Range Of Speed Ratios Research Articles

Improvement of aerodynamic performance of an offshore wind turbine blade by moving surface mechanism

In the present study, the effects of a moving surface on the aerodynamic characteristics of an offshore wind turbine blade were numerically examined. The S809 airfoil was considered as the blade section. A part of the airfoil surface was replaced with a moving surface as a flow control mechanism. To achieve the highest mechanical performance of the airfoil at each angle of attack, the effects of location and speed of the moving surface on the flow characteristics were studied. The flow simulation was carried out using a computational fluid dynamics technique. The results of this study indicated that the use of a moving surface with appropriate speed and location could enhance the airfoil performance. As the angle of attack increased, a stronger moving surface was needed. The performance improvement for α = 8°, 11°, 14°, and 17° was 30%, 62%, 131%, and 152%, respectively. In addition, the performance of a three-bladed horizontal axis wind turbine was numerically analyzed in the range of tip speed ratio, 4 ≤ TSR ≤ 9. It was observed that the moving surface significantly improved the torque and power generated by the turbine, especially at low TSRs. This improvement was over 90% for TSR < 5.

Blade design and optimization of a horizontal axis tidal turbine

Tidal turbine is a device which converts the kinetic energy of water into electric energy. The blade element momentum theory (BEM) is used to design the blade in this paper. Although tidal power resources are abundant, the actual operation of hydraulic turbines is not very good due to the limitations of turbine conversion efficiency and production cost. Therefore, this paper establishes a neural network model for variables and objective functions, and then applies multi-objective optimization algorithm to genetic optimization of the power coefficient, the main index of hydraulic performance of tidal turbines. The optimized results are verified by model test and sea trial. The results show that after optimizing the blade chord length distribution and pitch Angle distribution, the power coefficient of the turbine increases by 2%, and the optimal tip speed ratio range is also expanded, which is more conducive to the actual tidal turbine power generation, and has certain engineering significance.

Design approaches of performance-scaled rotor for wave basin model tests of floating wind turbines

The Froude scaling law is usually utilized in the wave basin model tests of Floating Wind Turbines (FWTs). However, the Froude-Scaled Rotor (FSR) cannot generate desired aerodynamic loads due to the Reynolds-Number Scaling Effect (RNSE). To mitigate the adverse effects of RNSE, two approaches are proposed to design Performance-Scaled Rotors (PSRs) in this paper. Taking DTU 10 MW baseline wind turbine as an example, the SD2030 airfoil is selected to replace the original FFA-W3-xx airfoils. Maximum Lift Tracking (MLT) and Load Distribution Matching (LDM) algorithms are proposed to assign the chord lengths and twist angles. Herein, MLT leads all airfoils to operate at the optimal angle of attack that corresponds to the maximum lift coefficient and afterwards increasing the chord lengths. LDM simultaneously adjusts the chord length and twist angle, aiming to match the span-wise distribution of normal force at the design point. Results show that both approaches can generate desired rotor thrusts in a range of tip speed ratios, which seems to outperform prior PSRs in the existing publications. The blade mass and inertia can be preserved with careful manufacturing procedures. The redesigned PSRs are helpful to improve the accuracy and reliability of FWT model tests in the wave basin.

A novel real-time feedback pitch angle control system for vertical-axis wind turbines

Modern horizontal-axis wind turbines improve their performance by controlling the blade pitch, which is a well-developed technique in practical use. However, the application of this method in vertical-axis wind turbines (VAWTs) is restricted, due to continuous changes in both the relative wind velocity and angle of attack of the blade. In the study, through an in-depth aerodynamic analysis between the blade pitch and performance of VAWTs, a real-time feedback blade pitch control system is developed, which is based on the real-time flow velocity around the blade. Subsequently, a computational fluid dynamics method with FLUENT is constructed to evaluate the performance of the pitch control system on improving VAWT performance. The calculation method is verified via wind tunnel experimental data. Results indicate that real-time feedback control of the pitch angle increases the average power coefficient in a wide range of tip speed ratios (TSRs). At low TSRs, it significantly improves the starting performance by decreasing the occurrence of vortex shedding and flow separation on the blade surface. At high TSRs, it effectively increases the power coefficient of VAWTs. Additionally, at high TSRs, the power coefficient generally exceeds 0.3 and the maximal power coefficients of the VAWTs corresponds to 0.441. At high TSRs, when compared to the zero pitch, the power coefficient relatively increases by at least 12.7%.

A modified implementation of actuator line method for simulating ducted tidal turbines

In this numerical study, the standard actuator line method is modified for simulating ducted tidal turbines and validated against experimental data for two turbines with different geometries. The necessary modifications, which are based on the unique features of ducted turbines, include using a grid-based distribution of blade elements, employing the local chord length as the length scale for the projection factor, and implementing a cylindrical projection. This study also provides a guideline to facilitate choosing the projection factor, the only parameter in the actuator line method which is chosen empirically. The first ducted tidal turbine studied in this work is the Cresswell turbine, the operation of which is simulated at its design tip speed ratio in axially aligned and yawed onset flows. The second turbine, developed by Clean Current Power Systems, is simulated over a range of tip speed ratios in axially aligned onset flow. In order to study the effect of the turbulence model on the actuator line method’s predictions, for this turbine, both the detached eddy simulation and k−ω turbulence models are employed and the results are compared. Importantly, the actuator line methodology presented here is implemented in a heterogeneous CPU/GPU architecture making in-situ simulations of tidal turbines feasible.

Synergistic analysis of a Darrieus wind turbine using computational fluid dynamics

The negative implications of indiscriminate usage of fossil fuels lead to many problems such as high carbon dioxide emissions that pollute the atmosphere and change the global climate. Wind energy is promising clean energy and it is ample supply and a nonpolluting power generation method. Wind turbines are renewable energy devices but are fraught with challenges such as low wind speeds and high turbulence intensity. In the present work, a comprehensive study is introduced to enhance the aerodynamic performance of a Darrieus turbine by enclosure it in a flanged diffuser. The conventional blade shapes of NACA 0015 and DU 06-W-200 profiles as well as non-conventional blade shapes such as J-shape design are investigated. The geometric modifications and different parametric studies conducted are also described in details with the relevant discussion of results obtained. This all followed by a numerical prediction scheme for noise is accomplished using the Ffowcs Williams & Hawkings process. The results revealed that the modification on the classical profile of the aerofoil has an important role in the enhancement of the power generated at a certain diffuser location. The optimum configurations improved the power coefficient by a factor of 2.24 in the case of NACA0015 and by factor 1.866 in case of DU06-W-200 aerofoil compared with conventional three-bladed Darrieus turbine. Furthermore, the J-shaped profile is promising from performance and aeroacoustics point of view in this low-moderate tip speed ratio range but is totally collapsed in front of the low solidity.

Aerodynamic Sensitivity Analysis for a Wind Turbine Airfoil in an Air-Particle Two-Phase Flow

In this paper, the Navier-Stokes equations coupled with a Lagrangian discrete phase model are described to simulate the air-particle flows over the S809 airfoil of the Phase VI blade, the NH6MW25 airfoil of a 6 MW wind turbine blade and the NACA0012 airfoil. The simulation results demonstrate that, in an attached flow, the slight performance degradation is caused by the boundary layer momentum loss. After flow separation, the performance degradation becomes significant and is dominated by a more extensive separation due to particles, since the aerodynamic coefficient increments and the moving distance of separation point present similar variation trends with increasing angle of attack. Unlike the NACA0012 airfoil, a most particle-sensitive angle of attack is found in the light stall region for a wind turbine airfoil, at which the lift decrement and the drag increment reach their peak values. For the S809 airfoil, the most sensitive angle of attack is about 3° higher than that for the maximum lift-to-drag ratio. Hence, the aerodynamic performance of a wind turbine is very susceptible to particles. Based on the most sensitive angles of attack, the more sensitive scope of angles of attack of a blade airfoil and the more sensitive range of rotor tip speed ratios are predicted sequentially. The present study clarifies the principles for the performance degradation of a wind turbine airfoil due to particles and the conclusions are useful for the wind turbine design reducing the particle influences.

A tip loss correction model for wind turbine aerodynamic performance prediction

The tip loss is an important phenomenon in wind turbine aerodynamics and has to be modelled separately in the blade element momentum (BEM) method for predicting the wind turbine aerodynamic performance. Instead of following the conventional trend of thought, the present study for the first time regards the interference factors as a summation of two parts. One is determined by the downwash due to the three-dimensional effect and the other is determined by rotation. Two corresponding factors of FS and FR are proposed to represent the effects of the two parts, respectively. This way of consideration implies a new model for the BEM method, providing appropriate tip loss predictions. The new model is validated in BEM computations of the NREL Phase Ⅵ rotor, the Swedish WG 500 rotor, and the NREL 5 MW reference rotor, with a radius of 2.675 m, 5.029 m and 63 m, respectively. It shows a good accuracy in a wide range of tip speed ratios from 1.58 to 11.3, and presents a good robustness in the cases involving high angles of attack and in the cases involving high axial interference factors. That makes the new model attractive for tip loss correction in BEM.

Power split transmission with continuously variable planetary ratio

The key component of a power split transmission is the planetary gear train that is used to split the input power into two power flows. The transmission characteristic of the planetary gear train is strictly related to a parameter, namely, the planetary ratio whose value is constant. This study proposes an innovative design of a half-toroidal planetary traction train (half-toroidal PT) with continuously variable planetary ratio. Using the half-toroidal PT as a substitute for the planetary gear train in power split transmissions, we propose six configurations of the power split transmission. Kinematic analysis indicates that the proposed half-toroidal PT provides the transmission with wider speed ratio range and higher efficiency in particular working conditions. This study also finds that there is always a trade-off between wide speed ratio range and high output torque in power split transmissions. The continuously variable planetary ratio is verified to be a solution to the trade-off.

Effect of blade pitching on power coefficient of small-scale vertical axis wind turbine at different tip speed ratios

This article analyses the effect of best blade pitching positions on the power coefficient of vertical axis wind turbine at different tip speed ratios. Analysis of the power coefficient of the vertical axis wind turbine is carried out for optimized six blade pitching curves. The first three pitching curves are designed for the tip speed ratios below 1.5 (0 &lt; λ &lt; 1.5) and other three are for the tip speed ratios in the range of 0 &lt; λ &lt; 3. The double multiple stream tube model is used for the present analysis. The results are compared between the optimized and sinusoidal pitching curves. It is concluded that the best optimized pitch position blade method improves the power coefficient than the typical sinusoidal blade pitching in the range of tip speed ratios 0 &lt; λ &lt; 3.

Numerical study into horizontal tidal turbine wake velocity deficit: Quasi-steady state and transient approaches

The results of a computational fluid dynamics (CFD) study of a three-bladed horizontal axis tidal turbine (HATT) using both quasi-steady and transient numerical approaches are presented. The wake from the turbine is studied numerically and compared to experimental results carried out by a French research group at IFREMER. All results provided incorporate the Reynolds Averaged Navier Stokes (RANS) Shear Stress Transport (SST) turbulence model. Simulations cover a range of tip speed ratios (TSR = 0.5–8) with a constant inflow velocity of 0.8 m/s. The resulting power and thrust coefficients, Cp and Ct, are compared to experimental results for validation purposes. The results show a good level of agreement with further discussions of differences addressed herein. In order to garner meaningful information on which numerical methods should be used for wake studies, two families of simulations are performed at TSR = 3.67, using either the frozen-rotor quasi-steady approach, or the fully transient one. For both approaches, near and far field wake propagation is investigated and discussed. Results from both series of simulations are compared to experimental ones.

内部にバッハ・勾玉型ロータを有する 直線翼ダリウス型風車の性能評価

The straight Darrieus type wind turbine, which is expected to use for dispersed power source in urban site, has low performance especially at low tip speed ratio. In this paper, we suggested new type inner rotor to increase the power coefficient over wide range of tip speed ratio. The new type inner rotor, called “Bach – Comma shaped rotor”, is consist of a combination with cylindrical outer shape and Bach type inner shape. The Bach type shape has straight part to gain more torque from the wind than the classical Savonius bucket. The outer cylindrical shape inhibit the flow separation on the blade and rise the power coefficient. It was verified that the “Bach – Comma shaped rotor” is valid for increase the power coefficient over wide range of tip speed ratio by performing both two-dimensional numerical simulations of flow and the torque measurement experiments under the wind-tunnel.

Spanwise flow corrections for tidal turbines

Actuator line computations of two different tidal turbine rotor designs are presented over a range of tip speed ratios. To account for the reduction in blade loading on the outboard sections of these rotor designs, a spanwise flow correction is applied. This spanwise flow correction is a modified version of the correction factor of Shen et al. (Wind Energy 2005; 8: 457-475) which was originally developed for wind turbine rotors at high tip speed ratios. The modified correction is described as ‘directionally dependent’ in that it allows a more aggressive reduction in the tangential (torque producing) direction than the axial (thrust producing) direction and hence allows the sectional force vector to rotate away from the rotor plane (towards the streamwise direction). When using the modified correction factor, the actuator line computations show a significant improvement in the accuracy of prediction of the rotor thrust and torque, when compared to similar actuator line computations that do not allow the sectional force vector to rotate. Furthermore, the rotation of the sectional force vector is attributed to the changing surface pressure distribution on the outboard sections of the blade, which arises from the spanwise flow along the blade. The rotation of the sectional force vector can also be used to explain the reduction in sectional lift coefficient and increase in sectional drag coefficient that has been observed on the outboard blade sections of several rotors in the literature

A comprehensive analysis of aerodynamic flow around H-Darrieus rotor with camber-bladed profile

Improving the H-Darrieus rotor is often followed by the investigation of the influence of the turbine’s parameter design, notably, the aspect ratio, the solidity ( σ), the tip speed ratio, and the airfoil profile shape. In this work, we are interested in both the aerodynamic flows around a straight cambered blade profile and the rotor turbine wake separation of a Darrieus vertical axis wind turbine. The aim of this study is to better understand the evolution of the instantaneous torque and the generated-separated blade vortex during full rotation. Indeed, a three-dimensional computational fluid dynamics model of a vertical axis wind turbine with a straight cambered blade profile NACA4312 operating over a large range of tip speed ratio is considered. The flows are governed by Reynolds-averaged Navier–Stokes equations and the turbulence is modeled with shear stress transport formulations k- ω. This research revealed a high correlation between the evolution of the torque coefficient and the generated-separated blades vortex. In particular, a good correlation between the maximum tip vortices size and the torque coefficient peak is demonstrated.

Microstructure and mechanical property improvement of X70 in asymmetrical thermomechanical rolling

Asymmetrical thermomechanical rolling (ATMR) process of API X70 microalloy steel was investigated to evaluate of rolling force and distribution of material property in the roll gap. Applying a user-defined VUMAT subroutine, the equations of material flow and microstructure kinetics, as material genome package, were implemented into finite element (FE) solver of ABAQUS for simulation of X70 multi-pass rolling. A rough rolling schedule was carried out to verify the accuracy of the material genome by evaluation of the computed rolling force of FE model and experimental results. Considering the interaction of the asymmetrical parameters such as rolls speed ratio (VA), pass height (PH) and rolls diameter ratio, it was found that coinciding of the high level PH with high range of rolls speed ratio and diameter ratio lead to a rise of the rolling force. Strain distribution of the roll gap indicated that an increase of the rolls speed ratio can improve the dislocation density and the strength of rolled material. The study of strain distribution contour in the length of slab thickness revealed that the homogeneity of deformation can be enhanced in the mechanism of asymmetrical rolling of microalloy steel. Experimental test reveals that in asymmetry condition in TMR process of X70, increase of the speed ratio (1.02–1.05) in roughing passes leads to a considerable grain refinement and growth of yield strength and ultimate tensile strength of 3.12 and 2.05% of final plate structure, respectively.

CFD performance enhancement of a low cut-in speed current Vertical Tidal Turbine through the nested hybridization of Savonius and Darrieus

This paper aims to enhance through two-dimensional numerical CFD simulations the hydrodynamic performance of a low cut-in speed current Vertical Axis Tidal Turbine (VATT) by means of the nested hybridization of Savonius and Darrieus. In fact, the hybrid turbine offers several coupling scenarios. In this work, the nested coupling has been used for Darrieus and Savonius to design the hybrid turbine model. Moreover, the effect of the rotational velocities of Savonius and Darrieus turbines, weather synchronized/desynchronized, identical/inverse rotational direction of the assessed rotors has been investigated.OpenFoam code was used to evaluate the hydrodynamic performance of the nested hybrid turbines, and to investigate their torque and power coefficients. This code is based on an Unsteady Reynolds Averaged Navier Stocks (URANS) approach coupled with k-ω Shear Stress Transport (SST) turbulence model. URANS models were validated against the literature results by performing 2D simulations on Savonius and Darrieus wind turbines. Therefore, the turbine’s behavior was studied and the overall performance efficiency of the nested hybrid design was enhanced over a defined Tip Speed Ratio (TSR) range. The main founding of this study was that the hybrid asynchronous coupling of Savonius and Darrieus (Identical rotational direction for the two rotors) is providing the effective performances compared to the other hybrid studied configurations.

Characteristics of the gait adaptation process due to split-belt treadmill walking under a wide range of right-left speed ratios in humans.

The adaptability of human bipedal locomotion has been studied using split-belt treadmill walking. Most of previous studies utilized experimental protocol under remarkably different split ratios (e.g. 1:2, 1:3, or 1:4). While, there is limited research with regard to adaptive process under the small speed ratios. It is important to know the nature of adaptive process under ratio smaller than 1:2, because systematic evaluation of the gait adaptation under small to moderate split ratios would enable us to examine relative contribution of two forms of adaptation (reactive feedback and predictive feedforward control) on gait adaptation. We therefore examined a gait behavior due to on split-belt treadmill adaptation under five belt speed difference conditions (from 1:1.2 to 1:2). Gait parameters related to reactive control (stance time) showed quick adjustments immediately after imposing the split-belt walking in all five speed ratios. Meanwhile, parameters related to predictive control (step length and anterior force) showed a clear pattern of adaptation and subsequent aftereffects except for the 1:1.2 adaptation. Additionally, the 1:1.2 ratio was distinguished from other ratios by cluster analysis based on the relationship between the size of adaptation and the aftereffect. Our findings indicate that the reactive feedback control was involved in all the speed ratios tested and that the extent of reaction was proportionally dependent on the speed ratio of the split-belt. On the contrary, predictive feedforward control was necessary when the ratio of the split-belt was greater. These results enable us to consider how a given split-belt training condition would affect the relative contribution of the two strategies on gait adaptation, which must be considered when developing rehabilitation interventions for stroke patients.

A study on flow fields and performance of water wheel turbine using experimental and numerical analyses

In this paper, an analysis of the performance and flow fields of water wheel turbines for tidal energy extraction is carried out using experimental and numerical methods. The purpose of this work is to develop a water turbine suitable for sites, where fast and shallow surface flows are available, such as rivers or tidal currents. For both methods, the water wheel turbine is tested over a range of tip speed ratios with a differing number of rotor blades, ranging between three and twelve. The results indicate that the numerical simulation shows agreement with the experiment in most cases. Also, the water wheel turbine operates effectively at a range of small tip-speed ratios, where the highest turbine efficiency is produced. Under the same working conditions, the turbines using between six and nine blades generate a greater efficiency and cause lesser reverse flows than others when submerged in water. In contrast, the 3-bladed turbine is the least efficient design as it produces the lowest amount of energy and causes intense vibrations and noises. These noises are a result of a collision between the incoming flow of the channel and the wheel blades during the experimentation, especially at high load conditions. By adding more blades, the torque generated is improved considerably; however, the upstream and downstream depths of the turbine, in this case, are also elevated significantly. Furthermore, in the inlet region, the 3-bladed and 6-bladed turbines have a smaller shock loss and a lower resistance to the main flow from the inlet than the others. Meanwhile, it is found that the flow in the outlet region on the turbines with between nine and twelve blades is in the opposite direction to the wheel’s rotation, significantly obstructing the main flow from the inlet.

Optimization of blade pitch in H-rotor vertical axis wind turbines through computational fluid dynamics simulations

Blade pitch control is a well-developed and widely-used approach in modern horizontal axis wind turbines in operation. However, its application in vertical axis wind turbines (VAWTs) is restricted by the ambiguities in its functional mechanism. A generic formulation that uses five governing parameters to represent the solution space of the optimal blade pitch control is developed through an in-depth analysis of the relationship between blade pitch and the output power of VAWTs. Subsequently, a variable blade pitch automatic optimization platform (VBPAOP) composed of genetic algorithm and computational fluid dynamics (CFD) simulation modules is built to search for optimal blade pitches that can maximize turbine power. A 2D unsteady CFD model is used as a performance evaluation tool because of its high computational efficiency, and its accuracy is validated through wind tunnel experiments prior to its application in optimization. Results show that in a wide range of tip speed ratios (TSRs), the optimized blade pitches can increase the average power coefficients by 0.177 and 0.317, respectively, in two simulated VAWT models with different chord lengths. At stages below the rated TSR, stall-induced torque losses are delayed or even avoided by the proposed optimized pitch control. At stages beyond the rated TSR, energy extraction in the downwind zone is improved due to increased upwind wake velocity.

Multiobjective aerodynamic optimization of a microscale ducted wind turbine using a genetic algorithm

A two-objective aerodynamic optimization of a microscale ducted wind turbine was performed using a genetic algorithm. Two different fitness function pairs were considered for this purpose. In the first alternative the algorithm maximized the power produced while minimizing the drag force at a given wind speed and tip speed ratio. In the second alternative, however, the annual energy production was maximized while minimizing the maximum drag force developed between the cut-in and cut-off wind speeds. Computational uid dynamics solutions performed for selected best designs showed that optimizations performed using the second alternative yielded better turbines, which could produce more power at lower drag. The best design of the second alternative was also observed to operate efficiently at a larger tip speed ratio range compared to the second alternative.