Aerodynamic Analysis of Blunt Trailing Edge Airfoils

Abstract

The adoption of blunt trailing edge airfoils for the inboard region of large wind turbine blades has been proposed. Blunt trailing edge airfoils would not only provide a number of structural benefits, such as increased structural volume and ease of fabrication and handling, but they have also been found to improve the lift characteristics of airfoils. Therefore, the incorporation of blunt trailing edge airfoils would allow blade designers to more freely address the structural demands without having to sacrifice aerodynamic performance. Limited experimental data make it difficult for wind turbine designers to consider and conduct tradeoff studies using these section shapes and has provided the impetus for the present analysis of blunt trailing edge airfoils using computational fluid dynamics. Several computational techniques are applied, including a viscous/inviscid interaction method and three Reynolds-averaged Navier-Stokes methods.

Aerodynamic and Aeroacoustic Properties of a Flatback Airfoil: An Update

“…The issue of acoustic emission from the flatback airfoils should be mitigated by their inboard location, where airspeeds are low. Given that these airfoils are designed for use on the inner portion of the blade (r/R<0.5) and sound intensity scales as V 5 (due to the interaction of a turbulent boundary layer with a sharp trailing edge), according to Brooks, Pope and Marcolini 6 , any noise produced will be attenuated by at least a factor of 2 5 (=32) compared to noise produced at the tip. Brooks, et al 6 do present an empirical relationship for estimating the effects of trailing edge thickness (called "bluntness" by Brooks, et al) on noise generation, but the maximum degree of trailing edge thickness for which they have data is approximately 1% of chord; an order of magnitude lower than what is apt to be encountered with the use of flatback airfoils.…”

“…This is in distinct contrast to truncated airfoils where the trailing edge of the airfoil is simply cut off, changing the camber and degrading the aerodynamic performance. Compared to thick conventional, sharp trailing-edge wind turbine airfoil, such as those designed by Delft 2,3 or Riso 4 , a flatback airfoil with the same thickness exhibits increased lift and reduced sensitivity to soiling, according to Standish 5 . In addition, the use of flatback airfoils permits the use of increased thickness airfoils without the chord increases that would occur if conventional thick airfoils were used.…”

Flow Control over the Blunt Trailing Edge of Wind Turbine Airfoils Using Circulation Control

“…The fact that the trailing edge has some thickness poses a problem in terms of how one treats the closure of the trailing edge. Standish et al [4] deemed reasonable to extend the chord of the airfoil by 0.2% and fit a quadratic polynomial to both the top and bottom surfaces for a blunt trailing edge airfoil using the C-grid topology, although this treatment actually changes a little the shape of the trailing edge. An alternative way is to use the O-grid topology for such a blunt trailing edge airfoil, which is popular since it can provide better orthogonality in grid lines near the blunt trailing edge, thus generating a better grid than a C-grid.…”

Numerical Analysis of NACA64-418 Airfoil with Blunt Trailing Edge

“…Standish and van Dam [5] investigated the aerodynamic characteristics of the baseline airfoil, the truncated version (cutting off method), and the blunt trailing edge version (adding thickness method) by numerical analysis. The baseline airfoil, TR-35, has a value of t/c=0.35 with a critical angles of attack than the cutting off method.…”

“…The lift, drag, and pitching moment are measured at high Reynolds numbers, Re=3×10 6 and Re=6×10 6 , for various angles of attack, -10 o < α < 20 o . In general, the cutting off and adding thickness method are used for creating a blunt trailing edge airfoil [5,6]. The cutting off method involves truncating the original chord from the trailing edge and rescaling the shortened chord to a unit chord length.…”