Abstract
Abstract. Dynamic stall phenomena carry the risk of negative damping and instability in wind turbine blades. It is crucial to model these phenomena accurately to reduce inaccuracies in predicting design driving (fatigue and extreme) loads. Some of the inaccuracies in current dynamic stall models may be due to the fact that they are not properly designed for high angles of attack and that they do not specifically describe vortex shedding behaviour. The Snel second-order dynamic stall model attempts to explicitly model unsteady vortex shedding. This model could therefore be a valuable addition to a turbine design software such as Bladed. In this paper the model has been validated with oscillating aerofoil experiments, and improvements have been proposed for reducing inaccuracies. The proposed changes led to an overall reduction in error between the model and experimental data. Furthermore the vibration frequency prediction improved significantly. The improved model has been implemented in Bladed and tested against small-scale turbine experiments at parked conditions. At high angles of attack the model looks promising for reducing mismatches between predicted and measured (fatigue and extreme) loading, leading to possible lower safety factors for design and more cost-efficient designs for future wind turbines.
Highlights
Wind turbines operate in highly unsteady aerodynamic environments (Leishman, 2002)
Snel (1997) derived a dynamic stall model based on the work of Truong (1993), who proposed that the dynamic lift coefficient can be distinguished into two parts, namely cl1 and cl2 : cl,dyn = cl,steady + cl1 + cl2
The improved Snel model from this paper will be tested against the New Model Experiments In Controlled Conditions (MEXICO) data sets of Table 3 (β = wind direction, θ = pitch angle) to see if the additions and changes to the model have a positive influence on the accuracy
Summary
Wind turbines operate in highly unsteady aerodynamic environments (Leishman, 2002). For design and certification, design load cases (DLCs) have been set which describe the conditions that wind turbine designs have to withstand (DNV GL, 2016). Some of the design driving DLCs are those for parked and idling conditions where wind turbine blades will experience high angles of attack (AoAs), leading to (dynamic) stall behaviour (Schreck et al, 2000). Dynamic stall is a phenomenon leading to larger variations in lift, drag, and pitching moments on the aerofoil than would be observed during steady operation (Choudry et al, 2014) This creates larger aerodynamic forces on the blades than expected during steady conditions (Leishman, 2002). A single large dynamic stall vortex will no longer be shed, but rather multiple periodic vortices from both the leading and trailing edge will be shed This will induce time-varying loads on the blades (Riziotis et al, 2010).
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