On the low drag regime of flatback airfoils

TL;DR

This paper investigates the low-drag regime of flatback airfoils at high angles of attack using advanced simulations and analysis techniques, revealing organized wake structures and secondary instabilities that could improve wind turbine efficiency.

Contribution

It introduces a detailed analysis of the aerodynamics of flatback airfoils in low-drag conditions using DES and data-driven modal analysis, highlighting the role of secondary instabilities.

Findings

Increased base pressure and wake organization in low-drag regime.

Secondary instability (Mode S') observed at 12° angle of attack.

Dominance of primary vortex street with distinct secondary structures at high angles.

Abstract

Flatback airfoils, characterized by a blunt trailing edge, are used at the root of large wind turbine blades. A low-drag pocket has recently been identified in the flow past these airfoils at high angles of attack, potentially offering opportunities for enhanced energy extraction. This study uses three-dimensional Detached Eddy Simulations (DES) combined with statistical and data-driven modal analysis techniques to explore the aerodynamics and coherent structures of a flatback airfoil in these conditions. Two angles of attack - one inside $\left(12^{\circ}\right)$ and one outside $\left(0^{\circ}\right)$ of the low-drag pocket - are examined more thoroughly. The spanwise correlation length of secondary instability is analyzed in terms of autocorrelation of the $\Gamma_{1}$ vortex identification criterion, while coherent structures were extracted via the multiscale Proper Orthogonal Decomposition (mPOD). The results show increased base pressure, BL thickness, vortex formation length, and more organized wake structures inside the low-drag regime. While the primary instability (B\'enard-von K\'arm\'an vortex street) dominates in both cases, the secondary instability is distinguishable only for the $12^{\circ}$ case and is identified as a Mode S$^{\prime}$ instability.