Preprints
https://doi.org/10.5194/wes-2021-12
https://doi.org/10.5194/wes-2021-12

  05 Mar 2021

05 Mar 2021

Review status: a revised version of this preprint is currently under review for the journal WES.

UNAFLOW: a holistic experiment about the aerodynamics of floating wind turbines under imposed surge motion

Alessandro Fontanella1, Ilmas Bayati2, Robert Mikkelsen3, Marco Belloli1, and Alberto Zasso1 Alessandro Fontanella et al.
  • 1Mechanical Engineering Department, Politecnico di Milano, Milano, Via La Masa 1, 20156, Italy
  • 2Maritime Research Institute Netherlands (MARIN), Wageningen, 6708 PM, The Netherlands
  • 3Technical University of Denmark (DTU), Department of Wind Energy, Lyngby, Denmark

Abstract. Floating offshore wind turbines are subjected to large motions because of the additional degrees of freedom offered by the floating foundation. The rotor operates in highly dynamic inflow conditions and this is deemed to have a significant effect on the aerodynamic loads, as well as on the wind turbine wake. Floating wind turbines and floating farms are designed by means of numerical tools, that have to model these unsteady aerodynamic phenomena to be predictive of reality. Experiments are needed to get a deeper understanding of the unsteady aerodynamics, and hence leverage this knowledge to develop better models, as well as to produce data for the validation and calibration of the existing tools. This paper presents a wind-tunnel scale-model experiment about the unsteady aerodynamics of floating wind turbines that followed a radically different approach than the other existing experiments. The experiment covered any aspect of the problem in a coherent and structured manner, that allowed to produce a low-uncertainty data for the validation of numerical model. The data covers the unsteady aerodynamics of the floating wind turbine in terms of blade forces, rotor forces and wake. 2D sectional model tests were carried to study the aerodynamics of a low-Reynolds blade profile subjected to a harmonic variation of the angle of attack. The lift coefficient shows an hysteresis cycle that extends in the linear region and grows in strength for higher motion frequencies. The knowledge gained in 2D sectional model tests was exploited to design the rotor of a 1/75 scale model of the DTU 10MW that was used to perform imposed surge motion tests in a wind tunnel. The tower-top forces were measured for several combinations of mean wind speed, surge amplitude and frequency to assess the effect of unsteady aerodynamics on the response of the system. The thrust force, that plays a crucial role in the along-wind dynamics of a floating wind turbine mostly follows the quasi-steady theory. The near-wake of the wind turbine was studied by means of hot-wire measurements, and PIV was utilized to visualize the tip vortex. It is seen that the wake energy is increased in correspondence of the motion frequency and this is likely to be associated with the blade-tip vortex, which travel speed is modified in presence of surge motion.

Alessandro Fontanella et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wes-2021-12', Anonymous Referee #1, 05 Apr 2021
    • AC1: 'Reply on RC1', Alessandro Fontanella, 14 Apr 2021
      • RC2: 'Reply on AC1', Anonymous Referee #1, 16 Apr 2021
        • AC2: 'Reply on RC2', Alessandro Fontanella, 16 Apr 2021
          • RC3: 'Reply on AC2', Anonymous Referee #1, 16 Apr 2021
  • RC4: 'Comment on wes-2021-12', Anonymous Referee #2, 27 Apr 2021
    • AC3: 'Reply on RC4', Alessandro Fontanella, 04 May 2021
      • RC5: 'Reply on AC3', Anonymous Referee #2, 04 May 2021
        • AC4: 'Reply on RC5', Alessandro Fontanella, 05 May 2021
          • RC6: 'Reply on AC4', Anonymous Referee #2, 05 May 2021

Alessandro Fontanella et al.

Alessandro Fontanella et al.

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Short summary
Floating wind turbines undergoes complex aerodynamic phenomena that are not completely understood. This paper presents an scale-model wind tunnel experiment about the investigated the unsteady aerodynamics of floating turbines with a radically different approach than previous experiments. The problem was studied under different aspects, from airfoil aerodynamics to wake.