Numerical investigation of rotational augmentation for S822 wind turbine airfoil

Abstract

Direct numerical simulations were carried out for an S822 wind turbine blade section at a chord Reynolds number of Re D 100;000 and an angle of attack of˛D 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.Accurate blade performance predictions are crucially needed during wind turbine development. A survey of numerical methods for analysing the aerodynamic and aeroelastic properties of wind turbines was provided by Hansen et al. 1 Blade element momentum theory (BEMT) is one of the main tools for computing the performance of entire turbines. This approach was found to perform well for attached flow conditions but underestimate the stall onset and hence the peak power output when two-dimensional (2D) airfoil data are used that are not corrected for rotational effects. For example, the National Renewable Energy Laboratory (NREL) Combined Experiment (Phase II) turbine power exceeded predictions by approximately 15-20%. 2 Almost all research suggests that this effect can be attributed to stall delay caused by blade rotation. 3The survey paper by Leishman et al. 4 points out that many details of the...

An Exploration of Radial Flow on a Rotating Blade in Retreating Blade Stall

“…In addition, recent numerical studies on a S822 wind turbine airfoil (Ref. 33) showed the effect of cross-flow on a rotating blade. The blade rotation resulted in a radial velocity component toward the blade tip in the separated flow region.…”

Large-Eddy Simulations for Wind Turbine Blade: Dynamic Stall and Rotational Augmentation

“…Gross et al's [2] method was applied for the rotational augmentation study. They derived the governing equations in a rotating reference frame by using an orderof-magnitude analysis for the blade sectional simulation with a periodic boundary condition in the spanwise direction.…”

“…This acceleration acts as favourable pressure gradient thus increases the lift. Gross et al [2] further scrutinized that the spanwise flow provides cross-flow instability which triggers early transition thus it delays separation and suppresses the boundary layer. They excluded the mass depletion effect by adopting the periodic boundary condition in the spanwise direction but did not discuss on the contribution of the Coriolis effect on rotational augmentation.…”

“…They excluded the mass depletion effect by adopting the periodic boundary condition in the spanwise direction but did not discuss on the contribution of the Coriolis effect on rotational augmentation. In order to further specifically identify the problem, we excluded considering the mass depletion as in [2] and used a NACA 0012 airfoil rather than a S833 airfoil. This airfoil is a leading edge separation type and the transition point is very close to the leading edge.…”

Comments on the research article by Grosset al.(2012)

“…There being different explanations proposed over the years to understand the rotational augmentation effect, it is necessary to critically review any work in order to achieve the most accurate understanding of the problem. In this regard, one recent work by Gross et al is reviewed in this document, and some of their conclusions are discussed. But first, it is useful to briefly review some of the relevant existing literature on the same subject.…”