Rotor Blade Research Articles

Sand Erosion Resistance and Failure Mechanism of Polyurethane Film on Helicopter Rotor Blades.

Polyurethane is widely used on the surface of composite materials for rotor blades as sand erosion protection materials. The failure mechanism investigation of polyurethane film under service conditions is useful for developing the optimal polyurethane film for rotor blades. In this article, the sand erosion test parameters were ascertained according to the service environment of the polyurethane film. The sand erosion resistance and failure mechanism of polyurethane film at different impact angles were analyzed by an infrared thermometer, a Fourier transform infrared spectrometer (FTIR), a differential scanning calorimeter (DSC), a field emission scanning electron microscope (FESEM), and a laser confocal microscope (CLSM). The results show that the direct measurement method of volume loss can better characterize the sand erosion resistance of the polyurethane film compared to traditional mass loss methods, which avoids the influence of sand particles embedded in the polyurethane film. The sand erosion resistance of polyurethane film at low-angle impact is much lower than that at high-angle impact. At an impact rate of 220 m/s, the volume loss after sand erosion for 15 min at the impact angle of 30° is 57.8 mm3, while that at the impact angle of 90° is only 2.6 mm3. The volume loss prediction equation was established according to the experimental data. During low-angle erosion, the polyurethane film damage is mainly caused by sand cutting, which leads to wrinkling and accumulation of surface materials, a rapid increase in roughness, and the generation of long cracks. The linking of developing cracks would lead to large-scale shedding of polyurethane film. During high-angle erosion, the polyurethane film damage is mainly caused by impact. The connection of small cracks caused by impact leads to the shedding of small pieces of polyurethane, while the change in the roughness of the film is not as significant as that during low-angle erosion. The disordered arrangement of the soft and hard blocks becomes locally ordered under the action of impact and cutting loads. Then, the disordered state is restored after the erosion test finishes. The erosion of sand particles leads to an increase in the temperature of the erosion zone of the polyurethane film, and the maximum temperature rise is 6 °C, which does not result in a significant change in the molecular structure of the polyurethane film. The erosion failure mechanism is cracking caused by sand cutting and impact.

Nonlinear free vibration analysis of internal thickness-tapered multi-layered composite rectangular plates undergoing moderately large deflections

Thickness-tapering in multi-layered composites is achieved by terminating the plies at discrete locations. This allows for effective material utilization and significant weight savings and is adopted in applications involving multi-layered composites that demand variation in thickness such as helicopter yokes, flexbeams and rotor blades, aircraft wing-skin structures and turbine blades. In the present work, the nonlinear free vibration of such tapered composites undergoing moderately large deflections is investigated for the first time. A general nonlinear finite element formulation for tapered multi-layered composite plates based on Reddy's higher-order shear deformation theory and considering Von Karman nonlinearity is developed. Newton–Raphson scheme in conjunction with Newmark time-integration technique is employed to obtain the time response. The developed model is validated against available results in the literature and through experimentally obtained linear natural frequencies of a honeycomb sandwich plate with tapered face sheets. Parametric studies are conducted to deeply understand the nonlinear dynamic behaviour of tapered composite structures. The investigation reveals that tapered multi-layered composite plates exhibit higher nonlinearity compared to uniform plates thus necessitating the consideration of nonlinear effects in the analysis of such structures.

Effect of the rotor blade geometry on particle size and energy demand for knife-milled beech chips: Experimental identification and mathematical modelling

Rotor blade geometry (linear and screw) was experimentally identified for particle size characteristics and specific energy demand for knife-milled beech chips (7.8 wt % moisture content). The operational conditions of peripheral speed (6.8–20.4 m s−1) and screen sieve openings (0.75–6 mm) were considered for the knife milling of beech chips. The Rossin-Rammler-Sperling-Bennet model precisely described particle size distribution of comminuted beech chips with its parameters depending on screen sieve, rotor speed and blade geometry in a linear relationship. This relationship allowed the development and calibration of a unique empirical linear model. The model predicted particle size under the conditions considered for each blade geometry with accuracy. Energy demand values between 6.97 and 101.6 kW h t−1 were identified for D50 particle sizes between 0.29 and 2.60 mm. Energy demand was independent of the rotor blade geometry, and it was mainly affected by particle size characteristics, even though speed was also impactful. The linear model obtained allowed the prediction of energy requirements using speed and final particle size with an R2 value of 0.97.

Flow visualization by means of 3D thermography on yawing wind turbines

Thermographic flow visualization is an established tool for analyzing the actual flow behavior of real wind turbines in operation. While the laminar-turbulent flow transition as well as the beginning of flow separation can be localized in the thermographic image of a rotor blade, the corresponding positions on the 3d rotor blade surface are not yet known. To compensate the disturbing cross-influence of the wind turbine motion such as the yawing on the projected chord position of an identified flow feature, a geometric mapping algorithm of the 2d thermographic image on the 3d rotor blade is presented. With no geometric mapping, a significant error can occur depending on the camera location and orientation with respect to the location and orientation of the wind turbine. For the considered example, the maximal chord position error occurs for flow features at a relative chord location between 30% and 40%. If the yaw angle changes between ±30° and no correction is applied, the position error amounts up to ±17% of the chord length when the blade is observed above the nacelle. This example illustrates the necessity for an error correction. After its verification, the geometric mapping approach is applied on a thermographic image series from a field measurement campaign on a yawing wind turbine. For this purpose, the yaw angle is additionally measured with a laser scanner. In comparison with no geometric mapping, the corrected flow visualization of the laminar-turbulent transition during yawing reveals the actual mean chord location that is 20% of the chord length larger, a shift of the chord location that is almost one order of magnitude larger, and a chordwise location increase instead of a decrease. As a result, the geometry mapping is therefore considered applicable to advance thermographic flow visualization for the analysis of flow dynamics on yawing and pitching wind turbines, and in future even during one rotor revolution.

Propeller design and verification studies for 30–35 meter tugboats

The invention of the propeller, the breakthrough of modern shipping, has supported a much more efficient use of the seas. The propeller consists of helical surfaces placed at equal intervals around a cylinder and can be characterized as ship elements that work like a screw in water. As a result of the rotation of the propeller, the ship to which the propeller is attached makes a translational movement due to the effect-response of the fluid on the propeller.This study includes propeller design and verification as part of a comprehensive project. The scope is the design of a 2m diameter aluminum bronze propeller with 4 blades of 50° backward inclination used on 25–30 m tugboats. Both static and CFD (Computational Fluid Dynamics) analyses were performed and design verification studies were carried out. The forces due to seawater, geometry of the blades, seawater flow rate and blade rotation speed were taken into consideration.The maximum deformation value of 1.7616 mm was obtained at the applied mode 4 value. When all the results obtained were evaluated, there was no need to go back to the propeller design. First of all, the angle value in the propeller geometry was found to be ideal for 50° angle in terms of both strength and fluid mechanics.

Mode I fracture of thick adhesively bonded GFRP composite joints for wind turbine rotor blades

This article investigates the effects of voids, joint geometry, and test conditions on the quasi-static Mode I fracture performance of thick adhesive Double Cantilever Beam (DCB) joints such as those prevailing in wind industry and shipbuilding. The specimens were made by glass fiber reinforced epoxy adherend and SikaPower®-830 epoxy adhesive in the cm thickness range. Side-grooved shape guided the crack propagation direction and assisted stable propagation, while lower cross-head displacement rates reduced the occurrence of unstable crack propagation and prevented crack deflection. Porosities, which are inevitable due to the high viscosity of the adhesive, led to unstable propagation and promoted crack path deviations. They could also decrease the apparent fracture energy release rate (SERR) since the crack surface is reduced. In conclusion, grooved DCB joints with low void content tested at low displacement rates showed stable crack propagation without significant crack path deviation. This method enables comparison with future adhesive formulations and the refinement of full-scale blade models.

Numerical analysis of flow and heat transfer characteristics of unsteady film cooling of HPT shroud under high-speed rotation sweeping of the rotor blade

3D URANS simulations were performed to investigate the influence of high-speed blade rotation sweeping effect on the unsteady multi-hole film cooling performance of a high-pressure turbine (HPT) shroud. The blade rotation speed was 10,000 rpm and 15,000 rpm, the associated Strouhal numbers (St) were 2.35 and 3.52, and the blow ratio (M) was varied from 0.5 to 1.5. The instantaneous and time-averaged characteristics of film outflow mass rate, pressure distribution, film cooling effectiveness (FCE), and convective heat transfer coefficient (CHTC) of the HPT shroud with and without impinging jets were determined. The results show that the St and M have a substantial impact on the distribution of the FCE and CHTC on the shroud surface. At a high St and M, the FCE on the shroud surface is greater and more sensitive to the rise in M. As St increases, the suppression effect of the tip leaking vortex (TLV) on film and FCE on the shroud surface improved. The increased impingement jet effect has minimal impact on the FCE of the shroud. Under a high-speed blade rotation sweep, the net heat flux on the shroud wall with impingement/film cooling falls by approximately 22% when compared to the uncooled shroud surface.

Prediction of crack growth at trailing edge bondlines of a wind turbine rotor blade for the assessment of remaining service life

Debonding along trailing-edge joint of a wind turbine rotor blade is one of the often reported failure modes during the rotor blade maintenance inspections. This study focuses on buckling-induced debonding along the trailing edge under in-plane compressive loads due to the gravity forces on the rotor blade. With a priori known debond crack length, an analytical model is used for predicting debond growth rate and assessing the time that takes the crack to reach a critical length. For the growth rate prediction, required input parameters are size of the initial debonding, laminate and adhesive layer thicknesses, elastic and interfacial fracture mechanics properties, gravitational load distribution and position of the initial debonding. The damage model is implemented in excel spreadsheet tool. The predictions of the analytical model were compared with the results from 3D finite element simulations and the difference between the results (energy release rate) was 8%. This study demonstrates how damage specific tools can be developed for predicting remaining useful life of rotor blades.

A novel OPR-free method for blade tip timing based on adaptive variable reference blades

Monitoring rotor blade vibrations is imperative in aero-engine testing and service. Blade tip timing (BTT), a promising technique for such monitoring, assesses the operational status of rotor blades through non-intrusive measurement of blade tip displacement. Traditional BTT technology relies on the Once-Per-Revolution (OPR) probe, which poses installation challenges and introduces measurement uncertainties in practical applications. This paper introduces a novel OPR-free approach, employing Adaptive Variable Reference Blades (AVRB) for non-intrusive measurement of rotor blade vibrations. The method assumes a stochastic mistuning of blades within the same stage, resulting in non-simultaneous resonance of blades at a given speed. In each revolution, the vibration level of individual blades is determined by assessing the deviation between the average speed of a blade, as measured by two adjacent BTT probes, and the average speed across the entire circumference. Consequently, a non-vibrating blade is adaptively identified and selected as the reference blade. The relative vibration levels among blades vary dynamically during different revolutions, influenced by mistuning features, leading to variability in the reference blades. These adaptive variable reference blades can then replace the OPR probe, aiding in the computation of vibration displacement and further parameter identification of blades. To validate the effectiveness of the proposed method, a five-degree-of-freedom numerical model is utilized for simulation. Additionally, experiments involving both a five-bladed disk and an eight-bladed disk are conducted to validate the proposed approach. Numerical simulations and two sets of test results demonstrate a fundamental agreement between the blade vibration displacement calculated using the proposed method and the outcomes obtained through the traditional BTT method with OPR. The relative error in the identified blade vibration frequency derived from the blade vibration displacement, as determined by both methods, does not exceed 1 %. Additionally, the relative error in the amplitude of the blade vibration displacement is within the range of 10 %.

Flutter Measurement of a Linear Turbine Blade Cascade with an Angular Position Deviation of One Blade

Abstract Experimental flutter testing of high-aspect ratio rotor blades is a mainstay of turbomachinery research. However, rotor blades are never identical, and geometrical errors between actual and nominal geometries exist due to limited machining precision or assembly imperfection. The paper presents the initial phase of the controlled flutter research of a linear turbine blade cascade with a geometric deviation in one blade position. A subsonic wind tunnel with four flexibly mounted blades in an otherwise rigid blade cascade is employed at one angle of incidence and three low reduced frequencies. Measurements are performed with an angular position deviation (±1.5°) of one blade in pure bending and torsion modes. A tangible effect of one blade’s slight incidence angle offset on the vibrating blade cascade aerodynamic stability is demonstrated, and this research effort opens the door to a more extensive testing campaign.

Quantification of process-induced effects on fatigue life of short-glass-fiber-filled adhesive used in wind turbine rotor blades

Cracks in the adhesive joints of wind turbine blades can often cause severe structural blade damage. Effects induced by the manufacturing process can have a significant impact on the cycle number toward crack initiation in the bonding material. This research compared two experimental series of neat adhesive, dog-bone-shaped specimens with different degrees of cure, fiber orientation, and porosity content, which were subjected to static and cyclic tension-tension loading. To enhance the comparability of the two coupon series, the experimental data was normalized. Normalization factors were introduced to compensate for the different properties of the two series. These factors were derived from correction models. After normalization, the S-N values of the two data series were in good agreement in the high-cycle fatigue regime. In the very-low-cycle fatigue regime, however, there was still a lack of agreement in some cases, including the static strength. Moreover, the failure mechanisms were investigated by analyzing images from the fracture surfaces and dedicated samples using X-ray microscopy. The correction models presented can be applied to estimate the design strength of adhesives. This knowledge can then be used to make rotor blade structures more resistant to crack initiation.

Constraining effect of load frames during full-scale rotor blade fatigue testing

In rotor blade fatigue tests used for the certification process, load frames are used to introduce the loads to the blade. These load frames have a constraining effect on the cross-sectional deformation. This work investigates these constraining effects using a finite element contact analysis on a current-generation commercial rotor blade. Two different load frame variants are considered: a conventional load frame covering most of the cross-section and a reduced load frame covering only the main spar caps. The interaction between blade and load frame is implemented via contact formulations, which allows pretensioning in combination with a flap and a lead-lag load case. Strains on the outer surface of the blade are evaluated and compared to an artificial reference loading. The area of influence of the load application through the load frame, in which the strains deviate significantly due to clamping effects, mostly corresponds to the 0.75 times the chord length assumed for the certification [1]. The strains in the trailing edge are significantly less affected by the reduced load frame variant than by the conventional one, thus potentially making it possible to consider the trailing edge at the position of the load frame as being properly tested for certification purposes.

Numerical study of airfoil used for helicopter rotor tip to investigate the enhancement in the aerodynamic characteristics using active flow control at forward flight conditions

Helicopter forward flight is the most important state of flight due to time spent in this mode during the complete helicopter trip. In this study, a main rotor blade tip airfoil was simulated to obtain the effect of applying active flow control on the tip part of the rotor blade. A blowing excitation jet is applied to the leading edge of the airfoil just before the separation point with small momentum and high frequency to enhance the near wall boundary layer characteristic. This jet is used with different amplitude, frequency and excitation location from a leading edge to study the effect of this parameter. A numerical model is used to solve two different angles of attack and all results are compared with NACA report that used the same helicopter hub and airfoil. Results show good agreement with the experimental results and the chosen excitation parameter gives good enhancement to the performance of the airfoil used in the tip part of the helicopter blade in this mode of flight. Performance represented in lift and drag and lift to drag ratio shows improvement, reaching about 10% increase in lift and 30% decrease in drag, which means more than 50% change in lift to drag ratio. Furthermore, this study gives a good estimation of the best excitation location that is required for real application on the helicopter blade to improve of forward flight performance.

Residual stresses in thick composites considering through-thickness temperature gradients during manufacturing

In the composite materials of wind turbine rotor blades, residual stresses form during manufacturing due to the different thermo-chemo-mechanical behaviors of the fibers and the matrix. These residual stresses decrease the ultimate and fatigue strength of the blade because the failure stress and cycles to failure are reached at lower levels of external loading. It is thus of utmost importance to consider residual stresses in the design stage to obtain reliable blade designs.This work deals with the prediction of residual stresses in a thick composite representative for a spar cap in a wind turbine rotor blade. A model is formulated for the thermo-chemo-mechanically coupled problem. An emphasis is given on the non-uniform and time-variable temperature distribution across the thickness of the composite. The different sources of residual stresses (e.g., chemical shrinkage, non-uniform distribution of different thermal expansion coefficients of fibers and matrix, exotherm, etc.) are analyzed, quantified, and compared. The impact of the time-dependent temperature distribution on the formation of residual stresses is highlighted by means of a representative numerical example.

A novel study on forecasting the airfoil self-noise, using a hybrid model based on the combination of CatBoost and Arithmetic Optimization Algorithm

One of the important challenges in aircraft design is the light, inexpensive, and self-noise-free design of airfoil. Since the self-noise generated by rotor blades interacting with turbulent flow produced in their boundary layer can impact how well the system works. Predicting aerofoil self-noise can help shorten computational times and improve cost-effectiveness because traditional computational fluid dynamics methods flow has more time consumption. Therefore, this paper proposes a forecasting approach using the hybrid model of the CatBoost algorithm. In this way, the hyperparameters of the CatBoost are tuned by twelve optimizers to increase precision and speed up forecasting. The precision of the hybrid models is evaluated by statistical evaluation indices such as Coefficient of determination (R2), Root Mean Square Error (RMSE), Mean Absolute Error (MSE), and relative absolute error (MAE). The obtained results show that the CatBoost combination with the Arithmetic Optimization Algorithm better operates optimally and depicts lower error values compared to other optimizers. Also, the values of the coefficient of determination for training and testing data with 0.9999 and 0.9706, respectively, demonstrate that Arithmetic Optimizer is the most precise optimizer among studied metaheuristic algorithms.

Design of an element test specimen for fatigue delamination growth of a thick laminate

Following the observation of multiple delaminations growing from a tunnelling crack in the spar cap of a wind turbine rotor blade subjected to cyclic loading, an element test specimen is proposed. Design of the element test specimen to study fatigue delamination of a thick laminate is presented. Complexities inherent to a full structure e.g., curvatures, ply drops, manufacturing defects, etc. are removed to study the fatigue delamination damage mechanism. The element test specimen is made of unidirectional layers with two embedded artificial delamination cracks connected by a tunnelling crack. In this paper, analytical and numerical (2D) models are used to design the element test specimen.

Failure mode and effects analysis using an improved pignistic probability transformation function and grey relational projection method

Failure mode and effects analysis (FMEA) is an important risk analysis tool that has been widely used in diverse areas to manage risk factors. However, how to manage the uncertainty in FMEA assessments is still an open issue. In this paper, a novel FMEA model based on the improved pignistic probability transformation function in Dempster–Shafer evidence theory (DST) and grey relational projection method (GRPM) is proposed to improve the accuracy and reliability in risk analysis with FMEA. The basic probability assignment (BPA) function in DST is used to model the assessments of experts with respect to each risk factor. Dempster’s rule of combination is adopted for fusion of assessment information from different experts. The improved pignistic probability function is proposed and used to transform the fusion result of BPA into probability function for getting more accurate decision-making result in risk analysis with FMEA. GRPM is adopted to determine the risk priority order of all the failure modes to overcome the shortcoming in traditional risk priority number in FMEA. Applications in aircraft turbine rotor blades and steel production process are presented to show the rationality and generality of the proposed method.

Characterization of a short fiber-reinforced adhesive designed for wind turbine rotor blades

Wind turbine rotor blades are usually made of two half-shells and shear webs that are joined together by structural adhesives. The adhesive joints suffer from longitudinal strains from blade bending combined with shear strains from torsion and shear forces acting on the blade. It is thus important to provide an accurate and reliable characterization of the adhesive as a bulk material to account for cohesive failure.This work focusses on the characterization of a short fiber-reinforced adhesive used in wind turbine rotor blades under uniaxial tension, compression, and shear as well as biaxial tension-compression/shear for both static and fatigue loading. To obtain small scatter in measurement results, a specimen geometry was designed that ensures a clear maximum stress in the test section while minimizing stress concentrations in the transition between the load introduction regions and the test section. The manufacturing process was then designed in such a way that an optimum mixture of resin and hardener was obtained without the inclusion of voids. The latter was verified after manufacturing by means of micro-CT scanning for all specimens. A very extensive test campaign was then carried out in order to quantify stiffness, static strength, fatigue strength, and fatigue-related stiffness degradation. Excerpts of this test campaign are presented in this paper.

Impact of shell structure stiffness on aero-structural coupling in wind turbine rotor blades

Wind turbine rotor blades are heavily loaded composite structures that experience a mixture of aerodynamic, inertial, gravitational, and gyroscopic forces during their operation life. Due to the high loads, the cross-sections of the blades are subjected to in-plane and out-of-plane deformations. The out-of-plane deformations are referred to as shear warping while the in-plane deformations are also called blade breathing. Blade breathing depends on the magnitude of the mechanical loads, which are expressed by means of internal forces and moments, and the stiffness of the blade shell.In this work, the relationships between in-plane cross-sectional deformations and internal loads are investigated. For the quantification of the deformation, a reference blade is studied via 3D finite shell element simulations for different loading scenarios. The cross-section of interest is located at the radial position of maximum chord. To compare the shape of the cross-sections in the undeformed and the deformed configurations, a procedure is proposed to relate the positions of nodes associated with the cross-section of interest in both configurations to a joint coordinate system. The shape of the deformed cross-section is then extracted and compared with the undeformed configuration. The comparison is executed for the individual internal forces and moments, namely flapwise and edgewise bending moments, normal force, shear forces, and torsion moment, respectively. The deformation patterns are discussed and it is addressed how these may influence the aerodynamic behavior of the cross-section under consideration.

Reliability analysis of 15MW horizontal axis wind turbine rotor blades using fluid-structure interaction simulation and adaptive kriging model

Over the course of the last four decades, the rotor diameter of Horizontal Axis Wind Turbines (HAWTs) has undergone a substantial increase, expanding from 15 m (30 kW) to an impressive 240 m (15MW), primarily aimed at enhancing their power generation capacity. This growth in blade swept area, however, gives rise to heightened loads, stresses and deflections, imposing more rigorous demands on the structural robustness of these components. To prevent sudden failure and to plan effective inspection, maintenance, and repair activities, it is vital to estimate the reliability of the rotor blades by considering all the forces (aerodynamic and structural dynamics) acting on them over the turbine's lifespan. This research proposes a comprehensive methodology that seamlessly combines fluid-structure interaction (FSI) simulation, Kriging model/algorithm and Adaptive Kriging Monte Carlo Simulation (AKMCS) to assess the reliability of the HAWT rotor blades. Firstly, high-fidelity FSI simulations are performed to investigate the dynamic response of the rotor blade under varying wind conditions. Recognizing the computationally intensive nature and time-consuming aspects of FSI simulations, a judicious approach involves harnessing an economical Kriging model as a surrogate. This surrogate model adeptly predicts blade deflection along its length, utilizing training and testing data derived from FSI simulations. Impressively, the Kriging model predicts blade deflection 400 times faster than the FSI simulations, showcasing its enhanced efficiency. The optimized surrogate model is then used to estimate the flap wise blade tip deflection for one million wind speed samples generated using Weibull distribution. Thereafter, to evaluate the reliability of the blades, statistical modeling using methods such as Monte Carlo Simulation (MCS), AKMCS is performed. The results demonstrate the faster convergence of AKMCS requiring only 21 samples, as opposed to 1 million samples for MCS with minimal reduction in the precision of the estimated probability of failure (Pf) and reliability index (β). Demonstrated on the backdrop of an IEA-15MW offshore reference WT rotor blade, the proposed methodology underscores its potential to be seamlessly incorporated into the creation of WT digital twins, due to its near real-time predictive capabilities for Pf and β assessments.