Abstract
Understanding the characteristics and effects of the laminar separation bubbles (LSBs) is important in the aerodynamic design of wind turbine airfoils for maximizing wind turbine efficiency. In the present study, numerical simulations using the γ-Reθ transition model were performed to analyze the flow structure of LSBs around a 21% thick NREL S809 airfoil. The simulation results obtained from the γ-Reθ transition model and the standard k-ε model for the aerodynamic coefficients at various angles of attack (AoAs) were compared with the wind tunnel data acquired from the Delft University 1.8 m × 1.25 m low-turbulence wind tunnel. When the AoA increased, the bubble on the suction airfoil surface was found to move closer to the leading edge owing to an earlier laminar separation (LS). Furthermore, the transition onset (TO) points were shown to occur right after separation, thus causing an abrupt increase in turbulence intensity (TI) and forming different bubble extents with increasing AoAs. Consequently, the transition model-based approaches can provide a clear understanding of the characteristics and effects of the LSB on airfoil aerodynamic performance. The findings of this study can provide important insights into redesigning an airfoil with a reduced bubble length causing the improved aerodynamic performance.
Highlights
Small wind turbine blades, which comprise a single airfoil, often experience relatively low Reynolds numbers in comparison with multi-MW turbine blades, which operate at high Reynolds numbers of above 10 million [1]
Numerical simulations using the γ-Reθ transition model coupled with stress transport (SST) k-ω transport equations around the 21% thick NREL S809 airfoil were performed to achieve a better understanding of the characteristics and effects of laminar separation bubbles (LSBs) existing at a low Reynolds number and low turbulence intensity (TI)
The transition model case showed very good agreement with the experimental case in terms of aerodynamic lift and drag coefficients, and characteristics of LSBs owing to the higher predictive accuracy relative to the standard k-ε model case
Summary
Small wind turbine blades, which comprise a single airfoil, often experience relatively low Reynolds numbers in comparison with multi-MW turbine blades, which operate at high Reynolds numbers of above 10 million [1]. At the low Reynolds numbers, laminar flow is separated at the upper surface of the airfoil under various angles of attack (AoAs). The bubble changes the boundary layer characteristics downstream and deteriorates the aerodynamic performance by loss of lift and an increase in drag [2,3]. The leading-edge, stall which occurs at a high AoA, causes abrupt loss of lift and increase of drag [5,6]. At a low AoA, which causes the formation of an LSB, the strong amplification of disturbances within the laminar separated boundary layer gives rise to a transition to turbulence in the separated shear layer and may have a significant impact on airfoil performance [6].
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