Blade Root Research Articles

Numerical investigation into performance of Savonius wind turbine equipped with inner blades: Overlap ratio effect

This research focuses on the effect of overlap ratio on the performance of Savonius rotor in the presence of inner blades in such a way that inner blade tip is parallel to the main blade tip and root. Findings reveal that at low overlap ratios, using inner blade that the inner blade tip is parallel to the main blade root leads to a higher power and torque coefficients than the other inner blade configuration and also conventional rotor. This research indicates that for the overlapped rotor and for the two inner blade configurations, the higher power and torque coefficients could be obtained at high and low tip speed ratios, respectively. Results show that for both inner blade configurations, the overlapped rotors overcome the negative torque generated by the non-overlapped one. It is also shown that at high overlap ratio, using inner blade has no advantage over the conventional rotor.

Independent Pitch Controller based on Fuzzy Adaptive Tuning PI for Load Mitigation of Large Land Wind Turbine

With the development of wind power generation units towards the mega-scale, the blade root and tower have large load and vibration problems during the operation of wind turbines. In order to extend the operational life of wind turbines, a PI independent pitch control (IPC) strategy with fuzzy adaptive tuning (FAT) is introduced based on the existing wind turbine model and control strategy to achieve optimal control of wind turbine loads. Simulations are carried out by the FAST software of the US Renewable Energy Laboratory to compare the independent pitch and the cooperative pitch. The results show that IPC reduces the load on the blade roots and improves the quality of the electrical energy.

Effects of blade torsion on IEA 15MW turbine rotor operation

The increase in the size of wind turbine rotors in recent decades necessitates investigations into aeroelastic behavior and its influence. This work proposes the computational modeling and the subsequent dynamic study of the rotor of the IEA Wind TCP Task 37 reference wind turbine of 15MW, considering different torsional stiffness and wind speed conditions. Sensitivity studies regarding the wind conditions and the torsional stiffness of the blades on the overall operation were carried out. It was verified that at nominal wind speed, the original torsional stiffness led to a reduction in the thrust on the rotor, the flapwise displacement at the blade tip, and the peaks of flapwise curvature at the blade root compared to the amplified stiffness. Referring to different wind speeds, simulations using the original torsion stiffness revealed a considerable decrease in generator torque with the expected in the high wind speed region, in contrast to the amplified stiffness. It is also noteworthy that the peak shaving process is affected by the amplified blade torsional stiffness, as it can lead to excessive adjustments due to smaller cut regions and thrust values. Consequently, a new pitch curve was proposed, taking into account the desired generator torque.

The influence of the lobed ejector on the horizontal axial tidal turbine and flow characteristic analysis

The power coefficient of the horizontal axis tidal turbine (HATT) is limited to 0.593 by the Betz limitation, however, the turbine's efficiency can be improved by incorporating a diffuser or lobe ejector. In this study. In this paper, water tunnel experiments were conducted to obtain the power and thrust coefficients of the original turbine and the turbine with a lobe ejector at different tip speed ratios (TSR). The accuracy of the numerical simulation was then verified by comparing it with the experimental results. The results reveal that the power coefficient of the turbine with a lobe ejector is up to 623% higher than that of the original turbine; however, it also leads to a negative thrust coefficient higher than the original turbine by 94.64%. To determine the optimal position of the lobe ejector, a numerical simulation of the turbine with a diffuser, which consists of a single inner lobe channel, was conducted under the same conditions. The findings demonstrate that the power coefficient of the diffuser is 123% higher than that of the original turbine, and the negative thrust is higher at 26.64%, significantly less than that of the lobe ejector. This indicates that the effect of the lobe ejector largely depends on the ejector effect of the outflow channel. Subsequently, to investigate the ejection effect of the lobe ejector, various aspects, including the vortex structure of the flow field, pressure coefficient of the blade section, velocity vector field of the rotor's center plane, and wake velocity curve, were analyzed. Moreover, the changes in the position of flow separation points along different sections of the blade from the blade root to the blade tip, the variation of the spiral structure of the blade root vortex, the evolution of the velocity vector field in the low-velocity region of the turbine's wake flow field and the alteration of the velocity curve of the wake flow field were further examined.

Analysis of fatigue fracture incident of centrifugal compressor blades in a small turbojet engine

In this study, performance verification tests were conducted on a small turbojet engine, during which a failure occurred at the root of the centrifugal compressor blades, which are made of aluminum alloy. A detailed disassembly and inspection was carried out to reveal the reason of the failure. It was found that the failure of one blade at the root led to jamming of other blades. Then the jammed blades experience sever rubbing and contact with the intake duct. Experimental measurements of material properties and a retrospective examination of the manufacturing process revealed that the yield strength of the material was lower than the design value, and the fillet radius at the blade root was smaller than specifications. The root cause of the fatigue failure of the compressor blades in this incident was attributed to substandard machining quality and material properties, which leads to crack initiation and fatigue propagation until failure occurred. The experience of identifying the root cause would provide engineering guidance for other small turbojet engines.

Passive aeroelastic study of large and flexible wind turbine blades for load reduction

In this paper, passive aeroelastic coupling of wind turbine blades for load reduction is realized by introducing bending-twist coupling (BTC) and geometric sweep coupling (GSC). The BTC is implemented by composite fiber off-axis of the blade spar cap, and the GSC is achieved by defining sweep curves of the blade reference axis. To investigate the passive coupling effect and load reduction of the large and flexible blades, the IEA 15 MW blades are considered. BTC are introduced into the three-dimensional finite element (FE) models and the residual stiffness coefficient is obtained. Meanwhile, geometrically exact beam theory (GEBT) is used to build the flexible blade. Both BTC and GSC are introduced into the beam model, and the effect elastic deformation on blade load is also considered. Multi-objective optimization (MOO) process of rotating blades for load reduction is proposed under steady-state rated conditions. The results show that the highest coupling efficiency can be obtained when the fiber off-axis angle is between 12° and 15°. The BTC of the blade can reduce the load of blade root with a small fiber off-axis angle. Meanwhile, the GSC can further reduce the axis torque. Compared with the initial design of IEA 15 MW steady-state rotating blade, the maximum thrust, flapwise moment and axis torque of the blade can be reduced by 8.8 %, 13.4 % and 22.9 % respectively.

FSI Application in the Parametric Analysis of Blade–Disk Connection of Turbojet Engine Compressor

The following paper evaluates the strength properties of the critical area of the turbine rotor stage of a jet engine. This is where the blade is attached to the rotor disk. Important aspects of this issue are characteristic geometrical dimensions and operating factors, e.g., changes in rotor speed. In the article, the strength properties of the rotor stage with a trapezoidal blade–disk connection were analyzed using parametric analysis considering fluid–structure interaction (FSI). Virtual models of the compressor rotor stages were developed, including parametric blade–disk connections and discrete models for FEM (finite element method) and CFD (computational fluid dynamics) analyses. The parametric analysis was aimed at assessing selected geometric parameters in terms of their impact on the maximum stresses in the blade root. The analysis performed shows that by changing the blade’s width and thickness angles at the root, we can influence the value of the maximum stresses in the rotor stage. It was noticed that some combinations of these two parameters may negatively affect the value of the stresses in the rim.

The performance of two control strategies for floating wind turbines: lidar-assisted feedforward and multi-variable feedback

In this paper, we analyze the performances of two control strategies and their combination on a floating turbine through OpenFAST simulations. The floating wind turbine is modeled based on the demonstrative FLOATGEN, which consists of a 2MW wind turbine mounted on the Damping Pool platform designed by BW Ideol. The lidar-assisted control utilizes lidar wind preview to achieve blade pitch feedforward control. The multi-variable feedback additionally uses the platform pitch rate to determine blade pitch. By simulations with the above-rated wind and irregular wave conditions, both control strategies and their combination has the potential to reduce turbine vibrations. Especially, combining lidar-assisted control and multi-variable feedback control brings the most significant reduction in the standard deviations of rotor speed (>36%), low-speed shaft torque (>30%), and blade root moment (>15%).

Analysis on influence of self-fan cooling motor on fan aerodynamics

With the development of multi electric/all electric aircraft, the degree of aircraft secondary energy electrification is becoming higher and higher. As the core component of the electrical system, the motor presents the trend of small volume and high-power density. The heat dissipation problem of compact motor is becoming more and more serious. At present, the self-driven fan cooling system is widely used in aircraft motors. In this paper, the axial fan cooling system of an aero generator is taken as the research object, and the changes of fan flow field and aerodynamic characteristics in the fan motor coupling model of the isolated fan and the installed motor are analyzed by using three-dimensional computational fluid simulation. At the same time, the influence of the change of the axial distance between the fan and the motor on the aerodynamic performance is also studied. The analysis of the simulation results shows that the close coupling between the motor and the fan components greatly affects the flow structure in the fan, resulting in the distortion potential effect on the fan outlet section. The fan flow field appears the blade root reflux, the channel vortex in the blade and the expansion of the corner separation area. With the increase of the axial distance between the motor and the fan, the influence of the motor on the flow in the fan gradually approaches the flow blocking effect.

Identifying cadmium and lead co-accumulation from living rice blade spectrum

Neither cadmium (Cd) nor lead (Pb) is necessary for crop growth, but they both can accumulate in soil and crop tissues, resulting in land degradation and crop reduction. Few researchers have explored how to detect Cd–Pb co-accumulation in leaves using proximal sensing techniques, especially by low-cost, easy-to-use leaf clips that capture hyperspectral reflections at suitable foliar positions. In this study, a hyperspectral imager was employed to collect images of the rice canopy from a designed greenhouse experiment that included 16 pretreatments of Cd–Pb co-accumulation, followed by spectral extractions from 3 foliar positions: the blade root, the middle of the leaf, and the leaf apex. A support vector machine with leave-one-out cross-validation was performed to diagnose the contaminative levels based on the feature wavelengths selected by an improved successive projection algorithm. Partial least squares regression was used to predict Cd–Pb concentrations in rice blades. The results indicated that diagnostic accuracies were varied using spectra of different foliar positions. The blade root and leaf apex of rice blades were the optimal foliar position for detecting Cd and Pb contamination, respectively. At the optimal foliar positions, diagnostic accuracies exceeded 0.80 for distinguishing whether the rice is subject to Cd–Pb contamination. The Cd prediction performed ‘very good’ with a residual prediction deviation (RPD) of 2.21, a R2 of 0.79, and a root mean square error (RMSE)of 6.14, while that of Pb was 1.62, 0.61, and 186.54. Important wavelengths were identified at 659–694 nm and 667–694 nm to detect Cd and Pb contamination. In summary, our results verified the feasibility and clarified the optimal foliar positions of rice blades to detect Cd–Pb contamination. The wavelengths selecting have the great potential in the design of future leaf clips, and the optimal foliar position can provide suggestions to improve diagnostic performances in field applications.

Fluid–Structure Interaction Analysis of a Wind Turbine Blade with Passive Control by Bend–Twist Coupling

The idea of improving the energy output for small wind turbines without compromising the remaining aspects of the technology, such as costs and structural integrity, is very appealing in the context of the growing concern for global warming and the goal of providing electricity to remote and isolated regions. This work aims to contribute to the development of distributed wind generation by exploring the effects of bend–twist coupling on the performance of a wind turbine with a focus on a small rotor based on the NREL Phase VI blade geometry. After defining a structure in composite materials exhibiting the coupling behavior along with a reference counterpart, a comparative numerical analysis is performed using a Fluid–Structure Interaction (FSI) analysis. The main numerical framework is based on commercial software and consists of a finite-volume solver for fluid physics, a finite-element solver for solid physics, and a coupling interface for the interaction problem. The results, complemented with the predictions from a one-way analysis based on the blade-element momentum theory are used to define the increments in rotor torque. The analysis of the annual energy yield shows a 3% increase due to the bend–twist coupling used as a passive pitch mechanism, considering a Rayleigh distribution with an 11 m/s average wind speed. Simultaneously, the coupling causes increments of 0.2% and 0.3% for the blade root flapwise moment and the rotor thrust force, respectively, when considering parked conditions and a simplified extreme wind model.

Characterization of energy transfer and triadic interactions of coherent structures in turbulent wakes

Turbulent wakes are often characterized by dominant coherent structures over disparate scales. Dynamics of their behaviour can be attributed to interscale energy dynamics and triadic interactions. We develop a methodology to quantify the dynamics of kinetic energy of specific scales. Coherent motions are characterized by the triple decomposition and used to define mean, coherent and random velocity. Specific scales of coherent structures are identified through dynamic mode decomposition, whereby the total coherent velocity is separated into a set of velocities classified by frequency. The coherent kinetic energy of a specific scale is defined by a frequency triad of scale-specific velocities. Equations for the balance of scale-specific coherent kinetic energy are derived to interpret interscale dynamics. The methodology is demonstrated on three wake flows: (i) ${Re}=175$ flow over a cylinder; (ii) a direct numerical simulation of ${Re}=3900$ flow over a cylinder; and (iii) a large-eddy simulation of a utility-scale wind turbine. The cylinder wake cases show that energy transfer starts with vortex shedding and redistributes energy through resonance of higher harmonics. The scale-specific coherent kinetic energy balance quantifies the distribution of transport and transfer among coherent, mean and random scales. The coherent kinetic energy in the rotor scales and the hub vortex scale in the wind turbine interact to produce new scales. The analysis reveals that vortices at the blade root interact with the hub vortex formed behind the nacelle, which has implications for the proliferation of scales in the downwind near wake.

Numerical study of the unsteady blade root aerodynamics of a 2 MW wind turbine equipped with vortex generators

Abstract. In order to design vortex generators for modern multi-megawatt wind turbines accurately, the 3D behaviour of the boundary layer has to be considered. Due to the rotation of the blade, the lift-enhancing rotational augmentation has a considerable impact, especially in the inner blade sections. To investigate the interaction of vortex generators and rotational augmentation, high-fidelity computational flow simulations by means of unsteady Reynolds-averaged Navier–Stokes methods are presented for a rotating blade of a generic 2 MW horizontal-axis wind turbine. The inner blade section is analysed with and without vortex generators for two different pitch settings, including one causing largely separated flow. Two ways of placing the vortex generators on the blade with different radial starting positions are investigated in order to find out if the coexistence of the two lift-enhancement methods (i.e. rotational augmentation and vortex generators) is beneficial. All simulations are performed with the flow solver FLOWer, and the vortex generators are modelled by the introduction of source terms into the computational domain through a so-called BAY (Bender–Anderson–Yagle)-type model. For the case without vortex generators, it is found that the strength of rotational augmentation largely depends on the effective angles of attack (i.e. the pitch setting). For the case with lower effective angles of attack, rotational augmentation is a cyclic phenomenon, whereas for the case with higher effective angles of attack, it generates large loads in the inner root section due to a constant centrifugal pumping mechanism in time. The results from the cases with vortex generators display a rather destructive interaction of vortex generators and rotational augmentation on the torque. For low effective angles of attack and thus attached flow conditions, vortex generators exhibit slight losses compared to the case without vortex generators, as they inhibit spanwise flow through rotational augmentation. For high effective angles of attack, the vortex generators placed over 30 % of the blade produce an increase of 3.28 % in torque compared to the case without vortex generators and high centrifugal pumping.

Aerodynamic analysis of TLP offshore wind turbine under wind and waves

Based on the real-scale NREL 5MW wind turbine as the analysis object, combined with the TLP floating platform, using the CFD software fluent and the Volume of Fluid (VOF) multiphase flow model, this paper simulates and analyzes the aerodynamic characteristics of the TLP floating offshore wind turbine under the wind and wave conditions by applying surge motion through UDF, and carries out an in-depth analysis of its power, axial thrust change, and blade pressure distribution. The power and axial thrust will fluctuate and change more violently as the amplitude and frequency of surge movement increase corresponding to waves. Low wave height, low amplitude, and low frequency are more obvious for local disturbance of power and axial thrust. After the surge motion is applied, the blade pressure at different times changes significantly. With the increase of amplitude, the pressure difference at the same time increases at the entire blade root, blade center, and blade tip, which focus mainly on the leading edge of the blade suction surface, and is most obvious at the blade root and least obvious at the blade tip. When the surge motion is not applied, with the increase of wave height, the front edge of the blade suction will obtain a greater negative pressure, and the pressure difference of each part will increase. The research results provide a technical reference and theoretical basis for the optimization design of the aerodynamic performance.

Assessment of Crack Growth and Fatigue Life of an Axial Fan Blade Based on a Co-Simulation Approach

Abstract This paper presents static and dynamic stress analyses of an axial fan blade, which were carried out under real-life centrifugal and aerodynamic loading conditions using the Abaqus software. The location of the crack was identified on the pressure side of the blade at the conjunction between the blade and the blade root. It reveals a high agreement between the predicted location of stress distribution and the real origin of the crack location. Furthermore, a fracture mechanics criterion was adopted to simulate fatigue crack growth. This was performed using a fracture analysis FRANC3D code for three-dimensional problems. As a result, the calculated stress intensity factors (SIFs) were presented for the first steps, and the fatigue life of the fan blade was evaluated using the Forman de Koning model at different stress ratios.

Study on erosion wear characteristics of aero-compressor blades considering distortion degree

A Tabakoff model of a certain type of turboshaft engine compressor blade was established, and the effects of no, mild, moderate, and severe distortions on blade erosion pattern were obtained. Based on particle velocimetry and erosive wear experiments, the Tabakoff model parameters (k12, V1, V2, V3, and θ0) were obtained. The effect of four distortion degrees on blade erosion pattern and erosion rate was studied. With an increase in the distortion degree, the erosion area of the rotor blade pressure surface is found to move toward the leading edge and blade tip. The erosion area of the stator blade suction surface moves toward the leading edge and blade root, whereas that of the pressure surface moves toward the blade root.

Investigation of Structural Strength and Fatigue Life of Rotor System of a Vertical Axial-Flow Pump under Full Operating Conditions

Axial-flow pumps consider both the conventional pump mode and the pump as turbine (PAT) mode operation and put forward higher requirements for long-term operation stability and structural strength; therefore, it is of great engineering significance to evaluate the structural strength and fatigue life of the rotor under full operating conditions. In this study, based on computational fluid dynamics and the one-way fluid-structure interaction algorithm, the structural strength and fatigue life of the rotor system of a large vertical axial-flow pump under full operating conditions were evaluated and studied. The results show that blade deformation and equivalent stress are generally higher in the PAT mode than in the pump mode. The maximum deformation in both modes occurs at the tip of the blade, while the area of stress concentration is at the root of the blade. Both the deformation and the equivalent stress increase with increasing flow rate. The minimum safety factor occurs at the blade root in both modes, and the safety factor in the PAT mode is relatively smaller than that in pump mode. Therefore, when designing and manufacturing axial flow pumps for turbine duties, priority should be given to material strength at the blade root during PAT mode operation to ensure safe and stable operation. The aim of this study is to provide technical references and theoretical foundations for evaluating the service cycle of axial-flow pumps and the influence on pump life under different operation modes.

Effect of the integral length scales of turbulent inflows on wind turbine loads

As wind turbines become larger, the fluctuations in the inflow become increasingly influential in the turbine structural loading. These fluctuations are characterized by the integral length scale, which represents the average size of the largest energy-containing turbulent eddies. Current design standards neglect the varying integral length scales that characterize inflows of wind turbines in operation. Using large-eddy simulations, we generate turbulent inflows of varying integral length scales and quantify the loads of the IEA 15-MW reference wind turbine. Results illustrate that the impact of turbulence on rotor and tower loads is up to 10 times greater than the impact of the mean shear profile. Increasing integral length scales from 0.3x to 0.5x the rotor diameter reduces blade root flapwise moments and rotor and tower loads. Increasing integral length scales from 0.5x to 1.7x the rotor diameter increases the rotor aerodynamic thrust force and the blade root flapwise shear loads and decreases the tower base torsional moment and the tilting and yawing rotor aerodynamic moments. Additionally, turbulence intensity has a greater impact on wind turbine loads than integral length scales. Findings indicate that design standards should consider varying integral length scales for accurate wind turbine loading characterization in turbulent inflow conditions.

LES simulation study of wind turbine aerodynamic characteristics with fluid-structure interaction analysis considering blade and tower flexibility

As wind turbine blades become longer and more flexible, aerodynamic characteristics become increasingly complex and require complete investigations. In this study, a two-way fluid-structure interaction (FSI) model is proposed to analyze the effects of inflow conditions, including uniform and atmospheric boundary layer (ABL) winds, as well as blade and tower flexibility, on the aerodynamic characteristics of wind turbine blades. A 4.5 MW real wind turbine with the rotor diameter of 152 m and the hub height of 94 m is considered. Results show that the edgewise blade motion can be disregarded, while the motion of the tower and the flap-wise motion of blade significantly influence the aerodynamic characteristics of blades. The separation flow at the blade root suction side is more distinct and becomes less significant with an increase in spanwise distance, with stagnation points primarily distributed along the flow separation boundary at 0.45–0.95L (L is the blade length). The three-dimensional rotational effect of the blade induces notable fluctuations in the pulsating pressure coefficient, particularly within this region. Notably, a prominent peak is observed at the location of 0.65L. Consequently, it is recommended to allocate additional focus on this specific region to mitigate potential fatigue loads.

Aerodynamic load evaluation of leading edge and trailing edge windward states of large-scale wind turbine blade under parked condition

The safety and reliability of wind turbine blades are increasingly challenged by extreme wind conditions such as typhoons, as wind turbines tend to become larger. Under these conditions, most units will be shut down and the blades will be pitched to around 90° to minimize the loads. This paper aims to compare a new strategy for the parked condition, i.e., the trailing edge windward state, with the traditional leading edge windward state to verify its technical feasibility. The aerodynamic loads of a 30%-thickness airfoil and a commercial wind turbine blade are comprehensively evaluated by wind tunnel experiment, CFD simulation and engineering analytical model. The two-dimensional airfoil cases indicate that the airfoil resultant force is lower in the trailing edge windward state than in the leading edge windward state for a wide range of angles of attack. Consequently, the three-dimensional blade cases shows that the low load region of the trailing edge windward state is relatively wider than that of the leading edge windward state. The averaged blade root load is reduced by 41.4% ~ 57.8% through trailing edge windward state with prescribed error bounds of windward angular. Besides, it is suggested that the traditional engineering analytical model should improve the precision of the extrapolated airfoil data around AOA = 180 deg. to ensure the accurate load evaluation under the trailing edge windward state. This study suggests a new control strategy for wind turbine blades under the parked condition, which offers significant benefits for load reduction and has a good potential for future applications.