Unsteady lift of thick airfoils in incompressible turbulent flow

nuThe unsteady lift forces that act on an airfoil in turbulent flow are an undesirable source of vibration and noise in many industrial applications. Methods to predict these forces have traditionally treated the airfoil as a flat plate. At higher frequencies, where the relevant turbulent length scales are comparable to the airfoil thickness, the at plate approximation becomes invalid and results in overprediction of the unsteady force spectrum. This work provides an improved methodology for the prediction of the unsteady lift forces that accounts for the thickness of the airfoil. An analytical model was developed to calculate the response of the airfoil to high frequency gusts. The approach is based on a time-domain calculation with a sharp-edged gust and accounts for the distortion of the gust by the mean flow around the airfoil leading edge. The unsteady lift is calculated from a weighted integration of the gust vorticity, which makes the model relatively straightforward to implement and verify. For routine design calculations of turbulence-induced forces, a closed-form gust response thickness correction factor was developed for NACA 65 series airfoils. The model was then validated by measuring the unsteady lift spectrum using piezoelectric force gages in a water tunnel with grid-generated turbulence. A series of four airfoils with thickness-to-chord ratios ranging from 8 to 16 percent were tested over a wide range of speeds. In addition, the turbulence spectrum was measured using Laser Doppler Velocimetry. The experimental results confirmed that the analytical model accurately predicts the attenuation of the high frequency gust response due to the airfoil thickness.