Design of a passive flow control solution for the mitigation of vortex induced vibrations on wind turbines blade sections as a response to extreme weather events

Highlights

• Passive control methods mitigate ViV on wind turbines due to extreme weather events.

• The interaction between the microcylinders and the airfoil wake reduces ViV.

• The microcylinders size of 10% of the chord length maximizes the ViV mitigation.

Abstract

The goal of the present work is to perform exploratory assessments on the suitability of microcylinder devices to mitigate wind turbine blade vortex induced vibration in small cross flow regimes, specially occurring under adverse weather events. For this purpose, the deep stall behavior of the NACA0021 has been evaluated for the first time on a wide range of high angles of attack (50°–130°) by means of CFD assessments, and the effects of microcylinders have been studied in terms of the reduction of aerodynamic coefficient magnitudes and fluctuations. In a first step, the passive flow control solution at 90° has been analyzed for a total number of investigated cases equal to 15, keeping constant the diameter of the microcylinders and varying the relative positions of these devices with respect to the blade. Mitigations of the load standard deviations between 63% and 97% across all the tested angles of attack have been found. Additionally, genetic programming (GP) was used to obtain a more general viewpoint of the effect of the microcylinders on the aerodynamics forces. Assessments of the flow fields confirm that properly located microcylinders disrupt the coherent vortical structures in the wake, responsible for periodic loading on the blade section.

Introduction

To face climate change, the interest towards sustainable energy sources has risen dramatically. Sources of environmental friendly power generation range from sunlight [1], water [2], wind [3], biogas [4] up to disruptive technologies such as microbial fuel cells [5], [6]. In this sense, the accurate modeling and forecasting of the energy consumption are challenging tasks that may help to overcome the current problematics [7]. Due to the exceptional situation, some more efforts are still required to guarantee the energy availability, considering the world energy consumption [8].

In terms of wind power, to face the rise in demand of wind energy harvesting, the size of wind turbines has undergone a continuous growth during the last few years, the wind-turbine blades being more susceptible to aeroelastic instabilities (especially in parked or non-operational conditions), such as Vortex-induced Vibrations (ViV), which represent a major risk for the structural integrity of wind turbine components [9], [10], [11].

In non-operational conditions (e.g. under extreme weather conditions), the flow features a small crossflow angle (i.e. the flow primarily parallel to the blade sections, at $AoA\simeq \pm$90°), and it is massively disrupted compared to the attached regime [9], [10], [11], [12], [13], [14], [15], resulting in a mechanism of alternate coherent vortex shedding similar to that occurring in 2D bluff bodies [11], [12], [13], [14], [15]. As a consequence, lock-in phenomena with the shedding frequency coupled to the oscillation frequency [12], [15], can yield large oscillations of the blades [9], [10], [11] and potentially severe structural damage to the rotors.

For the prediction of the lock-in phenomenon, numerical simulations have become an essential tool. For instance, Skrzypiński et al. [11] perform 2D and 3D RANS, and 3D DES unsteady CFD computations with non-moving, prescribed motion and elastically mounted DU96-W-180 airfoil at $AoA=90^{\circ }$, highlighting the risk of the lock-in phenomenon for the actual wind turbine blade sections. Large-scale, 2D-URANS sensitivity studies on a NREL S809 section at deep stall $AoA$ in [12], [13], [14] provide maps of the lock-in frequencies at different mean $AoA$ for oscillating motions of the blade sections. Hu et al. [15] observed the dependency of the lock-in features on the oscillation frequency and amplitude, and on the Reynolds number, through URANS on a DU96-W-180 airfoil oscillating in edge direction about $AoA=90^{\circ }$. On the other hand, Garcia-Ribeiro et al. [16] conduct URANS three-dimensional CFD simulations to analyze the effects of taper ratio and root chord ratio of winglets on the performance of wind turbines concluding that winglets can be optimized for enhancing the performance of the horizontal axis wind turbines.

While some work has been done to assess the origin of ViV on wind turbine blades, (passive) control solutions to mitigate these phenomena have been investigated very limitedly. For instance, Zaki et al. [17] numerically investigated the use of a slat near the leading edge of a S809 airfoil to delay or completely cancel separation, leading to 12% saving of material and 6% of the total cost of the turbine. Acarer [18] performed a parametric optimization of the slot geometry on a DU12W262 airfoil, which enabled the location of the slot both on the suction and on the pressure side, reaching the 16% peak $C_L/C_D$ improvement and an overall $\alpha -C_L/C_D$ rise.

Alternatively to vortex generators (VGs) Mostafa et al. [19] studied the impact of microcylinders on straight bladed NREL Phase II, horizontal axis wind turbine at four different positions around the leading edge. It is found that the microcylinders placed on the pressure side has less effect on the output power compared to when it is positioned in front of the blade leading edge. Similarly, a small rod in front of the leading edge of a symmetrical airfoil was also used in [20] to control the dynamic stall of the Darrieus vertical-axis wind turbine (DVAWT). The formation of dynamic stall vortices was prevented by the counter-rotating vortex shedding from the rod that continuously transmit kinetic energy into the boundary layer of the airfoils. Wang et al. [21] also investigated the impact of microcylinders in front of the blade leading edge as a flow control method to suppress the flow separation. With this configuration, the blade torque can greatly rise up to a 27.3%.

Even if the use of microcylinders seems very promising to improve the aerodynamic performance of wind turbines, there is a lack in the application of this passive control method to control the aeroelastic response.

The goal of the present work is to assess the suitability of microcylinders to prevent and mitigate ViV that might occur under extreme (e.g. critical wind directions and wind speed) weather environments, when wind turbines are in nonoperating conditions. The deep stall behavior of the NACA0021 has been evaluated on a wide range of angles of attack (50°–130°) and the effects of microcylinders have been evaluated in terms of the reduction of the aerodynamic coefficients and their fluctuations.

A parametric study was performed to assess the effects of the microcylinders diameters and locations on the unsteady aerodynamic response of the blade section. Finally, the optimization of both diameter and layout was carried out. The effectiveness of the microcylinders in wind turbines might be of interest for the forthcoming blade designs or as add-on devices temporarily installed during maintenance tasks under nonoperational conditions.

The manuscript is articulated as follows. Section 2 describes the problem and the numerical settings and discretization. Section 3 discusses the effect of the microcylinders on the mean values and on the standard deviations of the lift and drag forces at different angles of attack. The phase portraits of different force components are described in Section 4, followed by a genetic programming fitting based on the main variables of the problem (say, angle of attack, position and the microcylinder diameter). Some flow field maps are also included in this section for the analysis of the loads. Finally, in Section 5 the conclusions of this study and some future works are presented.