Figure - available from: Wind Energy
This content is subject to copyright. Terms and conditions apply.
![Schematic of a blade cross section of a downwind turbine passing through a turbulent wake of the tower (wake defined by u and v). The wake (u and v) is a function of time and space. The nondimensional clearance (XRE/Dtower) is a function of span wise location [Colour figure can be viewed at wileyonlinelibrary.com]](publication/338198469/figure/fig1/AS:11431281172921306@1688707105932/Schematic-of-a-blade-cross-section-of-a-downwind-turbine-passing-through-a-turbulent-wake.png)
Schematic of a blade cross section of a downwind turbine passing through a turbulent wake of the tower (wake defined by u and v). The wake (u and v) is a function of time and space. The nondimensional clearance (XRE/Dtower) is a function of span wise location [Colour figure can be viewed at wileyonlinelibrary.com]
Source publication
The downwind rotor configuration provides a structural advantage compared with an upwind design. However, tower shadow has long been a concern for downwind systems. The tower shadow negatively affects the blade by introducing a load impulse during the wake passage. An aerodynamic fairing could shroud the tower reducing the wake. However, there is n...
Similar publications
![Fig. 1. Diagram of tower shadow parameters from AeroDyn theory manual [23]](profile/Eric-Loth-3/publication/348239091/figure/fig1/AS:1030567793549315@1622717917786/Diagram-of-tower-shadow-parameters-from-AeroDyn-theory-manual-23_Q320.jpg)
Downwind wind turbines may be the future direction of large wind turbines, given their ability to have flexible, lightweight blades with much lower risk of tower strike. However, downwind turbines must deal with the “tower shadow” effect, whereby blades experience a velocity deficit as they pass through the tower wake, resulting in unwanted bending...
Citations
... Moreover, downwind wind turbines performed worse than upwind cases in reducing blade root bending moment at large velocities. Therefore, for the past few years, a multitude of scholars have been engaged in investigating strategies to optimize the aerodynamic characteristics of downwind wind turbines with the aim of minimizing safety hazards and enhancing output power [9][10][11]. ...
The vibrissae of harbor seals exhibit a distinct three-dimensional structure compared to circular cylinders, resulting in a wave-shaped configuration that effectively reduces drag and suppresses vortex shedding in the wake. However, this unique cylinder design has not yet been applied to wind power technologies. Therefore, this study applies this concept to the design of downwind wind turbines and employs wind tunnel testing to compare the wake flow characteristics of a single-cylinder model while also investigating the output power and wake performance of the model wind turbine. Herein, we demonstrate that in the single-cylinder test, the bionic case shows reduced turbulence intensity in its wake compared to that observed with the circular cylinder case. The difference in the energy distribution in the frequency domain behind the cylinder was mainly manifested in the near-wake region. Moreover, our findings indicate that differences in power coefficient are predominantly noticeable with high tip speed ratios. Furthermore, as output power increases, this bionic cylindrical structure induces greater velocity deficit and higher turbulence intensity behind the rotor. These results provide valuable insights for optimizing aerodynamic designs of wind turbines towards achieving enhanced efficiency for converting wind energy.
... The aerodynamics of the SUMR-D blade used in OpenFAST take into account the changes in Reynolds numbers (e.g., have a lower lift-to-drag ratio than that of the SUMR-13 blade). It should be noted that the OpenFAST predictions herein neglect the tower shadow effects since this has been shown to have a negligible impact on the unsteady blade moment in comparison to the effects of incoming turbulence based on predictions by Noyes et al. [44] and field test measurements by Simpson et al. [33]. ...
... Another important result of Fig. 9c is the lack of a significant drop in the flapwise blade moment as the blade passes downstream of the tower (for Ψ of about 160 • ). This experimental result is consistent with previous predictions that the downwind load-aligned SUMR coned design does not suffer significantly from tower shadow effects [33,44]. The tip deflection data sets are shown in Fig. 10. ...
... 4 The UAE employed rigid blades so the aeroelastic behavior is very different than what one would expect for an extreme-scale turbine, where the blades tend to be quite flexible. 17 To date, there has been no subscale parked testing that has sought to replicate the aeroelastic behavior of extreme-scale turbines (neither for upwind nor downwind rotor configurations). As a result, there has been no assessment of current computational capabilities for predicting blade deflections and loads in high gust conditions for flexible rotors. ...
Aeroelastic parked testing of a unique downwind two‐bladed subscale rotor was completed to characterize the response of an extreme‐scale 13‐MW turbine in high‐wind parked conditions. A 20% geometric scaling was used resulting in scaled 20‐m‐long blades, whose structural and stiffness properties were designed using aeroelastic scaling to replicate the nondimensional structural aeroelastic deflections and dynamics that would occur for a lightweight, downwind 13‐MW rotor. The subscale rotor was mounted and field tested on the two‐bladed Controls Advanced Research Turbine (CART2) at the National Renewable Energy Laboratory's Flatiron Campus (NREL FC). The parked testing of these highly flexible blades included both pitch‐to‐run and pitch‐to‐feather configurations with the blades in the horizontal braked orientation. The collected experimental data includes the unsteady flapwise root bending moments and tip deflections as a function of inflow wind conditions. The bending moments are based on strain gauges located in the root section, whereas the tip deflections are captured by a video camera on the hub of the turbine pointed toward the tip of the blade. The experimental results are compared against computational predictions generated by FAST, a wind turbine simulation software, for the subscale and full‐scale models with consistent unsteady wind fields. FAST reasonably predicted the bending moments and deflections of the experimental data in terms of both the mean and standard deviations. These results demonstrate the efficacy of the first such aeroelastically scaled turbine test and demonstrate that a highly flexible lightweight downwind coned rotor can be designed to withstand extreme loads in parked conditions.
... Since there have been no previous field experiments for flexible rotors, the influence of tower shadow for large downwind systems has been based on wind tunnel tests and simulations. In such simulations, tower shadow only appears to marginally increase blade fatigue, 12 and the trends indicate that the effect is reduced for lighter and more flexible blades. 13 To provide experimental data to compare with numerical tower shadow models, a 1/5th-scale gravo-aeroelastic model was developed based on the Segmented Ultralight Morphing Rotor 13-MW (SUMR-13) turbine and is denoted as the SUMR Demonstrator (SUMR-D). ...
... Noyes et al. found the stiff blades of the UAE Phase VI resulted in a strong tower shadow effect, while Noyes et al. predicted that damping ratio of a blade may determine the magnitude of the tower shadow effect.12,43 It was hypothesized that the flexible blades of the SUMR-D rotor would reduce the tower shadow effect compared to stiff blades. ...
As wind turbine rotors become larger, the blades become more flexible, requiring extra stiffness and cost to avoid the risk of tower strike. Wind turbines in a downwind configuration have a reduced risk of tower strike because the rotor thrust acts away from the tower. However, downwind blades pass through the wake of the tower, and the resulting load variation may contribute to blade fatigue. To date, there have been no field tests to quantify this tower shadow effect on unsteady blade moments. The present study reports on the first field testing of a flexible, downwind, coned rotor and compares the experimental data against simulations run in OpenFAST. The tower shadow effect is simulated using the conventional Powles model and a new Eames model (developed herein), which includes the influence of upstream turbulence. Both models reasonably predict the blade root out‐of‐plane bending moment data and the tower shadow dip magnitude when compared to field test data in Region 3. Tower shadow was found to increase the short‐term Damage Equivalent Loads (DELs) by less than 10% compared to other effects (gravity, shear, and turbulence), and the predictions were consistent with experiments. These results indicate that the tower shadow effect can be reasonably modeled with the simpler Powles model and that the tower shadow effect can be small compared to the effect of turbulence. However, long‐term fatigue due to tower shadow should be included in detailed structural analysis and design of the rotor and the tower.
... The field data gathered is enabling us to better understand the performance of such highly flexible, lightweight rotor designs (Phadnis, Zalkind, & Pao, 2020). Based on our experimental results, improved blade structure models for highly flexible blades and models of unsteady aeroloads via tower shadow effect (Noyes, Qin, & Loth, 2020b;Noyes, Qin, Loth and Schreck, 2018)) are now being integrated into software modules and tools (such as FAST (Jonkman & Buhl, 2005)) at NREL that are ultimately made freely available and will thus help to advance control co-design for wind turbines. Automatic controller tuning methods are also being integrated into these software tools (Abbas et al., 2020;Zalkind et al., 2020) to allow faster controller tuning which will in turn enable more wind turbine designs to be explored more efficiently. ...
Wind energy is recognized worldwide as cost-effective and environmentally friendly and is among the fastest-growing sources of electrical energy. To further decrease the cost of wind energy, wind turbines are being designed at ever larger scales, which is challenging due to greater structural loads and deflections. Large-scale systems such as modern wind turbines increasingly require a control co-design approach, whereby the system design and control design are performed in a more integrated fashion. We overview a two-bladed downwind morphing rotor concept that is expected to lower the cost of energy at wind turbine sizes beyond 13 megawatts (MW) compared with continued upscaling of traditional three-bladed upwind rotor designs. We describe an aero-structural-control co-design process that we have used in designing such extreme-scale wind turbines, and we discuss how we were able to achieve a 25% reduction in levelized cost of energy for our final turbine design compared to a conventional upwind three-bladed rotor design.
... If this load unsteadiness is severe, it can cause increased blade fatigue. The degree of influence of tower shadow on blade loads is a strong function of blade flexibility [13]. Therefore, it is important to understand this effect for flexible blades of large turbines so that they can be effectively designed. ...
Downwind wind turbines may be the future direction of large wind turbines, given their ability to have flexible, lightweight blades with much lower risk of tower strike. However, downwind turbines must deal with the “tower shadow” effect, whereby blades experience a velocity deficit as they pass through the tower wake, resulting in unwanted bending moments. The tower shadow effect is studied herein using field test data from a novel demonstrator turbine as well as simulations in OpenFAST using the Powles wake model. A method for simulating the turbulent wind field conditions using meteorological tower data is developed and applied to the turbine in operational conditions. The results show that the tower shadow effect is reasonably captured by OpenFAST, and that the strongest effects on flapwise operational loads generally occur at wind speeds above the turbine rated wind speed.
