M. Doosttalab
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4 records found
1
Ensuring reliable turbine operation requires accurate load assessment, which depends heavily on aeroelastic and dynamic load simulations across a broad range of operating conditions. As blades become larger and more flexible, they are subjected to stronger inflow turbulence, more pronounced unsteady variations in angle of attack, and an increased likelihood of transient stall phenomena. These factors make accurate unsteady aerodynamic modeling essential. In this context, dynamic stall models are frequently employed to predict unsteady aerodynamic forces. Historically, however, most dynamic stall models were developed for thin airfoils in helicopter applications, and despite subsequent adaptations for wind turbine use, their suitability for thick airfoils remains limited. The aerodynamic response of thick airfoils, particularly those typical of large modern blades, still lacks comprehensive validation, resulting in uncertainties in load predictions. Moreover, thick mid‑span airfoils experience large unsteady angles of attack and are more susceptible to early stall, necessitating active or passive flow‑control strategies. Flatback airfoils and vortex generators have emerged as promising solutions, offering increased lift, delayed flow separation, and improved stall characteristics. Nevertheless, their combined impact on unsteady aerodynamics is not yet fully understood, revealing a critical gap in current aeroelastic modeling capabilities.
This knowledge gap compromises the reliability of current aeroelastic models and poses risks for the design, optimization, and structural assessment of large modern turbines. Consequently, this dissertation is motivated by the need to advance the understanding and modeling of dynamic stall phenomena in thick wind turbine airfoils, with particular emphasis on the combined effects of vortex generators under unsteady flow conditions. By addressing these gaps, the work aims to contribute improved physical insight and enhanced predictive capability for the aerodynamic behavior of next generation wind turbine blades.
Therefore, several wind tunnel campaigns were conducted to investigate the unsteady aerodynamic characteristics of two $35\%$ thick airfoil configurations, comprising a conventional airfoil and a flatback airfoil, tested in different setups in Low-speed, low-Turbulence wind Tunnel (LTT) of the Delft University of Technology.
The results show that flatback airfoils exhibit inherently more stable hysteresis loops, as evidenced by reduced force fluctuations, larger lift overshoots, and delayed separation compared to sharp trailing edge airfoils, primarily due to their more favorable suction side pressure gradients under the studied operating conditions. Dynamic stall on these thick airfoils is governed not by leading edge vortex formation, as seen in thin airfoils, but by a gradual trailing edge separation mechanism that produces moderated load transients. Reduced frequency strongly influences dynamic stall, i.e, higher frequencies delay separation, enhance lift overshoot, and stabilize flow evolution for both airfoils. The addition of vortex generators substantially improves flow attachment during the upstroke phase, increases lift overshoot, and suppresses force fluctuations, particularly when placed near the leading edge. Their effectiveness, however, depends sensitively on chordwise placement, the underlying airfoil geometry, and the oscillation regime. While flatback airfoils generally benefit from improved stall delay, they experience larger force excursions during fully separated flow, indicating both the potential and the risks of employing blunt trailing edge designs. The evaluation of the Beddoes–Leishman and Risø dynamic stall models demonstrates that both models are not capable to predict separated flow behavior on thick airfoils, especially regarding lift overshoot, lift drop, post-stall loads, drag, and pitching moment. Lack of modeling accuracy lead to systematic discrepancies in model predictions, even when tuning time constants. These findings highlight the need for new or revised separation and reattachment modules tailored specifically for thick airfoils for reliable load prediction in the inboard and mid-span regions of large modern wind turbine blades. ...
Ensuring reliable turbine operation requires accurate load assessment, which depends heavily on aeroelastic and dynamic load simulations across a broad range of operating conditions. As blades become larger and more flexible, they are subjected to stronger inflow turbulence, more pronounced unsteady variations in angle of attack, and an increased likelihood of transient stall phenomena. These factors make accurate unsteady aerodynamic modeling essential. In this context, dynamic stall models are frequently employed to predict unsteady aerodynamic forces. Historically, however, most dynamic stall models were developed for thin airfoils in helicopter applications, and despite subsequent adaptations for wind turbine use, their suitability for thick airfoils remains limited. The aerodynamic response of thick airfoils, particularly those typical of large modern blades, still lacks comprehensive validation, resulting in uncertainties in load predictions. Moreover, thick mid‑span airfoils experience large unsteady angles of attack and are more susceptible to early stall, necessitating active or passive flow‑control strategies. Flatback airfoils and vortex generators have emerged as promising solutions, offering increased lift, delayed flow separation, and improved stall characteristics. Nevertheless, their combined impact on unsteady aerodynamics is not yet fully understood, revealing a critical gap in current aeroelastic modeling capabilities.
This knowledge gap compromises the reliability of current aeroelastic models and poses risks for the design, optimization, and structural assessment of large modern turbines. Consequently, this dissertation is motivated by the need to advance the understanding and modeling of dynamic stall phenomena in thick wind turbine airfoils, with particular emphasis on the combined effects of vortex generators under unsteady flow conditions. By addressing these gaps, the work aims to contribute improved physical insight and enhanced predictive capability for the aerodynamic behavior of next generation wind turbine blades.
Therefore, several wind tunnel campaigns were conducted to investigate the unsteady aerodynamic characteristics of two $35\%$ thick airfoil configurations, comprising a conventional airfoil and a flatback airfoil, tested in different setups in Low-speed, low-Turbulence wind Tunnel (LTT) of the Delft University of Technology.
The results show that flatback airfoils exhibit inherently more stable hysteresis loops, as evidenced by reduced force fluctuations, larger lift overshoots, and delayed separation compared to sharp trailing edge airfoils, primarily due to their more favorable suction side pressure gradients under the studied operating conditions. Dynamic stall on these thick airfoils is governed not by leading edge vortex formation, as seen in thin airfoils, but by a gradual trailing edge separation mechanism that produces moderated load transients. Reduced frequency strongly influences dynamic stall, i.e, higher frequencies delay separation, enhance lift overshoot, and stabilize flow evolution for both airfoils. The addition of vortex generators substantially improves flow attachment during the upstroke phase, increases lift overshoot, and suppresses force fluctuations, particularly when placed near the leading edge. Their effectiveness, however, depends sensitively on chordwise placement, the underlying airfoil geometry, and the oscillation regime. While flatback airfoils generally benefit from improved stall delay, they experience larger force excursions during fully separated flow, indicating both the potential and the risks of employing blunt trailing edge designs. The evaluation of the Beddoes–Leishman and Risø dynamic stall models demonstrates that both models are not capable to predict separated flow behavior on thick airfoils, especially regarding lift overshoot, lift drop, post-stall loads, drag, and pitching moment. Lack of modeling accuracy lead to systematic discrepancies in model predictions, even when tuning time constants. These findings highlight the need for new or revised separation and reattachment modules tailored specifically for thick airfoils for reliable load prediction in the inboard and mid-span regions of large modern wind turbine blades.
The accuracy of the Beddoes–Leishman and Risø dynamic stall models is evaluated against experiments on thick wind turbine airfoils with a relative thickness of 35% and trailing edge thicknesses of 10% and 2%, both with and without vortex generators. The dynamic lift, drag, and pitching moment coefficients simulation results are compared with the measurements, obtained in the TU Delft LTT wind tunnel at a Reynolds number of Re=1×106 and dynamic reduced frequency of 0.064. The study revealed that while the aforementioned models successfully predicted the direction of the dynamic cycles, they inaccurately captured the dynamic stall behavior of thick flatback and non-flatback airfoils in all configurations, particularly in separated flows. There was no significant difference observed in the performance of the two models. The reasons for modeling failure are thoroughly examined from both fundamental and mathematical perspectives, and suggestions for improvements are provided. The findings raise concerns regarding the accuracy and reliability of the dynamic load assessment and aeroelasticity analysis for modern large wind turbines, using current dynamic stall models and underscore the necessity for enhancing the existing models.
This study examined the effect of vortex generators on the dynamic stall characteristics of thick wind turbine airfoils with a relative thickness of 35% and trailing edge thickness of 10% and 2%. The experiments were conducted in the TU Delft LTT wind tunnel at a Reynolds number of Re=1×106 and dynamic reduced frequency ranging from 0.032 to 0.096. The study investigated the impact of various factors on the dynamic stall characteristics of the airfoils, including the vortex generator's chord position, trailing edge gap, roughness, mean angle of attack, and reduced frequency. The study found that vortex generators delay dynamic stall for thick airfoils by stabilizing the flow during the upstroke phase. However, this can increase the maximum lift overshoot, particularly with flatback airfoils, resulting in a higher drop in lift during dynamic stall. This can potentially increase the dynamic loads on a wind turbine blade due to stall-induced vibrations. The study noted a significant difference in dynamic stall behavior between flatback and non-flatback airfoils. Overall, this research provides valuable insights into the dynamic stall and flow physics characteristics of thick wind turbine airfoils using vortex generators, aiding in more accurate rotor blade design.
This paper studies the dynamic stall characteristics of thick flatback and nonflatback wind turbine airfoils. Two airfoils with a maximum thickness of (Formula presented.) were studied, with trailing edge thicknesses of (Formula presented.) and (Formula presented.), respectively. The static and dynamic experimental measurements were performed in the wind tunnel using surface pressure measurements for clean and tripped airfoils at the Reynolds number of (Formula presented.) and dynamic reduced frequency ranging from (Formula presented.) to (Formula presented.). The effects of the trailing edge gap, roughness, mean angle of attack, and reduced frequency on the dynamic stall characteristics of the airfoils were investigated. The results show that increasing the trailing edge gap delays the onset of dynamic stall. However, the lift loss after the onset of dynamic stall is for the flatback airfoil higher than the sharp trailing edge airfoil. Moreover, the flatback airfoil show higher lift overshoot compared to the sharp trailing edge airfoil in the dynamic stall condition. Increasing the reduced frequency affects the dynamic behavior both airfoils differently.