Abstract
The dynamic inflow effect describes the unsteady aerodynamic response to fast changes in rotor loading, due to the inertia of the wake. For pitch actuation and fast rotor speed changes this effect leads to load overshoots. The effect is suspected to be also relevant for gust situations, however this was never shown. The objective of the paper is to proove the dynamic inflow effect due to gusts and compare dynamic inflow engineering models to corresponding measurements. A 1.8 m diameter model turbine is used in the large wind tunnel of ForWind – University of Oldenburg with an active grid to impress rotor uniform gusts on the flow. The campaign features load and velocity measurements of the axial flow in the rotor plane. The unsteady dynamic inflow effect is investigated by comparing two experimental cases. Firstly, a dynamic measurement during a gust situation is performed. Secondly, quasi-steady loads and axial velocities are interpolated from a steady characterisation experiment according to the gust wind speed. By comparing both cases, the influence attributed to the dynamic inflow effect is isolated. Further comparisons to a typical Blade Element Momentum code and a higher fidelity Free VortexWake Model are performed. Based on analytical considerations an improvemed formulation of the Øye dynamic inflow model is proposed. The experiment shows a dynamic inflow effect due to gusts in the loads and axial velocity measurements. It leads to a reduction in load and axial velocity amplitudes and consequently also lower fatigue loading. The higher fidelity model shows a similar impact of the dynamic inflow effect. In contrast, the commonly used Øye engineering model in the BEM code predicts an increase in load amplitude and thus higher fatigue loads. The improved Øye engineering model however catches the observed dynamic inflow effect due to gusts in accordance to the experiment and FVWM simulations. An amplification of induced velocities, seen in the experiment and FVWM simulation, causes the reduced load amplitudes. Therefore, classic dynamic inflow models, which filter the induced velocity, cannot predict the effect. The proposed improvement to additionally consider the wake velocity for the filter of the dynamic inflow engineering model, proves to be a straight forward but also effective modification. In conclusion, these new experimental findings on dynamic inflow due to gusts and improvements to the Øye model enable improvements in wind turbine design by catching the related lower fatigue loads.
Highlights
The dynamic inflow phenomenon is an unsteady aerodynamic effect relevant for helicopters (Peters, 2009) and wind turbines (Snel and Schepers, 1994)
A comparison of thrust and induced velocity for the sine gust of 325 Blade Element Momentum (BEM) and Free Vortex Wake Methods (FVWM) simulations to the experiment is presented
The uncertainty band around the wind field 330 shows the 95% Confidence Intervals (CI) of the sine wind field altered to higher error at few positions to enclose the mean wind vector based on the three standstill measurements in the rotor plane
Summary
The dynamic inflow phenomenon is an unsteady aerodynamic effect relevant for helicopters (Peters, 2009) and wind turbines (Snel and Schepers, 1994) It is considered for a fast load variation through a blade pitch step or fast change in rotor speed. Used examples are the ECN model (Snel and Schepers, 1994) in Phatas and the ECN ( TNO) Aero-Module, the recent DTU model (Madsen et al, 2020) in HAWC2 and the Øye model (Snel and Schepers, 1994) in GH Bladed and OpenFAST. They all have the main working 35 mechanism that they filter the induced velocitiy based on time constants. One time constant is used for the ECN model and two time constants for the DTU and Øye model
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