Dynamic stall landmark events and time scales on a vertical-axis wind turbine blade*

Vertical-axis wind turbines (VAWT) enable urban and high-density offshore wind power applications. The occurrence of dynamic stall on the blades of VAWT has hampered their industrial deployment. To improve the efficiency and lifespan of these turbines, we aim to develop data-driven models and control strategies that take into account the timing and duration of subsequent events in the unsteady flow development. In this talk, we present the chain of events that leads to dynamic stall on a VAWT blade and quantify the influence of the turbine operation conditions on the duration of the individual flow development stages. These timescales are characterised using time-resolved flow and unsteady load measurements of a wind turbine model undergoing dynamic stall for a wide range of tip-speed ratios. We use proper orthogonal decomposition to identify dominant flow structures and distinguish six characteristic stall stages: the attached flow, shear-layer growth, vortex formation, upwind stall, downwind stall and flow reattachment stage. Dynamic stall stages are also identified based on aerodynamic force measurements. These findings can help improve low-order modelling of dynamic stall on vertical-axis wind turbines and inform flow control strategies.