Aerodynamic Optimization of the Sizing and Blade Designs of Hovering Corotating Coaxial Rotors

Corotating coaxial rotors are seeing renewed interest in distributed electric propulsion systems and electric vertical take-off and landing (eVTOL) aircraft. The recent literature reports many interesting investigations, using prescribed rotor blades, into the flow phenomena as well as aerodynamic and aeroacoustic benefits of corotating rotors. However, the subject of the design of blade geometries, optimized to a design goal, for corotating rotors is currently lacking in the literature. This paper is written from such a design perspective, by extending a previous generalized approach to the aerodynamic optimization of counterrotating rotors to corotating rotors. The previous requirement for upper and lower counterrotating rotor torques to be equal can now be lifted in the case of corotating rotors, enabling improved versatility in the optimization of corotating blade designs. The optimization is demonstrated on an application example to address the conflicting conditions that index angles (high) for aeroacoustic benefits of reduced noise are at odds with those (low) for aerodynamic efficiency. The approach demonstrated in this paper is to set the index angle for reduced noise and then recover back the aerodynamic efficiency by using the newly developed aerodynamic optimization technique.

Impact of the ground clearance on the annual energy production and tower cost of an offshore wind turbine

This article analyzes the impact of the ground clearance on the Annual Energy Production (AEP) and tower cost of a 20 MW offshore wind turbine. In addition, the influence of the rated wind speed on the analysis result will be considered. The AEP is computed by considering wind speed variation over the swept area of the rotor blades. The tapered tubular steel tower is considered for mass and cost calculation. The tower is considered as a fixed-free cantilever beam with concentrated mass at the free end. The analysis shows that the ground clearance only has a minor impact on the AEP but it has a remarkable impact on the tower mass. Specifically, when the ground clearance reaches 50 meters, the AEP only increases by roughly 3% while tower mass is nearly doubled compared to the case with no ground clearance. The results also reveal the significant impact of the rated speed on both the AEP and tower mass.

Design and Performance Evaluation of Low-Speed Vertical Axis Wind Turbines with Wind Boosters using CFD Analysis

The use of wind based energy is quickly expanding over the planet. The goal of this study is to use computational methods of fluid dynamics to develop a novel model of VAWT including Windbooster for various rotor blades like two, three, and four blades in order to enhance effectiveness. CAD modelling approaches of vertical axis wind turbines including and excluding booster are created. Including all vertical axis wind turbine blade designs including and excluding booster, torque, power, and Coefficient of performance are compared.The performance of three blades on the basis of mechanical properties includingi wind amplifier is 29.9% greater than two blades using wind amplifier, and four blades using wind amplifier is 21.5 percent greater than three blades using wind amplifier, according to the findings. Because the mechanical energy created by a four-blade wind booster wasn't as great as it is including three blades, VAWT employing three-blade wind booster seems to be more effective than VAWT with a two- or four-blade wind booster. For improved mechanical durability, VAWT with three-blade wind amplifier is recommended.

Flutter Behavior of Highly Flexible Two- and Three-bladed Wind Turbine Rotors

With the progression of novel design, material, and manufacturing technologies, the wind energy industry has successfully produced larger and larger wind turbine rotor blades while driving down the Levelized Cost of Energy (LCOE). Though the benefits of larger turbine blades are appealing, larger blades are prone to aero-elastic instabilities due to their long, slender, highly flexible nature, and this effect is accentuated as rotors further grow in size. In addition to the trend of larger rotors, new rotor concepts are emerging including two-bladed rotors and downwind configurations. In this work, we introduce a comprehensive evaluation of flutter behavior including classical flutter, edgewise vibration, and flutter mode characteristics for two-bladed, downwind rotors. Flutter speed trends and characteristics for a series of both two- and three-bladed rotors are analyzed and compared in order to illustrate the flutter behavior of two-bladed rotors relative to more well-known flutter characteristics of three-bladed rotors. In addition, we examine the important problem of blade design to mitigate flutter and present a solution to mitigate flutter in the structural design process. A study is carried out evaluating the effect of leading edge and trailing edge reinforcement on flutter speed and hence demonstrates the ability to increase the flutter speed and satisfy structural design requirements (such as fatigue) while maintaining or even reducing blade mass.

Dynamic-stall measurements using time-resolved pressure-sensitive paint on double-swept rotor blades

The study presents an optimized pressure-sensitive paint (PSP) measurement system that was applied to investigate unsteady surface pressures on recently developed double-swept rotor blades in the rotor test facility at the German Aerospace Center (DLR) in Göttingen. The measurement system featured an improved version of a double-shutter camera that was designed to reduce image blur in PSP measurements on fast rotating blades. It also comprised DLR’s PSP sensor, developed to capture transient flow phenomena (iPSP). Unsteady surface pressures were acquired across the outer 65% of the rotor blade with iPSP and at several radial blade sections by fast-response pressure transducers at blade-tip Mach and Reynolds numbers of $$\mathrm {M}_\mathrm{tip} = 0.282-0.285$$ M tip = 0.282 - 0.285 and $$\mathrm {Re}_\mathrm{tip}= 5.84-5.95 \times 10^5$$ Re tip = 5.84 - 5.95 × 10 5 . The unique experimental setup allowed for scanning surface pressures across the entire pitch cycle at a phase resolution of $${0.225}\,{\mathrm{deg}}$$ 0.225 deg azimuth for different collective and cyclic-pitch settings. Experimental results of both investigated cyclic-pitch settings are compared in detail to a delayed detached eddy simulation using the flow solver FLOWer and to flow visualizations from unsteady Reynolds-averaged Navier–Stokes (URANS) computations with DLR’s TAU code. The findings reveal a detailed and yet unseen insight into the pressure footprint of double-swept rotor blades undergoing dynamic stall and allow for deducing “stall maps”, where confined areas of stalled flow on the blade are identifiable as a function of the pitch phase. Graphical abstract

Assessment of wind field generation methods on predicted wind-turbine power production using a free vortex filament wake approach

The generated power and thrust of a wind turbine strongly depend on the flow field around the turbine. In the present study, three different inflow methods, i.e., a time series (TS) from Large-Eddy Simulation (LES) of atmospheric boundary layer flow field, a synthetic turbulent flow field using the Mann model (MM) and a steady-state mean wind profile with shear (PL), are integrated with the free vortex filament wake method to investigate the effect of wind field generation methods on the wind turbine performance where the impact of the turbine and the trailing wake vortices on the turbulent flow fields are ignored. For this purpose, an in-house Vortex Lattice Free Wake (VLFW) code is developed and used to predict the aerodynamic loads on rotor blades. The NREL 5-MW reference wind turbine is used for the VLFW simulations. For a fair assessment of different inflow generation methods on power production of a wind turbine, it is not sufficient that the generated wind fields employed in the TS and MM methods, have the same streamwise mean velocity and turbulence intensity at hub height. Instead, the generated inflows must have equivalent power-spectral densities especially at low frequencies since the rotor blades essentially respond to the large-scale fluctuations (macroscopic scales) rather than small-scale fluctuations (microscopic scales). A faster energy decay rate of LES inflow leads to a higher en-ergy content in the TS method at low frequencies (associated with the macroscopic dynamics of the rotor blades). This extra kinetic energy results in a slightly higher mean power production while using the TS method although the inflow conditions at hub height/rotor plane are the same for both the TS and MM methods. Moreover, the impact of simulation time (the length of time integration) on the power production of a wind turbine (exposed to an unsteady inflow) must be taken into account. A short simulation time remarkably affects the mean wind speed over the rotor area for identical turbulent inflows. For Taylor’s hypothesis application using a single LES flow field, the results show a significant difference in the mean powers corresponding to the different realizations due to large turbulent fluctuations.