Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session F17: Aerodynamics: Wind Energy IIAerodynamics Energy

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Chair: Mark Miller, Princeton University Room: 605 
Monday, November 20, 2017 8:00AM  8:13AM 
F17.00001: On the Evolution of the Integral Length Scale in the Wake of Wind Turbines and within Wind Farms Huiwen Liu, Yaqing Jin, Imran Hayat, Leonardo P. Chamorro Wind tunnel experiments were performed to characterize the evolution of integral length scale in the wake of a single turbine, and around wind farms. Hotwire anemometry was used to obtain highresolution measurements of the streamwise velocity fluctuation at various locations. Negligible and high freestream turbulence levels were considered in the case of single turbine. The integral length scale along the rotor axis is found to grow nearly linearly with distance independent of the incoming turbulence levels, and appears to reach the incoming level in the high turbulence case at about 3540 rotor diameters downstream. In the wind farm, results suggest that the distribution of integral length scale can be roughly described by a powerlaw growth with distance within consecutive turbines. Approximately past the third row, the integral length scale appears to reach equilibrium of the spatial distribution. [Preview Abstract] 
Monday, November 20, 2017 8:13AM  8:26AM 
F17.00002: Velocity field and coherent structures in the near wake of a utilityscale wind turbine Jiarong Hong, Teja Dasari, Yue Wu, Yun Liu Superlargescale particle image velocity (SLPIV) and the associated flow visualization technique using natural snowfall have been shown as an effective tool to probe turbulent velocity field and coherent structures around utilityscale wind turbines (Hong et al. \textit{Nature Comm.} 2014). Here we present a followup study using the data collected during multiple deployments from 2014 to 2016 around the 2.5 MW turbine at EOLOS field station. The data include SLPIV measurements in the near wake of the turbine in a field of view of \textasciitilde 120 m (height) x 60 m (width), and the visualization of tip vortex behavior near the bottom blade tip over a broad range of turbine operational conditions. SLPIV results indicate a highly intermittent flow field in the near wake, consisting of both intense wake expansion and contraction events. Such intermittent states of the near wake are shown to be influenced by both the incoming wind conditions and the turbine operation. The visualization of tip vortex behavior demonstrates the presence of the state of consistent vortex formation as well as various types of disturbed vortex states. The occurrence of these states is statistically analyzed and is shown to be correlated with turbine operational and response parameters under different field conditions. [Preview Abstract] 
Monday, November 20, 2017 8:26AM  8:39AM 
F17.00003: Horizontal Axis Wind Turbine Experiments at FullScale Reynolds Numbers Mark Miller, Janik Kiefer, Tara Nealon, Carsten Westergaard, Marcus Hultmark Achieving high Reynolds numbers on a wind turbine model remains a major challenge for experimentalists. Since Reynolds number effects need to be captured accurately, matching this parameter is of great importance. The challenge stems from the large scale ratio between model and fullsize, typically on the order of 1:100. Traditional wind tunnels are limited due to finite tunnel size, with velocity as the only freeparameter available for increasing the Reynolds number. Unfortunately, increasing the velocity 100 times is untenable because it violates Mach number matching with the fullscale and results in unfeasible rotation rates. Present work in Princeton University’s high pressure wind tunnel makes it possible to evaluate the Reynolds number sensitivity with regard to wind turbine aerodynamics. This facility, which uses compressed air as the working fluid, allows for adjustment of the Reynolds number, via the fluid density, independent of the Tip Speed Ratio (TSR) and Mach number. Power and thrust coefficients will be shown as a function of Reynolds number and TSR for a model wind turbine. The Reynolds number range investigated exceeds $10 \times 10^6$ based on diameter and freestream conditions or $3 \times 10^6$ based on the tip chord, matching those of the fullscale. [Preview Abstract] 
Monday, November 20, 2017 8:39AM  8:52AM 
F17.00004: Determination of windturbinewake centerline for the analysis of the wakemeandering phenomenon Nicolas Coudou, Philippe Chatelain, Jeroen van Beeck, Laurent Bricteux The oscillatory motion of wind turbine wakes, also known as wake meandering, is crucial in wind farms as it increases unsteady loading, in particular yawing moments, on downstream turbines. The study of this phenomenon requires, as a first step, the determination of the position of the wake. Therefore, the aim of this work is to compare different techniques to detect the wake centerline based on the velocity/momentum deficit inside the wake or on the estimation of azimuthal vorticity centroids. These techniques are applied to the data obtained from LargeEddy simulations of the NREL 5MW wind turbine. The computations were performed with a vortexparticle mesh code with the wind turbine rotor modeled by means of immersed lifting lines. This study constitutes a first step towards the understanding of meandering mechanisms and its accurate operational modeling. [Preview Abstract] 
Monday, November 20, 2017 8:52AM  9:05AM 
F17.00005: Aeroelastic impact of aboverated waveinduced structural motions on the nearwake stability of a floating offshore wind turbine rotor Steven Rodriguez, Justin Jaworski The impact of aboverated waveinduced motions on the stability of floating offshore wind turbine nearwakes is studied numerically. The rotor nearwake is generated using a liftingline free vortex wake method, which is strongly coupled to a finite element solver for kinematically nonlinear blade deformations. A synthetic time series of relatively highamplitude/highfrequency representative of aboverated conditions of the NREL 5MW referece wind turbine is imposed on the rotor structure. To evaluate the impact of these aboverated conditions, a linear stability analysis is first performed on the near wake generated by a fixedtower wind turbine configuration at aboverated inflow conditions. The platform motion is then introduced via synthetic time series, and a stability analysis is performed on the wake generated by the floating offshore wind turbine at the same aboverated inflow conditions. The stability trends (disturbance modes versus the divergence rate of vortex structures) of the two analyses are compared to identify the impact that aboverated waveinduced structural motions have on the stability of the floating offshore wind turbine wake. [Preview Abstract] 
Monday, November 20, 2017 9:05AM  9:18AM 
F17.00006: Impact of tower modeling on wind turbine wakes Elektra Kleusberg, Philipp Schlatter, Dan Henningson Recent research suggests the importance of modeling the support structure (tower and nacelle) when investigating the wake development behind wind turbines. These investigations are however mostly limited to low ambient turbulence levels which seldomly occur in field conditions. We present numerical simulations of wind turbine wakes using the actuator line method under different inflow conditions including varying turbulence levels and sheared inflow. The wind turbine, which employs the NREL S826 airfoil, is modeled after experiments conducted at the Norwegian University of Science and Technology. The rotor is investigated when perpendicular to the inflow and at a yaw angle of 30 degrees. The support structure is modeled using lift and drag body forces based on tabulated data. The simulations are performed with the spectralelement code Nek5000. After discussing the setup of the numerical domain and the turbulent inflow boundary condition, the influence of the tower model is characterized under turbulent, sheared and uniform inflow and the impact on downstream turbines is evaluated. [Preview Abstract] 
Monday, November 20, 2017 9:18AM  9:31AM 
F17.00007: Large eddy simulation of turbine wakes using higherorder methods Georgios Deskos, Sylvain Laizet, Matthew D. Piggott, Spencer Sherwin Large eddy simulations (LES) of a horizontalaxis turbine wake are presented using the wellknown actuator line (AL) model. The fluid flow is resolved by employing higherorder numerical schemes on a 3D Cartesian mesh combined with a 2D Domain Decomposition strategy for an efficient use of supercomputers. In order to simulate flows at relatively high Reynolds numbers for a reasonable computational cost, a novel strategy is used to introduce controlled numerical dissipation to a selected range of small scales. The idea is to mimic the contribution of the unresolved smallscales by imposing a targeted numerical dissipation at small scales when evaluating the viscous term of the NavierStokes equations. The numerical technique is shown to behave similarly to the traditional eddy viscosity subfilter scale models such as the classic or the dynamic Smagorinsky models. The results from the simulations are compared to experimental data for a Reynolds number scaled by the diameter equal to $Re_D$=1,000,000 and both the timeaveraged stream wise velocity and turbulent kinetic energy (TKE) are showing a good overall agreement. At the end, suggestions for the amount of numerical dissipation required by our approach are made for the particular case of horizontalaxis turbine wakes. [Preview Abstract] 
Monday, November 20, 2017 9:31AM  9:44AM 
F17.00008: Verticalaxis wind turbine experiments at full dynamic similarity Subrahmanyam Duvvuri, Mark Miller, Ian Brownstein, John Dabiri, Marcus Hultmark This study presents results from pressurized (upto 200 atm) wind tunnel tests of a selfspinning 5blade model VerticalAxis Wind Turbine (VAWT). The model is geometrically similar (scale ratio 1:22) to a commercially available VAWT, which has a rotor diameter of 2.17 meters and blade span of 3.66 meters, and is used at the Stanford university field lab. The use of pressurized air as working fluid allows for the unique ability to obtain full dynamic similarity with field conditions in terms of matched Reynolds numbers ($Re$), tipspeed ratios ($\lambda$), and Mach number ($M$). Tests were performed across a wide range of $Re$ and $\lambda$, with the highest $Re$ exceeding the maximum operational field Reynolds number ($Re_{\textrm{max}}$) by a factor of 3. With an extended range of accessible $Re$ conditions, the peak turbine power efficiency was seen to occur roughly at $Re = 2\,Re_{\textrm{max}}$ and $\lambda = 1$. Beyond $Re > 2\,Re_{\textrm{max}}$ the turbine performance is invariant in $Re$ for all $\lambda$. A clear demonstration of Reynolds number invariance for an actual fullscale wind turbine lends novelty to this study, and overall the results show the viability of the present experimental technique in testing turbines at field conditions. [Preview Abstract] 
Monday, November 20, 2017 9:44AM  9:57AM 
F17.00009: Wind tunnel study of helical and straightbladed verticalaxis wind turbine wakes Maryam Bagheri, Daniel Araya It is hypothesized that blade curvature can serve as a passive means to control fluid entrainment and wake recovery in verticalaxis wind turbine (VAWT) arrays. We test this experimentally in a wind tunnel using two different VAWT configurations, one with straight blades and another with helical blades, keeping all other experimental parameters fixed. A smallscale, commercially available VAWT (15W max power) is used as the baseline wind tunnel model in each case. The commercial VAWT blades are replaced with either straight or helical blades that are 3Dprinted extrusions of the same airfoil crosssection. Results from smoke flow visualization, threecomponent wake velocity measurements, and turbine power data are presented. These results give insight into the potential use of VAWTs with curved blades in utilityscale wind farms. [Preview Abstract] 
Monday, November 20, 2017 9:57AM  10:10AM 
F17.00010: Improving the performances of HDarrieus crossflow turbines through proper detached end plate designs Thierry Villeneuve, Matthieu Boudreau, Guy Dumas Previous studies on HDarrieus crossflow turbines have highlighted the fact that their performances are highly sensitive to the detrimental effects associated with the blades’ tips. Wingtip devices could be designed in order to attenuate these effects, but the benefits of such devices are always impaired by their added viscous drag since they are moving with the turbine’s blades. In this context, the development of fixed and detached end plates, i.e., which are not in contact with the turbine’s blades, could reduce the tip losses without the undesirable added drag of typical wingtip devices moving with the blades. The case of a single stationary blade with detached end plates has first been investigated with RANS simulations in order to understand the mechanisms responsible for the increase of the blade’s lift. An analysis of the vorticity lines’ dynamics provides crucial insights into the effects of the gap width between the blade and the detached end plate on the blade’s loading and on the intensity of the tip vortices. Based on these observations, various configurations of detached end plates are tested on crossflow turbines via RANS and DDES simulations. Preliminary results show that appropriate detached end plates can significantly increase the turbines’ efficiency. [Preview Abstract] 
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