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

. 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 5 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 10 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 15 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. a high-ﬁdelity 1/75 scale model of is used to study the of imposed and the are compared to simulations to assess the prediction capabilities of AeroDyn with respect to FOWTs. and experimental some suggested the to study the further. In Bayati et al. (2017b), a second test campaign is carried out to study by means hot-wire measurements the near-wake of the same scale model under imposed surge motion.


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Floating offshore wind is receiving a growing interest as it makes it possible to harvest the wind energy resource of deep waters, which cannot be exploited a cost competitive price with conventional bottom-fixed wind turbines. Floating offshore wind turbines (FOWTs) are subjected to large and low-frequency motions that are the cause of unsteady aerodynamics effects.
The rotor of an FOWT operates in dynamic inflow conditions de Vaal et al. (2014), mainly for two reasons: the platform motion 2D sectional model tests that were carried out at the Technical University of Denmark (DTU) Red wind tunnel, to characterize the aerodynamic coefficients of the SD7032 airfoil, used in the scale model turbine blades. Section 4 describes the full-turbine experiment, with emphasis on the wind turbine scale model and the measurements that were carried out. Section 5 reports the main findings of the full-turbine experiments with surge motion, in particular those about the rotor thrust force, the energy content of the near-wake, and the tip-vortex structure. Section 6 draws the conclusions and gives some recommendations for 65 future research.

Concept and design of the experiment
The rotor of an FOWT is often working in strong unsteady conditions, because of the large rigid-body motions that arise because the low-compliance of the floating platform and wave excitation. The UNAFLOW projects studied the unsteady behavior of an FOWT rotor, and the core of the experimental activity was an extensive wind tunnel test campaign with a high-70 fidelity wind turbine scale model subjected to imposed surge motion. The wind turbine was a 1/75 model of the DTU 10MW Bak et al. (2013), with a 2.38m diameter rotor designed to match the thrust and power coefficient of the reference wind turbine.
The purpose of the wind tunnel experiment was tp provide a large dataset of rotor integral loads and wake measurements for several wind-turbine operating and motion conditions, selected to be realistic for a multi-megawatt FOWT. 2D sectional airfoil experiments were carried out prior to the full-turbine tests, to guide the selection of the motion conditions for the turbine scale 75 model, and to support the creation of numerical models of the experiment.

Wind conditions
The experiment considered three operating conditions which are reported in table 1. No closed-loop control strategy is utilized, and the rotor speed and the collective pitch angles were fixed. In the first two conditions, the wind turbine is operated at the optimum value of tip-speed ratio (TSR) and power is extracted with maximum efficiency (i.e. the maximum power coefficient 80 is achieved). Being the TSR the same, the angle of attack (AoA) along the blade is the same in the RATED1 and RATED2 conditions. In the second condition, the TSR is lower and the collective pitch angle is increase, to get a lower power coefficient. Experiments were carried out in smooth flow conditions, and the turbulence index across the test section height was approximately 2%.  Figure 1 shows the Reynolds number along the span of the wind-turbine scale-model blade in the three operating conditions of Table 1. The Reynolds number is over 80k for most of the blade span in RATED2 and ABOVE conditions, and it drops to 85 50k in RATED1 wind speed. 2D airfoil sectional model experiments were carried out to measure the aerodynamic coefficients of the blade airfoil for a range of Reynolds number close to those experience by the blade in full-turbine tests.

Motion conditions
The aim of the experiment was to investigate the unsteady aerodynamics of an FOWT rotor associated with the rigid-body motion of the support platform. The unsteady aerodynamic problem is a complex multi-physics subject: the platform motion 90 is driven by the wave excitation and depends on the characteristics of the platform itself. To keep the focus on the aerodynamic problem, some simplifying assumptions were made for what concerns the wave simulation and the resulting motion. The wind turbine model was forced to move in the surge direction and the other platform motions were not considered. The surge motion was selected because it produces an along-wind motion of the wind turbine, which is in turn cause of a large variation of the wind speed seen by the rotor. Moreover, in the surge motion, any point of the wind turbine rotor moves with the same velocity.

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This simplifies the modeling of the aerodynamics as the effective wind speed is uniform across the rotor.
The surge motion x considered in the experiments is mono-harmonic: where A s and f s are the amplitude and frequency of motion respectively. The experiment investigated several mono-harmonic motions, obtained from the combination of different values of amplitude and frequency.

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Seven frequencies were selected in the range [0.125 -2] Hz (model scale) which corresponds to the low-frequency range for semi-sub and spar platforms. Large motions are expected in this range of frequencies, as the rigid-body motion modes are excited in resonance. The maximum frequency investigated in the full-turbine experiment was limited to 2 Hz to avoid exciting the first tower fore-aft flexible mode. The effect the frequency of motion has on the rotor aerodynamics is qualitatively described by the wake reduced-velocity parameter introduced in Bay (2017): where D is the rotor diameter, which is adopted as reference length, since it is widely used to describe the wake interaction in wind turbines. A high V * w means the air particles flow across the wind turbine in a short time, its path is not influenced by the wind turbine motion and the flow is quasi-steady. The lower the wake reduced velocity, the higher the unsteady effects.
Four amplitudes were tested for each combination of frequency and mean wind speed V . The selection of the values of 110 amplitude was based on the maximum surge velocity: The surge motion causes a variation of the angle of attack along the blades that is, in first approximation, proportional to: The four amplitude values were initially selected to achieve, for any pairing of wind speed and motion frequency, ∆V * = 115 1/20, 3/80, 1/40, 1/80. At low frequencies, the desired amplitude of motion was bigger than the stroke of the hydraulic actuator and it was reduced in reason of this constraint. Moreover, the amplitudes of the 1 Hz-frequency cases were increased by 50% to investigate a larger range of ∆V * . Figure 2 reports the average AoA along the blade span in the operating conditions of Table 1 and the maximum variation caused by the unsteady inflow associated with harmonic surge motion. Most of the blade sections work far from the stall front, 120 and the surge motion causes a variation of the AoA of some degrees. An additional set of 2D sectional model tests was carried out to characterize the unsteady aerodynamic behavior of the blade airfoil for a harmonic variation of the AoA. The amplitude and frequency of the latter covered the AoA variation experience by the full-turbine blades because of the surge motion.  design of the 3D experiment. In the UNAFLOW project there was not a specific effort to reproduce numerically the unsteady airfoil behavior, however, a wide dataset of unsteady polars are provided as project output. This data could be used both to validate unsteady airfoil aerodynamic models Boorsma and Caboni (2020) or unsteady CFD computation aiming to catch lift and drag oscillation due to dynamic variation of AoA.
The setup for 2D experiments is depicted in Figure 3. The 2D wing model, of 130mm chord, was fitted with a pressure loop 135 (with 32 taps) at midspan that was used to measure the lift force from the pressure distribution, and a single-component force transducer that provided an additional lift force gage. The profile drag was obtained by means of a down-stream wake rake.
The profile was mounted on a turning table that set the angle of attack.

Steady force coefficients
The force coefficients were measured for chord Reynolds number equal 50k, 60k, 75k, 100k, 150k, 200k and stepping through 140 the AoA range from -10 • to 25 • . The Reynolds range covers the flow condition experienced by the blade of the wind turbine scale model (see Figure 1). Measurements were repeated in smooth flow (turbulence intensity lower than 0.1%), and with an increased free-stream turbulence that was obtained placing three thin wires (0.15mm diameter) about four chords upstream the profile. The slight increase in turbulence intensity, avoids the formation of a laminar separation bubble by tripping the boundary layer. This inflow condition is deemed to be more realistic and closer to what is experienced by the wind turbine scale model.

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The lift and drag force coefficients are shown in Figure 4. The lift coefficient shows a non-linear behavior in correspondence of the stall AoA, which is clearly present at Re 50k, and becomes less evident for increasing Re values. The increased turbulence results in a smoother drag coefficient for any Re value. The effect on the lift coefficient is to smear out the non-linearity, and this is specially evident for Re values lower than 100k.

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The lift and drag force coefficient were also measured with an unsteady pitching of the airfoil. The conditions of the 2D experiment reflected those of the full-turbine blade. The experiments investigated the profile behavior for chord-Reynolds number of 50k, 100k, 150k, and a static AoA of 0, 3, 6, 9, 10, 12, 15 degrees. The amplitude and frequency of the sinusoidal pitching reflects the AoA variation produced by the imposed surge motion in the wind turbine scale model. The AoA amplitude was 0.5, 1, 2, 5 and the frequency of 0.25, 0.5, 1, 2, 3 Hz.

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An example of the results is reported in Figure 5, with reference to the inflow with increased turbulence, a chord Re of 50k, and a sinusoidal variation of the AoA of 5 degrees amplitude and different frequencies. An hysteresis cycle is always present when the airfoil is pitched in correspondence of the stall AoA, and increasing the motion frequency, the strength of this effect is increased. The amplitude of the hysteresis cycle is instead small in the linear region (i.e. for AoA lower than the stall value), where most of the wind turbine model blade operates (see Figure 2).

The full-turbine experiments
The UNAFLOW experiments were carried out at the Politecnico di Milano wind tunnel (Galleria del Vento Politecnico di Milano, GVPM GVP (2019)). The facility is a closed-loop subsonic wind tunnel and the flow is generated by 14 fans. UNAFLOW tests were carried out in the low-speed test chamber, which has a cross section of 3.84x13.84 m.
The wind turbine scale model was mounted on the test rig shown in Figure 6. The test rig is formed by a slider which is 165 driven by a first hydraulic actuator, and is utilized to simulate the surge motion. On top of the slider, there is a second hydraulic actuator which is connected to the base of the wind turbine tower through a slider-crank mechanism. The second actuator was utilized to tilt the wind turbine so to have the rotor plane normal to the ground. In this way, the periodic effects due to the rotor tilt angle were avoided.
The experiment investigated three wind turbine operating conditions and several types of surge motion. The tested conditions 170 are reported in Tables A1-A3, while the rationale behind their selection is explained in the rest of this section.

The wind turbine scale model
The wind turbine is a 1/75 scale model of the DTU 10MW Bak et al. (2013), that was designed within the LIFES50+ EU H2020 project Bayati et al. (2017). The rotor was designed based on the performance-scaling approach Kim (2014) Table 2.

Measurements
Several measurements were carried out during the experiments. The undisturbed wind velocity V was measured by a Pitot tube that was located 5 m upstream the wind turbine, at 1.5 m height from the floor. An LVDT sensor provided the feedback for the control system of the surge hydraulic actuator. In parallel, the wind turbine surge motion was measured by means 180 of a MEL M5L/200 laser sensor. The tower-top forces were measured by a 6-components force transducer. Two PCB MEMS accelerometers were fixed in correspondence of the tower-base to measured the x and z acceleration; another two were mounted on the nacelle to measure the x and y acceleration. All instruments were sampled synchronously with a frequency of 2000 Hz.
In few selected test cases, the wake of the wind turbine was scanned by tri-axial hot-wire probes. In an even smaller sample of test cases, PIV measurements were carried out to describe the wake flow structure.

Tower-top forces
The six-components constraint force at the tower-top was measured by an ATI Mini45 SI-145-5 force transducer. Such measurements cannot be used directly to evaluate the aerodynamic forces because they also include the rotor-nacelle assembly (RNA) weight and inertia. To isolate the aerodynamic fraction of the force measurement, another set of tests was carried out.
Each motion condition of the wind tests (SIW) of Tables A1-A3 was tested without wind and with fixed rotor (NOW). In the 190 NOW tests, only inertia and weight forces were measured.
The experiment focused on the thrust force, because it is a driving load in FOWTs. The aerodynamic thrust force was obtained according to this procedure, which is based on the assumption that the structural loads depend only on the type of motion and are the same in the NOW and SIW tests: 1. for any given motion condition, the SIW time histories are synchronized with the corresponding NOW. The reference 195 signal for the procedure is the LVDT position; 2. SIW and NOW time histories are trimmed, keeping the maximum number of full periods of motion; 3. the aerodynamic forces are obtained subtracting the NOW time series from the SIW time series: The procedure strongly relies on the assumption that the tower and blades behave as rigid bodies. This is in general true 200 around the frequency of the imposed surge motion, which was in any case lower than the natural frequencies of the wind turbine components and, in particular, of the first tower fore-aft mode (6.5 Hz). For higher frequencies, the dynamic amplification associated with tower flexibility cannot be neglected anymore, and the results obtained based on the inertia-subtraction procedure are not reliable. In Mancini et al. (2020), an alternative inertia-subtraction algorithm is proposed to better deal with tower flexibility. 4.4 Hot-wire wake measurements An automatic traversing system was used to measure the three-component velocity in the wake of the wind turbine. The system, visible in Figure 7, consists of a moving arm mounting two hot-wire probes. Measurements were carried out with the traversing system spanning across the Y-Z plane (cross-wind, CW) or across the X-Z plane (along-wind, AW).
In the CW case, the measurement plane was 2.3D (5.48 m) downwind the wind turbine; this was the furthest distance given 210 the size of the test chamber and it is considered to be part of the near-wake region Vermeer et al. (2003). One of the probe was mounted at hub-height, the other 0.2 m below. The probes were moved in the cross-wind direction, ranging from -1.6 m to 1.6 m with respect to the hub position, with a distance of 0.1 m between subsequent points. CW measurements were carried out both for the Rated2 and Above conditions, with and without surge motion.
In the AW case, the probes were mounted at hub-height, one next to the other: the first at y = 0.7 m, the second at y = 0.9 215 m. The probes were moved in the along-wind direction, ranging from 2.18 m to 5.48 m downwind the hub location, with a distance of 0.33 m between subsequent points. AW measurements were carried out only for the Rated2 condition, with and without surge motion. 5 Key findings of the full-turbine experiments

Rotor thrust force
The analysis of the thrust force is based on a simplified description of the wind turbine rotor, that focus on the integral forces rather than considering the single blades. The rotor produces a thrust force: where ρ is the air density, R the rotor radius, C T the thrust coefficient and V the undisturbed wind speed. The thrust coefficient is set by the wind turbine operating condition, which is defined by the TSR λ and the collective pitch angle β: The thrust force can be linearized based on a first-order Taylor expansion: where T 0 is the steady-state thrust force; ∆V , ∆β and ∆ω are the variation of rotor speed, collective pitch angle and wind speed from their respective steady-state value; K V T , K βT and K ωT are the partial derivatives of thrust with respect to wind speed, collective pitch and rotor speed Bianchi et al. (2007). In the present case, collective pitch and rotor speed are fixed, so: 245 with: where λ 0 is the steady-state TSR and C T,0 the steady-state thrust coefficient. The wind speed seen by any point of the rotor when the flow is smooth and the wind turbine moves in the surge direction is:

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where V 0 is the mean wind speed.
The thrust force is The thrust force variation induced by the surge motion is predicted based only on the wind turbine steady-state operational data. Equation 12 is therefore herein referred to as quasi-steady theory. According to quasi-steady theory (QST), the thrust 255 force variation depends only on the surge velocity.
The focus of the experimental force measurements is the surge-frequency thrust force. At the surge frequency, the effects of tower flexibility are small, and the thrust force is extracted from the tower-top force measurements based on the inertiasubtraction procedure presented in Section 4.3. The surge-frequency thrust force is:

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where |∆T | is the amplitude of the thrust force at the surge frequency and φ is the phase with respect to the surge displacement.
In general, the surge-frequency thrust force has a component in opposition of phase to the surge velocity, and one in opposition of phase to the surge acceleration. According to the QST model of Equation 12, the thrust force is perfectly in aligned to the surge velocity.
The adherence of the thrust force measurements to the QST model is studied based on the unsteady thrust force coefficient: This non-dimensional representation is useful to make the outcomes of the experiment comparable to other studies. According to QST: and the unsteady thrust coefficient is: where f r = 1/V * is the reduced surge-frequency, A r = A s /D the reduced surge-amplitude, and: The unsteady thrust coefficient for the motion conditions investigated in the experiment is shown on the left of Figure 8. In the figure C ∆T is divided by A r and is reported as function of f r . In this plot, the QST prediction corresponds to a straight increases above -90 • , and by C ∆T , which shifts away from the QST line. A trend appears in C ∆T , but it is less evident in the phase φ, which is scattered in a range of ±10 • around -90 • . This uncertainty is related to the fact that part of the thrust force opposed to the surge acceleration is in any case very small, and difficult to measure.

Hot-wire wake measurements
The wake shape at hub-height is captured by the mean velocity deficit. The deficit for all the conditions that were investigated 285 with hot-wire measurements is shown in Figure 9. The reduction of axial velocity is always higher in RATED2, where the wind turbine is operated at the maximum power coefficient, compared to ABOVE. The wake is also slightly asymmetric with respect to the hub. For any condition the velocity deficit is larger on the left side compared to the right. When the wind turbine moves, the wake is slightly narrower, meaning there is more energy in its outer region. Outside the boundaries of the rotor, a certain speed up is observable, which is caused by the wind tunnel blockage. Concerning the surge motion, it appears evident that it 290 does not significantly change the mean wake deficit. Even if there are some major differences in the experiment (a porous disk was used to emulate the wind turbine rotor, measurements were carried out at a distance of 4.6D, the inflow was turbulent), this is in agreement with Schliffke et al. (2020), where it is evidenced that the surge motion does not affect the shape of the vertical wake profile. The frequency content of the wake at hub-height is studied plotting the PSD of the three velocity components measured in 295 different points across the rotor. Figure 10 compares the spectral plots for the RATED2 case without and with surge motion, in particular with reference to the case of f = 1 Hz, A = 0.035 m. The energy content is concentrated in the outer region of the rotor and it is reasonably related to the blade-tip vortex. This distribution of energy is common also to any other RATED2 case.
The asymmetry seen in the velocity deficit is found also in the spectra, and it is particularly evident in the vertical component W , which is associated with the rotor angular speed. Looking at the unsteady case, a strong harmonic component is visible 300 at the frequency of motion, which is absent in the steady case. The surge-frequency harmonic is more evident in the axial velocity, compared to the other two velocity components. Another strong harmonic component is visible close to f = 4 Hz, the 1P frequency, and it is associated with a slightly different pitch angle setting for the three blades.
The same analysis is carried out in Figure 11 for what concerns the ABOVE condition. In this case, energy is concentrated in the inner region of the rotor, witnessing the presence of a strong blade-root vortex. Also in this case, the 1P component is 305 visible, at f = 4.417 Hz and the wake is slightly asymmetric. In the case with surge motion, an harmonic becomes evident at the surge frequency. The harmonic is equally present in the three velocity components.  summing the PSD for all the measurement points: where U y,f is the PSD of the axial velocity at point y evaluated at frequency f , n y is the number of points where the wake speed is measured, and n f the number of discrete frequencies where the PSD is computed. U 0 y,f denotes the PSD for the steady case with the same mean wind speed of U y,f . This choice, allows to understand how the wake energy content changes 315 because of the surge motion. The space-averaged PSD U f for the investigated conditions is shown in Figure 12. In the steady case, energy is evenly spread below 1 Hz, and decreases smoothly increasing frequency. A peak is always present at the 1P frequency. The energy is greater in RATED2 compared to ABOVE, and also the 1P peak for the above cases is much lower than for the RATED2 cases. The spectrum for any of the unsteady cases is similar to the spectrum of the corresponding steady case, except for a peak at the surge frequency: energy is transferred in the wake by the surge motion. Similar findings, but for 320 the far-wake of a porous disk, are reported in Schliffke et al. (2020). Looking at the PSD of Figure 12 it is also interesting to notice that, for a surge frequency up to 1 Hz, the amplitude of the surge-frequency peak is proportional to ∆V , but not linearly.
The energy increment in the 2 Hz case is much lower than for any other motion condition with similar ∆V . The surge motion also amplifies the 1P harmonic and the amplification in RATED2 is much higher than in ABOVE conditions. The frequency-averaged PSD defines how energy is distributed across the rotor and it is computed, for any measurement 325 point, as the frequency-integral of the corresponding PSD: In this case, U y,f is used for normalization. The frequency-averaged PSDs U y are reported in Figure 13. The energy space distribution is not affected by the type of motion, and it is strictly characteristic of the operating condition. In RATED2 conditions, energy is concentrated in the outer region of the rotor and it is associated with the blade-tip vortex. In ABOVE 330 conditions, most of the energy is in the central part of the rotor, where the blade-root vortex is, whereas the contribution of the tip vortex is lower. More energy is present on the left than on the right of the hub and this is particularly evident in the ABOVE cases. The fact the shape of U y remains the same, suggests the surge motion adds energy evenly across the rotor. In detail, it increases the axial travel velocity of vortices.

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PIV combined with a realistic wind turbine scale model, makes it possible to appreciate the structure of the wind turbine wake and investigate how it is affected by the surge motion. The focus is on the blade-tip vortex as it holds most of the wake energy.
The blade-tip vortex is visualized by means of the magnitude of the vorticity, obtained from the in-plane velocity components (u, v).  conditions. The blade-1 azimuth is always zero degrees. x/R and z/R are the axial and vertical distance from rotor rotor apex normalized by rotor radius. The origin of the axes is coincident with the rotor apex when the wind turbine is in the zero-surge position.
Hub-height hot-wire anemometry and PIV surveys were carried out to investigate how the wake of the wind turbine is affected by the surge motion. Hot-wire measurements show that the mean hub-height velocity deficit with surge motion is the 365 same as with still wind turbine. On the other side, the spectral content of the wake clearly shows the trace of the imposed surge motion. In particular, the surge motion adds energy to the wake, and the energy increment is proportional to the maximum surge velocity. PIV measurements, which are phase-locked to the wind turbine position in the surge cycle and to the rotor azimuth, show that the surge motion modifies the travel speed of the blade-tip vortex, that varies periodically at the frequency of the surge motion.

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The experiment highlighted some research questions that are still opened and could be answered with further investigations.
In detail: Because of this coupled dynamics, closed-loop pitch-to-feather control strategies may lead to an unstable response of the system Larsen and Hanson (2007) et al. (2020). In order to improve the current control practice, it would be useful to develop a control-oriented model of the unsteady thrust force. Experimental data would be useful to calibrate and validate such a model.
the platform pitch mode is very sensitive to the interaction with rotor, and a correct description of this phenomenon is 380 essential in model-based wind turbine control strategies. Moreover, numerical simulations have shown that pitch motion has a strong influence on the vertical wake deflection Wise and Bachynski (2020), which has the potential to be exploited for farm control purposes. The pitch shares some similarities with the surge mode, as it causes an along-wind motion of the rotor. However, in the surge case, the wind speed variation across the rotor is uniform, whereas in the pitch case, the flow seen by the rotor is skewed. It may be worth carrying out experiments for the pitch motion;

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unsteady aerodynamic effects appear to be more relevant at the higher reduced frequencies. It would be interesting to investigate surge-frequencies higher than those considered in this experiment. This is in general complex because of the risk of exciting the flexible modes of the wind turbine scale model. A potential solution is represented by numerical experiments. Numerical models could be validated based on the experimental data that are already available, and then used to study the other conditions, which may be unpractical to explore with experiments;

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the interaction between surge motion and wind turbine wake deserves further attention. A better understanding of the wake inflow may help explaining the unsteady behavior of the rotor thrust force. Moreover, it would lead to improved engineering wake models, which are needed for the design and optimization of future floating wind farms.
Data availability. All the data of the UNAFLOW experiment, from 2D and full-turbine tests, stored in the ftp ftp (2018). Credentials for the access are available upon request to the authors.