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.

The accuracy of airfoil polar predictions is limited by the usage of imperfect turbulence models. Can machine-learning improve this situation? Will airfoil polars teach the effect of turbulence on skin-friction? We try to answer these questions by refining turbulence treatment in the Rfoil code: boundary layer closure relations are learned from airfoil polar data. Two turbulent closure relations, for skin friction and energy shape factor, are parametrized with a class-shape transformation. An experimental database is then used to define code inaccuracy measures that are minimized with an interior point gradient algorithm. Results show that airfoil polars contain exploitable information about turbulent phenomena. Inferred closures agree with direct numerical simulation results of skin friction and the new code predicts drag more accurately. Maximum lift remains under-predicted but Rfoil maintains its robustness and suitability for optimization of wind energy airfoils.

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.

Ducted Wind Turbines are characterized by a strong interaction between the duct and the rotor. In this study, the effect of the duct cross-section geometry on the flow across the rotor is investigated. The latter is modelled as an actuator disc with constant thrust coefficient. The study is carried out by means of a vortex panel code and Reynolds Averaged Navier Stokes (RANS) simulations. The vortex panel method shows that airfoil geometry with large camber leads to flow augmentation at the rotor plane. Three duct geometries are then investigated with RANS computations, showing a separation region at the trailing edge suction side of the duct that might lead to a reduction of the aerodynamic performances of the overall system.

The feasibility of employing a passive tip jet for enhancing the diffusion of the concentrated vorticity of blade-tip vortices is studied on a small-scale two-bladed propeller. The rotor employs hollow blades with slots for flow suction at the root and ejection at the tip. The centrifugal acceleration of the fluid created inside the blade by the rotational motion of the propeller is used to displace a considerable amount of flow momentum from the most inboard part of the blade to the tip. The particular technology is quite attractive because it does not require additional external power sources to drive the fluid. Test are performed in a low-speed wind tunnel with the propeller mounted at zero angle of attack. Oil-flow visualizations are used to characterize the change of the flow on the blade surface, whereas particle image velocimetry measurements are carried out to quantify the diffusion of the vorticity field obtained for three propeller advance ratios. Results show that the application of tip blowing considerably reduces the blade-tip vortex peak swirl velocity (33% for 0.24 propeller advance ratio) with a considerable increase in the vortex core radius (64% for 0.24 propeller advance ratio) and vortex diffusion. Although the fluid inside the propeller blade experiences viscous losses, no significant change of the blade performance is measured.

This study describes a methodology for designing airfoils suitable to employ actuation in a wind energy environment. The novel airfoil sections are baptized wind energy actuated profiles (WAP). A genetic algorithm-based multi-objective airfoil optimizer is formulated by setting two cost functions: one cost function for wind energy performance and the other representing actuation suitability. The wind energy cost function compares the candidate airfoils' performance with 'reference' wind energy airfoils, considering a probabilistic approach to include the effects of turbulence and wind shear. The actuation suitability cost function is developed considering horizontal axis wind turbines active stall control, including two different control strategies designated by 'enhanced' and 'decreased' performance. Two different actuation types are considered, namely, boundary layer transpiration and dielectric barrier discharge plasma. Results show that using WAP airfoils provides much higher control efficiency than adding actuation on reference wind energy airfoils, without detrimental effects in non-actuated operation. The WAP sections yield an actuator employment efficiency that is two to four times larger than those obtained with reference wind energy airfoils, at equivalent wind energy performance. Regarding geometry, and compared with typical wind energy airfoils, WAP sections for decreased performance display an upper surface concave aft region, while for increased performance, a convex upper surface aft region is obtained. The present study emphasizes that there is much to gain in designing airfoils from the beginning to include actuation effects, especially compared with employing actuation on already existing airfoils. The results demonstrate the potential of including actuation effects in the airfoil design process, thus enabling novel horizontal axis wind turbines control strategies.

The framework of self-similar laminar boundary layer flow solutions is extended to include the effect of actuation with body force fields resembling those generated by DBD plasma actuators. The deduction line is similar to previous work investigating the effect of porous wall suction on laminar boundary layers. The starting point of the analysis is a generalised form of the Boundary Layer Partial Differential Equations (BL-PDEs) that includes volume force terms. Actuation force distributions are defined such that the volume force term of the BL-PDE equations conforms to the requirements of similarity. New similarity parameters for the plasma strength and thickness are identified. The procedure yields a general similarity equation which includes the effect of pressure gradients, wall transpiration and DBD plasma actuation. Select numerical solutions of the new similarity equation are presented to develop instinctive understanding and prompt a discussion on the construction of new closure relations for integral boundary layer models.