The design of ducted propellers is much more challenging than that of isolated propellers, as the complexity of the interference between the ductand the propeller makes it difficult to determine the induced velocity at the propeller. In the present study, a momentum model of ducted propellers is established. Based on the model, the ratio of the propeller thrust to the total thrust of ducted propellers, denoted as k, is analyzed, and its relationship with the induced velocity at the propeller is obtained. Furthermore, the variation law of k in the hoveringand forward flight states is revealed by numerical analysis based on Computational Fluid Dynamics (CFD) simulations. For a given duct configuration, the following variation law of k is found: in the hovering state, k hardly varies with the change of the propeller's rotational speed; in the forward flight state, the k valueand the thrust coefficient of the ducted propeller show a linear relationship. The linear relationship is independent of both the rotational speedand the incoming wind velocity, which is very beneficial for establishing an engineering model about the value of k. In addition, the uniform law of k values for ducted propellers with different number of blades is revealed. The present study enriches the theoretical understandings about ducted propellersand has potential values for application in the ducted propeller design.

The unsteady aerodynamic characteristics of parafoils are a matter of flight stability and flight safety and are worthy of in-depth study. In this paper, the computational fluid dynamics (CFD) method was used to carry out an unsteady numerical simulation of the parafoil airfoil under the dynamic change of the angle of attack, so as to analyze the influence of the average angle of attack, the amplitude of the angle of attack, and the reduced frequency on the unsteady aerodynamic force and flow field pattern of the parafoil airfoil. The results show that an increase in any one of the above three parameters leads to a significant increase in the aerodynamic unsteadiness of the parafoil airfoil. The average angle of attack and the amplitude of the angle of attack amplitude jointly determine the dynamic range of the angle of attack of the airfoil, which significantly affects the dynamic stall characteristics of the parafoil airfoil; the change in reduced frequency affects the lift hysteresis effect of the parafoil airfoil, with the maximum lift coefficient increasing linearly with increasing reduced frequency. Further flow field analysis shows that the unsteady effect of the angle of attack of the parafoil airfoil in the dynamic increase stage delays the occurrence of airfoil flow separation on the one hand, and on the other hand, it flattens the separating vortex against the upper airfoil, which together contributes to a significant increase in the critical angle of attack and maximum lift coefficient of the parafoil airfoil relative to the constant case. The results of this essay will help to enhance the understanding of the unsteady aerodynamic force and flow field patterns of parafoil airfoil, support the prediction of unsteady aerodynamic forces and safety assessment of parafoil in complex wind environments.

Aerodynamics is the primary and key issue faced by wind energy engineering, which determines its economy, stability and safety. Based on the development requirement and trend of large-scale, clustering, oceanic, intelligence and digital for wind engineering, the present review is mainly focused on the aerodynamics issues faced by wind energy, such as the atmospheric boundary layer (ABL), wind turbine wake flow, onshore/offshore/complex-terrain wind farm flow and its interactions. As the second review of a successive work, special interests here are put on the atmospheric inflow and wake characteristics of wind turbines. For these two aspects, this paper summarizes recent research efforts in field measurement, wind tunnel experiment, theoretical analysis, numerical simulation, engineering modelling and artificial intelligence predictions, and also provides some views on the corresponding flow distribution characteristics, evolution laws and key mechanisms. Some discussions and suggestions are made on the aerodynamic challenges faced by wind power development under our country’s specific atmospheric/geographical conditions. Finally, a non-exhaustive perspective on the future of wind energy engineering research is presented. It is expected to provide an important reference for wind power industry planning, technological development and project implementation.

With the establishment of national policies around the world, the development of wind industry has been entering its heyday. Wind turbine systems are continuously upsized from megawatt level to multi-megawatt class, deployed for both onshore wind power and offshore wind power in diverse sea conditions, intelligentized with smart materials, structures and control strategies, and digitized with precise prediction and state-aware control system. These mainstream trends for wind energy development, in the meanwhile, raise tremendous challenging issues for wind energy research and development. Among them, wind turbine aerodynamics will face new problems and great challenges. As the first review of a sucessive work, this study is mainly focused on the aerodynamics of horizontal-axis wind turbines. An explanation for the aerodynamic complexity is addressed at first. Then, research progress on key aerodynamics issues, such as the aerodynamic characteristics of wind turbine airfoils and blades, as well as modern wind turbine design (special focus is put on the offshore wind power technology and the inevitable typhoon phenomenon, together with the experienced aeroelastic problem caused by the increasement in wind turbine size) and new flow control strategies are discussed. In this process, recent theoretical, experimental, and numerical studies that have contributed to improve our understanding of wind turbine aerodynamic performance, such as aerodynamic forces, flow fields and flow mechanism, are summarized. Additionally, the strength and limitation of these research approaches are discussed. Possible future research trend is analyzed and prospected, providing some references for large-scale wind turbine blade design.