This paper presents a detailed investigation on the flow characteristics and the bed-shear-stress distribution around a finite circular cylinder at a fixed Reynolds number (Re = 2 × 104) and seven aspect ratios (AR = 0.5 ∼ 6) by solving the Reynolds Averaged Navier–Stokes (RANS) equations. It is found that, for a moderate Reynolds number and a relatively large boundary-layer thickness, the time-averaged streamwise vortex structure will be transformed from the ‘Dipole Type’ at AR ≤ 3 to the ‘Three-Pairs Type’ at AR ≥ 4. It is the first time that the ‘Three-Pairs Type’ vortex structure has been reported, which consists of Time-Mean Streamwise Tip Vortices, Time-Mean Streamwise Base Vortices and Time-Mean Streamwise Bottom Vortices. Besides, this study indicates that, with increasing AR, the maximum time-averaged bed-shear-stress amplification magnitude upstream of the cylinder will first decrease and then increase, with the minimum occurring at approximately AR = 3.0, due to the influence of AR on the horseshoe vortices. Additionally, this study manifests that different critical AR values can be obtained for various flow variables. For instance, in terms of the total lift coefficient of the cylinder and the transverse fluctuating velocity in the symmetry plane at the mid-height of the cylinder, the critical AR is equal to AR c = 3 . 0 and AR c = 2 . 0 , respectively.

Flow around a surface-mounted finite circular cylinder completely submerged within the bottom boundary layer

\n A double multiple streamtube model coupled with variable pitch methodology is used to analyze the performance characteristics of a small-scale straight-bladed Darrieus type vertical axis wind turbine (SB-VAWT). The numerical study revealed that a fixed pitch of −2.5 deg could greatly enhance the performance of the wind turbine. However, no improvement is observed in the starting torque capacity. Furthermore, the performance of upwind and downwind zones has been investigated, and it is found that the VAWT starting capacity is improved by increasing/decreasing the pitch angle upwind/downwind of the turbine. To optimize the performance, four cases of variable pitch angle schemes of sinusoidal nature were examined. The parameters of the sinusoidal functions were optimized using differential evolution (DE) algorithm with different cost functions. The results showed improvement in the power coefficient, yet with low starting capacity enhancement. Among the objective functions used in DE algorithm, the negative of the average power coefficient is found to lead to the best starting capacity with moderate peak power coefficient.

Numerical Investigation of a Vertical Axis Wind Turbine Performance Characterization Using New Variable Pitch Control Scheme

The airfoil design optimizations are often limited due to the high computational cost and complex algorithm selections. However, such computational requirement has not been fulfilled well because s...

Performance analysis of three heuristic algorithms for airfoil design optimization

Making full use of wind energy can effectively alleviate the global energy shortage and environment contamination problems. Nevertheless, how to significantly improve the performance of the wind turbine airfoil and blade is a crucial issue. As the novel flow control method, the co-flow jet (CFJ) technology is one of the most potential methods to solve this problem. Thus, the effects of the CFJ technology on the performance enhancement of the S809 airfoil and Phase VI wind turbine blade are explored in this study. Furthermore, the effects of the injection location and jet momentum coefficient are studied, and an adaptive jet momentum coefficient strategy of the CFJ technology is proposed. Results demonstrate that the CFJ technology can significantly improve the maximum lift coefficient and maximum corrected lift-to-drag ratio of the S809 airfoil. Moreover, the power coefficient of the Phase VI wind turbine blade at the low tip speed ratio is greatly enhanced as well. In particular, the maximum lift coefficient and maximum corrected lift-to-drag ratio of the typical S809 CFJ airfoil with adaptive Cμ are improved by 119.7% and 36.2%, respectively. The maximum power coefficient of CFJ blade can be increased by 4.5%, and the power coefficient of CFJ blade can be boosted by 226.7% when the tip speed ratio is 1.52.

Numerical Investigation of Performance Enhancement of the S809 Airfoil and Phase VI Wind Turbine Blade Using Co-Flow Jet Technology

The flow characteristics and the lift and drag behavior of a thick trailing-edged airfoil that was provided with fixed trailing edge flaps (Gurney flaps) of 1% to 5% height right at the back of the airfoil were studied both experimentally and numerically at different low Reynolds numbers (Re) and angles of attack for possible applications in wind turbines suitable for the wind speeds of 4-6 m/s. The flap considerably improves the suction on the upper surface of the airfoil resulting in a higher lift coefficient. The drag coefficient also increased; however, the increase was less compared to the increase in the lift coefficient, resulting in a higher lift-to-drag ratio in the angles of attack of interest. The results show that trailing-edge flaps can improve the performance of blades designed for low wind speeds and can directly be applied to small wind turbines that are increasingly being used in remote places or in smaller countries.

Epeli Nabolaniwaqa

Papers

Performance Studies on a Wind Turbine Blade Section for Low Wind Speeds with a Gurney Flap