Session Z14: Energy: Wind Power - Wakes, Control & Fluctuations

Superstatistical random fields from point-wise atmospheric turbulence measurements

We present an advanced model for the generation of synthetic wind fields that can be understood as an extension of the well-known Mann model of the wind energy sciences. In contrast to such Gaussian random field models which control second-order statistics (i.e., velocity correlation tensors or spectra), we demonstrate that our extended model incorporates the effects of higher-order statistics as well. In particular, the empirically observed phenomenon of small-scale intermittency, a key feature of atmospheric turbulent flows, can be reproduced with high accuracy and at considerably low computational cost. Our method is based on a recently developed multipoint statistical description of a turbulent velocity field [J. Friedrich et al., J. Phys. Complex. 2 045006 (2021)] and consists of a superposition of multivariate Gaussian statistics with fluctuating covariances. Furthermore, we explicitly show how such superstatistical Mann fields can be constrained on a certain number of point-wise measurement data. We give an outlook on the relevance of such surrogate wind fields in the context of the wind energy sciences.   

On the power fluctuations of wind turbines fitted with a hydrostatic transmission

In this study, we analyzed a hydrostatic transmission that replaces the heavy, stiff current drive train. The energy converted is transmitted through the less stiff fluid system from a pump coupled to the rotor and delivered to a hydraulic motor at ground level that drives the electric generator. The low stiffness of the hydrostatic transmission is achieved by integrating a gas accumulator so that the amplitude of the load fluctuations can be reduced compared with a base configuration, diminishing the fatigue loads on the wind turbine. A kW scale hydraulic wind turbine placed alongside a standard counterpart setup was used to compare the power generation response under the same atmospheric conditions. They are assessed by changes in the turbine transfer function of the power spectrum (Tobin et al., 2015). The overall efficiency of the hydraulic transmission reached roughly 80% and the torque amplitude fluctuations lowered by about 40% with the hydraulic architecture also tested under controlled conditions using a cyber-physical system. Hydrostatic transmission for offshore applications has the potential to reduce LCOE by an order of 5 to 18%.

    

Volumetric velocimetry of the near wake of a model wind turbine

For the purpose of commercial power production, wind turbines are often clustered together in wind farms. In such situations, some of the turbines will frequently operate in the wakes generated by the upstream turbines. As wake operation changes the inflow conditions of downstream turbines, characterisation of the wind turbine wake is important. Wind turbine wakes are inherently three-dimensional flows. Therefore, employing three-dimensional measurement techniques to capture the full wake structure would be insightful. This work presents volumetric velocity measurements of the near wake of a freely rotating, two-bladed model wind turbine at a Reynolds number of 18000. The measurements were obtained using the state-of-the-art velocimetry technique Shake-the-Box in a free surface recirculating water channel. The volume of interest measured 13.5 cm by 13.5 cm by 10 cm and was illuminated by high-power pulsed LEDs. The goal of the study is to characterise the three-dimensional flow features and flow structures generated by the rotor. We also investigate how these structures interact with the wind turbine tower.   

Resolvent analysis for predicting energy-containing structures in the far wake of a wind turbine

Turbulence generated by turbine wakes can significantly alter the power production and aerodynamic loads of a wind farm. However, the prediction of such turbulence by wake models is often hindered by the complex interplay among wake-induced shear, incoming turbulence and near-wake structures. We demonstrate the use of resolvent analysis to predict energy-containing structures in the far wake of a wind turbine subjected to a turbulent inflow. We assume that the mean flow is axisymmetric and that the effect of small-scale flow structures can be modeled with an eddy viscosity term. We find that resolvent analysis can capture both the dominant and subdominant modes educed from the spectral proper orthogonal decomposition of large-eddy simulation data. We also find that the amplitude gain peaks at a Strouhal number of around 0.2 and at an absolute azimuthal wavenumber of 1, demonstrating that resolvent analysis can capture the role of convective shear instability in generating wake turbulence. Although data driven, the present analysis is linear, making it computationally efficient and thus suitable for adoption in wake models used to design and optimize wind farms.

    

Effect of complex terrain topography on wind farm reliability and performances

Terrain topography affects the atmospheric boundary layer over a vertical region where wind turbines are usually located. It is commonly believed that placing wind turbines on the top of a ridge is beneficial because of the more energetic flow impinging on the turbines. However, the presence of topography with even modest altitude upstream of the wind farm may generate turbulent structures that significantly increase fluctuations in blade loads and power production.

In this study, we analyze a 9-turbine wind farm located on a ridge in the State of Bahia in Brazil. For the main wind direction, a hill with a lower altitude respect to the turbine locations is located upstream of 3 turbines of the farm. This allows a direct comparison of the impact of the upstream topography based on the turbine location. Large Eddy Simulations are used to reproduce the prevalent wind conditions at the wind farm site. Numerical simulations are then compared with the wind farm SCADA data. Turbines are found to be significantly affected by the dynamics of the large turbulent structures that detach from the upstream hill. This study shows the importance of considering the dynamics of the wind flow when assessing wind resources in presence of complex terrain topography.   

Large eddy simulations of turbulent flows in arrays of helical- and straight-bladed vertical axis wind turbines

Wind turbines are often placed in large arrays with limited spacings to achieve high power density in wind energy harvesting. This limited spacing between turbines results in considerable wake-turbine interactions that can affect the performance of wind turbines. In this study, the turbulent flows in large arrays of vertical axis wind turbines (VAWTs) are modeled using large eddy simulation. The VAWT forces are applied to the flow by the actuator line method. A concurrent precursor simulation is used to generate the atmospheric boundary turbulent inflow conditions for the turbine domain simulation. Two different VAWT shapes (i.e., helical- and straight-bladed) and various streamwise and spanwise turbine spacings are considered, and their effects on the turbine array boundary layer flow are investigated systematically.   

Qunatification of Dynamic Interactions of DBD Plasma Discharges with Water and Ice Pertinent to Wind Turbine Icing Mitigation

An experimental study was performed to characterize the Dielectric-Barrier-Discharge (DBD) plasma discharges interacting with a complex multiphase system (i.e., air, water and ice) associated with ice accretion process over blade surfaces in the context of wind turbine icing mitigation. The experimental study was carried out in the Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT) to generate typical wind turbine icing conditions with adequate liquid water content (LWV) levels in the frozen cold incoming airflows.  An array of DBD actuators were embedded over the surface of an airfoil/blade model were supplied with high voltages in either alternating current for AC-DBD plasma actuation or nanosecond pulses for ns-DBD plasma actuation. During the experiments, in addition to using a high-resolution imaging system to record the dynamic anti-/de-icing operation over the airfoil/wing surface upon switching on the DBD plasma actuators, a high-speed Infrared (IR) thermal imaging system is also utilized to quantitatively map the temperature distributions over the surface of the airfoil/wing to characterize the effects of DBD plasma actuations on the coupled heat and mass transfer of the ice accretion process. The findings derived from the present study are very helpful to explore/optimize design paradigms for the development of novel plasma-based anti-/de-icing strategies tailored specifically for wind turbine icing mitigation to ensure safer and more efficient wind turbine operation in atmospheric icing conditions.

 

    

Proportional Aerodynamic Performance Modulation using an Active Gurney Flap on a Wind Turbine Blade Section

Flow control devices can provide promising improvements in wind turbine blade performance by actively modulating the aerodynamic performance of the blade, so that the system can better adapt to varying flow conditions, achieve higher overall efficiency and be more robust to off-design operation. We present wind tunnel measurements using an Active Gurney Flap (AGF) located near the trailing edge of the wind turbine blade on the pressure surface that can be raised and lowered as needed, thus enabling flow control with additional lift, but avoiding the drag penalty when that lift boost is not required. Real-time adjustment of the active gurney flap (AGF) could be used to adaptively modulate the aerodynamic performance according to flow conditions. Lift, drag and pitching moment measurements are reported over a range of Reynolds number from 160,000 to 414,000, angle of attack from -10 to 15 degree and AGF deployment positions from 0 to 135 degrees. We show that the AGF provides proportional control to the aerodynamic performance, and improves the lift and pitch moment performance with an increased lift to drag ratio. The dynamic response of the force coefficients as the AGF is deployed is also discussed.