Experimental analysis of radially resolved dynamic inflow effects due to pitch steps

Abstract

Abstract. The dynamic inflow effect denotes the unsteady aerodynamic response to fast changes in rotor loading due to a gradual adaption of the wake. This does lead to load overshoots. The objective of the paper was to increase the understanding of that effect based on pitch step experiments on a 1.8 m diameter model wind turbine, which are performed in the large open jet wind tunnel of ForWind – University of Oldenburg. The flow in the rotor plane is measured with a 2D laser Doppler anemometer, and the dynamic wake induction factor transients in axial and tangential direction are extracted. Further, integral load measurements with strain gauges and hot-wire measurements in the near and close far wake are performed. The results show a clear gradual decay of the axial induction factors after a pitch step, giving the first direct experimental evidence of dynamic inflow due to pitch steps. Two engineering models are fitted to the induction factor transients to further investigate the relevant time constants of the dynamic inflow process. The radial dependency of the axial induction time constants as well as the dependency on the pitch direction is discussed. It is confirmed that the nature of the dynamic inflow decay is better described by two rather than only one time constant. The dynamic changes in wake radius are connected to the radial dependency of the axial induction transients. In conclusion, the comparative discussion of inductions, wake deployment and loads facilitate an improved physical understanding of the dynamic inflow process for wind turbines. Furthermore, these measurements provide a new detailed validation case for dynamic inflow models and other types of simulations.

A comparison of dynamic inflow models for the Blade Element Momentum method

“…The 2D nature of the BL correction combined with the fact that it is only applied to the airfoil coefficients and not on induced velocities are indicated as the most likely causes for this. As a result of the coupling between the axial and tangential BEM equations, a tangential induction peak has been noticed as well confirming recent experimental evidence from Berger et al (2021). For the first time, the impact of dynamic inflow models on a fatigue design load case has been assessed quantitatively.…”

“…2.3). A tangential induction peak was also observed experimentally by Berger et al (2021), and a similar trend can be recognized in the AWSM results as well once the initial oscillations induced by the shed vorticity are damped out.…”

Experimental analysis of the dynamic inflow effect due to coherent gusts

“…For this axial and uniform flow the wake induction factors thus are equal to the induction factors. The method was developed for steady operation, however, it was shown in a prior study in Berger et al (2021a) to be also applicable to study the transient changes in induction factors, maintaining axial and uniform conditions.…”

“…The dynamic inflow effect due to a pitch step should be described by two time constants (Pirrung and Madsen, 2018;Yu et al, 2019;Berger et al, 2021a). The faster time constant τ fast can be attributed to the sudden change in the trailed vorticity and the slower time constant τ slow to the effect of the wake inertia.…”

“…The wake of twelve revolutions is considered and the azimuthal discretisation is 10 • . Both model setups were already used in Berger et al (2020) and showed a good match to an experimental dynamic inflow focused pitch step experiment(Berger et al, 2021a) with MoWiTO. For both models neither unsteady profile aerodynamics nor structural flexibility are considered, but only the aerodynamic degrees of freedom at constant rotation.…”