This article was excerpted from Sandia’s Wind and Water Power Technologies Program Newsletter and republished with permission.
The National Rotor Testbed team has been looking into the effect of airfoil choice on the final design of the new rotor for SWiFT, Sandia National Labs Scaled Wind Farm Technology. The airfoil family would ideally be designed to operate in the relevant SWiFT-scale Reynold’s number range, 1 to 2 million, and should have been experimentally verified. Two leading options have been identified; one option is the classic utility-scale wind turbine airfoil family, the DU-series. A second option is a combination of two experimentally verified NREL S-series airfoils, the S814 (inboard) and the S825 (outboard).
The DU-series airfoil family suffers from a sharp decrease in lift beyond the stall point and there is a lack of experimental data at the lower Reynolds numbers. The S-series airfoil family has only experimental data for the two airfoils and also has a significantly lower maximum thickness percentage than the typically utilized DU-series airfoils; this lower maximum thickness adversely affects the structural load carrying capability of the rotor.
To fully understand the implications of the airfoil family selection, it is helpful to look at the integrated dynamic effects along the rotor. FAST with IEC normal turbulence was used to simulate unsteady operation. The analysis was performed at wind speeds corresponding to Region 2 with IEC class “C” normal turbulence. The plot in Figure 1 reveals the variability in the circulation profile, i.e. airfoil lift, of the S-series airfoil family during operation in normal turbulent inflow.
This analysis will also be performed for a rotor design using the DU-family of airfoils. The two analyses will be compared to predicted dynamic performance of a full-scale rotor in order to determine the better option for upcoming research. This result will be combined with a broader set of overall airfoil decision criterion to inform the final selection of the airfoil family to be used for the NRT rotor.