High-solidity Vertical Axis Wind Research Articles

Flow control for high-solidity vertical axis wind turbine based on adaptive flap

Adaptive flap as a new flow control technique with adaptability to the changing flow separation has recently attracted much attention. This study uses the adaptive flap to mitigate the flow separation of a vertical axis wind turbine with a high solidity of 0.75 and investigate its performance and flow control mechanism by considering different flap lengths and locations. The fluid flow is simulated using computational fluid dynamics with the shear-stress transport k-ω model, and the flap motion is calculated based on the fluid–solid interaction methodology. The results show that the flap can be adaptively raised by the backflow caused by flow separation and used to block the backflow. The blocking of the backflow alleviates the flow separation problem and increases the blades’ aerodynamic torque. However, the long flap causes a negative effect due to its inability to retract timely when the flow tends to the attached state at high tip speed ratio scenario. It is observed that the short flap can avoid this problem when it is located far from the blade leading edge. Also, the short flap located closer to the blade leading edge performs better at low tip speed ratios, even though the performance is observed to be weakened by the trailing edge vortices suppressing the flap from deployment. This study provides a technical approach and theoretical basis for better alleviating the flow separation problems in vertical axis wind turbine.

The Effects of Airfoil Thickness on Dynamic Stall Characteristics of High‐Solidity Vertical Axis Wind Turbines

AbstractThe flow physics of high solidity vertical axis wind turbines (VAWTs) is influenced by the dynamic stall effects. The present study is aimed at investigating the effects of airfoil thickness on the unsteady characteristics of high solidity VAWTs. Seven different national advisory committee for aeronautics (NACA) airfoils (0008, 0012, 0018, 0021, 0025, 0030, 0040) are investigated. A high fidelity computational fluid dynamics (CFD) approach is used to examine the load and flow characteristics in detail. Before the study is undertaken, the CFD simulation is validated with experimental data as well as large eddy simulation results with sound agreement. The investigation demonstrates that increasing the airfoil thickness is actually beneficial not only for suppressing the dynamic stall effects but also to improve the performance of high solidity turbines. Interestingly this is accompanied by a slight reduction in thrust component. The strength and radius of the dynamic stall vortex decrease with increasing airfoil thickness. The airfoil thickness strongly influences the pressure distributions during dynamic stall process, which is driven by the suction peak near the leading edge. The knowledge gained might be used by blade engineers for designing future turbines and for improving the accuracy of engineering models.

Airfoil optimization to improve power performance of a high-solidity vertical axis wind turbine at a moderate tip speed ratio

The relatively low power coefficient restricts the wide application of vertical axis wind turbines (VAWTs). An effective solution to this problem is to design specific airfoil profiles which directly influence the capture ratio of wind power. The main aim of the present study is to develop an automatic airfoil profile optimization system to improve the power performance of a VAWT. A three-bladed high-solidity VAWT is adopted as the research object with its chord length, blade span and rotor diameter being 0.2 m, 0.8 m and 0.8 m, respectively. The optimization is conducted at a moderate tip speed ratio (TSR) with a value of 1.0 and the method of coupled CFD simulations with genetic algorithms is employed. The following points make this paper different from previous studies: (a) introducing Multi-Island Genetic Algorithm to optimize airfoils for VAWTs; (b) investigating the airfoil as part of the VAWT rather than as a single isolated body with 3D simulations. The results show that the power coefficient of the VAWT equipped with the optimized blades sensibly improves at all TSRs from 0.4 to 1.5 and the maximum growth rate of it occurs at TSR = 0.9 with a value of 26.82%. The integrated optimization system used in this paper provides an effective way to generate suitable airfoil profiles for given VAWTs with the goal to achieve higher power efficiency.

Effects of wind gusts on a vertical axis wind turbine with high solidity

We present a time-dependent, two-dimensional computational fluid dynamics (CFD) model coupled with the dynamics of the rotor for a height-normalized, small-scale high-solidity vertical axis wind turbine (VAWT) that consists of three straight blades, which are cambered to fit the circular path. The model employs the k-ε turbulence model to approximate unsteady Reynolds-averaged Navier-Stokes equations. The CFD model is validated with data from field measurements. The angular velocity, wind torque, power output and the power coefficient of the rotor are obtained for different wind velocities and gusts from the CFD simulations. A simple rotor velocity feedback control is designed to maximize the power output of the VAWT and implemented in simulations for wind transients. Furthermore, local angle of attack and net incident velocity on the blades are calculated from the local velocity field. Oscillatory transient power coefficient has a very large amplitude owing to the oscillations in the wind torque, but the fluctuations in the generator torque are much smaller despite the large solidity of the VAWT. Overall results show that the proposed coupled modeling approach is an effective tool in evaluation of the transient performance of VAWT systems including the inertial effects of the rotor and the feedback control.

Measurement of high solidity vertical axis wind turbine aerodynamic loads under high vibration response conditions

A series of full-scale experimental wind tunnel tests were performed to determine the aerodynamic loading behaviour on the airfoils of a high solidity, H-type, vertical axis wind turbine. During the course of this investigation, high amplitude vibration of the turbine was observed over a wide range of test conditions. The primary vibration excitation mechanism was resonance of the dominant whirling mode of the turbine with the operating blade pass frequency. In addition, for a significant number of test cases, resonance of the airfoil support struts at higher frequencies was also observed. This large vibration response resulted in conditions that made it difficult or impossible to measure the underlying aerodynamic loading. As a result, in order to gain a greater understanding of the aerodynamic loading, a vibration isolation methodology was developed to remove the effect of vibration from the measured aerodynamic forces on the blades. This included tests with two different support shaft end conditions over a range of flow velocities from 8 to 11m/s, and the use of band-stop filtering to remove residual small amplitude vibrations. In this way, an accurate and complete measurement of the aerodynamic loading on the turbine blades could be obtained to better understand the effects of dynamic stall and validate the results of numerical model predictions.

Flow separation on a high Reynolds number, high solidity vertical axis wind turbine with straight and canted blades and canted blades with fences

The aerodynamics of a high solidity vertical axis wind turbine was investigated through wind tunnel tests of a full size turbine operating at Reynolds numbers of Re ≥ 500,000 (where the power production has been shown to be Re independent). Flow visualization using light-weight tufts attached to the inner surface of a blade was used to gain insight on the operating aerodynamics, which is related to the measured power performance. The tufts proved to be an effective and simple technique to observe the flow characteristics on the large scales involved where other flow measurement techniques like particle image velocimetry or laser Doppler velocimetry would be challenging and expensive. Straight blades on the H–Darrieus turbine operating at peak power showed large regions of flow separation on the upwind blade pass extending from early in the upwind pass from an azimuthal angle θ = 40–50° (where the blade is moving directly upwind at θ = 0°) to over 60° into the downwind pass. The effect of preset blade pitch was also investigated where a 6° leading edge toe-out pitch showed a delay in separation initiation and a reduction in the maximum fraction of chord with flow reversal as well as increased power performance. The overall separation behaviour of the straight blades shows the importance of dynamic stall and the interaction of the separated vortex with the blade as mechanisms in lift generation. Performance and flow separation results are presented for canted blades. The flow separation behaviour was considerably different from straight blades, with canted blades experiencing less flow reversal on their upwind pass, and recovering attached flow before θ = 180°. The installation of fences on the canted blades increased the power and reduced the blade speed ratio at which peak power occurred, suggestive of a reduction in spanwise flow on the swept blades (so that they locally behave more like straight blades). However, the location and amount of flow separation did not trend toward that seen with the straight blades, but rather the fences further reduced the amount of separation.

Vertical axis wind turbine performance enhancement using plasma actuators

A small high-solidity vertical axis wind turbine, with dielectric barrier discharge plasma actuators installed on the blade leading-edges, was built and tested in a low-speed wind tunnel. The main objectives were to assess turbine performance enhancements resulting from the control of dynamic flow separation and project the viability of up-scaling the turbine. The actuators employed were configured to control separation only on the upstream half of the turbine azimuth. Turbine power was measured using a specially designed dynamometer that allowed full characterization of its performance. A parametric study showed that actuator duty cycle dependence observed previously on static airfoils was also observed on the turbine. Optimum reduced frequencies, however, showed substantially different dependence and this was traced to the importance of the plasma pulsation frequency relative to the turbine rotational frequency. When considering overall performance improvements, increases in turbine power of up to 38% were measured. Based on the data acquired, up-scaling the turbine by a factor of 5 and 10, the percentage of plasma power required to produce comparable improvements was conservatively estimated at 3.3% and 1.7% respectively. Further work will address phase-locking the plasma pulsation and turbine frequencies, assessing the performance benefits of downwind actuation, employing combined upwind and downwind actuation and measuring the controlled flowfield using particle image velocimetry.

Blade Offset and Pitch Effects on a High Solidity Vertical Axis Wind Turbine

A high solidity, small scale, 2.5m diameter by 3m high Vertical Axis Wind Turbine (VAWT) consisting of three NACA 0015 profile blades, each with a span of 3m and a chord length of 0.4m, was tested in an open-air wind tunnel facility to investigate the effects of preset toe-in and toe-out turbine blade pitch. The effect of blade mount-point offset was also investigated. The results from these tests are presented for a range of tip speed ratios, and compared with an extensive base data set obtained for a nominal wind speed of 10m/s. Results show measured performance decreases of up to 47% for toe-in, and increases of up to 29% for toe-out blade pitch angles, relative to the zero preset pitch case. Also, blade mount-point offset tests indicate decreases in performance as the mount location is moved from mid-chord towards the leading edge, as a result of an inherent toe-in condition. Observations indicate that these performance decreases may be minimized by compensating for the blade mount offset with a toe-out preset pitch. The trends of the preset blade pitch tests agree with those found in literature for much lower solidity turbines.