Transient Loads Occurring over a Thin Airfoil Subjected to Large-Scale Freestream Turbulence

Abstract

In order to identify the influence of large-scale freestream turbulence on the transient loads occurring over an airfoil, surface pressures were measured at various chordwise and spanwise stations over a thin flat–plate airfoil. The turbulence intensity was varied from nominally smooth flow conditions (1.2%) to 12.3% while the integral length scale ranged from 0.15 m up to 1.3 m. The individual influence of turbulence intensity and integral length scale on the transient lift, pressure drag, pitching and rolling moments experienced by the airfoil was identified from these representative turbulence conditions. Increase in turbulence intensity resulted in a significant increase in the magnitude of force and pitching moment fluctuations experienced by the airfoil. Similar to the effect of changing turbulence intensity, increase in integral length scale also rendered an increase in lift and pressure drag fluctuations. However the rolling moment fluctuations were smaller at longer length scales. While the increase in force fluctuations at longer length scales has been attributed to the higher peak energy present at lower frequencies at longer length scales, the reduction in rolling moment was due to the increased lateral correlation of the oncoming turbulent eddies.