Bend Twist Research Articles

Structural design of a novel aeroelastically tailored wind turbine blade

The structural design for a recently presented aeroelastically-tailored wind turbine blade is produced. Variable elastic twist has been shown to improve performance in response to load variation across different wind conditions. This load variation is exploited as a source of passive structural morphing. Therefore, the angle of attack varies along the blade and adjusts to different operating conditions, hence improving both energy harvesting and gust load alleviation capability, below and above rated wind speed, respectively. The twist variation is achieved by purposefully designing spatially varying bend–twist coupling into the structure via tow steering and using a curved blade planform. This process enables the blade sections to twist appropriately while bending flapwise.To prove the feasibility of the proposed adaptive behaviour, a complete blade structure is analysed by using refined finite element models, with structural stability and strength constraints imposed under realistic load cases. Nonlinear structural effects are analysed as well as modal dynamic features. In addition, the weight penalty due to aeroelastic tailoring is assessed using structural optimisation studies.

An analytical model for composite tubes with bend–twist coupling

A simple analytical model is derived for composite tubes subjected to bending and torsion. The model is applied to a novel design of composite tube that exhibits bend–twist coupling such that the shear center of the cross section is away from the tube axis. Analytical expressions are derived for the shear center distance – distance of the shear center from the tube axis. Finite element analysis of the tubes is performed using shell elements. The agreement between analytical and numerical results is excellent. It is found that the shear center distance is independent of the radius of the tube but proportional to the length of the tube and it is also a function of coefficient of mutual influence of the composite material.

Single crystal plasticity with bend–twist modes

In this work a formulation is proposed and computationally implemented for rate dependent single crystal plasticity, which incorporates plastic bend–twist modes that arise from dislocation density based poly-slip mechanisms. The formulation makes use of higher order continuum theory and may be viewed as a generalized micromechanics model. The formulation is then linked to the burgers and Nye tensors, showing how their material rates are derivable from a newly proposed third-rank tensor Λp, which incorporates a crystallographic description of bend–twist plasticity through selectable slip-system level constitutive laws. A simple three-dimensional explicit finite element implementation is outlined and employed in three simulations: (a) bi-crystal bending; (b) tension on a notched single crystal; and (c) the large compression of a microstructure to induce the plastic buckling of secondary phases. All simulation are transient, for computational expediency. The results shed light on the physics resulting from dynamic inhomogeneous plastic deformation.

Design of shape-adaptive wind turbine blades using Differential Stiffness Bend–Twist coupling

The design of horizontal axis wind turbines (HAWT) has faced limitations in size and uncontrollable wind behaviour. Shape-adaptive wind turbine blades are proposed as one of the solutions to these design constraints, as they can improve the performance of the blades under varying operating conditions without increasing the size of the blades. A novel Differential Stiffness Bend–Twist coupling (DSBT)11Differential Stiffness Bend–Twist. concept is proposed in this paper to control the deformation behaviour of the blades. The DSBT concept achieves bend–twist coupling in the global structure while allowing the main deformation of the structural sub-components to remain in pure bending. By eliminating the need to design bend–twist coupled parts, this simplifies the material design and potentially enhances the manufacturability. An analytical conceptual model was developed to study the fundamental behaviour of DSBT structures under bending loads. The feasibility analysis demonstrated that the DSBT concept could achieve bend–twist coupling by altering the flexural stiffness distribution of stiffeners attached to the skin of the structure. A preliminary design scheme for the DSBT blades was also developed with the material design of stiffeners identified as an area to be optimised. The genetic algorithm (GA) was coupled with the Finite Element Method to search for the optimal flexure stiffness values. GA was reused to evaluate the required composite layup based on the stiffness requirements. Thus the design satisfies the requirements for strength and stiffness to enable DSBT.

Finite element modeling and effects of material uncertainties in a composite laminate with bend–twist coupling

Finite element models (FEMs) are used in the design of composite bend–twist (BT) coupled structures such as tidal turbine blades and marine propellers. However, such design tools must be verified experimentally. This paper presents the experimental verification of a composite FEM by static bending tests of 500×200mm BT coupled laminate plates. It was found that the FEM accurately predicted the test sample deformation, and the bending and twisting response of the samples were linear within the range of applied loads. The sensitivity of the FEM to model inputs and manufacturing accuracy was explored, and it was found that the laminate thickness and accuracy of ply angles were highly influential on the ability of the FEM to predict accurate deformations.

Experimental study of the steady fluid–structure interaction of flexible hydrofoils

This paper presents results from an experimental study of the hydrodynamic and hydroelastic performance of six different flexible hydrofoils of similar geometry; four metal hydrofoils of stainless steel (SS) and aluminum (AL), and two composite hydrofoils of carbon-fiber reinforced plastic (CFRP). The two CFRP hydrofoils had differing layups, one with fibers at 0° and the other at 30° relative to the spanwise axis of the hydrofoil. All hydrofoil models have the same unswept trapezoidal planform of aspect ratio 3.33. Two section profiles were chosen, a standard NACA0009 (Type I) and a modified NACA0009 (Type II) with a thicker trailing edge for improved manufacture of CFRP hydrofoils. Hydrofoils were tested in a water tunnel mounted from a six-component force balance. Forces and deformations were measured at several chord-based Reynolds numbers up to Rec=1.0×106 and incidences beyond stall. Hysteresis, force fluctuations, and the natural frequency of the hydrofoils in air and in water were also investigated. Pre-stall forces on the metal hydrofoils were observed to be Reynolds number dependent for low values but became independent at 0.8×106 and greater. Forces on the CFRP hydrofoils presented an increasing or decreasing lift slope for all Rec depending on the orientation of the carbon unidirectional layers. The change in loading pattern is due to the coupled bend–twist deformation experienced by the hydrofoils under hydrodynamic loading. Forces and deflections in the Type I hydrofoils were observed to be stable up to stall and non-dimensional tip deflections were found to be independent of incidence and Rec. Type II metal hydrofoils had a mild Rec dependence, attributed to the blunt trailing edge, and Type II CFRP hydrofoils had a stronger incidence and Rec dependence. The natural frequency under stall conditions of all but one of the CFRP hydrofoils was in agreement with added mass and finite element analysis estimates. The disagreement was observed in the CFRP hydrofoil with layers aligned at 30° and is attributed to the complex behavior of the carbon layers and to the coupled bend–twist deformation experienced under hydrodynamic loading of the hydrofoil.

Uncertainty quantification of aeroelastic stability of composite plate wings using lamination parameters

An approach is presented for modelling the aeroelastic stability of composite laminate wings with ply orientations subject to uncertainty. An aeroelastic model is constructed using the Rayleigh–Ritz technique coupled with modified strip theory aerodynamics. Lamination parameters are used as inputs to a Polynomial Chaos Expansion (PCE), enabling efficient propagation of the uncertainty through the aeroelastic model for a composite laminate with any number of plies. The Rosenblatt transformation is used to adapt the lamination parameter distributions for use with the PCE. A modified approach for modelling the uncertain aeroelastic response near the boundaries of different regimes of behaviour is also presented. Two case studies demonstrate application of the techniques; the first applies a simple PCE to a number of example balanced and symmetric laminates, the second applies the modified approach in a parametric investigation of the effects of bend–twist coupling on response mechanism. The proposed techniques offer at least an order of magnitude reduction in computation time compared to baseline Monte Carlo Simulation for all of the cases investigated.

Trunk bend and twist coordination is affected by low back pain status during running

Recent literature has related differences in pelvis–trunk coordination to low back pain (LBP) status. In addition, repetitive motions involving bending and twisting have been linked to high incidence of LBP. The purpose of this study was to examine trunk sagittal motion – axial rotation (‘bend and twist’) coordination during locomotion in three groups of runners classified by LBP status (LBP: current low back pain; RES: resolved low back pain and CTR: control group with no history of LBP). Trunk kinematic data were collected as running speed was systematically increased on a treadmill. Within-segment coordination between trunk sagittal and transverse planes of motion (trunk lean and axial rotation, respectively) was calculated using continuous relative phase (CRP), and coordination variability was defined as the between stride cycle standard deviation of CRP (CRPvar). Bend–twist coordination was more in-phase for the LBP group than CTR (p = 0.010) regardless of running speed. No differences in CRPvar were found between the groups. The results from our coordination (CRP) analysis were sensitive to LBP status and suggest that multi-plane interactions of the trunk should be considered in the assessment of LBP. This analysis also has potential for athletically oriented tasks that involve multi-plane interactions of the trunk, particularly ones that contain asymmetric action, such as sweep rowing or a shot on goal in field hockey or ice hockey.

On an exact bending curvature model for nonlinear free vibration analysis shear deformable anisotropic laminated beams

Geometrically nonlinear free vibration analysis of shear deformable anisotropic laminated composite beams resting on a two-parameter elastic foundation is presented. The material of each layer of the beam is assumed to be linearly elastic and fiber-reinforced. A new nonlinear beam model involving the exact expression of bending curvature is introduced, and the nonlinear vibration analysis with exact nonlinear characteristics of the work done by axial loading is accordingly performed. The governing equations are based on higher order shear deformation beam theory with a von Kármán-type of kinematic nonlinearity and including the bending–stretching, bending–twisting, and stretching–twisting couplings. Two kinds of end conditions, namely movable and immovable, are considered, and a perturbation technique is employed to determine the linear and nonlinear frequencies of a composite beam with or without initial stresses. The frequency response of laminated beams with different geometric and material parameters, end conditions and effect on elastic foundation is numerically illustrated. The results reveal that the geometric and physical properties, end conditions, and elastic foundation effect have a significant influence on large amplitude vibration behavior of anisotropic laminated composite beams.

An overview of the state of the art technologies for multi‐MW scale offshore wind turbines and beyond

An overview of technological trends in the design of multi‐mega Watt wind turbines focused on the offshore sector is presented. The state of the art technologies for wind turbine design are multidisciplinary ranging from blade aeroelasticity, power transmission to the generator, to advanced control systems that ensure performance and the design of offshore support structures to minimize cost of energy. Light weight carbon fiber blades, aeroelastic tailoring using bend–twist coupling are discussed in coordination with a multitude of aerodynamic technologies for optimal power capture such as high‐lift airfoils, flaps, and flat‐back airfoil designs. The pitch control of the turbine responsible for rotating the blades about its axis to reduce loads is the primary load reducing mechanism of the wind turbine, for which technologies that enable wind sensing such as forward looking LIDARs and redundancy mechanisms such as individual blade pitch control offer promising advances. The lack of reliability of the gearbox has resulted in drive train technologies to move toward direct drives, whose benefits and liabilities are assessed in combination with generator concepts. The support structures are discussed within an offshore framework for shallow, moderately deep, and deep waters. Fixed substructures such as monopiles, tripods, jackets, as well as floaters such as spar buoys and tension leg platforms are brought forth. The advances in these different component technologies that enable the wind turbine system to be reliable and cost effective are described as a precursor to further in‐depth reviews.This article is categorized under: Wind Power > Science and Materials

Mechanical Flexibility of ZnSnO/Ag/ZnSnO Films Grown by Roll-to-Roll Sputtering for Flexible Organic Photovoltaics

The mechanical flexibility of ZnSnO (ZTO)/Ag/ZTO multilayer films prepared on flexible poly(ethylene terephthalate) (PET) substrates by roll-to-roll (R2R) sputtering was investigated for use in cost-efficient flexible organic solar cells (FOSCs). The change of resistance (ΔR) of the ZTO/Ag/ZTO films was measured by means of lab-made outer/inner bending, twist bending, and stretching test systems. The failure bending radii of the ZTO/Ag/ZTO film in the outer and inner bending tests were 3 and 4.5 mm, respectively. In addition, the twisting test showed that the resistance of the ZTO/Ag/ZTO multilayer began to increase at an angle of 38°. Furthermore, the stretching test showed that the strain failure of the ZTO/Ag/ZTO multilayer film was 3%. The superior flexibility of the ZTO/Ag/ZTO films is attributed to the existence of a ductile Ag layer and the mechanical stability of the amorphous ZTO film. Similar performances of the FOSCs with flexible ZTO/Ag/ZTO anodes to reference OSCs with ITO anodes indicate that the R2R sputter-grown ZTO/Ag/ZTO multilayer is promising as an indium-free flexible anode for cost-efficient FOSCs.

Mechanical Flexibility of ZnSnO/Ag/ZnSnO Films Grown by Roll-to-Roll Sputtering for Flexible Organic Photovoltaics

The mechanical flexibility of ZnSnO (ZTO)/Ag/ZTO multilayer films prepared on flexible poly(ethylene terephthalate) (PET) substrates by roll-to-roll (R2R) sputtering was investigated for use in cost-efficient flexible organic solar cells (FOSCs). The change of resistance (ΔR) of the ZTO/Ag/ZTO films was measured by means of lab-made outer/inner bending, twist bending, and stretching test systems. The failure bending radii of the ZTO/Ag/ZTO film in the outer and inner bending tests were 3 and 4.5 mm, respectively. In addition, the twisting test showed that the resistance of the ZTO/Ag/ZTO multilayer began to increase at an angle of 38°. Furthermore, the stretching test showed that the strain failure of the ZTO/Ag/ZTO multilayer film was 3%. The superior flexibility of the ZTO/Ag/ZTO films is attributed to the existence of a ductile Ag layer and the mechanical stability of the amorphous ZTO film. Similar performances of the FOSCs with flexible ZTO/Ag/ZTO anodes to reference OSCs with ITO anodes indicate that the R2R sputter-grown ZTO/Ag/ZTO multilayer is promising as an indium-free flexible anode for cost-efficient FOSCs.

Tapered hygro-thermally curvature-stable laminates with non-standard ply orientations

Stacking sequence configurations for hygro-thermally curvature-stable (HTCS) laminates have recently been identified in nine unique classes of coupled laminate with standard ply angle orientations +45°, −45°, 0° and 90°. All arise from the judicious re-alignment of the principal material axis of laminate classes with Bending–Twisting and/or Bending–Extension and Twisting–Shearing coupling; where off-axis material alignment of these parent classes gives rise to distinctly different mechanical coupling behaviour. However, for standard ply angle orientations, HTCS solutions were found in only 8-, 12-, 16- and 20-ply laminates.This article considers non-standard ply angle orientations +60°, −60°, 0° and 90°, which lead to solutions in all ply number groupings for 10 plies and above, thus offering a possibility for ply terminations and hence tapered HTCS laminate designs.

Optimization‐based study of bend–twist coupled rotor blades for passive and integrated passive/active load alleviation

ABSTRACTThis work is concerned with the design of wind turbine blades with bend‐twist‐to‐feather coupling that self‐react to wind fluctuations by reducing the angle of attack, thereby inducing a load mitigation effect. This behavior is obtained here by exploiting the orthotropic properties of composite materials by rotating the fibers away from the pitch axis.The first part of this study investigates the possible configurations for achieving bend‐twist coupling. At first, fully coupled blades are designed by rotating the fibers for the whole blade span, and a best compromise solution is found to limit weight increase by rotations both in the spar caps and in the skin. Next, partially coupled blades are designed where fibers are rotated only on the outboard part of the blade, this way achieving good load mitigation capabilities together with weight savings. All blades are designed with a multilevel constrained optimization procedure, on the basis of combined cross‐sectional, multibody aero‐servo‐elastic and three‐dimensional finite element models.Finally, the best configuration of the passive coupled blade is combined with an active individual pitch controller. The synergistic use of passive and active load mitigation technologies is shown to allow for significant load reductions while limiting the increase in actuator duty cycle, thanks to the opposite effects on this performance metric of the passive and active control solutions. Copyright © 2012 John Wiley & Sons, Ltd.

Segmental vertebral motion in patients with lumbar disc herniation: an in vivo study

Objective To observe in vivo segmental lumbar motion in patients with lumbar disc herniation (LDH) during functional weight-bearing activities.Methods Fifteen patients with LDH at L4-5 were studied as experimental group.Ten healthy volunteers were recruited as control group.Three-dimension(3D) lumbar spine models of L3,L4 and L5 were reconstructed from thin section CT scans.Spine motions were then reproduced by matching lumbar spine models and images got from dual fluoroscopic imaging system (DFIS)under different motion state (standing,flexion-extension,left-right twisting and left-right bending).From local coordinate systems at the end plates,the motion of the cephalad vertebrae relative to the caudal vertebrae was calculated for vertebrae levels:L4-5 and L3-4.Results The motion pattern at L4-5 was found to be altered.During flexion-extension,the migrations of the affected segments along the frontal axis,sagittal axis,vertical axis were similar with that of the control group,but the rotation angle along the frontal axis was significantly larger than that of the control group (P＜0.05).During left-right bending and left-right twisting,the migration and rotation angle along the frontal axis were significantly larger than those of control group.During flexionextension,the migrations of the neighboring segments (L3-4) along the three axes were larger than those of the control group,but there were no statistical significances.During left-right bending and left-right twisting,the migrations of the neighboring segments (L3-4) along the vertical axis were significantly larger than those of the control group (P＜0.05).Conclusion The 3D lumbar motion pattern in LDH patient is different with that of normal people.For the affected segment,compared with the normal people,the range of flexion-extension motion and the translocation in left-right direction were significantly larger,but the rotation range along the vertical axis was smaller. Key words: Intervertebral disk displacement; Lumbar vertebrae; Range of motion, articular

The challenge of achieving hygrothermal stability in composite laminates with optimal couplings

The necessary and sufficient conditions for hygrothermal curvature stability of composite laminates have been derived previously, and their development is summarized in this work. These conditions are shown to be material independent. A constrained optimization routine is implemented to arrive at hygrothermally stable stacking sequences that are optimal for each of bend-twist and extension-twist coupling. The necessary and sufficient conditions for hygrothermal stability are used as constraints, while the compliance coefficients from classical lamination Theory are used as the objective functions. Both sequential quadratic programming and ant-colony optimization routines are used to ensure global optimality. Laminates consisting of two through ten plies are considered. Laminates consisting of five through ten plies are manufactured and tested to demonstrate the achievable level of extension-twist or bend twist coupling. A geometrically nonlinear model is presented to predict the response of bend-twist-coupled laminates. Comparison with the previous state-of-the-art stacking sequence for achieving extension-twist coupling with hygrothermal stability demonstrates a nearly 80% increase in the level of coupling.

Performance-based design and analysis of flexible composite propulsors

Advanced composite propellers, turbines, and jet engines have become increasingly popular in part because of their ability to provide improved performance over traditional metallic rotors through exploitation of the intrinsic bend–twist coupling characteristics of anisotropic composite materials. While these performance improvements can be significant from a conceptual perspective, the load-dependent deformation responses of adaptive blades make the design of these structures highly non-trivial. Hence, it is necessary to understand and predict the dependence of the deformations on the geometry, material constitution, and fluid–structure interaction responses across the entire range of expected loading conditions. The objective of this work is to develop a probabilistic performance-based design and analysis methodology for flexible composite propulsors. To demonstrate the method, it is applied for the design and analysis of two (rigid) metallic and (flexible) composite propellers for a twin-shafted naval combatant craft. The probabilistic operational space is developed by considering the variation of vessel thrust requirements as a function of the vessel speed and wave conditions along with the probabilistic speed profiles. The performance of the metallic and composite propellers are compared and discussed. The implications of load-dependent deformations of the flexible composite propeller on the operating conditions and the resulting performance with respect to propeller efficiency, power demand, and fluid cavitation are presented for both spatially uniform and varying flows. While the proposed framework is demonstrated for marine propellers, the methodology can be generally applied for any marine, aerospace, or wind energy structure that must operate in a wide range of loading conditions over its expected life.

Influence of uncertainties on the response and reliability of self-adaptive composite rotors

Advanced composite structures are becoming increasingly popular because of their high specific strength and stiffness, as well as ability to provide improved performance through passive morphing via intrinsic bend–twist deformation coupling. Self-adaptive composite structures tend to be more susceptible to geometric, material, and loading uncertainties because of their complex configuration, manufacturing process, and dependence on fluid–structure interaction (FSI) response. The objective of this work is to quantify the effects of material, geometric, and loading uncertainties on the response of self-adaptive composite propellers and overall system reliability. A fully-coupled, 3-D boundary element method–finite element method is used to compute the dynamic FSI response. Variability in propeller performance is estimated by considering variations in operating conditions, as well as blade geometry and stiffness. Modeling uncertainties are considered by employing various mechanistic-based failure initiation models. Random variations in material strengths are implemented and an estimate of the structural reliability is determined. The results indicate that adaptive composite structures that depend on FSI are more sensitive to natural, random variations than equivalent rigid, isotropic structures. Therefore, it is necessary to quantify the effects of material, geometric, and loading uncertainties on the responses, safe operating envelopes, and reliability of self-adaptive composite structures.

Electrode Size Effect on the Resonance Spectrum of PZT Piezoceramic Resonator

The resonance spectrums of disc type piezoceramic resonators with different electrode sizes were measured in the frequency range from 100 kHz to 1 MHz. Besides the fundamental mode, some overtone and other vibration modes existed in this frequency range, depending on the electrode size. The series resonance frequency of fundamental modes of the resonators increased with the reduction of the electrode size. When the electrode size was decreased down to 80% of size, some small spurious signals will exist; these spurious signals may be caused by another vibration mode such as twist mode or bending mode. The planar electromechanical coupling factor increased first with the decreasing of the electrode size, and reached a maximum when the diameter of electrode was decreased down to 80% of size, and then decreased with the decreasing of the electrode size.

ChemInform Abstract: Models for Stilbene Photoisomerization: Experimental and Theoretical Studies of the Excited-State Dynamics of 1,2-Diphenylcycloalkenes

The initial dynamics in the photoisomerization of cis-stilbene, especially the dynamics leading to photocyclization, is explored by studying the high-resolution spectroscopy and implied dynamics of a series of 1,2-diphenylcycloalkenes. These molecules are isochromophoric to cis-stilbene but are structurally hindered from undergoing cis-trans isomerization. In particular, the high-resolution fluorescence excitation spectrum of 1,2-diphenylcyclobutane (DPC-4) reveals detailed information about the symmetric in-plane bend and symmetric phenyl ring twist motions, which are crucial to photocyclization. The DPC-4 spectrum is analyzed theoretically, and an empirical surface is constructed for the S 1 state that reproduces both line positions and relative intensities in the observed spectrum. Evidence for the importance of photocyclization in the dynamics of excited state cis-stilbene is presented and discussed