Mean Power Coefficient Research Articles

Numerical investigation on the energy extraction characteristics of parallel multi foils arranged in columns

Abstract The oscillating foil is a focus device in the clean energy utilization field. The presence of an oscillating foil structure within the flow field significantly impacts the energy extraction capabilities of the system. In this paper, three types of parallel multi-foil structures arranged in columns are proposed. The energy extraction characteristics of diverse multi-foil structures under various oscillation conditions are analyzed numerically. The flow field structure and the impact of parallel multi foils on the aerodynamic characteristics are investigated. Results demonstrate that the multi-foil structure with a phase difference of 180° achieves the highest level of energy extraction performance, and the maximal mean power coefficient achieves 0.894, which is 11.03% higher than that of the structure with a smaller phase difference. The size of the trailing edge vortex under the structure with a phase difference of 180° is significantly larger, leading to an obvious fluctuation in the aerodynamic lift.

Optimizing H-Darrieus Wind Turbine Performance with Double-Deflector Design

This study aims to improve an H-Darrieus vertical-axis wind turbine (VAWT) by imposing a novel double-deflector design. A computational fluid dynamics (CFD) model was implemented to examine the aerodynamic characteristics of the VAWT with double deflectors. Geometrics factors related to the locations of the two deflectors were considered, and the orthogonal array based on the Taguchi method was constructed for CFD simulation. The CFD results were further provided as the training data for the artificial neural network (ANN) to forecast the optimal configuration. The results indicate that the performance of a VAWT with a double-deflector design could exceed that of a bare VAWT or that of one using a single deflector. The mean power coefficient for a bare VAWT is 0.37, although it could be much higher with a proper setup using double deflectors. The prediction of ANN analysis is consistent with the result of CFD simulation, in which the difference between the ANN prediction and CFD simulation is generally less than 4.48%. The result confirms the accuracy of the prediction of the optimal VAWT performance with a double-deflector design.

Performance evaluation and shape improvement of a novel vertical-axis autorotation current turbine

A new type of vertical-axis turbine called the vertical-axis autorotation current turbine (VAACT) has been investigated experimentally and numerically. This novel turbine utilizes the autorotation phenomenon to extract kinetic energy and is characterized by a higher moment of inertia and a flapped blade. The main objective of the present work is to further improve the efficiency of this turbine type by improving the original shape. First, through a comparison of experimental and numerical results under specific conditions, the accuracy of the three-dimensional (3D) detached eddy simulation (DES) used in the present study is validated. The free surface effect on the average rotation velocity and efficiency is found to be insignificant when the fluctuation of the free surface is small. Then, shape improvement based on response surface methodology (RSM) incorporated into computational fluid dynamics (CFD) simulations is performed. The inner length and flapped angle are selected as shape variation parameters. Finally, the optimal parameters of an inner length of 10.8 cm and a flapped angle of 47.2°, corresponding to a maximum mean power coefficient of 0.1545, are found and verified. The improvement methodology adopted in this paper greatly reduces the number of required sampling points compared to other optimization algorithms, such as genetic algorithms, which confirms the potential of the resource-intensive 3D DES method as a computational tool.

Study of the performance change in digital mammography systems depending on the total number of examinations

Digital mammography equipment should operate at a high-performance level for detecting breast cancer over the lifetime of the systems. This study investigates the performance of the Fujifilm Amulet Innovality full-field digital mammography system in relation to the number of exposures. The performance of twelve systems, from new and up to 17 000 examinations, was compared. The x-ray output, half-value layer (HVL), contrast-detail (C-D) detectability, contrast-to-noise ratio (CNR), mean glandular dose (MGD), signal transfer property (STP), pre-sampled modulation transfer function (MTF), normalized noise power spectra (NNPS), detective quantum efficiency (DQE) tests were performed to determine the performance change. The noise sources were also analyzed in the spatial domain. The x-ray output and HVL values at 28 kV with tungsten anode and rhodium filter were 15 μGy/mAs ± 1.0 (coefficient of variation (cov) = 7.0%) and 0.54 mmAl ± 0.008 (cov = 1.5%), respectively. The average MGDs for 60 mm equivalent breast to reach the achievable image quality level for 0.1 mm and 0.25 mm detail diameters of circular discs on the CDMAM 3.4 phantom image were 1.17 mGy ± 0.13 (cov = 11%) and 1.40 mGy ± 0.09 (cov = 6%), respectively. The average MGD for 53 mm equivalent breast was 1.08 mGy ± 0.14 (cov = 13%) at dose setting Normal in automatic exposure control (AEC) mode. All tested devices demonstrated good linearity with R2 ≥ 0.999 in the STP curves. The average MTF at 5.0 mm−1 spatial frequency was 0.66 ± 0.007 (cov = 1.0%). The average NNPS was 2.38 × 10−6 mm2 ± 2.13 × 10−7 (cov = 9.0%) at 5.0 mm−1. The average DQE value at 5.0 mm−1 was 0.33 ± 0.02 (cov = 5.0%), and the mean peak DQE was 0.66 ± 0.03 (cov = 5.0%). The mean power coefficient (b), determined from the power relationship between linearised standard deviation and detector dose while analyzing noise sources, was 0.49 ± 0.007 (cov = 1.4%). There was no gradual change in the x-ray tube and detector performance by the number of exposures, and taking up to 68 000 images did not decrease the quality of the systems.

Numerical prediction of the power coefficient improvements of three laterally aligned Savonius wind turbines above a forward facing step

This paper examines the effects of applying three Savonius wind turbines with different mounting positions, including lateral spacing, on power coefficients by conducting high-fidelity three-dimensional numerical simulations, i.e., based on the Reynolds-Averaged Navier-Stokes (RANS) method. The first validation case of the single Savonius wind turbine located on the fixed step height, and a flat surface is found to be in good agreement with that reported in the literature. Then the model is expanded to study the performances of three laterally aligned Savonius wind turbines by placing them above a flat surface and a forward facing step. When the three laterally placed turbines are on a flat surface, an increase of approximately 5% of the maximum mean power coefficient is observed compared to that of a single turbine. Next, the effect of implementing the three turbines at different positions on a fixed step is investigated, where the maximum mean power coefficient improvement for the three turbines can be as high as 230%. Finally, the effect of the lateral spacing distance between the two adjacent turbines is examined. It has been shown that there is an optimal spacing if the maximum mean power coefficient is concerned.

Effect of leading-edge slot on energy capture performance of semi-active flapping airfoil

To rise the efficiency of energy capture for semi-active flapping airfoils power generator at low flow speed, a flow control approach via leading edge slots is carefully examined using numerical method. The energy capture performances of airfoil that affected by slot types formulated with different incline angles are analyzed. The results show that the energy capture power and efficiency of the semi-active flapping airfoils can be improved by cutting appropriate slots on the leading edge of airfoil. Among the carried-out calculations, when the incline angle is from 15° to 17°, better energy capture performances are obtained compared with original airfoil without slots, and 17° incline slots configuration is the best with 9.23% higher mean power coefficient. By further examination of the flow structure, it can be found that the 17° incline slot can make leading edge vortex reattached to the surface of flapping airfoil and evolve tardily along the airfoil surface, to keep flapping airfoil with high lift in the process of reciprocating flapping, which makes the flapping airfoil possessing better energy capture performances.

The effects of heave motion on the performance of a floating counter-rotating type tidal turbine under wave-current interaction

Wave induced motions due to actual sea state conditions will impact the performance of floating counter-rotating type tidal turbines. The wave-current interaction is a significant factor in determining the heave motion of turbines. The present study aims at investigating the effects of heave motion on the performance of a floating counter-rotating turbine under wave-current interaction. In this study, the numerical method is used to study how the heave amplitude and frequency affect the main hydrodynamic coefficients of the turbine, including the power coefficient and thrust coefficient. The form of sliding mesh combined with overset mesh is adopted, and the heave motion of the turbine is realized by the movement of the overset mesh. The simulation results show that the heave motion has an adverse effect on the hydrodynamic performance of the counter-rotating tidal turbine. The heave motion will reduce the mean power coefficient and the mean thrust coefficient by about 9% and 5% respectively. The reduction in mean thrust and power is mainly due to less optimal rotor operation caused by the hydrodynamic effects associated with oscillating inflow. The front propeller is more susceptible to heave motion than the rear propeller. Furthermore, the effects of wave on the hydrodynamic performance of turbine are studied by changing the immersion depth of the turbine. The wave will increase the power coefficient slightly, but at the same time, also increase the probability of heave motion. The results can provide data to study motion response of floating tidal turbines under wave-current interaction and control design of the output electricity.

Wake interactions of two tandem floating offshore wind turbines: CFD analysis using actuator disc model

Floating offshore wind turbines (FOWTs) have received great attention for deep water wind energy harvesting. So far, research has been focused on a single floating rotor. However, for final deployment of FOWT farms, interactions of multiple FOWTs and potential impacts of the floating motion on power performance and wake of the rotors need to be investigated. In this study, we employ CFD coupled with an Actuator Disc model to analyze interactions of two tandem FOWTs for the scenario, where the upstream rotor is floating with a prescribed surge motion and the downstream rotor is fixed and influenced by the variations in the incoming flow created by the oscillating motion of the surging rotor. We will investigate three different surge amplitudes and analyze the fluctuations in power performance of the two rotors as well as their wake interactions. The results show a light increase in the mean power coefficient of both rotors for the surging case, compared against the case with no surge motion. The standard deviation of the transient CP of the surging rotor linearly scales with the surge amplitude, while such impact for the downstream rotor is very limited. Surging motion of the upstream rotor is found to enhance flow mixing in the wake, which therefore, accelerates the wake recovery of the downstream rotor. This finding suggests prospects for research into redesigning wind farm layout for FOWTs, aiming for more compact arrangements.

Turbine design dependency to turbulence: An experimental study of three scaled tidal turbines

In this paper the turbulence effects are studied for three rotors mounted on the same instrumented hub. Two scaled models of industrial turbines and one open-geometry turbine are considered. The turbulence characteristics are obtained from 2D Laser Doppler Velocimeter measurements and the turbine behaviour is analysed from thrust and torque measurements. Three turbulence intensities and a large range of tip speed ratios and flow velocities are considered. The results are anonymised in order to ensure confidentiality. The rotors have different blade profiles, blade numbers and solidity. The rotor design largely modifies the mean power and thrust coefficients. The turbulence intensity only slightly changes these results but has a larger influence on the fluctuating loads than the different rotor designs. The spectral analysis of the rotor torque and thrust shows that, at low frequencies the load variations are correlated to those of the flow velocity with some differences due to the turbulence intensity levels. The coherences between the loads and the velocity seem to be not affected by the rotor type. At high frequencies, the load variations are correlated to the speed control unit of the scaled model and the rotor design has an impact on the rotational speed and loads coherences.

High-resolution numerical simulation of the performance of vertical axis wind turbines in urban area: Part II, array of vertical axis wind turbines between buildings

Vertical axis wind turbines can harvest wind energy from every direction, and they are suitable for the complex flow conditions in urban areas. The flow field around buildings consists some high speed regions, and the blockage effect can provide higher wind velocity. Meanwhile, they can be installed at a certain altitude with no interference to pedestrians and vehicles. In this paper, we investigate the characteristics of wind turbines in an array by arranging them between two buildings. For this aim, a high-resolution numerical simulation method is adopted to simulate the accurate flow field and force coefficients. The high-resolution numerical simulation method is composed of adaptive mesh refinement and overset grid techniques. Firstly, there are two array types with wind turbines uniformly arranged in a line, which is perpendicular to the free stream. The result shows that array type A with asymmetric wake achieves a greater mean power coefficient. It reveals that the wake matching phenomenon of array type B causes a loss of wind energy between each couple. Secondly, five column positions between the two buildings are arranged in different positions. The five positions correspond to different flow conditions, and they belong to three typical processes: contraction acceleration process, uniform velocity process, and expansion deceleration process. When mounting array type A in contraction acceleration or expansion deceleration regions, the velocity profile is non-uniform along with the array. The power coefficients of wind turbines in one array are significantly different from each other. The array in the contraction acceleration region reaches the maximum mean power coefficient. Thirdly, in order to evaluate the influence of wind directions in urban area, there are four cases with different wind directions. The simulation results show that the array at α∞=15∘ obtains the maximum mean power coefficient. In summary, the mean output power of the wind turbine array in urban areas is always greater than that of a single wind turbine.

Two-stage optimization of three and four straight-bladed vertical axis wind turbines (SB-VAWT) based on Taguchi approach

This research uses computational fluid dynamics (CFD) to perform a two-stage optimization of power output in multiple vertical-axis wind turbines (VAWT) with straight blades. In the first stage, four configurations are evaluated and optimized utilizing the Taguchi approach. Three operational factors, including the distance between the third turbine and the y-axis (H), the distance between the second turbine and the third turbine (B) and, placing the fourth turbine along with two levels, are considered to account for their effects on the output of the multiple turbine system. The impacts of the three factors on the performance are highlighted by H > B > placing the fourth turbine. Subsequently, based on the Taguchi-optimized combination, the analysis of the specific factor is conducted in the second stage by varying the B value. The results suggest that B = 6m without placing the fourth turbine can further intensify the power output of the VAWT system by around 3%, increasing the mean power coefficient (Cp¯) from 0.5026 to 0.5174. This is attributed to the Magnus effect, originating from the first and the second turbine. Furthermore, the results also show that changing the rotating direction of the third turbine from counterclockwise to clockwise deteriorates the output power, reducing the Cp¯ to 0.3339. Overall, the mean power coefficient of the optimized three-turbine system is higher than that of the single VAWT (Cp = 0.4473) by a factor of 15.7% under the wind speed of 8 m⋅s−1 and a tip speed ratio of 2. This reveals that the optimal design can effectively increase the output power performance of VAWT systems.

Improvement of Aerodynamic Performance of Savonius Wind Rotor Using Straight-Arc Curtain

A straight and an arc-shaped curtain are combined to enhance the aerodynamic performance of the Savonius wind rotor. The straight-arc curtain is placed in front of the Savonius wind rotor to reduce the negative moment on the convex blade and increase the positive moment on the concave blade. The static and dynamic performances of the Savonius wind rotor with and without the curtain are investigated based on the computational fluid dynamics method. The results show that the static torque is higher with the curtain than without it at the same angle-of-attack. The maximum mean power coefficient with the curtain is increased by about 50% compared with conventional Savonius wind rotor. Additionally, the flow field around the rotor with the straight-arc curtain is presented, and the flow control mechanics of the straight-arc curtain are discussed.

Effects of blade shape and its corresponding moment of inertia on self-starting and power extraction performance of the novel bowl-shaped floating straight-bladed vertical axis wind turbine

In this paper, the effects of the blade shape and its corresponding moment of inertia on the performance of a novel bowl-shaped floating straight bladed vertical axis wind turbine (VAWT) have been investigated. A numerical method was developed and validated to accurately simulate the interaction between the wind and VAWT. Three inferential parameters, the starting time, mean power coefficient, and standard deviation of the aerodynamic force, were employed to evaluate the performance of the turbine with different blade shapes. Performance indicators including the self-starting capability, power extraction efficiency, and fatigue characteristics, were assessed. The results indicated that the VAWT with S-1046 blade had better overall performance as compared to the one with NACA0018 blade, where the reduced self-starting time, slight more or identical mean power coefficient, and minor standard deviation of Ft and Fn were observed. Further, the flow structure around the blade was also analyzed, it was found that at low wind speed (less than 6.0 m/s) separated vortex around the turbine is observed, whereas at high wind speed, attached vortex is observed. Fluctuation curves of power coefficient variation with tip speed ratio ‘λ’ were also noted for the wind induced rotation of VAWT. At low wind speed, the highest power coefficient amplitude was obtained when the turbine was at maximum tip speed ratio, whereas at high wind speed, the highest power coefficient amplitude was obtained when the tip speed ratio of the turbine was slight less than the maximum value. Based on these results, it is strongly recommended to apply thinner symmetry (S-1046) blade for designing VAWT.

Effect of platform disturbance on the performance of offshore wind turbine under pitch control

AbstractThe aerodynamic performance of offshore floating wind turbines (OFWTs) is more complicated than onshore wind turbines due to 6‐degree of freedom (DOF) motion of the floating platform. In the current study, the aerodynamic analysis of a horizontal‐axis floating offshore wind turbine is performed with the aim of studying the effects of floating platform movement on the aerodynamic characteristics of the turbine in the presence of a pitch angle control system. The National Renewable Energy Laboratory (NREL) 5‐MW offshore wind turbine is selected as the baseline wind turbine. For this sake, the unsteady blade element momentum method with dynamic stall and dynamic inflow models have been employed to obtain the unsteady aerodynamic loads. The baseline pitch angle control system is assumed to be coupled with the aerodynamic model to maintain the rated condition of the wind turbine and also to approach a closer model of wind turbine. In case of pitching motion input, the reduction of mean power coefficient for tip speed ratios (TSRs) less that 7 is expected by an amount of 16% to 20% at pitch amplitude of 2° and frequency of 0.1 Hz. For high TSRs, the trend is reverse with respect to fixed‐platform case. The mean thrust coefficient is reduced for almost all range of TSRs with maximum loss of 37%. Moreover, the mean control pitch angle that is an index of control system effort is increased. The results also represent the importance of considering the pitch control system for aerodynamic analysis of disturbed OFWT.

Effects of inward and outward overlap ratios on the two-blade Savonius type of vertical axis wind turbine performance

ABSTRACTIn this paper, using numerical simulation, the effects of inward and outward overlaps on the performance of a conventional two-blade Savonius wind turbine and the complete analysis of the events caused by the rotation of the blades have been investigated. Computations have been carried out at several nondimensional overlap ratios such as G/d = 0, ±0.1, ±0.2, and ±0.3 for three wind speeds of 3, 5, and 7 m/s. On the other hand, in the present study, the values of the tip speed ratio (TSR) are considered in the range from 0.2 to 1.2. The obtained results are validated against the available data in the open literature and found excellent agreements between them. Several results such as the instantaneous velocity and pressure fields in the rotating zone, variation of torque coefficients in one complete cycle of the rotor, and mean power coefficient distributions as a function of TSR and wind speed have been presented in this numerical investigation. The results show that, regardless of the overlap ratio, the maximum power coefficient occurs at a tip speed ratio (TSR) of 0.8. Additionally, the maximum power coefficient is observed in the inwards overlap ratio of 0.2 and an increasing or decreasing the overlap ratio reduces the maximum value of the power coefficient in all wind speeds under consideration. Finally, it was concluded that at each TSR and G/D value, increasing the wind speed enhances the power coefficient of two-blade Savonius VAWT. It was hoped the obtained results arouse interest among the VAWT designers.

Optimization Study on the Blade Profiles of A Horizontal Axis Tidal Turbine Based on BEM-CFD Model

In order to increase the performance of horizontal tidal turbines, a multi-objective optimization model was proposed in this study. Firstly, the prediction model for horizontal tidal turbines was built, which coupled the blade element momentum (BEM) theory and the CFD calculation. Secondly, a multi-objective optimization method coupled the response surface method (RSM) with the multi-objective genetic algorithm NSGA-II was applied to obtain the optimal blade profiles. The pitch angle and the chord length distribution were chosen as the design variables, while the mean power coefficient and the variance of power coefficient were chosen as the objective functions. With the mean power coefficient improved by 4.1% and the variance of power coefficient decreased by 46.7%, results showed that both objective functions could be improved.

Effects of non-sinusoidal pitching motion on the propulsion performance of an oscillating foil.

Numerical simulations have been used in this paper to study the propulsion device of a wave glider based on an oscillating hydrofoil, in which the profile of the pitching and heaving motion have been prescribed for the sake of simplicity. A grid model for a two-dimensional NACA0012 hydrofoil was built by using the dynamic and moving mesh technology of the Computational Fluid Dynamics (CFD) software FLUENT and the corresponding mathematical model has also been established. First, for the sinusoidal pitching, the effects of the pitching amplitude and the reduced frequency were investigated. As the reduced frequency increased, both the mean output power coefficient and the optimal pitching amplitude increased. Then non-sinusoidal pitching was studied, with a gradual change from a sinusoid to a square wave as the value of β was increased from 1. It was found that when the pitching amplitude was small, the trapezoidal pitching profile could indeed improve the mean output power coefficient of the flapping foil. However, when the pitching amplitude was larger than the optimal value, the non-sinusoidal pitching motion negatively contributed to the propulsion performance. Finally, the overall results suggested that a trapezoidal-like pitching profile was effective for the oscillating foil of a wave glider when the pitching amplitude was less than the optimal value.

Numerical modeling of aeroacoustic characteristics of different savonius blade profiles

Purpose This study aims to carry out numerical modeling to predict aerodynamic noise radiation from four different Savonius rotor blade profile. Design/methodology/approach Incompressible unsteady reynolds-averaged navier-stokes (URANS) approach using gamma–theta turbulence model is conducted to obtain the time accurate turbulent flow field. The Ffowcs Williams and Hawkings (FW-H) acoustic analogy formulation is used for noise predictions at optimal tip speed ratio (TSR). Findings The mean torque and power coefficients are compared with the experimental data and acceptable agreement is observed. The total and Mono+Dipole noise graphs are presented. A discrete tonal component at low frequencies in all graphs is attributed to the blade passing frequency at the given TSR. According to the noise prediction results, Bach type rotor has the lowest level of noise emission. The effect of TSR on the noise level from the Bach rotor is investigated. A direct relation between angular velocity and the noise emission is found. Practical implications The savonius rotor is a type of vertical axis wind turbines suited for mounting in the vicinity of residential areas. Also, wind turbines wherein operation are efficient sources of tonal and broadband noises and affect the inhabitable environment adversely. Therefore, the acoustic pollution assessment is essential for the installation of wind turbines in residential areas. Originality/value This study aims to investigate the radiated noise level of four common Savonius rotor blade profiles, namely, Bach type, Benesh type, semi-elliptic and conventional. As stated above, numbers of studies exploit the URANS method coupled with the FW-H analogy to predict the aeroacoustics behavior of wind turbines. Therefore, this approach is chosen in this research to deal with the aeroacoustics and aerodynamic calculation of the flow field around the aforementioned Savonius blade profiles. The effect of optimal TSR on the emitted noise and the contribution of thickness, loading and quadrupole sources are of interest in this study.

The Effect of Damping Coefficient, Spring Coefficient, and Mass Ratio on the Power Extraction Performance of a Semiactive Flapping Wing

The effect of varying damping coefficient C∗, spring coefficient K∗, and mass ratio M∗ on the semiactive flapping wing power extraction performance was numerically studied in this paper. A numerical code based on Finite Volume method to solve the two-dimensional Navier-Stokes equations and coupled with Finite Center Difference method to solve the passive plunging motion equation is developed. At a Reynolds number of 3400 and the pitching axis at quarter chord from the leading edge of the wing, the power extraction performance of the semiactive flapping wing with different damping coefficient, spring coefficient, and mass ratio is systematically investigated. The optimal set of spring coefficient is found at a value of 1.00. However, the variation of mass ratio M∗ cannot increase the maximum mean power coefficient and power efficiency, but it can influence the value of damping coefficient C∗ at which the wing achieves the maximum mean power coefficient and power efficiency. Moreover, insensitivity of the mean power coefficient and power efficiency to the variation of damping coefficient C∗ is observed for the wing with smaller mass ratio, which indicates the wing with smaller M∗ has better working stability.

Optimal Design of Vertical Axis Wind Turbines for Mechanical and Aesthetic Performances under Green Lifestyles in Thailand

Although current Vertical Axis Wind Turbines (VAWTs) is highly potential to energy conversion of wind energy under low wind speed conditions, which are abundantly found in Thailand, it is clear that there are no strong attentions of usages from private investors and government. To resolve the problems, the optimal design of VAWTs is proposed to obtain the most efficient power generation and the most physical attractiveness to household users. For each three dimensional VAWT model, the mechanical performance index is determined from the mean power coefficient over tip speed ratios via CFD analysis while the aesthetic performance index is obtained from the mean vote of respondents about their preference to three dimensional VAWT models. The product t-norm of two indices is used to quantify the most efficient power generation and physical attractiveness. The greatest values are chosen as the best designs. It is confirmed that there are significant tradeoffs between engineering achievement and appearance satisfaction for given models in the survey experiment. The mechanical performance index and aesthetic performance index should be taken in account for commercialization design. This work aims at research communities for systematically exploring new wind turbine rotors with innovative shapes, sizes, and orientations for physical attractiveness.