Comparison of a radially resolved dynamic inflow pitch step experiment to mid-fidelity simulations and BEM

Berger, Frederik; Höning, Leo; Herráez, Iván; Kühn, Martin

doi:10.1088/1742-6596/1618/5/052055

Cited by 4 publications

(6 citation statements)

References 8 publications

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“…The largest difference between the blade and inter-blade position is seen for r=R ¼ 0:9. At this radial position, the inter-blade dynamic is close to the one observed at r=R ¼ 0:6, in agreement with the experimental results of Berger et al, 21 while the induced velocity at the blade position shows a large initial transient followed by a slower dynamic. The amplitude of the transient is related to the tip speed ratio, as shown in Figure 15, with the same ϵ and e cv as before.…”

Section: Radial Behaviorsupporting

confidence: 91%

“…This is possible by the fact that the two phenomena have different timescales: the local aerodynamics one is proportional to the local airfoil chord divided by the local relative velocity, while the wake dynamics one is proportional to the rotor radius divided by the free wind speed. The duration of the ramp is 0.47 R / U ∞ similar to the pitch duration in the experiment by Berger et al 21 …”

Section: Dynamics Study With Free Vortex Wake Modelsupporting

confidence: 70%

“…Comparison of tip vortex axial and radial positions for different tip speed ratios for the MEXICO wind turbine for experimental results28 and FVW results of CACTUS dynamics one is proportional to the rotor radius divided by the free wind speed. The duration of the ramp is 0.47R/U ∞ similar to the pitch duration in the experiment by Berger et al 21 Figure 2 shows the variation of the thrust coefficient after the three perturbations leading to the same final thrust coefficient. Both the rotor speed and blade pitch change induce a dynamic behavior of the thrust coefficient.…”

Section: Thrust Coefficient Increasementioning

confidence: 59%

“…Until recently, the experimental knowledge on the radial behavior of the dynamic inflow effect was only from blade pressure taps measurements, such as the NREL Phase VI blade pitch measurements 17 and the MEXICO rotor speed steps 22 . Recently, Berger and Kühn 20 have made measurements of the velocity in the near wake as well as phase‐locked flow measurements on a line between two blades in the rotor plane 21 for a rotor undergoing a blade pitch change. This gives a valuable insight into how the dynamic inflow effect develops radially.…”

Section: Analytical Model Of the Aerodynamicsmentioning

confidence: 99%

“…The velocity variations are slightly faster closer to the tip of the blade. More insight into the radial behavior of the dynamic inflow effect was given by Berger et al, 21 who measured the axial‐induced velocity between two blades in the rotor plane. Similar temporal dynamics were found for a position near the edge of the rotor disk and around half the blade span.…”

Section: Introductionmentioning

confidence: 99%

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The aerodynamics of a blade pitch, rotor speed, and surge step for a wind turbine regarding dynamic inflow

2022

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Floater motions introduce unsteadiness in the aerodynamics of floating offshore wind turbines. The aerodynamics of a wind turbine after three perturbations are studied: a blade pitch step, a rotor speed step for which dynamic inflow is expected, and a surge velocity step. The free vortex wake method and an analytical helical vortex model based on the Joukowsky rotor model are used to study the dynamic behavior of the induced velocity at the blades. As expected, the dynamic inflow effect is clear for the blade pitch and rotor speed changes, but for a surge velocity step, the models show that very little dynamic inflow effect takes place because the velocity induced by the vortex helix is not significantly modified: the tip vortex helix circulation change is partially compensated by the geometry change of the helix. For the rate of change, the velocities induced on the rotor by the vortex helix for the pitch and rotor speed changes show a rapid adjustment at the blade tip, with a slower change throughout the rest of the blade and at the center of the rotor.The convection velocity of the tip vortices is shown to be the main variable of the temporal evolution of the dynamic inflow effect.

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Section: Radial Behaviorsupporting

confidence: 91%

Section: Dynamics Study With Free Vortex Wake Modelsupporting

confidence: 70%

Section: Thrust Coefficient Increasementioning

confidence: 59%

Section: Analytical Model Of the Aerodynamicsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The aerodynamics of a blade pitch, rotor speed, and surge step for a wind turbine regarding dynamic inflow

2022

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

et al. 2021

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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.

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Experimental analysis of the dynamic inflow effect due to coherent gusts

et al. 2022

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Abstract. The dynamic inflow effect describes the unsteady aerodynamic response to fast changes in rotor loading due to the inertia of the wake. Fast changes in turbine loading due to pitch actuation or rotor speed transients lead to load overshoots. The phenomenon is suspected to be also relevant for gust situations; however, this was never shown, and thus the actual load response is also unknown. The paper's objectives are to prove and explain the dynamic inflow effect due to gusts, and compare and subsequently improve a typical dynamic inflow engineering model to the measurements. An active grid is used to impress a 1.8 m diameter model turbine with rotor uniform gusts of the wind tunnel flow. The influence attributed to the dynamic inflow effect is isolated from the comparison of two experimental cases. Firstly, dynamic measurements of loads and radially resolved axial velocities in the rotor plane during a gust situation are performed. Secondly, corresponding quantities are linearly interpolated for the gust wind speed from lookup tables with steady operational points. Furthermore, simulations with a typical blade element momentum code and a higher-fidelity free-vortex wake model are performed. Both the experiment and higher-fidelity model show a dynamic inflow effect due to gusts in the loads and axial velocities. An amplification of induced velocities causes reduced load amplitudes. Consequently, fatigue loading would be lower. This amplification originates from wake inertia. It is influenced by the coherent gust pushed through the rotor like a turbulent box. The wake is superimposed on that coherent gust box, and thus the inertia of the wake and consequently also the flow in the rotor plane is affected. Contemporary dynamic inflow models inherently assume a constant wind velocity. They filter the induced velocity and thus cannot predict the observed amplification of the induced velocity. The commonly used Øye engineering model predicts increased gust load amplitudes and thus higher fatigue loads. With an extra filter term on the quasi-steady wind velocity, the qualitative behaviour observed experimentally and numerically can be caught. 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 less conservative fatigue loads.

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Comparison of a radially resolved dynamic inflow pitch step experiment to mid-fidelity simulations and BEM

Cited by 4 publications

References 8 publications

The aerodynamics of a blade pitch, rotor speed, and surge step for a wind turbine regarding dynamic inflow

The aerodynamics of a blade pitch, rotor speed, and surge step for a wind turbine regarding dynamic inflow

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

Experimental analysis of the dynamic inflow effect due to coherent gusts

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