Measurement of the unsteady lift of thick airfoils in incompressible turbulent flow

Abstract

The unsteady lift spectrum for airfoils in turbulent flow has been measured in a water tunnel experiment. The results provide validation data for analytical models that account for the effect of airfoil thickness on the high frequency gust response. A series of four airfoils with elliptical leading edge profiles and thickness-to-chord ratios ranging from 8% to 16% were tested in grid-generated turbulence. The turbulent velocity spectrum was measured using Laser Doppler Velocimetry, and was found to be reasonably well approximated by an isotropic, homogeneous turbulence model. The unsteady force measurement setup and calibration procedure were designed to minimize the effect of system resonances, and contamination from facility vibration was reduced using a multiple coherence noise removal technique. Measurements of the unsteady lift spectrum were made at six speeds over the Reynolds number range 0.5–1.6million, and the results were collapsed into a non-dimensional force spectrum for the non-dimensional frequency range 1<fc/U∞<10. The experimental results agree very well with theory and confirm the importance of accounting for thickness to avoid overprediction of the forces in the high frequency range.