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 D28: Wind Turbines: Blade Design |
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Chair: Fotis Sotiropoulos, University of Minnesota Room: 309 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D28.00001: Instability of outer tip vortices for a 2.5 MW wind turbine: integrating snow PIV with LES Fotis Sotiropoulos, Xiaolei Yang, Jiarong Hong, Matthew Barone Recent field experiments conducted around a 2.5 MW wind turbine using super-large-scale PIV (SLPIV) using natural snow particles have revealed tip vortex cores (visualized as areas devoid of snowflakes) of complex shape, consisting of both round and elongated void patterns. Here we employ large-eddy simulation to elucidate the structure and dynamics of the complex tip vortices identified experimentally. The LES is shown to reproduce vortex cores in remarkable agreement with the SLPIV results, essentially capturing all vortex core patterns observed in the field in the tip shear layer. We show that the stretched elongated vortex cores observed in 2D planes are the footprints of a second set of counter-rotating spiral vortices that emanates along the tip shear layer immediately downwind of the blades and is intertwined with the tip vortices. We argue that this large-scale instability is of centrifugal type since the mean flow characteristics in the outer tip shear layer resemble those of the Taylor-Couette flow. This study highlights the feasibility of employing snow voids to visualize tip vortices and demonstrates the enormous potential of integrating SLPIV with LES as a powerful tool for gaining novel insights into the wakes of utility scale wind turbines. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D28.00002: Experimental Study on the Effects of Winglets on the Wake of a Model Wind Turbine Nicolas Tobin, Ali M. Hamed, Leonardo P. Chamorro Wind tunnel particle image velocimetry was used to investigate the effects of winglets on the wake dynamics of a model wind turbine. The behavior of a turbine with downstream-facing winglets was compared with a turbine without winglets. The turbines were placed in a turbulent boundary layer that reached up to the hub height, allowing for investigation of behavior in both turbulent and uniform flow. The winglets did not significantly change the strength of the tip vortices in the region of uniform incoming flow. The tip vortices in the more turbulent region, however, decayed much faster, diminishing to near-zero within the first $\sim$1.5 rotor diameters, whereas the upper tip vortices persisted potentially up to $\sim$4 rotor diameters. The winglets also increased the power coefficient by 7.5$\%$, while increasing the coefficient of thrust by 10.0$\%$. The higher coefficient of thrust created a region of enhanced mean shear in the outer portion of the wake, leading to increased turbulence statistics in the far wake. The wingletted turbine had a similar wake deficit at 5 rotor diameters as the base turbine did at 1.5 rotor diameters, with potential implications for using wingletted turbines in wind farms. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D28.00003: Fluid-structure coupling for wind turbine blade analysis using OpenFOAM Bastian Dose, Ivan Herraez, Joachim Peinke Modern wind turbine rotor blades are designed increasingly large and flexible. This structural flexibility represents a problem for the field of Computational Fluid Dynamics (CFD), which is used for accurate load calculations and detailed investigations of rotor aerodynamics. As the blade geometries within CFD simulations are considered stiff, the effect of blade deformation caused by aerodynamic loads cannot be captured by the common CFD approach. Coupling the flow solver with a structural solver can overcome this restriction and enables the investigation of flexible wind turbine blades. For this purpose, a new Finite Element (FE) solver was implemented into the open source CFD code OpenFOAM. Using a beam element formulation based on the Geometrically Exact Beam Theory (GEBT), the structural model can capture geometric non-linearities such as large deformations. Coupled with CFD solvers of the OpenFOAM package, the new framework represents a powerful tool for aerodynamic investigations. In this work, we investigated the aerodynamic performance of a state of the art wind turbine. For different wind speeds, aerodynamic key parameters are evaluated and compared for both, rigid and flexible blade geometries. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D28.00004: Dominant mechanism of load fluctuations on a wind turbine in a realistic atmosphere through Hybrid URANS-LES Ganesh Vijayakumar, Adam Lavely, Balaji Jayaraman, Brent Craven, James Brasseur Atmospheric turbulence causes load fluctuations on a wind turbine through various forcing mechanisms across a wide span of time scales relative to the rotation time scale. We identify the dominant mechanisms of load fluctuation through blade-boundary-layer-resolved hybrid URANS-LES of a single rotating blade of the NREL-5MW turbine in a daytime moderately convective atmospheric boundary layer (ABL) on flat terrain with surface heating simulated with high-fidelity LES. We find that the integral scale motions in the atmosphere cause the largest fluctuations over multiple rotations of the turbine, while the rotation of the turbine through eddies in the ABL cause fluctuations at the rotation time scale. Blade-boundary-layer dynamics—separation, dynamic stall and rotational augmentation, cause further load fluctuations at time scales much smaller than the rotation time scale. At all time-scales, however, we find that the dominant mechanism underlying load fluctuation on the blade is from local spatio-temporal fluctuations in the angle of attack (AoA) associated with atmospheric eddy passage. By integrating fundamental kinematic analysis with high-resolution CFD, we describe the fundamental role of ABL-turbulence-forced AoA fluctuations on nonsteady wind turbine loadings. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D28.00005: An Aeroelastic Perspective of Floating Offshore Wind Turbine Wake Formation and Instability Steven N. Rodriguez, Justin W. Jaworski The wake formation and wake stability of floating offshore wind turbines are investigated from an aeroelastic perspective. The aeroelastic model is composed of the Sebastian-Lackner free-vortex wake aerodynamic model coupled to the nonlinear Hodges-Dowell beam equations, which are extended to include the effects of blade profile asymmetry, higher-order torsional effects, and kinetic energy components associated with periodic rigid-body motions of floating platforms. Rigid-body platform motions are also assigned to the aerodynamic model as varying inflow conditions to emulate operational rotor-wake interactions. Careful attention is given to the wake formation within operational states where the ratio of inflow velocity to induced velocity is over 50\%. These states are most susceptible to aerodynamic instabilities, and provide a range of states about which a wake stability analysis can be performed. In addition, the stability analysis used for the numerical framework is implemented into a standalone free-vortex wake aerodynamic model. Both aeroelastic and standalone aerodynamic results are compared to evaluate the level of impact that flexible blades have on the wake formation and wake stability. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D28.00006: POD based analysis of three-dimensional stall over a pitching wind turbine blade Matthew Melius, Raul Bayoan Cal, Karen Mulleners Aerodynamic performance of a wind turbine blade is a predominant factor in its power production. Under dynamic loading conditions, predicted aerodynamic loads often do not match operational loads. In the interest of gaining understanding of the complex flow over wind turbine blades, a three-dimensional scaled blade model has been designed and manufactured to be dynamically similar to a rotating full-scale NREL 5MW wind turbine blade. Time resolved particle image velocimetry (PIV) measurements collected over the suction surface of an inboard section of the experimental turbine blade. Flow characteristics are analyzed using coherent structure identification techniques to capture dynamic stall behavior. Proper orthogonal decomposition (POD) is applied to the velocity field providing information about separation point and stall development time scales based on the associated time coefficients and modes. Additionally, continuity and circulation calculations are used to capture three dimensional effects within stalled volumes during developing stall and re-attachment phases of dynamic stall. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D28.00007: Reduced-order FSI simulation of NREL 5 MW wind turbine in atmospheric boundary layer turbulence Javier Motta-Mena, Robert Campbell, Adam Lavely, Pankaj Jha A partitioned fluid-structure interaction (FSI) solver based on an actuator-line method solver and a finite-element modal-dynamic structural solver is used to evaluate the effect of blade deformation in the presence of a day-time, moderately convective atmospheric boundary layer (ABL). The solver components were validated separately and the integrated solver was partially validated against FAST. An overview of the solver is provided in addition to results of the validation study. A finite element model of the NREL 5 MW rotor was developed for use in the present simulations. The effect of blade pitching moment and the inherent bend/twist coupling of the rotor blades are assessed for both uniform inflow and the ABL turbulence cases. The results suggest that blade twisting in response to pitching moment and the bend/twist coupling can have a significant impact on rotor out-of-plane bending moment and power generated for both the uniform inflow and the ABL turbulence cases. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D28.00008: Turbulent Potential Model Predictions of High Re Flow Around the S809 Airfoil Nathaniel DeVelder Utility scale wind turbines operate at a range of chord-based Reynolds numbers often between $10^6$ and $10^7$. Reynolds Averaged Navier-Stokes (RANS) models offer computational efficiency at high Reynolds numbers. As a model that avoids the eddy-viscosity hypothesis, the Turbulent Potential Model, a time-varying RANS model, is identified as an appropriate balance between computational resource usage and physical fidelity. Development of the Turbulent Potential Model is discussed. Comparisons are made between Turbulent Potential Model results and Moser's Direct Numerical Simulation $Re_{\tau}$=590 channel flow. S809 airfoil simulations at $\alpha=0.02 ^{\circ}$, $\alpha=4.03 ^{\circ}$, $\alpha=10.03 ^{\circ}$, and $\alpha=20.11 ^{\circ}$ are compared to results from the $k-\omega SST$, Spalart-Allmaras, and $v^{2}-f$ models, as well as wind tunnel results from Ohio State University. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D28.00009: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D28.00010: Effect of blade loading and rotor speed on the optimal aerodynamic performance of wind turbine blades Christopher Bryson, Fazle Hussain, Alan Barhorst Optimization of wind turbine torque as a function of angle of attack - over the entire speed range from start-up to cut-off - is studied by considering the full trigonometric relations projecting lift and drag to thrust and torque. Since driving force and thrust are geometrically constrained, one cannot be changed without affecting the other. Increasing lift to enhance torque simultaneously increases thrust, which subsequently reduces the inflow angle with respect to the rotor plane via an increased reduction in inflow velocity. Reducing the inflow angle redirects the lift force away from the driving force generating the torque, which may reduce overall torque. Similarly, changes in the tip-speed ratio (TSR) affect the inflow angle and thus the optimal torque. Using the airfoil data from the NREL 5 MW reference turbine, the optimal angle of attack over the operational TSR range (4 to 15) was computed using a BEM model to incorporate the dynamic coupling, namely the interdependency of blade loading and inflow angle. The optimal angle of attack is close to minimum drag during start-up phase (high TSR) and continuously increases toward maximum lift at high wind speeds (low TSR). [Preview Abstract] |
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