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G. Xu
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In a world with an urgent demand for sustainable energy, the wind energy industry plays a key role in accelerating this transition. Over the past decades, wind turbines have evolved from expensive, relatively inefficient machines into increasingly cost-effective and highly efficient technologies, supported and promoted by many countries worldwide. However, this rapid progress has come at a cost: larger and more efficient turbines require substantial investments in raw materials, research and development, component standardization, and supply chain optimization.
In recent years, the curtailment of wind power in Europe has increased, largely due to insufficient grid capacity and limited energy storage. As a result, wind turbines are more frequently operated in parked conditions, with their rotors brought to a standstill. Under these circumstances, one of the key challenges in scaling up turbine size is the risk of vortex-induced vibrations (VIV) in the blades. In parked conditions, the blades are often pitched to very high angles of attack (close to 90°) to cut out of the wind. If the vortex shedding frequency approaches the blade’s natural frequency, a lock-in phenomenon may occur, leading to strong vibrations. This vibration in the long term can contribute to the overall fatigue load of the wind turbine and reduce the structural life.
Although increasing attention has been given to VIV in wind turbine blades, significant gaps remain in understanding the fundamental flow physics that govern these vibrations, specifically the unsteady aerodynamics of airfoils at high angles of attack. This dissertation therefore investigates the unsteady aerodynamics of both static and oscillating airfoils under such conditions, with the aim of building a detailed physical understanding of VIV from an aerodynamic perspective.
The research was carried out through a series of wind tunnel measurements. First, a campaign on a static airfoil examined unsteady aerodynamics across a wide range of angles of attack (up to 310°). Aerodynamic forces, vortex shedding patterns, and shedding frequencies were compared between forward flow (leading edge upwind) and reverse flow (trailing edge upwind) conditions. Although reverse flow is uncommon in normal operation, it can occur during parked or installation phases; the insights gained in this research therefore form a critical foundation for subsequent studies on oscillating airfoils.
The main focus of the dissertation is the unsteady aerodynamics of oscillating airfoils, studied using the forced motion method to mimic VIV. Three motion types, namely surging, plunging, and pitching, were investigated. Particle Image Velocimetry (PIV) was employed to capture the flow fields, while surface pressure measurements provided aerodynamic forces. By correlating vortex dynamics with force responses, the study reveals how the mean angle of attack and motion parameters (such as frequency and amplitude) influence the overall unsteady aerodynamics of the airfoil and how lock-in is triggered under different motion kinematics. Comparisons between forward and reverse flow conditions further enrich the findings, where the reverse flow dynamic stall was thoroughly discussed—from vortex dynamics and aerodynamic forces to a newly proposed dynamic stall vortex and trailing edge vortex onset determination method.
Overall, the comprehensive experimental dataset and resulting conclusions advance the fundamental understanding of unsteady airfoil aerodynamics at large angles of attack. These findings not only clarify the underlying mechanisms causing VIV from the perspective of vortex dynamics and aerodynamic forces, but also provide a valuable basis for future aeroelastic VIV studies and the development of engineering models. ...
In recent years, the curtailment of wind power in Europe has increased, largely due to insufficient grid capacity and limited energy storage. As a result, wind turbines are more frequently operated in parked conditions, with their rotors brought to a standstill. Under these circumstances, one of the key challenges in scaling up turbine size is the risk of vortex-induced vibrations (VIV) in the blades. In parked conditions, the blades are often pitched to very high angles of attack (close to 90°) to cut out of the wind. If the vortex shedding frequency approaches the blade’s natural frequency, a lock-in phenomenon may occur, leading to strong vibrations. This vibration in the long term can contribute to the overall fatigue load of the wind turbine and reduce the structural life.
Although increasing attention has been given to VIV in wind turbine blades, significant gaps remain in understanding the fundamental flow physics that govern these vibrations, specifically the unsteady aerodynamics of airfoils at high angles of attack. This dissertation therefore investigates the unsteady aerodynamics of both static and oscillating airfoils under such conditions, with the aim of building a detailed physical understanding of VIV from an aerodynamic perspective.
The research was carried out through a series of wind tunnel measurements. First, a campaign on a static airfoil examined unsteady aerodynamics across a wide range of angles of attack (up to 310°). Aerodynamic forces, vortex shedding patterns, and shedding frequencies were compared between forward flow (leading edge upwind) and reverse flow (trailing edge upwind) conditions. Although reverse flow is uncommon in normal operation, it can occur during parked or installation phases; the insights gained in this research therefore form a critical foundation for subsequent studies on oscillating airfoils.
The main focus of the dissertation is the unsteady aerodynamics of oscillating airfoils, studied using the forced motion method to mimic VIV. Three motion types, namely surging, plunging, and pitching, were investigated. Particle Image Velocimetry (PIV) was employed to capture the flow fields, while surface pressure measurements provided aerodynamic forces. By correlating vortex dynamics with force responses, the study reveals how the mean angle of attack and motion parameters (such as frequency and amplitude) influence the overall unsteady aerodynamics of the airfoil and how lock-in is triggered under different motion kinematics. Comparisons between forward and reverse flow conditions further enrich the findings, where the reverse flow dynamic stall was thoroughly discussed—from vortex dynamics and aerodynamic forces to a newly proposed dynamic stall vortex and trailing edge vortex onset determination method.
Overall, the comprehensive experimental dataset and resulting conclusions advance the fundamental understanding of unsteady airfoil aerodynamics at large angles of attack. These findings not only clarify the underlying mechanisms causing VIV from the perspective of vortex dynamics and aerodynamic forces, but also provide a valuable basis for future aeroelastic VIV studies and the development of engineering models. ...
In a world with an urgent demand for sustainable energy, the wind energy industry plays a key role in accelerating this transition. Over the past decades, wind turbines have evolved from expensive, relatively inefficient machines into increasingly cost-effective and highly efficient technologies, supported and promoted by many countries worldwide. However, this rapid progress has come at a cost: larger and more efficient turbines require substantial investments in raw materials, research and development, component standardization, and supply chain optimization.
In recent years, the curtailment of wind power in Europe has increased, largely due to insufficient grid capacity and limited energy storage. As a result, wind turbines are more frequently operated in parked conditions, with their rotors brought to a standstill. Under these circumstances, one of the key challenges in scaling up turbine size is the risk of vortex-induced vibrations (VIV) in the blades. In parked conditions, the blades are often pitched to very high angles of attack (close to 90°) to cut out of the wind. If the vortex shedding frequency approaches the blade’s natural frequency, a lock-in phenomenon may occur, leading to strong vibrations. This vibration in the long term can contribute to the overall fatigue load of the wind turbine and reduce the structural life.
Although increasing attention has been given to VIV in wind turbine blades, significant gaps remain in understanding the fundamental flow physics that govern these vibrations, specifically the unsteady aerodynamics of airfoils at high angles of attack. This dissertation therefore investigates the unsteady aerodynamics of both static and oscillating airfoils under such conditions, with the aim of building a detailed physical understanding of VIV from an aerodynamic perspective.
The research was carried out through a series of wind tunnel measurements. First, a campaign on a static airfoil examined unsteady aerodynamics across a wide range of angles of attack (up to 310°). Aerodynamic forces, vortex shedding patterns, and shedding frequencies were compared between forward flow (leading edge upwind) and reverse flow (trailing edge upwind) conditions. Although reverse flow is uncommon in normal operation, it can occur during parked or installation phases; the insights gained in this research therefore form a critical foundation for subsequent studies on oscillating airfoils.
The main focus of the dissertation is the unsteady aerodynamics of oscillating airfoils, studied using the forced motion method to mimic VIV. Three motion types, namely surging, plunging, and pitching, were investigated. Particle Image Velocimetry (PIV) was employed to capture the flow fields, while surface pressure measurements provided aerodynamic forces. By correlating vortex dynamics with force responses, the study reveals how the mean angle of attack and motion parameters (such as frequency and amplitude) influence the overall unsteady aerodynamics of the airfoil and how lock-in is triggered under different motion kinematics. Comparisons between forward and reverse flow conditions further enrich the findings, where the reverse flow dynamic stall was thoroughly discussed—from vortex dynamics and aerodynamic forces to a newly proposed dynamic stall vortex and trailing edge vortex onset determination method.
Overall, the comprehensive experimental dataset and resulting conclusions advance the fundamental understanding of unsteady airfoil aerodynamics at large angles of attack. These findings not only clarify the underlying mechanisms causing VIV from the perspective of vortex dynamics and aerodynamic forces, but also provide a valuable basis for future aeroelastic VIV studies and the development of engineering models.
In recent years, the curtailment of wind power in Europe has increased, largely due to insufficient grid capacity and limited energy storage. As a result, wind turbines are more frequently operated in parked conditions, with their rotors brought to a standstill. Under these circumstances, one of the key challenges in scaling up turbine size is the risk of vortex-induced vibrations (VIV) in the blades. In parked conditions, the blades are often pitched to very high angles of attack (close to 90°) to cut out of the wind. If the vortex shedding frequency approaches the blade’s natural frequency, a lock-in phenomenon may occur, leading to strong vibrations. This vibration in the long term can contribute to the overall fatigue load of the wind turbine and reduce the structural life.
Although increasing attention has been given to VIV in wind turbine blades, significant gaps remain in understanding the fundamental flow physics that govern these vibrations, specifically the unsteady aerodynamics of airfoils at high angles of attack. This dissertation therefore investigates the unsteady aerodynamics of both static and oscillating airfoils under such conditions, with the aim of building a detailed physical understanding of VIV from an aerodynamic perspective.
The research was carried out through a series of wind tunnel measurements. First, a campaign on a static airfoil examined unsteady aerodynamics across a wide range of angles of attack (up to 310°). Aerodynamic forces, vortex shedding patterns, and shedding frequencies were compared between forward flow (leading edge upwind) and reverse flow (trailing edge upwind) conditions. Although reverse flow is uncommon in normal operation, it can occur during parked or installation phases; the insights gained in this research therefore form a critical foundation for subsequent studies on oscillating airfoils.
The main focus of the dissertation is the unsteady aerodynamics of oscillating airfoils, studied using the forced motion method to mimic VIV. Three motion types, namely surging, plunging, and pitching, were investigated. Particle Image Velocimetry (PIV) was employed to capture the flow fields, while surface pressure measurements provided aerodynamic forces. By correlating vortex dynamics with force responses, the study reveals how the mean angle of attack and motion parameters (such as frequency and amplitude) influence the overall unsteady aerodynamics of the airfoil and how lock-in is triggered under different motion kinematics. Comparisons between forward and reverse flow conditions further enrich the findings, where the reverse flow dynamic stall was thoroughly discussed—from vortex dynamics and aerodynamic forces to a newly proposed dynamic stall vortex and trailing edge vortex onset determination method.
Overall, the comprehensive experimental dataset and resulting conclusions advance the fundamental understanding of unsteady airfoil aerodynamics at large angles of attack. These findings not only clarify the underlying mechanisms causing VIV from the perspective of vortex dynamics and aerodynamic forces, but also provide a valuable basis for future aeroelastic VIV studies and the development of engineering models.
During wind turbine installation or idling, the blades often operate at large angles of attack, where vortex-induced vibration (VIV) can occur. This study experimentally investigates the aerodynamic characteristics of a plunging NACA0021 airfoil at a fixed angle of attack of 90∘ and amplitude of one chord length, focusing on vortex dynamics, lock-in effect, and unsteady force generation. Phase-locked particle image velocimetry (PIV) was conducted at two reduced frequencies of 0.19 and 0.38. At the lower reduced frequency, asymmetric vortex shedding prevents synchronization between shedding and plunge motion frequencies, whereas at the higher reduced frequency, lock-in occurs with periodic shedding of separated leading- and trailing-edge vortices. Compared with previously studied surging motion under identical conditions, plunging requires a higher frequency to achieve lock-in and produces weaker wakes that break down more quickly downstream. Additionally, the aerodynamic load is extracted from the PIV flow field. For the plunging motion, the aerodynamic loads are dominated by pressure forces, with a maximum streamwise coefficient of approximately four times the static value at 90∘ angle of attack. This contrasts with the surging motion, where higher force variations are observed, and both pressure and mean momentum convection play comparable roles in the overall force. These results indicate that lock-in behavior depends strongly on both motion frequency and kinematics, where the effective angle of attack variation and the resulting vortex dynamics also determine whether synchronization can occur.
...
During wind turbine installation or idling, the blades often operate at large angles of attack, where vortex-induced vibration (VIV) can occur. This study experimentally investigates the aerodynamic characteristics of a plunging NACA0021 airfoil at a fixed angle of attack of 90∘ and amplitude of one chord length, focusing on vortex dynamics, lock-in effect, and unsteady force generation. Phase-locked particle image velocimetry (PIV) was conducted at two reduced frequencies of 0.19 and 0.38. At the lower reduced frequency, asymmetric vortex shedding prevents synchronization between shedding and plunge motion frequencies, whereas at the higher reduced frequency, lock-in occurs with periodic shedding of separated leading- and trailing-edge vortices. Compared with previously studied surging motion under identical conditions, plunging requires a higher frequency to achieve lock-in and produces weaker wakes that break down more quickly downstream. Additionally, the aerodynamic load is extracted from the PIV flow field. For the plunging motion, the aerodynamic loads are dominated by pressure forces, with a maximum streamwise coefficient of approximately four times the static value at 90∘ angle of attack. This contrasts with the surging motion, where higher force variations are observed, and both pressure and mean momentum convection play comparable roles in the overall force. These results indicate that lock-in behavior depends strongly on both motion frequency and kinematics, where the effective angle of attack variation and the resulting vortex dynamics also determine whether synchronization can occur.
Wind turbine blades in standstill or parked conditions often experience large angles of attack (AoA), where vortex-induced vibrations (VIV) may occur that increase the risk of structural damage. To better understand the VIV of airfoils at high AoA from an aerodynamic perspective, we conducted experimental investigations into the vortex dynamics of a surging airfoil at a 90∘ incidence undergoing forced vibrations. Experiments were conducted at two reduced frequencies (k) to demonstrate the lock-in effect, where the vortex shedding frequency aligns with the motion frequency. Results indicate distinct vortex shedding behaviors: at higher k value of 0.38, downstream wake vortices form when the airfoil is moving upwind, while upstream vortices emerge during the downwind motion, interacting with the downstream vortices and leading to an outward flow. At lower k value of 0.19, the wake remains directed to the downwind side, regardless of the airfoil’s motion direction. Lock-in is evident in both cases, with one vortex pair shed per cycle at lower k and two pairs at higher k. Furthermore, the study examines the influence of vortex dynamics on unsteady aerodynamic loads. The results show that drag peaks when the airfoil moves upwind near the center position of its trajectory; at higher k, negative drag occurs as the airfoil moves downwind near the center, driven by the interactions among convection, turbulent momentum, pressure, and viscous forces. A reduced-order load estimation model for a flat plate is applied to the experimental data, showing good agreement during the upwind motion of the airfoil, which is the design condition for the original flat plate model. However, during the downwind motion, as the flow condition does not match the original flat plate design condition, the circulatory part of the model is modified to account for the presence of two pairs of vortices in the flow field, yielding improved agreement with the drag values determined from the measured flow field. The findings highlight distinct flow patterns and vortex interactions for the two motion cases, offering insights into their impact on aerodynamic loads.
...
Wind turbine blades in standstill or parked conditions often experience large angles of attack (AoA), where vortex-induced vibrations (VIV) may occur that increase the risk of structural damage. To better understand the VIV of airfoils at high AoA from an aerodynamic perspective, we conducted experimental investigations into the vortex dynamics of a surging airfoil at a 90∘ incidence undergoing forced vibrations. Experiments were conducted at two reduced frequencies (k) to demonstrate the lock-in effect, where the vortex shedding frequency aligns with the motion frequency. Results indicate distinct vortex shedding behaviors: at higher k value of 0.38, downstream wake vortices form when the airfoil is moving upwind, while upstream vortices emerge during the downwind motion, interacting with the downstream vortices and leading to an outward flow. At lower k value of 0.19, the wake remains directed to the downwind side, regardless of the airfoil’s motion direction. Lock-in is evident in both cases, with one vortex pair shed per cycle at lower k and two pairs at higher k. Furthermore, the study examines the influence of vortex dynamics on unsteady aerodynamic loads. The results show that drag peaks when the airfoil moves upwind near the center position of its trajectory; at higher k, negative drag occurs as the airfoil moves downwind near the center, driven by the interactions among convection, turbulent momentum, pressure, and viscous forces. A reduced-order load estimation model for a flat plate is applied to the experimental data, showing good agreement during the upwind motion of the airfoil, which is the design condition for the original flat plate model. However, during the downwind motion, as the flow condition does not match the original flat plate design condition, the circulatory part of the model is modified to account for the presence of two pairs of vortices in the flow field, yielding improved agreement with the drag values determined from the measured flow field. The findings highlight distinct flow patterns and vortex interactions for the two motion cases, offering insights into their impact on aerodynamic loads.
The airfoil DU91-W2-150 was investigated in the Low Speed Low Turbulence Tunnel at the Delft University of Technology to study unsteady aerodynamics. This experimental study tested the airfoil under a wide range of angles of attack (AoA) from 0° to 310° at three Reynolds numbers ((Formula presented.)) from (Formula presented.) to (Formula presented.). Pressure on the airfoil surface was measured and particle image velocimetry (PIV) measurements were conducted to capture the flow field in the wake. By examining the force coefficient and comparing the wake contours, it shows that an upwind concave surface provides a higher load compared to a convex surface upwind case, highlighting the critical role of surface shape in aerodynamics. When comparing separation at specific locations along the chord for all three (Formula presented.) values, it is observed that as (Formula presented.) increases, separation tends to occur at lower AoA, both for positive stall and negative stall. The examination of the aerodynamic force variation indicates that, during reverse flow, fluctuations are more pronounced compared to forward flow. This is owing to separation occurring at the aerodynamic leading edge (geometric trailing edge) in reverse flow. In terms of vortex shedding frequency, the study found a nearly constant normalized Strouhal number ((Formula presented.)) of 0.16 across various (Formula presented.) and AoA values in fully separated regions, indicating a consistent pattern under these conditions. However, a slight increase in (Formula presented.), between 0.16 and 0.20, was observed for AoA values exceeding 180°, possibly due to the convex curvature of the airfoil in the upwind direction. In conclusion, this research not only corroborates previous findings for small AoA values but also adds new data on the aerodynamic behavior of the DU91-W2-150 airfoil under large AoA values, offering various perspectives on the effects of surface curvature, (Formula presented.), and flow conditions on key aerodynamic parameters.
...
The airfoil DU91-W2-150 was investigated in the Low Speed Low Turbulence Tunnel at the Delft University of Technology to study unsteady aerodynamics. This experimental study tested the airfoil under a wide range of angles of attack (AoA) from 0° to 310° at three Reynolds numbers ((Formula presented.)) from (Formula presented.) to (Formula presented.). Pressure on the airfoil surface was measured and particle image velocimetry (PIV) measurements were conducted to capture the flow field in the wake. By examining the force coefficient and comparing the wake contours, it shows that an upwind concave surface provides a higher load compared to a convex surface upwind case, highlighting the critical role of surface shape in aerodynamics. When comparing separation at specific locations along the chord for all three (Formula presented.) values, it is observed that as (Formula presented.) increases, separation tends to occur at lower AoA, both for positive stall and negative stall. The examination of the aerodynamic force variation indicates that, during reverse flow, fluctuations are more pronounced compared to forward flow. This is owing to separation occurring at the aerodynamic leading edge (geometric trailing edge) in reverse flow. In terms of vortex shedding frequency, the study found a nearly constant normalized Strouhal number ((Formula presented.)) of 0.16 across various (Formula presented.) and AoA values in fully separated regions, indicating a consistent pattern under these conditions. However, a slight increase in (Formula presented.), between 0.16 and 0.20, was observed for AoA values exceeding 180°, possibly due to the convex curvature of the airfoil in the upwind direction. In conclusion, this research not only corroborates previous findings for small AoA values but also adds new data on the aerodynamic behavior of the DU91-W2-150 airfoil under large AoA values, offering various perspectives on the effects of surface curvature, (Formula presented.), and flow conditions on key aerodynamic parameters.
This study experimentally investigates the dynamic stall of a pitching NACA 643418 airfoil under reverse flow conditions, whereby the geometric trailing edge is located upstream of the geometric leading edge. The investigation focuses on the correlation between surface pressure, aerodynamic forces, and the evolution of the dynamic stall vortex (DSV) and the aerodynamic trailing edge vortex (TEV). A range of mean angles of attack from 5° to 25°, pitching amplitudes from 5° to 15°, and reduced frequencies between 0.05 and 0.21 were tested on an airfoil using a combination of particle image velocimetry (PIV) and surface pressure measurements. The results reveal a distinct dynamic stall mechanism in reverse flow conditions: multiple flow separations occur during both the upstroke and downstroke periods, yet the DSV maintains partial attachment. While the outer layer of the DSV sheds into the flow, its inner core remains anchored and is subsequently reinforced by shear layer feeding. This sustained vorticity injection leads to periodic DSV re-growth, which manifests in the force hysteresis loop as multiple distinct peak regions. In addition, it is also found that the largest difference between conventional and reverse flow dynamic stall lies in the initial stage of the DSV development. For reverse flow dynamic stall cases, the initial flow separation near the leading edge causes a gradual decrease in the pressure coefficient. However, for conventional dynamic stall cases, the laminar separation bubbles that occur before the DSV cause the maximum suction on the airfoil surface. Furthermore, by comparing the dominant modes from Proper Orthogonal Decomposition (POD) analysis, it is found that the type of flow regime depends mainly on the mean angle of attack within the tested range of pitching amplitudes and frequencies. In particular, the dominant vortex determines the flow dynamics. At low mean angles of 5° and 10°, the most energetic flow features are associated with DSV dynamics. At a mean angle of 15°, both DSV and TEV dynamics contribute. At higher mean angles of 20° and 25°, the flow is dominated more by TEV dynamics. Despite these mean-angle-dependent variations in modal dominance, all dominant POD modes share a consistent physical interpretation where they capture the growth or decay of DSV and TEV structures.
...
This study experimentally investigates the dynamic stall of a pitching NACA 643418 airfoil under reverse flow conditions, whereby the geometric trailing edge is located upstream of the geometric leading edge. The investigation focuses on the correlation between surface pressure, aerodynamic forces, and the evolution of the dynamic stall vortex (DSV) and the aerodynamic trailing edge vortex (TEV). A range of mean angles of attack from 5° to 25°, pitching amplitudes from 5° to 15°, and reduced frequencies between 0.05 and 0.21 were tested on an airfoil using a combination of particle image velocimetry (PIV) and surface pressure measurements. The results reveal a distinct dynamic stall mechanism in reverse flow conditions: multiple flow separations occur during both the upstroke and downstroke periods, yet the DSV maintains partial attachment. While the outer layer of the DSV sheds into the flow, its inner core remains anchored and is subsequently reinforced by shear layer feeding. This sustained vorticity injection leads to periodic DSV re-growth, which manifests in the force hysteresis loop as multiple distinct peak regions. In addition, it is also found that the largest difference between conventional and reverse flow dynamic stall lies in the initial stage of the DSV development. For reverse flow dynamic stall cases, the initial flow separation near the leading edge causes a gradual decrease in the pressure coefficient. However, for conventional dynamic stall cases, the laminar separation bubbles that occur before the DSV cause the maximum suction on the airfoil surface. Furthermore, by comparing the dominant modes from Proper Orthogonal Decomposition (POD) analysis, it is found that the type of flow regime depends mainly on the mean angle of attack within the tested range of pitching amplitudes and frequencies. In particular, the dominant vortex determines the flow dynamics. At low mean angles of 5° and 10°, the most energetic flow features are associated with DSV dynamics. At a mean angle of 15°, both DSV and TEV dynamics contribute. At higher mean angles of 20° and 25°, the flow is dominated more by TEV dynamics. Despite these mean-angle-dependent variations in modal dominance, all dominant POD modes share a consistent physical interpretation where they capture the growth or decay of DSV and TEV structures.
The topic of vortex-induced vibrations on a wind turbine blade has recently gained much attention due to its growing size and flexibility. To address this concern, a wind tunnel test was conducted to study the forced plunging and surging motion of a NACA0021 airfoil at 90° angle of attack. Results indicate that vortex lock-in occurred for a motion amplitude of one chord length even for a small frequency ratio (between motion frequency and static Strouhal frequency) of 0.39. Analysis of the drag coefficient, derived from the phase-averaged Particle Image Velocimetry (PIV) data, shows that a plunging airfoil experiences higher average loading than a surging airfoil, which is deemed to be more harmful considering the higher loading in the crossflow-direction due to the variation of effective angle of attack.
...
The topic of vortex-induced vibrations on a wind turbine blade has recently gained much attention due to its growing size and flexibility. To address this concern, a wind tunnel test was conducted to study the forced plunging and surging motion of a NACA0021 airfoil at 90° angle of attack. Results indicate that vortex lock-in occurred for a motion amplitude of one chord length even for a small frequency ratio (between motion frequency and static Strouhal frequency) of 0.39. Analysis of the drag coefficient, derived from the phase-averaged Particle Image Velocimetry (PIV) data, shows that a plunging airfoil experiences higher average loading than a surging airfoil, which is deemed to be more harmful considering the higher loading in the crossflow-direction due to the variation of effective angle of attack.
Machine learning method has always been popular to solve wind turbine related problems at a data level. However, with the limitation of the availability of relevant data, transfer learning has gained increasing attention. In this study, traditional machine learning method of artificial neural networks (ANN), together with parameter-based transfer learning method has been used to estimate wind turbine load. First, ANN load model was built for DTU 10MW wind turbine as well as NREL 5MW wind turbine. Then, parameter-based transfer learning has been applied to the above-mentioned models to estimate load for a different turbine type or two mixed turbine types. Results indicate that ANN method provides good estimation on wind turbine fatigue load. For DTU 10MW ANN model, the trend of accuracy becomes steady as the number of input samples increases and 1500 samples is deemed as the optimal number of samples for training DTU 10MW. In addition, with transfer learning, it was succeeded in building NREL 5MW model with corresponding DTU 10MW pretrained model but failed in establishing mixed dataset model neither with DTU 10MW nor with NREL 5MW pretrained model.
...
Machine learning method has always been popular to solve wind turbine related problems at a data level. However, with the limitation of the availability of relevant data, transfer learning has gained increasing attention. In this study, traditional machine learning method of artificial neural networks (ANN), together with parameter-based transfer learning method has been used to estimate wind turbine load. First, ANN load model was built for DTU 10MW wind turbine as well as NREL 5MW wind turbine. Then, parameter-based transfer learning has been applied to the above-mentioned models to estimate load for a different turbine type or two mixed turbine types. Results indicate that ANN method provides good estimation on wind turbine fatigue load. For DTU 10MW ANN model, the trend of accuracy becomes steady as the number of input samples increases and 1500 samples is deemed as the optimal number of samples for training DTU 10MW. In addition, with transfer learning, it was succeeded in building NREL 5MW model with corresponding DTU 10MW pretrained model but failed in establishing mixed dataset model neither with DTU 10MW nor with NREL 5MW pretrained model.