Effect of Periodic Longitudinal Gusts on Airfoil Performance Under Separated-Flow Conditions and Elevated Freestream Turbulence Levels*

Canonical studies have shown that kinematic forcing in separated flow conditions can produce a leading-edge vortex, that can grow to the order of the airfoil chord length and produce significant transient loading. The present research experimentally investigates separated-flow dynamics for an airfoil under the influence of periodic longitudinal gusts. Unsteady forces over an SD7003 airfoil are measured for a wide range of gust frequencies, angles of attack, and mean Reynolds number. The results show that the mean forces increase significantly when the natural vortex shedding frequency locks in with the gust frequency. Also, the lift amplitude increases considerably when the circulatory contributions are in phase with the unsteady freestream velocity and reduces drastically when out of phase. The effects of elevated freestream turbulence levels are also considered, where turbulence intensities below 1% have a weak effect on the unsteady force generation. However, significantly different trends are observed at larger turbulence intensities.  Smoke visualization and phase-averaged particle image velocimetry techniques are employed to study the associated flow physics. The final presentation will answer the following questions: 1) What is the impact of gust frequency, angle of attack and freestream turbulence levels on unsteady loads over an airfoil? 2) How are the dynamics associated with the unsteady vortical structures correlated with unsteady force generation?