Dynamic stall reattachment

Nikolai Skaare

Semester project

The reattachment of separated flow on a sinusoidally pitching airfoil recovering from dynamic stall has been studied. A wind tunnel test performed at Re =9.2e5 provided experimental data such as pressure, lift and time-resolved particle image velocimetry data. From the TR-PIV, finite-time Lyapunov exponents were calculated for the purpose of analyzing Lagrangian coherent structures and their migration over the airfoil. The initiation of the reattachment process was found to be independent of pitch rate and other motion parameters. After the process is initiated, it takes a finite amount of time and this time was observed to be dependent on the pitch rate of the airfoil. A hysteresis between the pressure along the chord and the lift was also observed, and showed to be approximately constant regardless of the airfoil dynamics. The separation line, dividing the separated fluid over the airfoil and the free stream flow, was observed to advance at a uniform velocity which differed little as pitch rate was varied.