is acquired over consecutive locations up to 10 diameters downstream of the turbine but the focus here is on the tip vortices identified in the instantaneous fields. Contrary to previous studies, both top and bottom tip vortices are clearly distinguishable in either ensemble fields or instantaneous realizations. The streamwise extent of these vortices stretches from the turbine till they merge into the expanding mid-span wake. The similarities and differences in the top and bottom tip vortices are explored through the evolution of their statistics. In particular, the distributions of the loci of vortex cores and their circulations are compared. The information will improve our understanding of near wake vortical dynamics, provide data for model validation, and aid in the devise of flow control strategies. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R26.00002: Effects of a three-dimensional hill on the wake characteristics of a model wind turbine Xiaolei Yang, Kevin B. Howard, Michele Guala, Fotis Sotiropoulos The spatial evolution of a turbine wake downwind of a sinusoidal hill is studied using large-eddy simulations and wind tunnel measurements. The computed flow fields behind the hill show good agreement with Particle Image Velocimetry measurements. It is observed that the turbine wake downwind of the hill recovers faster than the wake of the same turbine in the turbulent boundary layer flow (OT case) because of the increased entrainment of ambient flow into the turbine wake, which is due to enhanced turbulence convection in both the spanwise and vertical directions. It is also observed that the recovery rates of the available mean kinetic energy in the turbine wakes are nearly the same for turbine positions of 4D, 6D and 8D downwind of the hill (HT cases). The turbulence kinetic energy (TKE) in the turbine wake for all the HT cases exhibit significant increases as compared to that from the OT case. However, the profiles of the turbine-added TKE nearly collapse for all OT and HT cases (except in the turbine near wake region) when normalized by a characteristic velocity defined by the turbine thrust. A simple model for the turbine-added TKE in complex terrain is also proposed based on the new physical insights from the simulated cases. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R26.00003: Development of a Large Scale Field PIV System For Wake Measurement in a Wind Farm Larry Brock, Luciano Castillo, Jian Sheng Efficient utilization of wind energy requires detailed field measurements. Conventional techniques such as LIDAR and sonic anemometers can only provide low resolution point-wise measurement. Particle Image Velocimetry (PIV) is widely used in laboratory scale studies, however, has considerable difficulties for application in the field. The issues mainly arise due to the presence of background sunlight and the requirement of a large seeding volume. To address these issues, a novel, large-format, field PIV system is developed in this study. The PIV system is capable of measuring 2D velocity in a 1m X 1 m field of view with 0.2 mm spatial resolution and 7.6 mm vector spacing. The instrument achieves a three-decade measurement range, which enables the quantification of wide spectrum of wake structures as well as those in ABL. It can be applied to assess inflow conditions and to identify coherent structures in turbine wakes. The paper will present the principle of measurement and the development of optical, electrical and mechanical systems, as well as the preliminary measurement in an experimental wind farm. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R26.00004: Near-wake instability and sensitivity analysis of wind turbines immersed in the atmospheric boundary layer Francesco Viola, Giacomo Valerio Iungo, Simone Camarri, Fernando Port\'e-Agel, Fran\c{c}ois Gallaire In wind farms, the separation distance among wind turbines is mainly determined by the downstream recovery of wind turbine wakes, which affects in turn power production and fatigue loads of downstream turbines. Thus, the optimization of a wind farm relies on the understanding of the single wake dynamics and a better characterization of their interactions within the atmospheric boundary layer (ABL). This work is focused on the stability analysis of vorticity structures present in wind turbine wakes. In order to take into account the effects of a non-uniform incoming wind investing the turbine, a 3D local stability analysis is performed on the non-axisymmetric swirling wake prevailing at different downstream stations. Different wind shear and veer of the incoming wind can now be investigated, together with a 3D non-isotropic turbulent velocity field. This procedure enables to perform stability analysis of wind turbine wakes for wind conditions very similar to the ones experienced in reality. The present analysis is carried out on wind tunnel data acquired in the wake of a down-scaled three-bladed wind turbine. The Reynolds stresses are taken into account via eddy-viscosity models calibrated on the experimental data. Furthermore, the effect of an external perturbation in the wake flow is investigated through linear sensitivity. This analysis represents a preliminary step for control of wind turbine wakes, and optimization of wake interactions and power harvesting. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R26.00005: Hub vortex instability and wake dynamics in axial flow wind turbines Daniel Foti, Kevin Howard, Xiaolei Yang, Michele Guala, Fotis Sotiropoulos The near wake region of an axial flow wind turbine has two distinct shear layers: an outer tip vortex shear layer, which rotates in the same direction as the rotor, and an inner counter-rotating hub vortex shear layer. Recent simulations (Kang et al., J. Fluid Mech., vol. 744, 2014, pp. 376-403), corroborated with experiments (Chamorro et al., J. Fluid Mech., vol. 716, 2013, pp. 658-670), showed that the hub vortex can undergo spiral vortex breakdown immediately downstream of the turbine. The precessing hub vortex core intercepts and interacts with the tip vortex shear layer causing the large-scale wake meandering motions in the far wake to intensify. These results were obtained for an axial flow hydrokinetic turbine in a turbulent open channel flow. Here we integrate high-resolution LES with experiments to show that a hub vortex instability also occurs in the near wake of a wind turbine in a wind tunnel. We show that the interactions of the hub vortex with the outer flow have significant effects on the wake meandering amplitude and frequency. Our results reinforce the conclusions of Kang et al. (2014) that the hub vortex must be included in wake models to simulate wake interactions at the power plant scale and optimize turbine siting for realistic terrain and wind conditions. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R26.00006: Measuring wind turbine wakes and unsteady loading in a micro wind farm model Juliaan Bossuyt, Charles Meneveau, Johan Meyers Very large wind farms, approximating the ``infinite'' asymptotic limit, are often studied with LES using periodic boundary conditions. In order to create an experimental realization of such large wind-turbine arrays in a wind tunnel experiment including over 100 turbines, a very small-scale turbine model based on a 3cm diameter porous disk is designed. The porous disc matches a realistic thrust coefficient between 0.75-0.85, and the far wake flow characteristics of a rotating wind turbine. As a first step, we characterize the properties of a single model turbine. Hot-wire measurements are performed for uniform inflow conditions with different background turbulence intensity levels. Strain gage measurements are used to measure the mean value and power spectra of the thrust force, power output and wind velocity in front of the turbine. The dynamics of the wind turbine are modeled making it possible to measure force spectra at least up to the natural frequency of the model. This is shown by reproducing the -5/3 spectrum from the incoming flow and the vortex shedding signatures of an upstream obstruction. An array with a large number of these instrumented model turbines is placed in JHU's Corrsin wind tunnel, to study effects of farm layout on total power output and turbine loading. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R26.00007: Development of a wind turbine wake in the infinite turbine array characterized via wall-normal-spanwise planes and cylindrical coordinates Ra\'{u}l Bayo\'{a}n Cal, Nicholas Hamilton A wind turbine wake was investigated experimentally through a wind tunnel experiment. Velocity fields oriented normal to the mean convective flow were measured using stereo-PIV every half-rotor diameter (6cm) in the wake. The full Reynolds stress tensor is available through SPIV measurements and shows that very near to the turbine the presence of the mast has a large influence over the stress fields. Further downstream gradients in the mean velocity soften and the stress fields become roughly axisymmetric. In the far wake of the wind turbine, the flow is well mixed and becomes more homogeneous, with vertical and spanwise velocities an order of magnitude less than inlet velocity. Previous research indicates that the flux in the vertical direction is the dominant contributor to the flux tensor in a plane aligned with the hub of the turbine. Data from the current experiment indicate that spanwise components of the flux tensor make a significant contribution at the edges of the wake. In polar-cylindrical coordinates, flux is considered radially inward from outside of the wake. In this frame of reference turbulence phenomena are assessed in a more natural sense. In a polar coordinate system, the production and flux tensors show a single dominant component, emphasizing the suitability of the current analysis. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R26.00008: Characterization of Reynolds and deterministic stresses through phase-dependent measurements in the near wake of a wind turbine in an infinite turbine array Nicholas Hamilton, Murat Tutkun, Ra\'{u}l Bayo\'{a}n Cal The wake of a wind was investigated experimentally through stereo-PIV measurements made in planes parallel to the rotor. Phase-locked data were collected at four angles of rotor orientation beginning from one blade at top-dead-center. The phase angle of the turbine rotor was measured with a remote optical sensor detecting a reflective portion of the rotor blades. Measurements in the wind turbine array include turbulent effects and mixing from leading devices, making phase dependence in turbulent structures difficult to detect for small differences in phase angle of the turbine rotor. Analysis of the flow field from a polar-cylindrical reference frame (r,$\theta $,x) with the axial coordinate aligned with the hub of the rotor highlights differences in the Reynolds stress tensor not evident in the Cartesian frame. The axial normal stress becomes independent of the phase angle of the rotor for x/D $\ge $ 1.5. Stresses including the radial and azimuthal velocity fluctuations retain phase dependence throughout the near wake of the turbine. Deterministic stresses are approximately two orders of magnitude smaller than the turbulent stresses indicating that they can be neglected at first order. The flux of kinetic energy and production of turbulence composed with phase-locked turbulent stresses make periodic contributions to the time-averaged values. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R26.00009: Higher order moments, structure functions and spectral ratios in near- and far-wakes of a wind turbine array Naseem Ali, A. Aseyev, J. McCraney, V. Vuppuluri, O. Abbass, T. Al Jubaree, M. Melius, R.B. Cal Hot-wire measurements obtained in a 3 x 3 wind turbine array boundary layer are utilized to analyze higher order statistics which include skewness, kurtosis as well as the ratios of structure functions and spectra. The ratios consist of wall-normal to streamwise components for both quantities. The aim is to understand the degree of anisotropy in the flow for the near- and far-wakes of the flow field where profiles at one diameter and five diameters are considered, respectively. The skewness at top tip for both wakes show a negative skewness while below the turbine canopy, this terms are positive. The kurtosis shows a Gaussian behavior in the near-wake immediately at hub-height. In addition, the effect due to the passage of the rotor in tandem with the shear layer at the top tip renders relatively high differences in the fourth order moment. The second order structure function and spectral ratios are found to exhibit anisotropic behavior at the top and bottom-tips for the large scales. Mixed structure functions and co-spectra are also considered in the context of isotropy. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R26.00010: Meandering patterns in the wake of horizontal-axis wind and river turbines Michele Guala, Kevin Howard, Arvind Singh, Craig Hill, Mirko Musa, Christopher Feist, Fotis Sotiropoulos Energy harvesting devices with rotor axis oriented with the flow generate a wake which is unstable due to the complex interactions among turbulent structures from the incoming flow, root, hub and tip vortices (see Foti et al. APS/DFD 2014). Experiments in wind tunnel and open-channel flow with erodible surface show similar meandering patterns in the velocity field, which are responsible for the far wake expansion and the incoming turbulence experienced by down-wind/stream units. Wake meandering statistics were observed to depend on the operating turbine conditions (tip speed ratio), upstream device siting (turbine -- turbine interaction) or specific turbine kinematics (floating turbine under waves). In addition, for wall boundary conditions defined by an erodible surface, where sand grains respond to local shear stress by moving (erosion) or settling (deposition), turbines were observed to induce dynamic topographic perturbations also exhibiting meandering patterns. This occurred in limited mobility conditions and under migrating bedforms, with large scale topographic features amplified under specific asymmetric turbine configurations. The work opens up the possibility to place turbines in complex flows optimizing their performance while maintaining, or reshaping, the surrounding topography by specific control or siting strategies. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R26.00011: ABSTRACT WITHDRAWN |
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