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 L30: Wind Turbines: Blade Designs |
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Chair: Luciano Castillo, Texas Technical University Room: 2016 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L30.00001: Passive control of a dynamically pitching wind turbine airfoil under aeroelastic conditions using a Gurney flap Pourya Nikoueeyan, Andrew Magstadt, John Strike, Michael Hind, Jonathan Naughton To reduce the cost of energy, wind turbine design has moved towards larger blades that are heavier and have lower relative structural stiffness compared to shorter blades. To address the lower blade stiffness, different flow control techniques have been considered. The Gurney flap, a small, low-cost and effective control method, is a promising control actuator. Wind tunnel testing has been performed on a DU97-W-300 10\% flatback airfoil undergoing dynamic pitching relevant to flow conditions encountered by wind turbine blades. To mimic blade compliance, the airfoil is actively driven through a torsionally elastic element. Time-resolved surface pressure measurements have been acquired from which lift $C_l$ and moment $C_m$ coefficients were calculated. Changes in $C_l$ and $C_m$ in moderate and deep dynamic stall regimes for different Gurney flap heights were studied for different pitch drive conditions (amplitude and frequency). The results show the significant impact of compliance on the angle of attack ($\alpha$) range experienced by the airfoil. Shifts in $\alpha$ range result in different hysteresis behavior in both $C_l$ and $C_m$ and demonstrate the effectiveness of the Gurney flap in modifying the aerodynamics of wind turbine blades experiencing dynamic pitching. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L30.00002: Sub-scale Inverse Wind Turbine Blade Design Using Bound Circulation Christopher Kelley, Jonathan Berg A goal of the National Rotor Testbed project at Sandia is to design a sub-scale wind turbine blade that has similitude to a modern, commercial size blade. However, a smaller diameter wind turbine operating at the same tip-speed-ratio exhibits a different range of operating Reynolds numbers across the blade span, thus changing the local lift and drag coefficients. Differences to load distribution also affect the wake dynamics and stability. An inverse wind turbine blade design tool has been implemented which uses a target, dimensionless circulation distribution from a full-scale blade to find the chord and twist along a sub-scale blade. In addition, airfoil polar data are interpolated from a few specified span stations leading to a smooth, manufacturable blade. The iterative process perturbs chord and twist, after running a blade element momentum theory code, to reduce the residual sum of the squares between the modeled sub-scale circulation and the target full-scale circulation. It is shown that the converged sub-scale design also leads to performance similarity in thrust and power coefficients. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L30.00003: Two Key Discoveries on Atmospheric Turbulent Wind Forcing of Nonsteady Wind Turbine Loadings, from HPC James Brasseur, Ganesh Vijayakumar, Adam Lavely, Balaji Jayaraman, Eric Paterson, Peter Sullivan Loading transients on wind turbine blades underlie premature component failure. We research the underlying causes of nonsteady blade loadings from interactions with atmospheric eddies in the atmospheric boundary layer (ABL) using combinations of blade-boundary-layer-resolving HPC simulation and lower-order blade models (ALM, BEMT). A daytime ABL simulated with a 760 760 256 pseudo-spectral LES interacts with a 62 m rotating wind turbine blade, simulated with advanced finite-volume-based algorithms in two complex multi-grid/scale domains in relative motion. We focus on two key discoveries: (1) Whereas nonsteady blade loadings are generally interpreted as in response to nonsteadiness in wind speed, time changes in wind vector direction are a much greater contributor to load transients, and strongly impact boundary layer dynamics; (2) Large temporal variations in loadings occur within two disparate time scales, an advective time scale associated with atmospheric eddy passage, and a sub blade-rotation time scale associated with turbulence-induced forcings as the blades traverse internal atmospheric eddy structure. The latter generates the strongest transients; the former modulates the response. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L30.00004: Direct Numerical Simulations of a Full Stationary Wind-Turbine Blade Adnan Qamar, Wei Zhang, Wei Gao, Ravi Samtaney Direct numerical simulation of flow past a full stationary wind-turbine blade is carried out at Reynolds number, Re$=$10,000 placed at 0 and 5 (degree) angle of attack. The study is targeted to create a DNS database for verification of solvers and turbulent models that are utilized in wind-turbine modeling applications. The full blade comprises of a circular cylinder base that is attached to a spanwise varying airfoil cross-section profile (without twist). An overlapping composite grid technique is utilized to perform these DNS computations, which permits block structure in the mapped computational space. Different flow shedding regimes are observed along the blade length. Von-Karman shedding is observed in the cylinder shaft region of the turbine blade. Along the airfoil cross-section of the blade, near body shear layer breakdown is observed. A long tip vortex originates from the blade tip region, which exits the computational plane without being perturbed. Laminar to turbulent flow transition is observed along the blade length. The turbulent fluctuations amplitude decreases along the blade length and the flow remains laminar regime in the vicinity of the blade tip. The Strouhal number is found to decrease monotonously along the blade length. Average lift and drag coefficients are also reported for the cases investigated. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L30.00005: Bio-inspired turbine blades offer new perspectives for wind energy Benjamin Thiria, Vincent Cognet, Sylvain Courrech du Pont The efficiency of wind turbines is especially poor if the wind speed is too low for the working range of the rotor, or if the oncoming wind has a too large incident angle with respect to the rotor axis. The consequence is that a large amount of potential available wind energy is not converted by the turbines, leading to heavy energetic and economic losses. The present work introduces a solution to overcome this technological limitation, using new types of blades connected to the rotors. This new type of blades is inspired by recent studies showing how insects improve flight performance by taking benefit from the flexibility of their wings (Ramananarivo et al. PNAS, 2011). Here, we show that, by bending along the chord under the action of the wind, the deformable blade plays the role of a shape factor to reorientate the torque in the direction of the rotation of the rotor, an especially helpful feature for critical wind conditions. The flexibility of the wing can significantly extend the performance range of wind turbines to low wind speeds and high azimuthal incoming wind directions, solving the technological barrier specific to this type of machines. The consequences of the presented results are outstanding for renewable solutions. Our estimation based on real wind data predicts a large increase in energy production, which is drawn using passive, non-consuming mechanisms, from the reservoir of energy available at critical wind conditions. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L30.00006: The Role of Free Stream Turbulence on the Aerodynamic Performance of a Wind Turbine Blade Victor Maldonado, Adrien Thormann, Charles Meneveau, Luciano Castillo Effects of free stream turbulence with large integral scale on the aerodynamic performance of an S809 airfoil-based wind turbine blade at low Reynolds number are studied using wind tunnel experiments. A constant chord (2-D) S809 airfoil wind turbine blade model with an operating Reynolds number of 208,000 based on chord length was tested for a range of angles of attack representative of fully attached and stalled flow as encountered in typical wind turbine operation. The smooth-surface blade was subjected to a quasi-laminar free stream with very low free-stream turbulence as well as to elevated free-stream turbulence generated by an active grid. This turbulence contained large-scale eddies with levels of free-stream turbulence intensity of up to 6.14{\%} and an integral length scale of about 60{\%} of chord-length. The pressure distribution was acquired using static pressure taps and the lift was subsequently computed by numerical integration. The wake velocity deficit was measured utilizing hot-wire anemometry to compute the drag coefficient also via integration. In addition, the mean flow was quantified using 2-D particle image velocimetry (PIV) over the suction surface of the blade. Results indicate that turbulence, even with very large-scale eddies comparable in size to the chord-length, significantly improves the aerodynamic performance of the blade by increasing the lift coefficient and overall lift-to-drag ratio, L/D for all angles tested except zero degrees. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L30.00007: Stall behavior of a scaled three-dimensional wind turbine blade Karen Mulleners, Matthew Melius, Raul Bayoan Cal The power generation of a wind turbine is influenced by many factors including the unsteady incoming flow characteristics, pitch regulation, and the geometry of the various turbine components. Within the framework of maximizing energy extraction, it is important to understand and tailor the aerodynamics of a wind turbine. In the interest of seeking further understanding into 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. A wind tunnel experiment has been carried out in the 2.2m x 1.8m cross-section closed loop wind tunnel at DLR in G\"{o}ttingen by means of time-resolved stereoscopic PIV. An extensive coherent structure analysis of the time-resolved velocity field over the suction side of the blade was performed to study stall characteristics under a geometrically induced pressure gradient. In particular, the radial extent and propagation of stalled flow regions were characterized for various static angles of attack. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L30.00008: Dynamic stall development in the near-root region of a model wind turbine blade Matthew Melius, Raul Bayoan Cal, Karen Mulleners The dynamic behavior of atmospheric flows create highly variable operational conditions which affect the life expectancy of the turbine components and the power output of the turbine. To gain insight into the unsteady aerodynamics of wind turbine blades, wind tunnel experiments were conducted with a scaled three-dimensional NREL 5MW wind turbine blade model in the 2.2m x 1.8m cross-section closed loop wind tunnel DLR in G\"{o}ttingen. The development of dynamic stall in response to a sudden change in the blades angle of attack are studied by means of time-resolved stereoscopic PIV in span-wisely distributed planes capturing the suction side of the blade. The change in angle of attack was obtained by varying the blade pitch angle to simulate a sudden change in wind speed or pitch angle regulation. Resulting time scales associated with flow separation and reattachment are determined at different radial positions ranging from $r/R = 0.19$ to $r/R = 0.38$. The influence of the three-dimensionality of the blade geometry on the corresponding aerodynamic effects is captured by analyzing the radial flow component in neighboring measurement fields during stall development. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L30.00009: Dynamic Stall Patterns Phillip Davidson, Ashli Babbitt, Andrew Magstadt, Pourya Nikoueeyan, Jonathan Naughton The performance of helicopter and wind turbine blades is affected by dynamic stall. Dynamic stall has received considerable attention, but it is still difficult to simulate and not fully understood. Over the past seven years, many airfoils for helicopter and wind turbine use ranging from 9.5 to 30\% thick have been experimentally tested and simulated while dynamically pitching to further characterize dynamic stall. Tests have been run at chord Reynolds number between 225,000-440,000 for various reduced frequencies, mean angles of attack, and oscillation amplitudes. Characterization of stall has been accomplished using data from previous studies as well as the unsteady pressure and flow-field data available from our own work. Where available, combined surface and flow-field data allow for clear identification of the types of stall observed and the flow structure associated with them. The results indicate that thin airfoil stall, leading edge stall, and trailing edge stall are observed in the oscillating airfoil experiments and simulations. These three main stall types are further divided into subcategories. By improving our understanding of the features of dynamic stall, it is expected that physics-based simulations can be improved. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L30.00010: Mechanisms of Vortex Evolution in Unsteady Stalled Flows James Buchholz, Kevin Wabick, James Akkala, Azar Eslam Panah Formation of a leading-edge vortex is considered on plunging and rotating flat plates at a chord-based Reynolds number of $10^4$. In all cases, a concentrated leading-edge vortex is formed. The physical mechanisms of vorticity transport governing the growth and evolution of the vortex are investigated within selected spanwise regions. It is demonstrated that the net flux magnitude of (opposite-sign) secondary vorticity is often significant during formation of the leading-edge vortex, in comparison to that of the leading-edge shear layer, suggesting that the secondary flux plays a substantial role in regulating the growth and evolution of leading-edge vortex circulation. Other mechanisms of vorticity transport will also be discussed, including the importance of spanwise flow to vortex circulation, and the roles of vortex tilting and stretching on the evolution of the vorticity field. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L30.00011: Implementation of a Forth-Order Aeroelastic Coupling into a Viscous-Inviscid Flow Solver with Experimental Validation (for One Degree of Freedom) Sirko Bartholomay, N\'estor Ramos-Garc\'Ia, Robert Flemming Mikkelsen The viscous-inviscid flow solver Q$^3$UIC for 2D aerodynamics has recently been developed at the Technical University of Denmark [1]. The Q$^3$UIC solver takes viscous and unsteady effects into account by coupling an unsteady inviscid panel method with the integral boundary layer equations by means of a strong coupling between the viscous and inviscid parts, and in this respect differs from other classic panel codes e.g. Xfoil. In the current work a Runge-Kutta-Nystr\"{o}m scheme was employed to couple inertial, elastic and aerodynamical forces and moments calculated by Q$^3$UIC for a two-dimensional blade section in the time-domain. Numerical simulations are validated by a three step experimental verification process carried out in the low-turbulence wind tunnel at DTU. First, a comparison against steady experiments for a NACA 64418 profile and a flexible trailing edge flap is presented for different fixed flap angles, and second, the measured aerodynamic characteristics considering prescribed motion of the airfoil with a moving flap are compared to the Q$^3$UIC predictions. Finally, an aeroelastic experiment for one degree of freedom --airfoil pitching- is used to evaluate the accuracy of aeroelastic coupling. \\[4pt] [1] A strong viscous--inviscid interaction model for rotating airfoils.~Ramos-Garc\'{\i}a, N\'{e}stor; S{\o}rensen, Jens N{\o}rk{\ae}r; Shen, Wen Zhong. Wind Energy, 2013. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L30.00012: Control of wing-tip vortex using winglets at low Reynolds number Seunghyun Cho, Haecheon Choi Winglets are considered as one of the effective devices for reducing induced drag, and thus many studies have been conducted, but mainly at high Reynolds numbers (\textit{Re }$\approx 10^{6}\sim 10^{7})$ for commercial airplanes. However, small-size unmanned air vehicles (UAV), operating at low Reynolds numbers (\textit{Re} \textless $10^{5})$, become an important transportation system for different purposes. Therefore, in the present study, we experimentally investigate the effect of winglets on the aerodynamic performance of an UAV by varying the cant angle. The WASP UAV model is used and the Reynolds numbers considered are 110,000$\sim $140,000 based on the free stream velocity and mean chord length of the WASP wing. The lift and drag forces on UAV are measured, and PIV measurements are conducted at several cross-flow planes for a few different angles of attack ($\alpha )$. At high angles of attack ($7^{\circ}\sim 13^{\circ})$, the winglets with the cant angle of $70^{\circ}$ increase the aerodynamic performance, whereas at low angles of attack ($2^{\circ}\sim 6^{\circ})$, the wing-tip extension (cant angle of $0^{\circ})$ shows better performances. The velocity fields measured from PIV indicate that, with the winglet, the wing-tip vortex moves away from the wing surface at $\alpha =12^{\circ}$, and the downwash motion in the wake behind the trailing edge is decreased, reducing the magnitude of the induced drag. A concept of changing the cant angle during flight is also suggested at this talk. [Preview Abstract] |
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