A computational framework for the lifetime prediction of vertical-axis wind turbines:CFD simulations and high-cycle fatigue modeling

A novel computational framework is presented for the lifetime prediction of vertical-axis wind turbines (VAWTs). The framework uses high-fidelity computational fluid dynamics (CFD) simulations for the accurate determination of the aerodynamic loading on the wind turbine, and includes these loading characteristics in a detailed 3D finite element method (FEM) model to predict fatigue cracking in the structure with a fatigue interface damage model. The fatigue interface damage model allows to simulate high-cycle fatigue cracking processes in the wind turbine in an accurate and robust fashion at manageable computational cost. The FEM analyses show that the blade-strut connection is the most critical structural part for the fatigue life of the VAWT, particularly when it is carried out as an adhesive connection (instead of a welded connection). The sensitivity of the fatigue response of the VAWT to specific static and fatigue modeling parameters and to the presence of a structural flaw is analyzed. Depending on the flaw size and flaw location, the fatigue life of the VAWT can decrease by 25%. Additionally, the decrease of the fatigue resistance of the VAWT appears to be mainly characterized by the monotonic reduction of the tensile strength of the adhesive blade-strut connection, rather than by the reduction of its mode I toughness, such that fatigue cracking develops in a brittle fashion under a relatively small crack opening. It is emphasized that the present computational framework is generic; it can also be applied for analyzing the fatigue performance of other rotating machinery subjected to fluid–structure interaction, such as horizontal-axis wind turbines, steam turbine generators and multistage pumps and compressors

Unsteady Aerodynamic Load Control Using DBD Plasma Actuators: Various Trailing-edge shapes and Multi-DBDs

Big wind farms with big wind turbines are more cost effective and produce greener electricity compared to smaller ones. This is one of the reasons for the growth of wind turbine size during the last 3 decades. However, as wind turbines become bigger, their blades become longer, thicker and heavier. This results in larger unsteady loads on blades which is an important limitation for their life time and their size growth. Flow control has emerged as the promising solution both for improving the aerodynamic efficiency and controlling the unsteady loads on wind turbine blades. DBD plasma actuator is an active flow control mechanism that has shown high potentials for unsteady load control with the capability to change lift coefficient to a significant amount with a very fast response time. The current research first intends to identify the variations of angle of attack and lift coefficient on wind turbine blades as a result of gravitational loads, mass and aerodynamic imbalances, turbulence, wind shear, yawed inflow and tower shadow and investigate their corresponding frequencies and the fatigue damage from the blade root bending moments. Then, (single and multi) DBD plasma actuator with different configurations will be used on three different trailing-edge shapes (round, half-round and sharp) of modified version of ’NACA64-2-A015’ airfoil to control the aerodynamic loads via circulation control. This is managed either by manipulation of Kutta condition or acting as a virtual Gurney flap. Furthermore, it is intended to investigate the correlation between the frequency of actuation, frequency of vortex shedding and the amount of lift enhancement.Aerospace Engineerin

Towards optimal layout design of vertical-axis wind-turbine farms: Double rotor arrangements

Designing an optimal wind farm layout requires fundamental knowledge of the interaction of wind turbines in an arrangement. In this paper, extensive high-fidelity CFD simulations are performed to investigate the influence of relative spacing, i.e., distance (R) and angle (Φ), in double rotor arrangements of co-rotating Darrieus H-type vertical axis wind turbines (VAWTs) on their aerodynamic performance. The relative spacing varies within 1.25d ≤ R ≤ 10d (d: turbine diameter) and −90° ≤ Φ ≤ +90°. The turbines operate at their optimal tip speed ratio. The analysis is focused on the individual and overall power performance of the turbines and their aerodynamics. Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, validated with experiments, are employed. It is found that an optimal region exists in which a higher overall power coefficient (CPoverall) compared to the CP of an isolated solo rotor (CPSolo) can be achieved. This region corresponds to compact rotor arrangements, i.e. R/d ≤ 3d with Φ ≥ +45° and Φ ≤ −45°, yielding a maximum 1.8% increment in CPoverall/CPSolo at R/d = 1.25 and Φ = +75°. Detailed flow analysis reveals that in the optimal spacing, a narrow passage between the two rotors is formed within which the flow accelerates, forming a high-velocity region. The downstream turbine benefits from its blade(s) passing through this region and consequently yields higher CP values. The findings highlight the high potential for compact VAWT farms with high power density and support the optimal layout design of VAWT farms

Effect of pitch angle on power performance and aerodynamics of a vertical axis wind turbine

Due to growing interest in wind energy harvesting offshore as well as in the urban environment, vertical axis wind turbines (VAWTs) have recently received renewed interest. Their omni-directional capability makes them a very interesting option for use with the frequently varying wind directions typically encountered in the built environment while their scalability and low installation costs make them highly suitable for offshore wind farms. However, they require further performance optimization to become competitive with horizontal axis wind turbines (HAWTs) as they currently have a lower power coefficient (CP). This can be attributed both to the complexity of the flow around VAWTs and the significantly smaller amount of research they have received. The pitch angle is a potential parameter to enhance the performance of VAWTs. The current study investigates the variations in loads and moments on the turbine as well as the experienced angle of attack, shed vorticity and boundary layer events (leading edge and trailing edge separation, laminar-to-turbulent transition) as a function of pitch angle using Computational Fluid Dynamics (CFD) calculations. Pitch angles of −7° to +3° are investigated using Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations while turbulence is modeled with the 4-equation transition SST model. The results show that a 6.6% increase in CP can be achieved using a pitch angle of −2° at a tip speed ratio of 4. Additionally, it is found that a change in pitch angle shifts instantaneous loads and moments between upwind and downwind halves of the turbine. The shift in instantaneous moment during the revolution for various pitch angles suggests that dynamic pitching might be a very promising approach for further performance optimization.status: publishe

Vertical-axis wind-turbine farm design: Impact of rotor setting and relative arrangement on aerodynamic performance of double rotor arrays

The impact of rotor setting and relative arrangement on the individual and overall power performance and aerodynamics of double rotor vertical axis wind turbine (VAWT) arrays is investigated. Eight rotor settings are considered: two relative rotational directions (co-rotating, CO, and counter-rotating, CN), two relative positionings (downstream turbine positioned in the leeward, LW, and windward, WW, of the upstream rotor), and two phase lags (Δθ = 0° and 180°). For each of the eight rotor settings, 63 different relative arrangements are considered resulting in 504 unique cases. The arrangements are considered within 1.25d ≤ R ≤ 10d (d = rotor diameter) and 0° ≤Φ≤ 90°, where R and Φ are relative distance and angle of the rotors, respectively. Unsteady Reynolds-Averaged Navier–Stokes (URANS) CFD simulations, validated with experimental data, are employed. The results show that the power performance of the array is significantly influenced by the relative rotational direction and positioning, ∼8% in power coefficient (C P), while it is marginally dependent on relative phase lag. The different performance of the studied arrays is because of different parts of the downstream turbine revolution being affected by the wake of the upstream turbine and dissimilar strength/width of the shear layer created in the two rotors’ wake overlap. The preferred rotational direction for WW arrays is co-rotating while for LW arrays counter-rotating is favored. For the same arrangement, counter-rotating turbines with LW relative positioning have the highest C P due to their downstream turbine blade moving along the flow direction in the wake overlap region resulting in little energy dissipation and weak shear layer. In contrast, counter-rotating arrays with WW relative positioning have the lowest C P, because the downstream turbine blade moves against the flow in the wake overlap region, resulting in extensive velocity deficit and a thick, strong shear layer

Impact of relative spacing of two adjacent vertical axis wind turbines on their aerodynamics

The impact of relative spacing on the individual and overall performance of two adjacent co-rotating Darrieus H-type VAWTs is investigated through high-fidelity URANS simulations, validated with experimental data. The simulations cover relative distances of 1.25d ≤ R ≤ 10d (d: turbine diameter) and relative angles of 0° ≤ Φ ≤ 90°. The relative angles of 30° ≤ Φ ≤ 75 with relative distance range of 1.25d ≤ R ≤ 5d are identified as the optimal regime with the highest overall power performance for the array. In this regime, the downstream turbine has a maximum increase of 5.1% in CP (R = 1.5d and Φ = 45°) with respect to an isolated solo rotor with similar characteristics. Local flow characteristics including wake length, wake expansion, vorticity and velocity fields are also investigated. It is found that for azimuthal angles of 90° ≤ θ ≤ 160° in the optimal regime, regions of accelerated flow are created due to the contraction of the flow between the turbines which benefit the downstream turbine CP and thus the overall power performance of the array. This provides an opportunity for a compact placement of turbines within a vertical-axis wind turbine farm and consequently increasing the farm power density