Rain Erosion Tests Research Articles

Damage evolution simulation and lifetime prediction for composite blades under continuous droplet impacts

Offshore wind power generation is a promising technology in renewable energy applications due to its high reserves of wind energy in sea areas. To improve the energy transformation efficiency, the blade length of the offshore wind turbines has become larger and larger, and it has made rain erosion be one of the most frequent failures during the turbine operation. As the natural rainfall is stochastic in spatial and time domains, it is difficult to depict the damage evolution process caused by rain impact exactly. Therefore, regular and continuous droplet impact simulation and experiments present an alternative methodology for this issue. With the composite structure of the blade and deformability of the liquid droplet in consideration, a fluid solid interaction model will be established to investigate the impact response and subsequent damage evolution. In which, the Smooth Particle Hydrodynamics (SPH) model is utilized to depict the constitutive relationship within the droplet, and Finite Element Method (FEM) is used to construct the Representative Volume Element (RVE) model of the blade leading edge. The impact process is simulated first to obtain the impact pressure distribution at the contact center and velocity field in the droplet. Furthermore, the stress wave propagation in the blade multilayer structure can be analyzed. Owing to the multiaxial fatigue feature of the continuous droplet impact, the continuum damage mechanics is integrated with the fatigue criterion and the Jump-in-Cycle procedure is used to simulate the high-cycle fatigue process. The damage factor distribution on the blade coating surface and its influence on mechanical properties are analyzed. Thereafter, the droplet impact fatigue life can be accumulated based on Miner’s linear rules. The theoretical achievements are validated by experimental data provided by Rain Erosion Testing (RET), which shows a good agreement between each other. As a result, V-N curves and D-N curves, i.e. quantitative relationship between droplet falling conditions and impact fatigue life, are established. The achievements in this study can provide an effective tool for rain erosion mechanism analysis and life prediction in industrial applications.

Fatigue damage mechanics approach to predict the end of incubation and breakthrough of leading edge protection coatings for wind turbine blades

The state of the surface of the leading edge of wind turbine blades is important for optimal aerodynamic performance. Rain erosion leads to damage and roughness on the leading edge. In this study, we present an approach to predict the level of roughness of the leading edge based on fatigue damage accumulation due to impacts of rain droplets. Impact simulations of droplets on the protective coating layer are coupled with rain erosion test data and fatigue S-N curves. Fatigue damage values are then related to surface roughness levels. An approach to extract S-N curve parameters from a rain erosion test and then make predictions for other coatings based only on the output of impact simulations is presented and tested against test data. Both the end of incubation and coating breakthrough times are predicted with this approach. Stress, strain and energy density values were used as fatigue damage indicators and the respective predictions were compared to each other. It was found that the maximum principal strain gave the best predictions and matched the experimental trends.

The influence of acid rain on asphalt and its mixture performance, simulation and micro-mechanism exploration

Acid rain will destroy asphalt roads and affect human production and life. Therefore, this study has designed a new indoor simulated acid rain erosion test, and explored the effects of acid rain on the properties of 70# petroleum asphalt, SBS asphalt and their mixtures and the acid rain erosion mechanism from the micro and macro perspectives. In this paper, the influence of acid rain erosion on the microscopic properties of asphalt has been revealed through microscopic experiments. Through the combination with the basic performance test of asphalt, the effect of acid rain erosion on the conventional properties of asphalt was studied. The effects of acid rain on the properties of road asphalt mixtures were studied by Marshall immersion and high-temperature rutting tests. The results show that there is no obvious chemical reaction between the two kinds of asphalt after acid rain erosion, but the surface structure and morphology have changed significantly. There was a mass loss, the decreases of the softening point and ductility, and an increase of the penetration; the residual stability and high temperature resistance of both asphalt mixtures were reduced. In the worst case, the quality of petroleum asphalt was lost by 82.2%, the softening point and ductility decreased by 9.6% and 16.9% respectively, the penetration degree increased by 3.1% and the residual stability and dynamic stability of petroleum asphalt mixture decreased by 8.82% and 27.9% respectively. The mass loss of SBS asphalt was 82.5%, the softening point and ductility decreased by 5% and 3.3% respectively, the penetration increased by 2.6%, and the residual stability and dynamic stability of SBS asphalt mixture decreased by 4.82% and 25.4% respectively. In general, under the action of acid rain, the performance of 70# oil asphalt, SBS asphalt and their mixtures are affected to varying degrees. The specific performance is to destroy the original shape of asphalt, resulting in the loss of quality of asphalt, the reduction of deformation resistance, high temperature stability and low temperature stability of asphalt, weakening the high temperature stability and water stability of asphalt mixture, and leading to the decline of road performance.

Characterization of a pulsating water jet for rain erosion testing of blade coatings: Flow visualization, pressure investigation, and damage analysis

In the past decade or so, blade coatings have become state-of-the-art technology for the protection of wind turbine blades against particle and rain erosion. The present work considers the application of a high-frequency pulsating water jet, based on a passive acoustic-type nozzle, for accelerated rain erosion evaluation of blade coatings. Using high-speed image visualization and impact pressure investigation, the flow properties were systematically characterized, providing both qualitative and quantitative understanding of the key flow parameters. Continuous, pulsating and expanding flow zones, at velocities from 99 to 143 m/s, were classified for standoff distances within 120 mm. For a reference blade coating, the pulsating jet produced more realistic erosion patterns than the continuous flow. Furthermore, high-frequency impacts at around 7500 impacts/s led to fast fatigue accumulation, and enhanced lateral jetting effects, all of which shortened the coating erosion lifetime. In summary, for this work, a high-frequency pulsating water jet, for accelerated erosion studies on wind turbine blade coatings, was constructed and applied.

How leading edge roughness influences rain erosion of wind turbine blades?

Rain erosion of wind turbine blades is observed very commonly and is a concerning issue. Damage on the leading edge of blades leads to loss of aerodynamic efficiency. The mechanisms of damage initiation are not yet fully understood. This study focuses on the effect of existing surface damage or roughness of the leading edge of blades on impact stresses and erosion rates for the protective coating layer. The main mechanisms of interaction between impacting rain droplets and surface irregularities of a polyurethane-based coating are examined through finite element simulations and high-stress areas are identified. The results of the simulations reveal that areas inside erosion pits experience larger von Mises stresses than other undamaged areas. This could be explained by increased velocity as the water flows into erosion pits, observed in the simulations. At the same time, impacts on rough and damaged areas cause large stress values over a larger area than impacts on flat surfaces, due to the interaction with surface damage. These two observations could be the cause for faster erosion rates near damaged areas, as observed in experimental images during rain erosion testing. The predicted elevated stresses due to initial roughness or pre-existing damage could lead to faster damage growth, and to erosion pits growing wider and deeper.

Experimental campaign for the characterization of precipitation in a complex terrain site using high resolution observations

Precipitation has an effect on wind power at several levels. It affects the wind current, blade status, wake development and power production. Power production is affected by the harmful effect of precipitation on the blades eroding its surface and altering their aerodynamic performance. In the past decades, wind has been characterized using different techniques, but less effort has been devoted to precipitation measurement. In this work, the results of an experimental campaign performed at a high altitude complex terrain site to characterize precipitation using high resolution observations are presented. The campaign, carried out at CENER’s experimental wind farm (Alaiz) during 2023 within the framework of the Horizon Europe AIRE project, lasted nine months and different precipitation types (rain, snow, graupel) were recorded using a Micro Rain Radar (MRR), a Parsivel disdrometer and a rain gauge co-located with an instrumented wind mast with anemometers and wind vanes at different heights. Two case studies are selected to illustrate the wide range of variability found in precipitation conditions, particularly during the cool season. Precipitation characterization is very challenging at high temporal resolution, making necessary measurement campaigns with different precipitation equipment to optimize their performance and optimise its calibration. The study of precipitation profiles with MRR will support the study of precipitation impingement on wind turbine blades responsible of blade erosion. Moreover, these measurements will contribute to create the link between in-field wind farm data, laboratory experiments in rain erosion test rig and blade damage models necessary to improve wind turbine and wind farm design and operation.

Impact of meteorological data factors and material characterization method on the predictions of leading edge erosion of wind turbine blades

Leading edge erosion of wind turbine blades is a major contributor to wind farm energy yield losses and maintenance costs. Presented is a multidisciplinary framework for predicting rain erosion lifetimes of wind turbine blades. Key aim is assessing the sensitivity of lifetime predictions to: modeling aspects (material erosion model, blade aerodynamics), input data and/or their preprocessing (joint frequency distribution of wind speed and droplet size based on synchronous site-specific measurements versus frequency distribution generated with partly site-agnostic modeling standards, wind speed records of nacelle anemometer or extrapolated at hub height from met masts), and environmental conditions (UV weathering). The analyses consider a Northwest England onshore site where a utility-scale turbine is operational, focus on a reference 5 MW turbine assumed operational at the site, and use a typical leading edge coating material. It is found that the largest variations in erosion lifetime predictions are due to material erosion model (based on rain erosion test data or fundamental material properties) and wind and rain model (measurement-based joint wind speed and droplet size distribution or standard-based modeled distribution). The use of joint wind and rain distribution also enables identifying wind/rain states with highest erosion potential, knowledge paramount to deploying erosion-safe turbine control.

Fatigue S-N curve approach for impact loading of hyper- and visco-elastic leading edge protection systems of wind turbine blades

Computational evaluation of leading edge erosion remains challenging due to the high-strain rate loading conditions caused by impact of the wind turbine blade leading edge with rain droplets and other environmental particles. Here, a methodology is proposed for obtaining an S-N curve which can be used for impact fatigue evaluation of hyper- and viscoelastic leading edge protection systems for wind turbine blades, in the relevant strain rate domain. Two material systems (hard and soft polyurethane (PU)) are characterised experimentally by dynamic mechanical analysis (DMA) and static tensile tests. Time-temperature superposition is applied to the raw DMA data in order to obtain the material’s mastercurve, describing its visco-elastic behaviour in an expanded strain rate domain. The Yeoh (hyperelastic) and prony series (vis-coelastic) material model parameters are calibrated and form the input for a 2D-axisymmetric finite element model, in which Single Point Impact Fatigue Test (SPIFT) testing conditions are simulated. The stress field experienced by the coating during SPIFT testing is obtained and combined with the experimental measurements, allowing the determination of the material systems S-N curve, in the relevant strain rate domain. Results for a hard and soft PU coating system are compared with rain erosion test (RET) data. The RET data shows higher lifetime for the hard PU systems, a tendency that can be predicted when comparing the S-N curve for the hard and soft PU system. This methodology can be utilised in computational lifetime evaluation of leading edge coating systems. Furthermore, the methodology has the potential to partly alleviate the need of RET in the development and comparison of next-generation leading edge protection systems.

The Development of a Novel Thin Film Test Method to Evaluate the Rain Erosion Resistance of Polyaspartate-Based Leading Edge Protection Coatings

The relationship between the bulk thermomechanical properties and rain erosion resistance of development polyaspartate-based coatings as candidate leading edge protection (LEP) materials for wind turbine blades is investigated by the combined application of dynamic mechanical analysis (DMA) and rain erosion testing (RET) within a novel test method (DMA+RET). This method introduces the use of DMA+RET to both monitor the change in thermomechanical properties with respect to raindrop impact and subsequently rationalise differences in rain erosion resistance between coating formulations of comparable composition. The application of this combined process has demonstrated the importance of relatively high viscoelastic moduli at increased strain rates and creep recovery after RET as key material properties to be considered for LEP material development, whereas previous research presented in the scientific literature has primarily focussed on the use of routine characterisation procedures by tensile testing or stand-alone DMA to evaluate coating formulations prior to rain erosion testing. This journal article therefore presents a novel method to evaluate key material properties relevant to rain erosion resistance before and after subjection to raindrop impact using standard ASTM G73 RET equipment. The test method is demonstrated on a novel polyaspartate-based coating, PA-U, that exhibits notable rain erosion resistance in comparison to commercial LEP products. PA-U exhibited negligible mass loss after 30 h of rain erosion testing and favourable thermomechanical properties (E″ = 35 MPa at critical strain; equilibrium recoverable compliance of 0.05 MPa−1) in comparison to alternative formulations.

Adhesion and tribological characteristics of modified polyurethane coating on composite substrate

ABSTRACT In the present study, glass-epoxy composites were used as a substrate to study the performance of modified polyurethane coating. Two-layer coating (180–200 µm) wherein the epoxy primer as the first coat and polyurethane as the top coat was applied using an air-assisted spray gun to increase the service life of the composite. The analysis of coating was performed using gloss, contact angle, FTIR, TGA, tensile test and SEM. The bonding strength between coating and substrate was evaluated by cross-hatch cut, 180 deg peel strength and pull-off adhesion tests. Scratch, Taber abrasion and Pin-on-disc tests were also performed to evaluate the wear resistance. In addition, a rain erosion test was performed to check the stability of the coating. From the above tests, it is concluded that the applied coating well adhered to the composite substrate. The data generated from different tests can be used to estimate the service life of polymer composites and coatings.

Investigation of the Causes of Premature Rain Erosion Evolution in Rotor Blade-like GFRP Structures by Means of CT, XRM, and Active Thermography

Premature rain erosion damage development at the leading edges of wind turbine rotor blades impair the efficiency of the turbines and should be detected as early as possible. To investigate the causes of premature erosion damage and the erosion evolution, test specimens similar to the leading edge of a rotor blade were modified with different initial defects, such as voids in the coating system, and impacted with waterdrops in a rain erosion test facility. Using CT and XRM with AI-based evaluation as non-destructive measurement methods showed that premature erosion arises from the initial material defects because they represent a weak point in the material composite. In addition, thermographic investigations were carried out. As it shows results similar to the two lab-based methods, active thermography has a promising potential for future in-situ monitoring of rotor blade leading edges.

Mechanical and interfacial characterisation of leading‐edge protection materials for wind turbine blade applications

AbstractModern wind turbine blades experience tip speeds that can exceed 110 m/s. At such speeds, water droplet impacts can cause erosion of the leading edge, which can have a detrimental effect on the performance of the wind turbine blade. More specifically, rain erosion is leading to both reduced efficiency and increased repair costs. The industry is using polymeric coatings—leading‐edge protection (LEP) materials—to protect the blades but those are also prone to rain erosion. In this work, LEP materials that are currently used by the industry for the protection of wind turbine blades were selected and their performance assessed. The LEP materials were characterised in terms of mechanical properties by using different experimental methods, and they were also assessed in terms of durability by performing rain erosion testing (RET). Finally, the damage and failure mechanisms observed were further investigated using CT scanning. This paper provides an insight to the properties of LEP materials, their durability, and the damage and failure mechanisms they experienced during rain erosion.

Experimental study on the effect of drop size in rain erosion test and on lifetime prediction of wind turbine blades

Rain erosion of turbine blades causes loss of production and expensive repairs in the wind energy sector. There is a common consensus, that the size of impacting rain drops has a governing effect on the added damage. However, the literature lacks systematic experimental studies on the topic. In the present paper the effects of drop sizes in Rain Erosion Tests (RET) are studied for a commercial polyurethane based top coat applied to glass fiber-epoxy specimens. The tests are conducted applying a whirling arm RET at impact velocities ranging from 90 to 150 m/s and with four different rain field setups generating mean droplet diameters of 0.76 mm, 1.90 mm, 2.38 mm and 3.50 mm respectively. The time to damage at the end of incubation is determined by inspection of photographs captured inline at regular intervals through the test.Data sets of time to damage as function of local rotor velocity are extracted for each of the four different rain fields. From this data VQ curves (V for Velocity, Q for Quantity) are presented for different representations of quantity, namely specific impacts (DNV GL), specific impacts (ASTM) and accumulated impingement. The slope of the velocity-impingement (VH) curves vary with drop size.We propose a drop size dependent empirical model for impingement (H) to damage as function of impact velocity (v), H (v) = cv−m, where the scale parameter m and the shift parameter c are functions of the drop size.The drop size-dependent impingement model is then applied for computing the expected leading edge lifetime of a virtual 15 MW IEA reference turbine at 18 different meteorological stations in Northern Europe based on 10-min time series of rain intensity and wind speeds observed during several years. The drop size dependent model predicted on average 2.35 times longer lifetime compared to models based on the standard 2.38 mm drop size. Comparing the sites, the model shows a factor 3 variation of added damage per meter of accumulated rain between sites because of differences in concurrent wind during the rain events.

A spectral model generalising the surface perturbations from leading edge erosion and its application in CFD

Blade leading edge erosion (LEE) is a major cost driver in the wind energy sector. LEE is caused by the environmental conditions under which the blades operate. The impact energy of the airborne particles striking the leading edge determines the speed of erosion, thus LEE severity grows towards the blade tip and with a turbine’s tip speed. Currently there is no established method for assessing the aerodynamic impact of LEE, either numerically or experimentally caused by a lack of erosion topological data and its stochastic nature. Whilst previous studies investigated specific realisations of real-world erosion—modelling roughness, gouges, pinholes etc.—we propose a novel, reproducible representation of erosion, based on the superposition of waves with different frequencies and directions of propagation. Using lidar surface scans of a LE exposed to a rain erosion test, we demonstrate the possibility of representing surface perturbations from erosion by a simple spectrum, thereby allowing the mathematical representation of eroded surfaces. Furthermore, we demonstrate how the spectral model simplifies the analysis of LEE-affected aerofoils in CFD. Our study thus encompasses the workflow from rain erosion test → surface scan → spectral perturbation model → numerical erosion generation → 2D CFD → performance loss statistics.

Experimental Study on the Effect of Drop Size in Rain Erosion Test and on Lifetime Prediction of Wind Turbine Blades

Experimental Study on the Effect of Drop Size in Rain Erosion Test and on Lifetime Prediction of Wind Turbine Blades

Assessment of a Wind Turbine Blade Erosion Lifetime Prediction Model with Industrial Protection Materials and Testing Methods

Leading edge protection (LEP) coating systems are applied to protect turbine blade edges from rain erosion. The performance of a LEP system is assessed in an accelerated rain erosion test (RET) as a metric for industrial application, but these tests are expensive. Modelling methods are available to predict erosion, based on fundamental material properties, but there is a lack of validation. The Springer model (1976) is analysed in this work to assess it as a tool for using material fundamental properties to predict the time to failure in a rain erosion test. It has been applied, referenced and industry validated with important partial considerations. The method has been applied successfully for erosion damage by wear performance prediction when combined with prior material data from rain erosion test (RET), instead of obtaining it directly from fundamental properties measured separately as Springer proposed. The method also offers accurate predictions when coupled with modified numerical parameters obtained from experimental RET testing data. This research aims to understand the differences between the experimental data used by Springer and the current industry approach to rain erosion testing, and to determine how it may introduce inaccuracies into lifetime predictions of current LEP systems, since they are very different to those tested in the historic modelling validation. In this work, a review of the modelling is presented, allowing for the understanding of key issues of its computational implementation and the required experimental material characterisation. Modelling results are discussed for different original application issues and industry-related LEP configuration cases, offering the reader to interpret the limits of the performance prediction when considering the variation in material fundamental properties involved.

A novel solution for preventing leading edge erosion in wind turbine blades

ABSTRACT Over the past 30 years, wind energy has been established as one of the leading forms of renewable energy. As the industry grows so too does the size of the wind turbines themselves – large wind turbines can now generate up to 15 MW. However, with larger turbines comes additional structural challenges to overcome, where one such challenge is erosion along the leading edge of the blade due to water impingement at the higher tip speeds of the blade. Therefore, in this paper, the development of a novel solution for preventing leading edge erosion on wind turbine blades (LEP) is presented. Primarily, this paper describes the experimental testing campaigns that were performed during LEP development. Based on the results from the rain erosion testing of selected materials, their manufacturability and other mechanical properties, thermoplastic polyurethane has been selected as the most suitable material to manufacture the LEP. The LEP component was de-risked through demonstrator testing and then bonded to the leading edge of a full-scale wind turbine blade. Structural (dynamic, static and fatigue mechanical) testing was performed on the blade with no significant damage observed. The next stage of development is operational trials on a wind turbine in marine conditions.

A Staged Approach to Erosion Analysis of Wind Turbine Blade Coatings

The current wind turbine leading-edge erosion research focuses on the end of the incubation period and breakthrough when analysing the erosion mechanism. This work presented here shows the benefits of splitting and describing leading-edge erosion progression into discrete stages. The five identified stages are: (1) an undamaged, as-new, sample; (2) between the undamaged sample and end of incubation; (3) the end of incubation period; (4) between the end of incubation and breakthrough, and (5) breakthrough. Mass loss, microscopy and X-ray computed tomography were investigated at each of the five stages. From this analysis, it was observed that notable changes were detected at Stages 2 and 4, which are not usually considered separately. The staged approach to rain erosion testing offers a more thorough understanding of how the coating system changes and ultimately fails due to rain droplet impacts. It is observed that during microscopy and X-ray computed tomography, changes unobservable to the naked eye can be tracked using the staged approach.

An experimental study of rain erosion effects on a hydro-/ice-phobic coating pertinent to Unmanned-Arial-System (UAS) inflight icing mitigation

An experimental investigation was conducted to evaluate the variations of the surface wettability and ice adhesion strength on a typical hydro−/ice-phobic surface before and after undergoing continuous impingement of water droplets (i.e., rain erosion effects) at relatively high speeds (i.e., up to ~100 m/s) pertinent to Unmanned-Arial-System (UAS) inflight icing mitigation. The experimental study was conducted by leveraging a specially designed rain erosion testing rig available at Iowa State University. Micro-sized water droplets carried by an air jet flow were injected normally onto a test plate coated with a typical Super-Hydrophobic Surface (SHS) coating to simulate the scenario with micro-sized water droplets in the cloud impacting onto UAS airframe surfaces. During the experiments, the surface wettability (i.e., in the terms of static, advancing and receding contact angles of water droplets) and the ice adhesion strength on the SHS coated test plate were quantified as a function of the duration of the rain erosion testing. The surface topology changes of the SHS coated surface against the duration of the rain erosion testing were also measured by using an Atomic Force Microscope (AFM) system. The characteristics of the surface wettability and ice adhesion strength on the eroded SHS surface are correlated with the AFM measurement results to elucidate the underlying physics for a better understanding about the rain erosion effects on hydro−/ice-phobic coatings in the context of UAS inflight icing mitigation.

A two‐dimensional quantitative parametric investigation of simplified surface imperfections on the aerodynamic characteristics of a NACA 633‐418 airfoil

AbstractThe aerodynamic performance of a NACA 633‐418 airfoil has been analyzed with disturbances in approximately 1000 different configurations focused on the frontal 10% of the airfoil. The configuration parameters are based on field test samples and rain erosion test specimens. The most important trends are presented by 500 configurations each simulated for 6°, 8°, and 10° angle of attack. The simulations are performed with the DTU Wind Energy in‐house 2D CFD Reynolds‐averaged Navier–Stokes solver, EllipSys2D, combined with the eN transition model for the laminar‐turbulent boundary layer transition. The configurations are modeled by a direct geometrical modification of the airfoil shape. The results show that the most important parameters are the position and the depth/height of the disturbance, with up to 35% lift reduction and 90% lift/drag reduction within the specified angle of attacks and disturbance parameter ranges.