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Keywords:

  • wind turbine;
  • blade section;
  • direct numerical simulation;
  • rotational augmentation;
  • crossflow

ABSTRACT

Direct numerical simulations were carried out for an S822 wind turbine blade section at a chord Reynolds number of Re = 100, 000 and an angle of attack of α = 5°. Results for a stationary non-rotating blade section compare favorably with wind tunnel data by the University of Illinois at Urbana-Champaign and XFoil predictions. By adding volume forcing terms to the right-hand side of the Navier–Stokes equations, the Coriolis and centrifugal accelerations resulting from blade rotation are modeled in the blade section simulations. Blade rotation is shown to delay separation especially near the hub, resulting in a lift increase of up to 100% and a drag reduction. The simulations provide insight into a physical mechanism that offers an explanation for the lift increase observed for rotating blade sections when compared with stationary blade sections, which is commonly referred to as rotational augmentation. Rotation is shown to lead to a radial velocity component toward the blade tip in areas where the velocity is substantially different from its free-stream value, such as near the stagnation point and especially in the separated flow region, and to the appearance of stationary and traveling crossflow vortices. A linear stability theory analysis that compares favorably with the simulation data provides proof that the primary instabilities are of a mixed type, including both a two-dimensional mode (Tollmien–Schlichting and Kelvin–Helmholtz type) and a stationary and unsteady crossflow mode. The crossflow instabilities accelerate transition, leading to separation delay, lift increase and drag reduction. This effect is very pronounced at 20% blade radius and still present at 80% radius. Because periodicity conditions were applied in the spanwise direction, the present results provide an explanation for rotational augmentation that is not based on the transfer of fluid from the inboard region toward the blade tip (‘centrifugal pumping’). For the low Reynolds number conditions considered here, crossflow instabilities, which destabilize the flow leading to earlier transition and a separation delay, may contribute to rotational augmentation. Copyright © 2012 John Wiley & Sons, Ltd.