Aerodynamic optimization of variable-section J-shaped vertical axis wind turbine using feature-parameter microsegment dimension reduction method

Aerodynamic Optimization and AI-Based Performance Prediction of Vertical Axis Wind Turbines

A lift-driven vertical axis wind turbine (VAWT) generates peak power when it is rotating at high tip-speed ratios (TSR), at which time the blades encounter angles of attack (AOA) over a small range from zero to 30 degrees. However, its ability to self-start is dependent upon its performance at low TSRs, at which time the blades encounter a range of AOAs from zero to 180 degrees. A novel vented aerofoil is presented with the intention of improving the performance of a lift-driven VAWT at low TSRs without hampering the performance of the wind turbine at high TSRs. Computational fluid dynamics (CFD) simulation is used to predict the aerodynamic characteristics of a new vented aerofoil based on the well documented NACA0012 profile. Simulations are performed using the SST turbulence model. The results obtained show a reduction in the coefficient of tangential force (the force that generates torque on the wind turbine) at low AOAs (less than 90 degrees) of no more than 30%, while at high AOAs (more than 90 degrees) an improvement in the tangential force of over 100% is observed. Using a simple momentum based performance prediction model, these results suggest that this would lead to an increase in torque generation by a theoretical three-bladed VAWT of up to 20% at low TSRs and a minor reduction in coefficient of performance of up to 9% at TSR of 2 and closer to 1% at higher TSRs.

Research of asymmetric airfoil on aerodynamic characteristics of vertical axis wind turbines

We study the vortex dynamics of a two-dimensional H-rotor wind turbine using a Navier-Stokes solver. The turbulence model with the wall function is used as the turbulence closure. A sliding mesh technique is employed to handle the blade rotation. The vortex-blade interaction is systematically investigated and its influence on the force generation is discussed. Our simulations show that the vortex-blade interaction largely depends on the solidity and tip speed ratio. We further study the impact of solidity on the turbine performance. Our simulations show that the peak torque per blade decreases with the solidity while the peak torque azimuthal angle increases with the solidity. Our simulations also show that the increase in the azimuthal angle is more significant at low tip speed ratios than at high tip speed ratios. The impact of blade thickness is studied. Our simulations show that a thicker airfoil has a higher torque coefficient than a thinner airfoil. However, because for the thinner airfoil its peak torque occurs at a high tip speed ratio, the thinner airfoil has an overall higher power coefficient than the thicker airfoil.

When investigating limits of rotor aerodynamic models, the Betz limit serves as a solid marker of an upper limit which no model should be able to exceed. A century ago Joukowsky (1912) proposed a rotor aerodynamic model utilizing a rotating actuator disc with a constant circulation. This model has since then been the subject of much controversy as it predicts a power performance that for all tip speed ratios exceeds the Betz limit and which goes to infinity when the tip speed ratio goes to zero. Recently, it was demonstrated that the Joukowsky model is fully consistent with the inviscid Euler equations and that the apparent inconsistency partly can be explained by the lack of viscous effects (Sprensen and van Kuik [4]). However, even including a term to account for the effect of viscosity at small tip speed ratios, the model still predicts a power yield that exceeds the Betz limit. In the present work we study in detail, using a CFD actuator line model, the flow behavior for rotors at small tip speed ratios. It is shown that the excessive swirl appearing towards the rotor center at small tip speed ratios generates vortex breakdown, causing a recirculating zone in the wake that limits the power yield of the rotor. The appearance of vortex breakdown has a similar effect on the flow behavior as the vortex ring state that usually appears at higher tip speed ratios. Limits to where vortex breakdown might occur with tip speed ratio and rotor loading as parameter are investigated and presented in the paper. The limits found correspond to well-known criterion for vortex breakdown onset for swirling flows in general. By applying a criterion for vortex breakdown in combination with the general momentum theory, the power performance always stays below the Betz limit.

As the utilization of wind energy continues to expand as a prominent renewable energy source, the application of Darrieus Vertical Axis Wind Turbine (VAWT) technology has expanded significantly. Various passive modification methods have been developed to enhance efficiency and optimize the aerodynamic performance of the rotor through blade modifications. This study presents passive modification method utilizing Kline–Fogleman (KF) blades which incorporate step-like horizontal slats along the trailing edge. Through Computational Fluid Dynamics (CFD) simulations, this study evaluates ten distinct KF blade configurations, varying in step length and depth, with steps positioned on the inner side, outer side, and both sides of the airfoil. The results indicate that the KF blade with a shorter step on inner side, 20%c in length and 2%c in depth, enhances the average power coefficient (Cp) by 19% compared to the rotor with a clean blade. However, when horizontal slats are incorporated on both sides of the blade, with dimensions of 50%c in length and 5%c in depth, Cp decreases by 33% compared to the clean blade. This reduction occurs across both low and high tip speed ratio (TSR) ranges. It has been observed that the presence of a high-pressure zone of 200 Pa at the trailing edge disrupts the aerodynamic performance when the KF blade is in the upwind region between the azimuth angles of 45° and 135°.

Slotted airfoils mitigate the flow separation on the blades operating at high angles of attack in the upwind region, consequently augmenting the power coefficient and reducing the startup wind speed of Darrieus vertical axis wind turbines (VAWTs). Nonetheless, the presence of the slot structure alters the original flow dynamics, inducing flow separation when the blade operates in the downwind region and at elevated blade tip speed ratios (TSR), which leads to a reduction in the blade’s power coefficient. This study establishes an aerodynamic model of the flow field migration around the blade surface by utilizing the lattice Boltzmann method in conjunction with large eddy simulation to ascertain the influence of the inlet and outlet positions of the slot on the flow field structure across different wind regions. The simulations indicate that, under the downwind region and at high TSR, positioning the slot at the midsection of the blade, although it expands flow separation near the trailing-edge, does not disrupt the primary flow at the leading-edge. Unexpectedly, the slot optimizes the pressure distribution on the pressure side of the blade, thereby enhancing the blade’s performance in the downwind region. At a TSR of 3.3, the average power coefficient of the blades in the downwind region increases by up to 63.62%. These results offer valuable insights for the implementation of slotted airfoils to enhance energy conversion efficiency in VAWTs’ design optimization.

Nowadays the importance of renewable energy is growing, and the utilization of the low wind energy potential is getting crucial. There are turbines with low and high tip speed ratio. Turbines with low tip speed ratio such as the Savonius wind turbine can generate adequate amount of torque at low wind velocities. These types of turbines are also called drag machines. The geometry of the blade can greatly influence the efficiency of the device. With Computational Fluid Dynamics (CFD) method, several optimizations can be done before the production. In our paper the Savonius wind turbine blade geometry was based on the so-called Myring equation. The primary objective of this paper was to increase the power coefficient by modelling the effect of the wind on the turbine blade. For the sake of simplicity, a 2D cross-sectional area was investigated in the simulation with ANSYS CFX 19.1.

High-solidity straight-bladed vertical axis wind turbine: Aerodynamic force measurements

Comparison study of URANS and hybrid RANS-LES models on predicting vertical axis wind turbine performance at low, medium and high tip speed ratio ranges

The starting issue of the Darrieus vertical-axis wind turbine is a crucial challenge, particularly at low tip-speed ratios. This paper demonstrates a solution to overcome the self-starting issue for this turbine type by studying the influence of various blade airfoils in light of their kind and orientation. The proposed airfoils included symmetric airfoils NACA0012 (reference model), E474, and S1048 in addition to cambered airfoils S1210, NACA6712, DU-06-W-200, Clark Y, and FX 63–137. Numerical simulations based on a finite-volume method software, ANSYS Fluent, were executed utilizing the SST-k-ω as a turbulence model to solve unsteady Reynolds-averaged Navier–Stokes equations. The numerical model was validated against available published experimental data. The results indicated that the cambered-in-oriented NACA6712 airfoil was the most effective at low tip-speed ratios (TSRs) ranging from 1.2 to 2.4. At a TSR of 2.0, its power coefficient (Cp) increased by approximately 180% compared to the reference airfoil at the same TSR. Furthermore, the E474 airfoil performed efficiently at mid-to-high TSRs (2.0 to 3.3). Its peak power coefficient is enhanced by about 19.5% at TSR = 3.0 relative to the reference model at the same TSR. On the other hand, the S1210, Clark Y, and FX63-137 cambered-in-oriented airfoils performed poorly at all TSR ranges (1.2 to 3.5). Nevertheless, the flipping of the camber of these airfoils outward provided a significant improvement in the power coefficient and the torque coefficient relative to the cambered-in-oriented ones. The Clark Y in the flipped orientation performs best at all TSR ranges relative to other flipped airfoils. As a result, NACA6712 was the optimum blade profile chosen for low TSRs, whereas E474 was suitable for mid-to-high TSR zones.

ABSTRACTWith rapid advancements in computer hardware and numerical modeling methods, Computational Fluid Dynamics (CFD) has gained prominence in simulating complex flows. As parallel computation becomes an industry standard, the computational efficiency of simulations has become critical. The flow around a Vertical Axis Wind Turbine (VAWT), characterized by complex dynamics and challenging rotating geometry, serves as an intriguing case for CFD studies. This study employs the open‐source CFD solvers SU2 and OpenFOAM to simulate the incompressible, unsteady, and turbulent flow around an H‐type Darrieus VAWT in two dimensions. Spatial and temporal discretization parameters are examined to balance computational cost and accuracy, revealing notable effects on power predictions. Simulations conducted under identical conditions allow for a comparison of the predictions and parallel performances of SU2 and OpenFOAM across three distinct tip speed ratios (TSRs). The findings show that discretization parameters behave differently at various TSRs. While power predictions from SU2 and OpenFOAM generally align with experimental data and with each other, discrepancies arise at lower TSRs, with thrust predictions showing better consistency. Although OpenFOAM provides a faster solution across all parallel configurations, SU2 demonstrates superior parallel scalability, achieving higher speedup and efficiency.

The application of the Darrieus rotors in regions with lower wind potentials is one of the challenging issues in the operation of this type of vertical axis wind turbine (VAWT). To overcome this problem, various combinations of Darrieus and Savonius rotors, called the hybrid rotor, are proposed, which is one of the attractive subjects among wind turbine researchers. Using the computational fluid dynamics technique (CFD), this study investigates a new combination between the Savonius and Darrieus rotors at which a two-bladed Bach-type rotor is attached inside a two-bladed Darrieus rotor made by NACA 0018. In this way, various attachment angles (α) between the internal and external rotors such as 0°, 45°, and 90° are studied to find the optimal case. Computations are performed for various tip speed ratios (TSR) such as 1.5, 2.5, and 3.5, and free-wind speeds (U∞) of 5 and 10 m/s. The obtained results revealed that the application of a hybrid wind turbine is more beneficial under lower TSR and U∞ values. Additionally, it is demonstrated that the case with α = 90° is the optimal case in TSR = 1.5 and 2.5 providing a maximum 21.43% improvement in comparison to the Darrieus rotor for TSR = 1.5 and U∞ = 5 m/s.

Low-center-of-gravity wind turbines (LCGWTs) characterized by tapered blades whose chord length c increases nonlinearly from the top (where c = 0.11 m) to the bottom (where c = 0.17 m) of each blade. Further, turbines featuring these blades do not need any arms, or even a center pole in the rotor. Two experimental LCGWTs (diameter: 0.4 m; height: 0.25 m) with symmetrical blades (NACA 0018) and cambered blades were built. A dead band, which is a band of tip speed ratio (TSR) where the rotor has negative torque at TSR lower than that where the maximum power-coefficient condition is achieved, was observed when symmetrical blades were subjected to low wind speed. In contrast, no dead band was observed for the cambered blades. Under high wind speeds and over a wide range of TSR values, performance of the LCGWTs was better with cambered blades than with symmetrical blades. Computational fluid dynamics (CFD) analysis of 2-dimensional rotors whose blade sections corresponded to the blade sections at the equatorial planes of both types of LCGWTs showed the same tendency. Performance predictions by the blade element momentum (BEM) method using aerodynamic data on the NACA 0018 blades showed some agreement with the CFD analysis. For the cambered blade rotor, Wilson and Walker’s empirical correction of the thrust coefficient, a correction that is typically used in simulations of horizontal axis wind turbines, brought the BEM prediction closer to the CFD prediction than Glauert’s correction did. However, the agreement between the BEM prediction with Wilson and Walker’s correction and the CFD prediction of the cambered blade rotor was thought to be just a coincidence due to large difference on the torque variations between BEM and CFD. At least, the Wilson and Walker’s correction predicts larger torque than the Glauert’s correction at high TSR region. : Wind power, Vertical axis wind turbine, Low center of gravity, Cambered blade, Flow curvature, Conformal mapping, Blade element momentum method, Computational fluid dynamics

The growing integration of renewable energy, especially wind power, in urban settings necessitates the optimization and performance enhancement of small-scale vertical axis wind turbines (VAWTs). The major challenges in VAWT performance enhancement are impediments to self-starting, lower performance, flow instability due to blade wake interactions, and dynamic stability at lower tip speed ratios (TSRs). This work examines two aspects: the numerical investigation of the influence of TSR on the aerodynamic performance of the two-bladed H-rotor, and the analysis of the deep dynamic stall phenomenon involving interaction between blade and shaft vortices. It is observed that fluctuations increase to 2.6 times for the deep stall condition compared to no stall. It is also observed that between no stall and deep stall conditions, there is a significant difference (∼83%) in the peak-to-peak amplitude of the lift coefficient, indicating deterioration in rotor performance as it enters a deep stall. In the downwind zone, the vorticity contour indicates blade–vortex interaction, affirming the reduction in moment and its reduced recovery. The results of Fast Fourier Transform indicate that the peak amplitude decreases by a factor of 1.6 in the downwind zone, signifying the development of stronger vortical structures in the upwind region.