Flexible Blades Research Articles

Wave damping by flexible vegetation: Connecting individual blade dynamics to the meadow scale

Aquatic vegetation provides ecosystem services of great value, including the damping of waves, which protects shorelines and reduces resuspension. This study proposes a physically-based model to predict the wave decay associated with a submerged meadow as a function of plant morphology, flexibility, and shoot density. In particular, the study considers both the rigid (sheath) and flexible (blade) segments of the plant. Flexible plants reconfigure in response to wave orbital velocity, which diminishes wave decay relative to a rigid plant of the same morphology. The impact of reconfiguration on wave decay can be characterized using an effective blade length, le, which represents the length of a rigid blade that generates the same drag as the flexible blade of length l. The effective blade length depends on the Cauchy number, which defines the ratio of hydrodynamic drag to blade stiffness, and the ratio of blade length to wave orbital excursion. This laboratory study considered how the scaling laws determined for individual blades can be used to predict the wave decay over a meadow of multiple plants, each consisting of multiple blades attached at a rigid stem (sheath). First, the drag force on and motion of individual model blades (made of low-density polyethylene) was studied for a range of wave conditions to provide empirical coefficients for the theoretically determined scaling laws for effective blade length, le. Second, the effective blade length predicted for individual blades was incorporated into a meadow-scale model to predict wave decay over a meadow. The meadow-scale model accounts for both the rigid and flexible parts of individual plants. Finally, wave decay was measured over meadows of different plant density (shoots per bed area), and the measured decay was used to validate the wave-decay model. Wave decay was shown to be similar over meadows with regular and random arrangements of plants.

A theoretical analysis of blade coating for third-grade fluid

In this article, coating process for a third-grade fluid using a simple fixed blade on a moving substrate is discussed. The analysis is carried out for both exponential (flexible blade) and plane (stiff blade) coaters. Coupled partial differential equations are simplified by applying the lubrication approximation theory and assuming that the coating layer thickness is much less than the blade length. The obtained system is normalized using suitable scales. A numerical solution of the governing boundary value problem is developed for various third-grade parameters. In addition, a perturbation solution is also obtained for small third-grade parameters. Interesting quantities such as pressure, pressure gradient, velocity, and load are computed and shown graphically and in tables. How the involved parameters influence the results is examined. The load on the blade is the most important physical quantity in the present work as it controls the coating thickness and quality. We found that the blade load increases as the third-grade material parameter [Formula: see text] increases.

Development of a fully coupled aero-hydro-mooring-elastic tool for floating offshore wind turbines

A floating offshore wind turbine (FOWT) is a coupled system where a wind turbine with flexible blades interacts with a moored platform in wind and waves. This paper presents a high-fidelity aero-hydro-mooring-elastic analysis tool developed for FOWT applications. A fully coupled analysis is carried out for an OC4 semi-submersible FOWT under a combined wind/wave condition. Responses of the FOWT are investigated in terms of platform hydrodynamics, mooring dynamics, wind turbine aerodynamics and blade structural dynamics. Interactions between the FOWT and fluid flow are also analysed by visualising results obtained via the CFD approach. Through this work, the capabilities of the tool developed are demonstrated and impacts of different parts of the system on each other are investigated.

Health Monitoring and Safety Evaluation of the Offshore Wind Turbine Structure: A Review and Discussion of Future Development

With the depletion of fossil energy, offshore wind power has become an irreplaceable energy source for most countries in the world. In recent years, offshore wind power generation has presented the gradual development trend of larger capacity, taller towers, and longer blades. The more flexible towers and blades have led to the structural operational safety of the offshore wind turbine (OWT) receiving increasing worldwide attention. From this perspective, health monitoring systems and operational safety evaluation techniques of the offshore wind turbine structure, including the monitoring system category, data acquisition and transmission, feature information extraction and identification, safety evaluation and reliability analysis, and the intelligent operation and maintenance, were systematically investigated and summarized in this paper. Furthermore, a review of the current status, advantages, disadvantages, and the future development trend of existing systems and techniques was also carried out. Particularly, the offshore wind power industry will continue to develop into deep ocean areas in the next 30 years in China. Practical and reliable health monitoring systems and safety evaluation techniques are increasingly critical for offshore wind farms. Simultaneously, they have great significance for strengthening operation management, making efficient decisions, and reducing failure risks, and are also the key link in ensuring safe energy compositions and achieving energy development targets in China. The aims of this article are to inform more scholars and experts about the status of the health monitoring and safety evaluation of the offshore wind turbine structure, and to contribute toward improving the efficiency of the corresponding systems and techniques.

Impact of Blade Flexibility on Wind Turbine Loads and Pitch Settings

Along with the upscaling tendency, lighter and so more flexible wind turbine blades are introduced for reducing material and manufacturing costs. The flexible blade deforms under aerodynamic loads and in turn affects the flow field, arising the aeroelastic problems. In this paper, the impacts of blade flexibility on the wind turbine loads, power production, and pitch actions are discussed. An advanced aeroelastic model is developed for the study. A free wake vortex lattice model instead of the traditionally used blade element momentum (BEM) method is used to calculate the aerodynamic loads, and a geometrically exact beam theory is adopted to compute the blade structural dynamics. The flap, lead-lag bending, and torsion degrees-of-freedom (DOFs) are all included and nonlinear effects due to large deflections are considered. The National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine is analyzed. It is found that the blade torsion deformations are significantly affected by both the aerodynamic torsion moment and the sectional aerodynamic center offset with respect to the blade elastic axis. Simulation results further show that the largest bending deflection of the blade occurs at the rated wind speed, while the torsion deformation in toward-feather direction continuously increases along with the above-rated wind speed. A significant reduction of the rotor power is observed especially at large wind speed when considering the blade flexibility, which is proved mainly due to the blade torsion deformations instead of the pure-bending deflections. Lower pitch angle settings are found required to maintain the constant rotor power at above-rated wind speeds.

АНАЛИЗ ПРИВОДНОГО МЕХАНИЗМА ПИЛЫ С ПОПЕРЕМЕННЫМ В ОБЕ СТОРОНЫ ДВИЖЕНИЕМ ГИБКОГО РЕЖУЩЕГО ОРГАНА

АНАЛИЗ ПРИВОДНОГО МЕХАНИЗМА ПИЛЫ С ПОПЕРЕМЕННЫМ В ОБЕ СТОРОНЫ ДВИЖЕНИЕМ ГИБКОГО РЕЖУЩЕГО ОРГАНА

Innovative concept of compliant mechanisms made by additive manufacturing

The complete redesign for Additive Manufacturing of compliant mechanism structures enables CSEM to develop innovative concepts to drastically reduce the need of machining and assembly after additive manufacturing. Support structures under flexure blades are thus minimised and the overall process becomes more streamlined. Moreover, this concept allows us to easily design and produce monolithic cross flexure pivots with interlocked flexible blades. Based on this concept, CSEM is now developing new architectures of Compliant Mechanisms based on Additive Manufacturing (COMAM) for the European Space Agency (ESA) in the frame of a GSTP research project. The past and current work of design, 3D printing and testing on several compliant mechanisms are presented. These demonstrators will be used as use-case for future high-precision and harsh environment applications such as cryogenic and space.

EXPERIMENTAL AND NUMERICAL STUDY OF CONTROLLED FLUTTER TESTING IN A LINEAR TURBINE BLADE CASCADE

In this paper, experimental testing of flutter and numerical simulations using a commercial code ANSYS CFX and an in-house code TRAF are performed on an oscillating linear cascade of turbine blades installed in a subsonic test rig. Bending and torsional motions of the blades are investigated in a travelling wave mode approach. In each numerical approach, a rig geometry model with a different level of complexity is used. Good agreement between the numerical simulations and experiments is achieved using both approaches and benefits and drawbacks of each technique are commented in this paper. It is demonstrated that both used computational techniques are adequate to predict turbine blade flutter. It is concluded that validated numerical tools can provide a better insight of flutter phenomena of operationally flexible steam turbine last stage blades.

INTERACTION BETWEEN WAVES AND HANGING HIGHLY FLEXIBLE KELP BLADES

The interaction between waves and flexible blades has drawn recent attention because of the capacity of nature-based infrastructure, such as aquatic vegetation and kelp, to attenuate waves. In this study, a new numerical model was developed to study the wave-blade interaction for both bottom-fixed and suspended blades. The dynamics of the blades simulated by a cable model were coupled with OpenFOAM®-based wave model IHFoam with the immersed boundary method. The results showed that the distribution of the blade-induced vortices was asymmetric with more vortices upstream for the single bottom-fixed blade while more vortices downstream for the single suspended blade. For both submerged and suspended canopies, the vortex distribution is also asymmetric. More vortices concentrate upstream for the submerged canopy. For a suspended canopy, more vortices concentrate upstream and below the bottom of the suspended canopy. Yet near the surface above the suspended canopy, more vortices concentrate downstream. Understanding the distribution of vortices is important for predicting the sediment transport and nutrient distribution.

On aeroelastic response of an airfoil under dynamic stall using time delay neural network

To clarify the mechanism of the complex aeroelastic responses of flexible blades of helicopter rotors under dynamic stall, an aeroelastic system of a 2D airfoil forced to oscillate is investigated. A reduced-order model based on time delay neural network is introduced to cut down the cost in parametric study of the aeroelastic system. The model is validated against full-order model results both with and without elasticity, which is proved to be an accurate and computational efficient method. The model is then applied to study the influence of system’s natural frequency on the aeroelastic responses, and a phenomenon of secondary resonance is found in the results. Detailed investigation and full-order simulation are performed on the cases of secondary resonances. The phenomenon is believed to be associated with the strong aerodynamic nonlinearity under dynamic stall where the instability of the vortex shedding or interaction leads to subharmonics of the aerodynamic loads. When the structure’s natural frequency is coincident with the multiples of the subharmonic, secondary resonances happen.

Concept of flexible vertical-axis wind turbine with numerical simulation and shape optimization

Vertical-axis wind turbines (VAWT) are becoming popular solutions for electric power generation. Among them, Savonius-type VAWTs have low rotational speed, reduced noise and ability to self-start, but their disadvantage is the low energy conversion efficiency. This paper investigates a novel VAWT concept and whether it can provide a higher energy conversion efficiency. The novel concept is a Savonius-type VAWT with flexible blades that change their shape passively due to aerodynamic and blade inertial forces during rotation. An initial design was optimized by a customized shape optimization workflow based on genetic algorithms. The blade design evaluation was performed by CFD model with two-way fluid-structure interaction. The optimization objective was to determine the optimal blade shape, thickness distribution and location of the blade support arms by maximizing the power coefficient and keeping structural stresses below the design limit. The results show that 8% improvement of the power coefficient is possible while keeping the structural stress within the design limit. A detailed performance analysis in the paper shows that there is potential for further optimization considering annual energy production for a selected location. For that purpose, the proposed optimization workflow can serve as a tool for development of future designs.

A videogrammetric system for measuring the full-field blade deformation of a heavy-duty helicopter's rotating rotor.

In this paper, a videogrammetric system, based on three-dimensional digital image processing and industrial close range photogrammetry technology, is proposed to solve the three-dimensional motion and deformation parameters of a rotating blade by identifying a speckle pattern. For the high aspect ratio and flexible rotor blade design often used in heavy-duty helicopters, there are extremely complex aerodynamic, dynamic, and coupling problems that require solutions to measure the full-field deformation of the rotating blade in the test condition of a rotor test tower. The key issues involved include a two-step calibration method using separate internal and external parameters for a large field of view, and a reliable estimation of the seed point method for a large deformation and weak correlation speckle images match. The system was developed and a simulation test was conducted on a rotor test tower. The results show that the proposed system is practicable to measure the surface displacement and motion parameters of a high-speed rotating rotor blade.

Novel compliant wiper mechanism

Abstract. Conventional wiper mechanisms that are used in automotive vehicles comprise numerous linkages and joints. In this study, in order to obtain a simpler design, a novel compliant wiper mechanism is introduced. The wiper mechanism is essentially a partially compliant four-bar mechanism. To the best of our knowledge, this is the first compliant wiper mechanism in the literature. The wipers currently labeled “compliant” in the literature possess only a flexible wiper blade frame. However, these are still driven by conventional rigid body mechanisms. After introduction of the fundamental concept, a compliant wiper mechanism is designed for an L7e car. Finite element analysis is carried out for verifying analytical results and fatigue calculations are performed. Finally, a prototype is manufactured, and it is experimentally verified that a compliant wiper mechanism may have an infinite life.

A parametric study of flutter behavior of a composite wind turbine blade with bend-twist coupling

Now a days wind turbine blades are generally designed to have length as high as 60 m or more to maximize power production. Aeroelastic instabilities such as flutter are major concerns for these long, flexible and slender blades. Stiffness coupling between bending and twisting modes can be used to improve the aeroelastic performance of such blades. In composite blades bend twist coupling can be achieved by imparting unbalance in the lamination sequence. In the present work, a parametric study has been conducted to study the effect of unbalances in different parts of a wind turbine blade on flutter instability. An eigenvalue-based approach has been used for flutter analysis. It has been observed that flap-torsional stiffness has high impact on critical flutter speed. The critical flutter speed is increased by 40% with flap-torsional stiffness due to the unbalance in the entire section of the blade with symmetric skin, while for asymmetric skin; the achievable increment is 100%. The unbalance in the spar cap of the blade has less critical flutter speed as compared to the unbalance in the entire section of the blade. Unbalance in the entire section of the blade with asymmetric skin can lead to highest flutter speed.

Compensation of deformation errors in five-axis flank milling of thin-walled parts via tool path optimization

In flank milling of thin-walled parts, the profile tolerance would usually be violated by the excessive static deformations. This paper presents a comprehensive method to compensate deformation errors in five-axis flank milling from the aspect of tool path optimization. Firstly, the machined surface is constructed by imprinting the predicted tool/workpiece deformations on the cutter envelope surface. Secondly, the signed distances from the sample points on the design surface to the machined surface are calculated for the machining error evaluation. Their differential increments that characterize the variation of surface errors with respect to the adjustment of tool path are then derived. On this basis, the mathematical model and algorithm for minimizing the deformation-induced surface errors are developed through slightly optimizing the shape parameters of the tool path surface. Finally, five-axis blade milling experiments are conducted to validate the effectiveness of the proposed method. The results demonstrate that the surface errors mainly caused by machining deformations in flank milling of flexible blades can be largely reduced by using the developed algorithm.

Aeroelastic analysis of a floating offshore wind turbine in platform‐induced surge motion using a fully coupled CFD‐MBD method

AbstractModern offshore wind turbines are susceptible to blade deformation because of their increased size and the recent trend of installing these turbines on floating platforms in deep sea. In this paper, an aeroelastic analysis tool for floating offshore wind turbines is presented by coupling a high‐fidelity computational fluid dynamics (CFD) solver with a general purpose multibody dynamics code, which is capable of modelling flexible bodies based on the nonlinear beam theory. With the tool developed, we demonstrated its applications to the NREL 5 MW offshore wind turbine with aeroelastic blades. The impacts of blade flexibility and platform‐induced surge motion on wind turbine aerodynamics and structural responses are studied and illustrated by the CFD results of the flow field, force, and wake structure. Results are compared with data obtained from the engineering tool FAST v8.

Turbulent flow field in a stirred vessel agitated by an Impeller with flexible blades

In order to reveal the effect of the blades normal vibration on flow turbulence in the stirred vessel, we designed three kinds of blades: the flexible, flat‐rigid and curved‐rigid blades. The flow fields produced by the impellers with these three kinds of blades were measured by two‐dimensional particle image velocimetry. The results showed that the calculated turbulent kinetic energy (TKE) based on the pseudo‐isotropic assumption is slightly higher than that by the three fluctuating velocities for the flexible and curved‐rigid impellers, and the difference between above two calculations is smaller for the former impeller. For the flexible blades, the trailing vortices slightly move outwards in radial direction than those for the curved‐rigid blades, enhancing TKE transport from the blade to the bulk region of the vessel. For the flexible impeller, the phase‐averaged TKE differs slightly from that for the flat‐rigid impeller, but is higher than that for the curved‐rigid impeller. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4148–4161, 2018

Influence of wake asymmetry on wind turbine blade aerodynamic and aeroelastic performance in shear/yawed wind

Wind turbines operate in atmospheric shear layer and often in yawed flow condition, which, among others, produce cyclic fluctuating loads on the blades. The unsteady aerodynamics of wind turbine in both shear and yawed wind are studied in the present work. The rotor aeroelastic behaviors considering blade flexibility are also discussed. An advanced aeroelastic model based on free wake lifting surface model and geometrically exact beam theory is established to conduct the study. The aerodynamic simulations show that the wake is asymmetric in both shear and yawed conditions. Comparison is made between the free wake lifting surface model and the skewed wake correction model used in BEM theory for the induced velocity on blade at different azimuth positions. The correction model for yawed flow in BEM theory seems to overpredict the induced velocity variations. The skewed wake induction in yawed condition causes a phase shift of angle of attack variation as a function of azimuth angle, particularly on the outboard of the blade. In addition, the blade aeroelastic deformations are found to further change the phase of circumferential distribution of aerodynamic loads. Rotor moments are influenced by the phase shifts of aerodynamic loads.

Performance analysis of a small wind turbine equipped with flexible blades

Abstract Wind turbine efficiency can drop drastically away from design conditions, which is especially troublesome for small fixed-pitch, constant speed types of devices and those operating in highly variable winds. Recent advances in the design of adaptive structures gives rise to a new turbine concept, employing continuous shape morphing, allowing the turbine to adapt more effectively to variable conditions. Such morphing blades could increase energy capture, and help small wind turbines become more economically viable through increased efficiency over a wide range of wind speeds and tip-speed ratios. In this paper, we examine the practicality of a flexible or morphing bladed turbine through experimental and numerical analysis. Experiments are conducted comparing a prototype rigid bladed design to an identical flexible one, with a total of 18 data sets containing 230 data points. Experimental results show that the flexible design outperforms the rigid one, especially when experiencing unfavorable loading conditions. Maximal corrected power coefficients were increased in all cases, up to 32.6%. The operational range was also increased in most cases, to a maximum of 34.5% over the rigid bladed design. These results suggest that the flexible design could produce more power than a rigid one, especially when conditions are sub-optimal.

Role of Blade Flexibility on Cycloidal Rotor Hover Performance

In this paper, the aeromechanics of a cycloidal rotor (or cyclorotor) is investigated to understand the effect of blade flexibility on the performance of the rotor in hover. Toward this, experiments are conducted on cyclorotors using moderately and highly flexible blades. These studies show that, with increasing blade flexibility, the rotor thrust decreases; whereas the power consumption increases, leading to a significant drop in power loading (thrust/power). To investigate these phenomena, a coupled aeroelastic model of cyclorotor is developed by coupling an unsteady aerodynamics model with a beam-based structural model and is systematically validated with experimental results. The aerodynamic model contains rigorous modeling of various physics behind the operation of the cyclorotor, such as the nonlinear dynamic virtual camber, the effect of near and shed wakes, and the leading-edge vortex. In comparison with a second-order nonlinear structural model, inclusion of a geometrically exact structural model proves to be essential in accurate prediction of rotor deflections and performance. Based on a systematic analysis performed using the validated model, it is observed that the large torsional deflections of the blades are a key reason for the drop in thrust. The torsional deflections are mainly produced by centrifugal force and nonlinear moments due to bending curvatures.