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Concrete-filled double skin steel tubular columns with large hollow ratios (LHR-CFDST) have been commonly researched in the past few years. The excellent performance of this structure renders it promising as a novel form of wind turbine tower. Increasing the diameter-to-thickness ratio is one efficient way to reduce steel consumption as the size of this engineering structure continues to expand. There is a paucity of research on thin-walled LHR-CFDST. Meanwhile, these structures may be susceptible to local buckling without stiffening measures. Furthermore, the size effect of this type of structure is more complicated. This paper reports the first experimental investigation on super-sized thin-walled LHR-CFDST columns under combined compression-torsion-shear-bending loads. Two LHR-CFDST columns with varying stud spacings are discussed. Investigations are conducted into failure modes, local buckling behaviors, load-strain responses, and a few popular mechanical indices. According to the test results, the specimen with smaller stud spacing has an 8.1% greater ultimate bearing capacity. It is assumed that a more intensive stud arrangement can delay the onset of local buckling, thereby increasing the peak load. Besides, the arrangement of the studs leads to an obvious effect of section confinement, thus improving the overall performance of the specimens. The failure modes and load-displacement relationships simulated by the FE models agree well with the experimental results. Then, a modified method for predicting the ultimate strength of LHR-CFDST based on the previously summarized formulae for CFST under compression-bending-shear-torsion is proposed and utilized for the evaluation of two specimens in this research. Since the tested value of the specimen with narrower stud spacing is higher by 5.6% than the predicted value, it can be concluded that the large-scale thin-walled LHR-CFDST may be evaluated using this method, complemented by appropriate stiffened measures. • Two thin-walled CFDST columns were tested subjected to combined loads. • The stiffened studs greatly postpone the appearance of local buckling. • The studs produces a clear impact of section confinement, which enhances the specimen's overall performance. • This experiment utilizes a novel mutual verification model. • A modified method for CFDST under combined loads is proposed and utilized for the evaluation of two specimens. [ABSTRACT FROM AUTHOR]

• A novel vertical grouting joint in precast concrete structure for wind turbines was proposed, and six specimens were designed and subjected to shear tests. • A three-stage shear force model of the joints was proposed. • The shear capacities of specimens were calculated using the existing design methods, and correspongding design suggestions were proposed. Precast concrete structure (PCS) towers have been widely used as support structures for onshore wind turbines owing to their high stiffness, good stability, and low maintenance cost. Owing to the limitations of production, transportation and construction, the PCS towers must be vertically segmented, and the vertical segments of precast concrete are mainly connected by filling the joints with high-strength grouting materials (HSGMs). The shear performance of the vertical grouting joints significantly affects the mechanical behaviour of PCS towers for wind turbines. To study the shear performance of the vertical grouting joints of PCS towers, six vertical grouting joint specimens were designed and tested under shear loads, and the failure modes and shear force–slip curves of the specimens were examined. The experimental results indicated that the failure modes of the specimens involved the interface peeling failure of the new and old concrete and the shear failure of the HSGM. U-shaped shear reinforcement can increase the ultimate and residual shear capacities of joints and significantly improve the ductility of vertical grouting joints. Vertical reinforcement has a limited effect on the ultimate shear capacities of vertical grouting joints but can limit the development of internal cracks and improve the residual shear capacities of joints. According to the experimental results and force–slip curves of the specimens, the shear force mechanism of the joints was analysed, and a three-stage shear force model of the joints was proposed. Moreover, the distribution of shear strain on the concrete surface along the height of the specimens under different load levels was analysed. Finally, the shear capacities of all the vertical grouting joint specimens were calculated using the existing design methods, and correspongding design suggestions were proposed according to the calculation results. [ABSTRACT FROM AUTHOR]

Yaw control is a potential control strategy to redirect the wakes of wind turbines for the purpose of improving the wind farm performance. Because of its time-consuming nature, high fidelity numerical simulations are hard to be applied in engineering applications. Instead, a simple and efficient engineering wake model is preferable. In this paper, a novel yaw wake model is developed in the following three steps: a novel wake velocity model for non-yawed wind turbines is first developed; second, a novel wake deflection model is developed to overcome the shortcomings in existing models; and third, a novel wake model for yawed wind turbines is developed. To verify its accuracy, the results of the novel yaw wake model implemented in the in-house Analytical Wake Model Calculator (AWMC) are validated against experimental data and simulation results. Moreover, the comparisons with a few existing yaw wake models show its superiority. In terms of error, the overall accuracy of the wake deflection model is enhanced by more than 20 % compared to other models in all cases. In addition, the overall accuracy of the novel yaw wake distribution model is improved by at least 10 % in most cases as compared to state-of-the-art yaw wake models. • A novel Gaussian wake velocity model for non-yawed turbines is presented. • A novel wake deflection model for predicting the wake-center of yawed turbine is established. • A novel yaw wake model which includes a wake velocity model and a wake turbulence intensity model is developed. • The novel yaw wake model is compared to state-of-the-art yaw wake models. • Validations are performed against experimental data and numerical simulation results and good agreements are seen. [ABSTRACT FROM AUTHOR]

Prestressed concrete filled steel tubular (CFST) lattice tower is an innovative application to combine prestress and CFST composite structure to onshore tall wind turbines. In order to explore the seismic behavior of prestressed CFST lattice tower under a complex load state, there was pseudo-static testing carried out for prestressed four-chord CFST lattice columns under compression-bending-torsion combined load. The hysteretic performance of the specimens was analyzed by obtaining the failure mode, load-displacement hysteretic curve, energy dissipation, stiffness degradation, variation of strands stress and steel strain. The influence of prestress on seismic behavior of CFST lattice chords was investigated from prestress loss rate. Results showed that: the inclined battens of CFST lattice columns were seriously damaged with or without strands, while the inclined battens of non-prestressed specimen were damaged first and then the CFST chords. The existence of inclined battens had a great influence on the structural stiffness, while the application of strands significantly affected the initial stiffness and structural ductility of the prestressed specimen, but did not have any significant effect on ultimate bearing capacity. Post-tensioned strands could protect the main bearing chords and reduced the cracking of concrete in the tension area under the coupling action of compression, bending, and torsion. • Pseudo-static test of a massive structure under complex loading conditions was carried. • The hysteretic performance was investigated experimentally and analytically. • The influence mechanism of post-tensioned strands was investigated from the prestress loss ratio. • Some specific suggestions for engineering design were proposed. [ABSTRACT FROM AUTHOR]

Recent developments in the high-output wind turbine have heightened the need for CFDST support towers. Tapered Concrete-Filled Double Skin Steel Tubular columns with Large Hollow Ratios (TLHR-CFDSTs) are the typical members in the wind turbine towers, which are usually subjected to torsion during the life cycle. It is necessary to pay more attention to these members under torsion due to their tapered shapes and thin-walled cross sections. This paper thus presents an experimental study on the torsional behaviour of the TLHR-CFDST column. Specimens with different tapered angles, hollow ratios, tubes' thicknesses and axial compression ratios were tested under pure torsion and compression–torsion. The typical failure modes, torque versus deformation relations, strain distributions and the influence of important parameters are revealed. Based on the experimental results, the mechanism of TLHR-CFDST columns under torsion is discussed. The applicability of existing torsional capacity calculation methods is evaluated and finally a simplified method for predicting the torsional capacity of TLHR-CFDST columns is proposed. • Tests on TLHR-CFDST columns under pure torsion and compression–torsion​ are reported. • Behaviour and parametric effects of TLHR-CFDST columns under torsion are revealed. • Torsional mechanism of TLHR-CFDST columns is discussed. • Theoretical method for predicting the torsional capacity of TLHR-CFDST columns is proposed. [ABSTRACT FROM AUTHOR]

• Three hexagonal CFDST stub columns with varying stiffened measures were tested. • Stiffened measures such as ribs and studs can effectively increase the ultimate strength. • Mechanistic analysis further reveals the positive effect of the stiffening measures during axial compressive stress. • Extensive parametric analyses are carried out on the material strength, and so on. • Modified formulas based on previous studies for circular CFDST structures obtained more consistent predicted values. With the escalating height of wind turbine structures, wind turbine steel-concrete hybrid towers have gradually become a new type of structure to replace pure steel towers. As a component connecting the upper steel tower with the lower concrete tower, the transition piece assumes an indispensable role in the overall structure of the hybrid tower. The traditional reinforced concrete-type transition piece has necessitated enhancements owing to the complicated construction and concrete cracking problems. A hexagonal concrete-filled double skin steel tubular (CFDST) transition piece is proposed to fit the hexagonal steel-concrete hybrid tower. Studies that investigate the hexagonal CFDST structures are currently few. Based on this, three hexagonal CFDST stub columns were tested, with the variables in stiffened measures. The experimental portion included a thorough investigation of local buckling behaviors and failure modes in addition to the analysis of typical mechanical indexes. Further, the validated finite element (FE) model was then utilized for mechanism analysis about the contact stress between different interfaces, the stress distributions, and so on. Besides, a parametric study was done to identify the sensitive parameters affecting this kind of structure's peak loads. Ultimately, the peak load prediction formulas found in published studies and specifications were modified by incorporating the findings from this investigation. [ABSTRACT FROM AUTHOR]

• An inflatable structure is designed for Savonius wind turbine. • A double-layer fabrication process is proposed for the inflatable wind turbine. • Geometry parameters of the inflatable wind turbine are optimized based on CFD. • Static and dynamic performances are investigated through wind tunnel tests. • The inflatable wind turbine shows high portability and rapid deployment capability. Faced with the increasingly severe issue of the greenhouse effect, wind power emerges as a sustainable solution. However, the large size, heavy weight, and high risk associated with traditional wind turbines restrict their widespread deployment in urban, rural, and outdoor settings , wasting a vast amount of untapped wind energy resources. To address these challenges, this paper proposes an inflatable structure for the Savonius wind turbine and explores a double-layer fabrication process to manufacture the inflatable Savonius wind turbine (ISWT). 2D simulations are carried out on ANSYS-Fluent. Within predetermined parameter ranges, this paper successfully identifies the optimal geometric parameters for the ISWT model, with a blade thickness of 7 cm and a blade arc angle of 120°. This model exhibits the maximum power coefficient (C p) of 0.1912 at the tip speed ratio (λ) of 0.7. The manufactured ISWTs are fabricated based on the optimal geometric parameters. Due to deformation after inflation, the actual blade arc angle of the first ISWT sample is only 89.7°. Therefore, by adjusting the fabric template, an approximate blade arc angle of 120° is achieved for the second ISWT sample. Both ISWT samples are subjected to open-section wind tunnel tests. The results revealed that while the C p curves of Sample 1 and Sample 2 display trends similar to those in numerical results, the overall performance is lower. The reasons for the deviation include the simplification of vertical flow in 2D simulation and the lack of endplates. Specifically, Sample 2 also achieves the highest C p value of 0.1486 at λ = 0.7. Due to an inflation time within 60 s to the required pressure of 1.4 bar and a compressed volume lower than 0.021 m3, the ISWTs demonstrate rapid deployment capability during installation, portability during transportation, and storability during severe weather. [ABSTRACT FROM AUTHOR]

The precast concrete segments of wind turbine hybrid towers are usually lifted and installed using the pre-installed lifting anchors with supplementary reinforcements. The lifting anchors are made of steel plate and the supplementary reinforcements are often arranged near the lifting anchors to enhance their anchoring performance. This lifting system including the reinforcement configuration is presented in existing design codes, but the design recommendation and calculation method of ultimate capacity are not provided. It is necessary to explore the pull-out behavior of the lifting anchor considering the effects of reinforcements. Therefore, a comprehensive investigation on the pull-out behavior of the lifting anchor is conducted in this paper, by using the precast concrete segments of hybrid tower as a carrier. Four specimens based on the actual dimension of precast concrete segment of hybrid tower were tested under cyclic pull-out loading, and the failure modes as well as the ultimate capacity were analyzed. The verified finite element (FE) models were established based on the test results, and mechanical analysis were performed to reveal the pull-out mechanism of the lifting anchor. Subsequently, the parametric analysis was conducted to explore the sensitive parameters of the lifting anchor's pull-out performance. The accuracy and applicability of the calculation methods for ultimate pull-out capacity in existing design codes and literatures were evaluated based on the test results. Lastly, a practical calculation method for the ultimate pull-out capacity of lifting anchor was proposed based on theoretical model and regression analysis. The proposed method can provide a reference for the lifting design of precast concrete segments of wind turbine hybrid towers. • 4 specimens with the specialized lifting anchor based on the actual size of precast concrete segment were tested under cyclic pull-out loading. • The verified FE models were established, and mechanical analysis were performed to reveal the pull-out mechanism of the lifting anchor. • The parametric analysis was conducted to explore the sensitive parameters of the lifting anchor's pull-out performance. • The accuracy and applicability of the existing calculation methods for ultimate pull-out capacity were evaluated based on the test results. • A practical calculation method for the pull-out capacity of lifting anchor was proposed based on theoretical model and regression analysis. [ABSTRACT FROM AUTHOR]

• Typical failure patterns of TLHR-CFDST columns for wind turbine towers are investigated. • Coupled mechanisms of columns under compression-bending-torsion loads and parameter effects are analysed. • Ultimate strength correlations of TLHR-CFDST columns for wind turbine towers are revealed. • A strength prediction method of TLHR-CFDST columns under combined loads is proposed. Concrete-filled double-skin steel tubular (CFDST) tower has emerged as a promising type of wind turbine support structure with excellent strength and stiffness. As the basic member in CFDST towers, tapered concrete-filled double-skin steel tubular columns with large hollow ratios (TLHR-CFDSTs) are frequently subjected to complex combined compression-bending-torsion loads. This paper presents a comprehensive analytical study on the coupled behaviour of the TLHR-CFDST column under combined loads and the relevant ultimate strength prediction method. A FE model was established to conduct numerical analyses and extensive parameter studies while the data from the previous experimental research was used to provide a verification basis. It is found that the column subjected to combined loads has three typical failure patterns, which are mainly governed by the bending-to-torsion ratio (m t). Columns with moderate m t undergo the bending-torsion failure and show obvious coupled performances. The load-deformation relation, the ultimate characteristics and the effects of important parameters, including axial compressive ratio (n), tapered angle (θ), hollow ratio (χ) and height-to-diameter ratio (λ) were discussed. The ultimate strength correlations of the column were revealed by analysing strength data within the parameter scopes commonly used in wind turbine towers. Lastly, a practical and accurate prediction method for the ultimate strength of the TLHR-CFDST column under combined compression-bending-torsion loads was proposed. [ABSTRACT FROM AUTHOR]

• Six steel–concrete composite adapter (SCCA) specimens and one reinforcement concrete adapter (RCA) specimen were tested under axial compression. • The superiority of SCCA over RCA was proved. • The effects of stud spacing, the thickness of stiffened plates, and the amount of bottom circumferential rebars on SCCA were reveal. • Finite element models were established and validated by the test results. • Parametric analysis was conducted and the prime sensitive parameters affecting the ultimate compressive capacity were presented. Adapter is one of the most important components in wind turbine hybrid towers. It allows the upper steel segments to connect reliably with the lower concrete segments and thus they can work well as a whole structure. However, traditional reinforced concrete adapters (RCAs), which have been commonly used in engineering practice, encounter many shortcomings such as complicated construction processes and strength/stiffness degradation after the concrete cracking. In this paper, a novel type of steel–concrete composite adapter (SCCA) was proposed. Experimental and numerical studies on its compressive behavior were conducted. Six SCCA specimens and one RCA specimen were tested under axial compression. The tested parameters include the stud spacing, the thickness of stiffened plates, and the amount of bottom circumferential rebars. By revealing the failure modes, load–displacement relations, ultimate strength, deformability, and strain relations of SCCA, the superiority of SCCA over RCA was proved. To extend the scope of the experiment and conduct parametric analyses, finite element models were established and validated with test results. Sensitive parameters related to the ultimate capacity of SCCA were investigated. Additionally, reasonable studs spacing which is beneficial for enhancing the ultimate capacity of SCCA was obtained. [ABSTRACT FROM AUTHOR]

Steel tubes adopted in wind turbine towers generally sustain the combination of compression, bending, and torsion loads. The study of compressive behavior is crucial for investigating the combined bearing capacity of the tower and proposing the corresponding design method. Local buckling tends to affect compressive behavior due to the large diameter-to-thickness ratios of the tubes. Thus, the authors proposed a new form of tower based on longitudinally stiffened steel tubes, which were experimentally and analytically studied in this paper. A total of six tube specimens (four stiffened ones and two unstiffened ones) were tested under axial compression. The experimental results indicate that the local buckling of the tube can be substantially prevented by stiffeners and the T-type stiffener exhibits a superior effect. Finite element models were built and validated by the test results, and the parametric analysis was conducted, indicating that the diameter-to-thickness ratio of the steel tube, the yield strength of steel, and the proportion of stiffener area are the prime sensitive parameters affecting the ultimate strength. Ultimate compressive strength prediction methods in design specifications were evaluated. A practical method for calculating the ultimate strength of stiffened steel tubes was subsequently developed in this paper. • Six steel tube specimens with different stiffener types and diameter-thickness ratios were tested under axial compression. • Finite element models were established and validated by the test results. • Parametric analysis was conducted and the prime sensitive parameters affecting the ultimate strength were presented. • The applicability of calculation methods suggested by several specifications was evaluated. • The calculation method for the ultimate axial compressive strength was proposed by parametric and regression analysis. [ABSTRACT FROM AUTHOR]

Thin-walled steel tubes have been widely used in wind turbine towers. These towers are commonly subjected to combined compression-bending-torsion loads; thus, the steel tubes are prone to local buckling. Installing stiffeners has proven an effective method to improve the local stability of the thin-walled members. This paper presents an experimental and analytical study of thin-walled steel tubes strengthened by longitudinal stiffeners. Six specimens with different types of stiffeners and various diameter-thickness ratios were tested under a combined compression-bending-torsion load. The failure modes, load–displacement relationships, ultimate strength, and deformability were revealed. The validated finite element model was used to analyze the structural mechanisms of the stiffened steel tubes. The results show that stiffeners can substantially alleviate the local buckling of steel tubes, significantly improving their ultimate strength and plastic deformability. In addition, T-type stiffeners perform better than I-type stiffeners. The ultimate strength of the specimens is preliminarily evaluated using the methods recommended in existing design guidelines. The calculated ultimate strength of the unstiffened specimens shows a good agreement with the test results, whereas the ultimate strength of the stiffened specimens is underestimated. • Six specimens with different stiffener types and diameter-thickness ratios were tested under combined load. • The validated finite element model was developed to further analyze the structural mechanisms of the stiffened steel tubes. • The effects of stiffeners on the static behavior of steel tubes were revealed. • The applicability of the design methods recommended by the existing design guidelines was preliminarily evaluated. • The reasonable stiffener type and stiffening position were suggested. [ABSTRACT FROM AUTHOR]

Steel tubes with large diameter-thickness ratios are widely used in tubular steel towers of wind turbines. These steel tubes are susceptible to local buckling under compression-bending load, thus leading to a failure of the overall structures. In this paper, a new structural form of steel tubes with longitudinal stiffeners was proposed. Six specimens were tested to study the effects of stiffeners forms and diameter-thickness ratios of steel tubes. Finite element models were compared and validated by test results and used to further study the structural mechanisms. Lastly, the ultimate strength of specimens was evaluated by existing design guidelines preliminarily. According to the results, by setting stiffeners, especially the T-type stiffeners under the same consumption of the steel, the local buckling was prevented and the plastic deformability of the steel tubes was increased, resulting in the great enhancement of ultimate strength and ductility. The limits of quantity and width-thickness ratio of stiffeners to obtain a more stable and significant improvement effect were suggested through parametric analysis. The current methods agree well with the test results of unstiffened steel tubes but might be inaccurate for stiffened steel tubes. • Six specimens with different stiffener types and diameter-thickness ratios were tested under compression-bending load. • The finite element models were compared and validated by test results and used to further study the structural mechanisms. • The effects of stiffeners on the compression-bending behavior of steel tubes were revealed. • The limits of quantity and width-thickness ratio of stiffeners to obtain a significant effect were suggested. • The applicability and accuracy of the calculating methods in existing design guidelines were preliminarily evaluated. [ABSTRACT FROM AUTHOR]