Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D06: Energy Harvesting and Power Generation II |
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Chair: Kiran Bhaganagar, University of Texas at San Antonio Room: Georgia World Congress Center B208 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D06.00001: Vortex and Wake-Induced Vibrations of Vertical-Axis Wind Turbine Blades Bridget Benner, Daniel W Carlson, Banafsheh Seyed-Aghazadeh, Yahya Modarres-Sadeghi We present a series of experimental results to investigate vortex and wake-induced vibrations of an airfoil used in vertical-axis wind turbines (VAWTs). The experiments were conducted in both a re-circulating water tunnel and wind tunnel to study the response of a flexibly-mounted NACA 0021 airfoil, which is used in some designs of VAWTs. The airfoil was free to oscillate in the crossflow direction, and tests were conducted in a Reynolds number range of 600<Re<13,300 and reduced velocity range of 0.6<U*<13. The amplitudes of oscillations and flow forces acting on the airfoil were measured over the entire possible range of angles of attack, 0°<α<180°. In the VIV case, the airfoil was observed to oscillate in the range of 60°<α<130°, where α=90° exhibited the widest lock-in range and peak amplitude. For all cases where oscillations were observed, the oscillation frequency remained close to the structure's natural frequency, defining a lock-in range. Flow visualization tests were also conducted to study the changes in vortex shedding patterns, especially between symmetrical angles of attacks about α=90°. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D06.00002: Optimization Of Vertical Axis Wind Turbines Using Design-By-Morphing Haris Moazam Sheikh, Philip S Marcus Currently, Horizontal Axis Wind Turbines (HAWTs) driven by aerodynamic lift are the most common form of commercial wind turbines. However, many studies have shown that Vertical Axis Wind Turbines (VAWTs) may be more efficient because VAWTs can be packed more closely into arrays than HAWTs without losing efficiency and are in fact more efficient as compared to a standalone VAWT. Thus, per unit area of land (or sea), more energy can be produced with VAWTs than HAWTs, making them more economically viable. The present study focuses on optimization of a standalone VAWT and its supplementary structures as well as position optimization of VAWT triplets to optimize their power output. The study utilizes novel ‘Design-by-Morphing’ technique, using four hybrid lift and drag based blades as baseline shapes and morphing them to produce novel blade shapes by assigning weights to these baseline shapes. These shapes were then analyzed using Computational Fluid Dynamics (CFD) and machine learning was used to optimize a multi-objective performance function. These optimized turbine and supplementary structure were then used to analyze an optimum arrangement of VAWT triplets to increase array efficiency. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D06.00003: Using Artifical Neural Networks and the Rapid Refresh Model for Wind Energy Forecasting Jordan Nielson, Kiran Bhaganagar The goal of this research is to use machine learning to improve wind energy forecasting applications. The forecasting includes both Annual Energy Production (AEP) of a single wind turbine and short-term (1-18 hours ahead) of an 86 wind farm array. The basis of wind energy forecasting is the wind turbine power curve. The power curve is a powerful tool but does not take into account atmospheric effects. The study used Artificial Neural Networks (ANNs) to create an improved power curve that uses wind speed, turbulence intensity, and density which capture atmospheric effects. The ANNs were validated with previous research and reduced the peak Mean Average Error (MAE) of power by 40%. The reduction in power estimation improved the energy production from 5% to 1% error. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D06.00004: Geometric and Control Optimization of an Array of Two Cross-Flow Turbines Benjamin W Strom, Isabel Scherl, Steven L Brunton, Brian Polagye Cross-flow turbines, also known as vertical-axis turbines, convert kinetic energy in wind or flowing water to mechanical power via blades that rotate about an axis perpendicular to the flow. Interactions between two cross-flow turbine rotors operated in a flume were experimentally optimized. Under “tip-speed ratio control”, the rotation rate of each turbine was individually optimized to take advantage of the mean flow structure. Under “coordinated control”, periodic rotor interactions were exploited by operating each rotor at identical rotation rates. The rotation rate and blade position difference between the rotors is optimized. Control parameters were optimized to maximize array power output using a modified Nelder-Mead algorithm for Sixty-four array geometries. Both control schemes produced a maximum power output of 1.3 times the power generated by two isolated turbines. Tip-speed ratio control outperformed coordinated control where the downstream turbine was located directly in the wake of the upstream turbine, while coordinated control was more successful for other array geometries. Co- and counter-rotating configurations produced similar maximum power output, though beneficial array interactions occurred over a broader range of array geometries with counter-rotation. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D06.00005: Stochastic calibration of the actuator line model parameters Benedetto Rocchio, Umberto Ciri, Stefano Leonardi, Maria Vittoria Salvetti Since the numerical representation of wind turbine blades leads to simulations beyond the present computational resources, simplified models are usually employed. We focus on the Actuator Line Model (ALM), in which the blade is replaced by a rotating line divided into finite segments with representative aerodynamic coefficients. The Lift and Drag forces are distributed among the nearest grid points by using a Gaussian kernel. The standard deviation of this kernel and the reference velocity are two fundamental parameters that strongly affect the characteristics of the wind turbine wake. The sensitivity of the ALM predictions to these parameters is here investigated by comparing the wake features obtained in Large Eddy Simulations of the flow around a 2D airfoil (NACA 0009) with the results of simulations in which the body is modeled by ALM. We adopt a stochastic approach to the sensitivity analysis to obtain a continuous response surfaces of the quantities of interest in the parameter space, starting from a few simulations. Attention is focused on the influence of these parameters on the prediction of the mean velocity field and of the turbulent kinetic energy in the wake. This also permits to identify the values of the model parameters providing the best agreement with LES. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D06.00006: Kelvin-Helmholtz wavepackets in Actuator disk wake with turbulent co-flow Aditya S. Ghate, Aaron S. Towne, Sanjiva K Lele The interaction of a dragging, fixed thrust coefficient actuator disk with decaying homogeneous isotropic turbulence (HIT) of varying spatial integral length scales and dissipation rates is studied using Large Eddy Simulation performed at high resolution. The shear layer entrainment (and hence wake recovery) is shown to be strongly sensitive to the integral length scale of the upstream HIT (as opposed to its intensity) and this observation is reconciled using a space-time modal decomposition of the flow field to identify the dominant Kelvin-Helmholtz wavepackets that dictate the shear layer growth rate. This spectral proper orthogonal decomposition (SPOD) further suggests that due to the turbulent co-flow and core, the overall fluctuation fields lack any tonal behavior, and hence a low-rank projection-based representation is not possible. However, a truncated representation of the turbulent wake based on these SPOD modes can be enriched to its full spectral bandwidth using spectrally and spatially localized Gabor wavepackets, previously used to enrich LES of boundary layer turbulence (Ghate & Lele, J. Fluid Mech., 2017). |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D06.00007: Wake Modelling behind a Vertical-axis Tidal-current Rotor Ruiwen Zhao, Angus Creech, Alistair Borthwick, Stephen Salter, Takafumi Nishino A group of close-packed contra-rotating vertical-axis rotors has previously been designed by Stephen Salter to maximize the fraction of flow passage swept (Salter and Taylor, 2006, IMechE). We model a single vertical-axis turbine (VAT) using actuator line theory to capture important flow features contributing to the fast wake recovery behind one of the rotors (Bachant, 2016, Wind Energy). Tip-loss effects and dynamic stall are investigated and incorporated in the model as corrections applied to the force terms in the actuator line method. The spoked-ring wheel is a more efficient load-bearing structure than a tower, which experiences drag and suppresses tip vortices caused by adjacent foils at different angles. Proper pitch control could solve the problem of dynamic stall. The Wind and Tidal Turbine Embedded Simulator (WATTES) (Creech et al., 2015, Surveys in Geophysics) is used to predict the dynamic response to the flow, where lift and drag force components are calculated from tabular aerofoil data. Using this approach, we examine the vorticity magnitude distribution in order to investigate the effect of wake turbulence on VAT device performance. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D06.00008: CFD analysis on the hydrodynamic performance of an Oscillating Water Column Wave Energy device with variable bottom topography Piyush Mohapatra, Trilochan Sahoo, Anirban Bhattacharyya Oscillating Water Column (OWC) device is one of the various types of Wave Energy Converters developed so far to harness energy from the Ocean waves. In the present study, the effect of sea bed slope on the hydrodynamic performance of a floating OWC device is analyzed using Computational Fluid Dynamics (CFD). An incompressible, multiphase Volume of Fluid (VOF) model is adapted with the help of a numerical wave tank (NWT) for simulating the ocean waves. The dynamic mesh technique is used to capture the motion of the device. The free surface elevation and flow characteristics are analyzed inside and outside the OWC device. The hydrodynamic capture efficiency is investigated for waves of different time periods. Four different cases of seabed slopes are studied and the results indicate a certain influence of the seabed slope on the hydrodynamic performance of the OWC device. It was observed that there is a significant rise in peak efficiency for few cases of the bottom slopes and in general, the frequency band for wave energy capture widens due to the presence of a seabed slope. This is a very important analysis from the practical viewpoint, considering the installation and operation of such devices. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D06.00009: Energy harvesting from a cylinder undergoing vortex-induced vibrations Edgard Ballner, Stavroula Balabani, Ali Abolfathi, Neil Cagney Vortex-Induced vibrations (VIVs) occur throughout industry and nature, and are typically seen as a problem as they can lead to fatigue damage. However, VIV also represents a source of renewable energy if the kinetic energy of the structure can be harvested. This approach has many benefits over conventional wind turbine technologies, including low space requirements, potentially simpler design, leading to lower installation costs, and most notably, the oscillation frequency can be tuned such that the aerodynamic noise generated occurs outside of the range of human hearing, preventing noise pollution and allowing this technology to be employed in urban environments. We design a device that harvests energy from a cylinder free to move in the lift direction. Power extraction is achieved through induction, with magnets attached to a leaf spring. Our design allows us to control the magnetic damping and the natural frequency of the system in order to optimise the efficiency. We vary the reduced velocity throughout the VIV regime and measure both the amplitude response and the power extraction, exploring the influence of temporal variations in the amplitude of VIV and the magnitude of damping on the efficiency and the potential of this technology for large-scale power generation. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D06.00010: Optimal Mechanical Properties for Energy Extraction from Plates Subjected to Discrete Vortex Loading Alireza Pirnia, Katelyn M. Clemons, Sean D. Peterson, Byron Erath The optimization of energy extraction from coherent vortical structures using flexible membranes is a subject of interest in recent work. The focus of this study is to find the optimal properties of a plate that, when subjected to a discrete vortex passing tangentially over it, maximizes the plate strain energy. Vortex rings are generated with Re≈4300 and the vortex to plate spacing is varied. The plate mechanical properties are expressed in terms of a non-dimensional mass parameter (Π_{m}), and a Strouhal number (St), where the St number expresses the duration of pressure loading relative to the period of plate oscillation. It is hypothesized that a St≈0.5 will optimize the fluid to plate energy transfer. Four sets of plates with mass parameters that span the range of 3<Π_{m}<11 are investigated for Strouhal numbers that span the range of 0.2<St<1.0. It is observed that the energy transfer is optimized for St≈0.5, and that this value is largely insensitive to the plate mass parameter and vortex/plate separation distance. The results are applicable to predictive energy harvesting design efforts. |
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