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Abstract

In an effort to combat global greenhouse gas (GHG) emissions, sustainable energy harvesting alternatives like offshore wind turbines have garnered international attention across industrial, legislative, and academic sectors. Despite substantial efforts to enhance wind turbine efficiency and aggregate power output in large-scale farms, structural fatigue related downtime remains to be a considerable logistical and financial burden. Some authors have estimated between 4% and 10% average downtime for offshore wind turbines (Horn and Leira, 2019) due to a combination of electrical and mechanical failures. Cevasco et al. (2021), reported an appreciably larger downtime range, 0.9-19.8%, based on a comprehensive survey of various reliability studies in Europe. They also note a disparity in operational availability between mega-watt range, off and onshore wind turbines (onshore being less susceptible to failure) supported by a prior correlation of turbine failure rate and unfavourable environmental conditions (Tavner et al., 2012).

To broaden the mechanical lifetime of offshore wind turbines, modern research efforts have been focused towards structural fatigue mitigation by means of both active and passive flow control devices. Commonly employed devices/technologies include camber morphing, popup tabs, suction/blowing, and variations thereof (Gardner et al., 2023). The primary end-goal of such devices is to assist in load alleviation during transient conditions like gusts, crosswinds, or wake that can potentially excite a dynamic aeroelastic response of the wind turbine blades during operational and stand still conditions. When operating at high mean incidence angles, this response to volatile environmental conditions can be further augmented due to various dynamic stall related events. In general, these events consist of the formation, migration, and detachment of a strong leading-edge vortex (LEV) from the aerofoil which are essential for driving the single degree of freedom instability known as stall flutter (Dimitriadis and Li, 2009). Prolonged exposure to these conditions can lead to sustained oscillations causing premature wind turbine blade wear, mechanical failure, and eventual decommissioning. Thus, accurately characterising these dynamic stall related events is a crucial endeavour to inform design strategies and operational regimes that alleviate unwanted aerodynamic loading and vibratory modes.

The dynamic stall processes during high amplitude aeroelastic pitching cycles and its induced effects are highly nonlinear due to a complex coupling between inertial, restorative, and aerodynamic forces. This coupling has shown to be sensitive to various operational parameters such as the reduced frequency, aerofoil shape, and free stream turbulence intensity, for example. Consequently, the rich fluid dynamics associated with stall flutter has been mostly restricted to a phenomenological understanding of how the LEV contributes to unsteady aerodynamic loading and energy extraction rather than an analytical one. During transient conditions like aeroelastic starting cycles or one-shot pitching/plunging motions, this knowledge gap is even more evident. This is because stall flutter is known to approach a limit cycle oscillation (LCO) (Culler and Farnsworth, 2019) where initial transients are effectively ‘washed out’ at sufficiently long timescales. Though, the transient dynamic stall processes that precede stable orbits are critically important from the purview of topics such as subcritical bypass transition, non-normality, and stall flutter suppression. Specifically, the LEV saturation and detachment phenomenon are sources of nonlinearity that have proven difficult to incorporate in reduced order models. Thus, a step-change in our understanding of these processes through high fidelity simulations is warranted for informing industry relevant objectives such as modelling, mapping operational envelopes, and designing flow control methods.

One promising method to achieve active load attenuation is by using aerofoil surface blowing. Several groups (Chen et al., 2022), (Müller-Vahl et al., 2015), (Seidel, Fagley and McLaughlin, 2015) have implemented constant blowing via surface jet to successfully suppress dynamic stall events in high amplitude aeroelastic pitching cycles. One mechanism responsible for this achievement is attributed to an injection of chordwise momentum which aids in delaying trailing-edge flow separation (Towne and Buter, 1994). This can be achieved via ‘tangential’ or nearly tangential jet orientations relative to the aerofoil surface such that the boundary layer attachment is maintained during high amplitude pitching motions. By delaying stall, the leading-edge shear layer development and vortex rollup are absent which promotes positive aerodynamic damping. Consequently, the stall flutter instability can either be maintained at a lower amplitude or suppressed entirely making it an effective method for fatigue mitigation in large-scale wind turbines. Though, this capability requires careful tuning of the jet intensity/momentum injection to be effective. For example, the required momentum injection from a constant blowing jet increases when transitioning between shallow and deep dynamic stall regimes to maintain boundary layer attachment (Gardner et al., 2023). This potentially makes active blowing quite expensive to apply in practice due to control system requisites such as the jet influx rate. Thus, it is crucial to explore an active jet control authority that satisfies a conservative influent capacity. Current efforts in this faculty have sought the use of adaptive (Müller-Vahl et al., 2016) or scheduled jet pulses (Kim and Jee, 2023) to disrupt the dynamic stall process without the need for constant blowing. While both methods demonstrated potential for unsteady aerodynamic load alleviation in flexible structures, the parameter space encompassing this control authority remains largely unexplored. In particular, the role of the jet orientation angle in scheduled blowing has not been thoroughly examined within the current literature. Consequently, an investigation into conservative control authorities that can achieve unsteady aerodynamic load attenuation but at a reduced injection cost, i.e. lower momentum influx or jet intensity, is fundamentally necessary for deployment in real-world engineering systems.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TJ Mechanical engineering and machinery T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments:	School of Science & Technology > Department of Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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