Numerical Investigation of Active Flow Control for Delaying Dynamic Stall Onset

The onset of dynamic stall limits the operating envelope of helicopters and wind turbines. Dynamic stall is accompanied by large fluctuations of the lift, drag, and moment coefficient and has to be avoided. Recent studies have identified laminar separation bubble bursting as a key trigger for dynamic stall onset at Reynolds numbers up to one million and higher at compressible Mach numbers. This paper presents results from large-eddy simulations of a blade section with NACA 0012 airfoil for a Reynolds number of 100,000 and Mach numbers of 0.1 and 0.4 that is subjected to a harmonic pitching motion. Active flow control using harmonic blowing and suction through a leading edge slot is investigated in both two- and three-dimensional simulations. For the uncontrolled flow, large period-to-period variations of the aerodynamic coefficients are observed. Space-time proper orthogonal decompositions of the two-dimensional flow fields reveal that the period-to-period variations are reduced with active flow control. It is demonstrated that two-dimensional flow control can alleviate dynamic stall in three-dimensional simulations for Mach 0.1. For Mach 0.4, two-dimensional flow control is less effective but combined forcing of a two- and three-dimensional mode shows promise. These findings contribute to the development of effective strategies for controlling dynamic stall on helicopter rotor blades.