Dynamic stall of an airfoil under tailored three-dimensional inflow conditions

TL;DR

This study investigates the dynamic stall behavior of a wind turbine airfoil under three-dimensional gust conditions, revealing flow stabilization effects and lift enhancement due to spanwise flow coupling analyzed through wind tunnel experiments and POD.

Contribution

It provides new insights into how three-dimensional inflow affects dynamic stall, highlighting the role of spanwise flow coupling and flow stabilization mechanisms.

Findings

Lift overshoot of up to 16% observed during stall cycle

Delay of 1/8 period between local angle of attack and lift

Flow stabilization and stall inhibition due to spanwise flow coupling

Abstract

Rotor blades of wind turbines in the atmospheric boundary layer regularly experience the aerodynamic phenomenon of dynamic stall consisting of a temporary overshoot of lift and detrimental fatigue loads. Particularly the formation of dynamic stall under three-dimensional inflow conditions raises open questions. Aerodynamic behavior of a DU 91-W2-250 wind profile undergoing light dynamic stall is thus analyzed in a wind tunnel. Effects of a gust with streamwise and spanwise periodic variation are investigated by comparing total and local lift generation with flow formation above the airfoil. The observed stall cycle is divided into five stages of which one reveals lift overshoot of up to $16~\%$. The aerodynamic response of the airfoil shows a delay of about $1/8$ period between evolution of local angle of attack and lift giving a counterclockwise dynamic polar. A proper orthogonal decomposition (POD) analysis of the flow field contributes to understand aerodynamic consequences of the three-dimensional gust. Local inflow, total lift as well as certain lift events are captured by one POD eigenmode, respectively. Obtained results lead to the conclusion that the flow and particularly the stalled wake of an airfoil facing a three-dimensional gust are strongly coupled in the spanwise direction. This yields to flow stabilization, inhibition of stall, and in turn, counterclockwise dynamic polar along with augmented total lift.