Ducted wind turbines (DWTs) can take the advantage of ground effect (GE) when installed close to urban areas. To this aim, a parametric study to investigate the aerodynamic performance of DWTs in relation to three different ground distances are investigated. The flow around a commercial DWT model using a simplified duct actuator disc (AD) model based on Reynolds Averaged Navier-Stokes (RANS) equations is performed. The results indicate that DWTs placed close to the ground will lead to increased mass flow rate the turbine plane, and thereby improving the aerodynamic performance. However, the additional ground force leads to an asymmetric flow-field at the turbine plane, which will ultimately induce unsteady forces on the DWT system. The present analysis will serve as a strong recommendation to address siting issues for DWT manufacturers.

@en

FLORIDyn is a parametric control-oriented dynamic model suitable to predict the dynamic wake interactions between wind turbines in a wind farm. In order to improve the accuracy of FLORIDyn, this study proposes to calibrate the tuning parameters present in the model by employing a probabilistic setting using the UQ4WIND framework. The strategy relies on constructing a surrogate model (based on polynomial chaos expansion), which is then used to perform both global sensitivity analysis and Bayesian calibration. For our analysis, a nine wind turbine configuration in a yawed setting constitutes the test case. The results of sensitivity analysis offer valuable insight into the time-dependent influence of the model parameters onto the model output. The model parameter tied to the turbine efficiency appear to be the most sensitive parameter affecting the model output. The calibrated FLORIDyn model using the Bayesian approach yield predictions much closer to the measurement data, which is equipped with an uncertainty estimate.

@en

This paper presents an efficient strategy for the Bayesian calibration of parameters of aerodynamic wind turbine models. The strategy relies on constructing a surrogate model (based on adaptive polynomial chaos expansions), which is used to perform both parameter selection using global sensitivity analysis and parameter calibration with Bayesian inference. The effectiveness of this approach is shown in two test cases: calibration of airfoil polars based on the measurements from the DANAERO MW experiments and calibration of five yaw model parameters based on measurements on the New MEXICO turbine in yawed conditions. In both cases, the calibrated models yield results much closer to the measurement data, and in addition they are equipped with an estimate of the uncertainty in the predictions.@en

Wind farm flow control (WFFC) is a topic of interest at several research institutes and industry and certification agencies worldwide. For reliable performance assessment of the technology, the efficiency and the capability of the models applied to WFFC should be carefully evaluated. To address that, the FarmConners consortium has launched a common benchmark for code comparison under controlled operation to demonstrate its potential benefits, such as increased power production. The benchmark builds on available data sets from previous field campaigns, wind tunnel experiments, and high-fidelity simulations. Within that database, four blind tests are defined and 13 participants in total have submitted results for the analysis of single and multiple wakes under WFFC. Here, we present Part I of the FarmConners benchmark results, focusing on the blind tests with large-scale rotors. The observations and/or the model outcomes are evaluated via direct power comparisons at the upstream and downstream turbine(s), as well as the power gain at the wind farm level under wake steering control strategy. Additionally, wake loss reduction is also analysed to support the power performance comparison, where relevant. The majority of the participating models show good agreement with the observations or the reference high-fidelity simulations, especially for lower degrees of upstream misalignment and narrow wake sector. However, the benchmark clearly highlights the importance of the calibration procedure for control-oriented models. The potential effects of limited controlled operation data in calibration are particularly visible via frequent model mismatch for highly deflected wakes, as well as the power loss at the controlled turbine(s). In addition to the flow modelling, the sensitivity of the predicted WFFC benefits to the turbine representation and the implementation of the controller is also underlined. The FarmConners benchmark is the first of its kind to bring a wide variety of data sets, control settings, and model complexities for the (initial) assessment of farm flow control benefits. It forms an important basis for more detailed benchmarks in the future with extended control objectives to assess the true value of WFFC.

@en

Ducted wind turbines (DWTs) can be used for energy harvesting in urban areas where non-uniform flows are caused by the presence of buildings or other surface discontinuities. For this reason, the aerodynamic performance of DWTs in yawed-flow conditions must be characterized depending upon their geometric parameters and operating conditions. A numerical study to investigate the characteristics of flow around two DWT configurations using a simplified duct-actuator disc (AD) model is carried out. The analysis shows that the aerodynamic performance of a DWT in yawed flow is dependent on the mutual interactions between the duct and the AD, an interaction that changes with duct geometry. For the two configurations studied, the highly cambered variant of duct configuration returns a gain in performance by approximately 11 % up to a specific yaw angle (α= 17.5∘) when compared to the non-yawed case; thereafter any further increase in yaw angle results in a performance drop. In contrast, performance of less cambered variant duct configuration drops for α>0∘. The gain in the aerodynamic performance is attributed to the additional camber of the duct that acts as a flow-conditioning device and delays duct wall flow separation inside of the duct for a broad range of yaw angles.@en

Ducted Wind Turbines (DWTs) can be used for energy harvesting in urban areas where non-uniform inflows might be the cause of aerodynamic and acoustic performance degradation. For this reason, an aerodynamic and aeroacoustic analysis of DWTs in yawed inflow condition is performed for two duct geometries: a baseline commercial DWT model, DonQi®, and one with a duct having a higher cross-section camber with respect to the baseline, named DonQi D5. The latter has been obtained from a previous optimization study. A numerical investigation using Lattice-Boltzmann Very-Large-Eddy Simulations is presented. Data confirm that the aerodynamic performance improvement, i.e. increase of the power coefficient, is proportional to the increase of the duct thrust force coefficient. It is found that, placing the DWT at a yaw angle of 7.5°, the aerodynamic performances of the DonQi D5 DWT model are less affected by the yaw angle. On the other hand, this configuration shows an increase of broadband noise with respect to the baseline DonQi® one, both in non-yawed and yawed inflow conditions. This is associated to turbulent boundary layer trailing edge noise due to the turbulent flow structures developing along the surface of the duct.

@en

Ducted Wind Turbines (DWTs) are one of the many concepts that have been proposed to improve the energy extraction from wind in comparison to bare wind turbines. In reviewing the DWT studies, investigations based on the combined use of theoretical, computational, and experimental techniques have been presented. Although indicated in these studies that the power output of wind turbines can be significantly increased by using surrounding ducts, the factors influencing this power increase, like the duct shape, augmentation add-on’s and yawed inflow conditions, need further investigation. These topics have been addressed in this doctoral thesis. The study presents a computational investigation of DWTs, employing two-dimensional and three-dimensional CFD simulations. To this intent, solutions obtained using panel, RANS, URANS and LB-VLES methods are shown. For reliable solution accuracy, verification and validation assessments are performed when possible. Through parametric investigation, it is found that the aerodynamic performance of the DWT can be improved by increasing the duct cross-section camber and a correct choice of turbine thrust force coefficient, whilst maintaining the same duct-exit-area ratio. The aerodynamic performance improvement for a DWT directly corresponds to the dimensionless duct thrust force coefficient. Flow analysis showed that flow separation when detected inside of the duct, reduces the duct thrust force coefficient and ultimately the aerodynamic performance of the DWT model. In an effort to further improve on the aerodynamic performance of the DWT, the effect of multi-element ducts and Gurney flap on the existing DWT models are investigated. The aerodynamic performance improvement with multi-element ducts strongly depends on the installation settings of the secondary duct element with respect to the primary DWT geometry. On the other hand, a Gurney flap retrofitted at the trailing edge of the duct improves the aerodynamic performance of the DWT model by delaying inner duct wall flow separation, thus increasing the mass flow rate at the turbine. Finally, the effects of yawed inflow condition on the aerodynamic and aeroacoustic performance of DWT models are studied in detail. The analysis showed that DWTs can demonstrate yaw insensitivity up to a specific yaw angle. The yaw insensitivity for the DWT model, however, strongly depends on the aerodynamic mutual interaction between the duct and turbine, which changes with the duct geometry, turbine configuration and yaw angle. While assessing the aeroacoustic performance of the DWT models, it is found that the DWT model xiii xiv Summary with highly cambered duct cross-section generates higher broadband noise levels, which results from the turbulent flow structures convecting along the surface of the duct. @en

Ducted Wind Turbines (DWTs) are used for energy harvesting in urban areas where the flow is non-uniform in comparison to the free-field because of the presence of buildings or other surface discontinuities. For this reason, the aerodynamic performance and far-field noise of DWTs in yawed inflow conditions must be characterized. Both the aerodynamic and the acoustic fields are dependent on the geometry of the duct. In this study, the effect of the duct geometry is analysed with high fidelity numerical simulations carried out with the lattice-Boltzmann method.@en

Multi-element ducts are used to improve the aerodynamic performance of ducted wind turbines (DWTs). Steady-state, two-dimensional computational fluid dynamics (CFD) simulations are performed for a multi-element duct geometry consisting of a duct and a flap; the goal is to evaluate the effects on the aerodynamic performance of the radial gap length and the deflection angle of the flap. Solutions from inviscid and viscous flow calculations are compared. It is found that increasing the radial gap length results in an augmentation of the total thrust generated by the DWT, whereas a larger deflection angle has an opposite effect. Reasonable to good agreement is seen between the inviscid and viscous flow calculations, except for multi-element duct configurations characterized by large flap deflection angles. The viscous effects become stronger at large flap deflection angles, and the inviscid calculations are incapable of taking this phenomenon into account.

@en

The complex aerodynamic interactions between the rotor and the duct has to be accounted for the design of ducted wind turbines (DWTs). A numerical study to investigate the characteristics of flow around the DWT using a simplified duct–actuator disc (AD) model is carried out. Inviscid and viscous flow calculations are performed to understand the effects of the duct shape and variable AD loadings on the aerodynamic performance coefficients. The analysis shows that the overall aerodynamic performance of the DWT can be increased by increasing the duct cross-sectional camber. Finally, flow fields using viscous calculations are examined to interpret the effects of inner duct wall flow separation on the overall DWT performance.

@en

This paper aims to study the aerodynamic performance of ducted wind turbines (DWT) using inviscid and viscous flow calculations by accounting for the mutual interaction between the duct and the rotor. Two generalized duct cross section geometries are considered while the rotor is modelled as an actuator disc with constant thrust coefficient. The analysis shows the opportunity to significantly increase the overall aerodynamic performance of the DWT by a correct choice of the optimal rotor loading for a given duct geometry. Present results clearly indicate that the increased duct cross section camber leads to an improved performance for a DWT. Finally, some insights on the changes occurring to the performance coefficients are obtained through a detailed flow analysis.

@en

In this paper, a numerical investigation on the effect of gurney ap (GF) on the performance of a diffuser augmented wind turbine (DAWT) is presented. The flow-field around the DAWT is obtained by solving the Reynolds-averaged-Navier-Stokes (RANS) equations. The turbine is modelled as an uniformly loaded actuator disc (AD) that imposes a resistance to the passage of the flow. Comparison of the numerical results with experimental measurements in similar conditions shows that the numerical approach used satisfactorily reproduces the mean flowfield. GF heights equal to 2% and 4% of the diffuser chord length were investigated. Results show that separation induced by GF creates a low pressure region at the diffuser exit, that increases the mass ow through the diffuser and the power coeffcient of the DAWT. @en

The practice of ducting wind turbines has shown a beneficial effect on the overall performance, when compared to an open turbine of the same rotor diameter1. However, an optimization study specifically for ducted wind turbines (DWT’s) is missing or incomplete. This work focuses on a numerical optimization of the duct orientation and the ideal loading coefficient for the rotor. A 2D planar geometry was employed to model the DWT and the rotor is modelled as an uniformly loaded actuator disc (AD). The flow-field around the DWT is obtained through numerical solutions of Reynolds-averaged-Navier-Stokes (RANS) equations2 and a steady state Lagrangian approach based on vortex ring method3 . The study determines the optimal angle of attack for the duct corresponding to the AD loading, in order to achieve the optimal performance for a given DWT configuration. @en

In this paper, a computational approach, based on the solution of Reynolds-averaged-Navier–Stokes (RANS) equations, to describe the flow within and around a diffuser augmented wind turbine (DAWT) is reported. In order to reduce the computational cost, the turbine is modeled as an actuator disc (AD) that imposes a resistance to the passage of the flow. The effect of the AD is modeled applying two body forces, upstream and downstream of the AD, such that they impose a desired pressure jump. Comparison with experiments carried out in similar conditions shows a good agreement suggesting that the adopted methodology is able to carefully reproduce real flow features.@en

In order to harvest wind resources more efficiently and to the greatest extent possible, unconventional wind turbine designs have been proposed, but never gained any acceptance in the marketplace. A team of researchers from TU Delft plans to revisit the concept of ducted wind turbines, which have been around for decades, and provide some clarity on its potential.@en