Radiated Sound Generated By Airfoils in a Single Stream Shear Layer

The spanwise correlation length scale of lateral velocity and the gust response function are the quantities of interest in predicting the sound production from an airfoil. Typically, these quantities are taken to be a correlation length scale model based on isotropic turbulence and Sears' gust response function, respectively. The present study is an experimental investigation of the accuracy of these selections.

Detailed velocity measurements were obtained for a single stream shear layer, a flow known to be anisotropic at large turbulence length scales. The measured spanwise correlation length scale was found to have excellent agreement with the model correlation length scale down to frequencies where the flow was weakly anisotropic. Acoustic measurements were acquired for three airfoil geometries placed in the shear layer. It was observed that sound predictions using either the model correlation length scale or the measured correlation length scale did not match the measured acoustics unless the attenuation of sound due to airfoil thickness was accounted for by applying an appropriate correction to Sears' gust response function. It was also found that a gust response function correction for flat, elliptic leading edge airfoils can be used for a thick, non-elliptic leading edge airfoil if the thickness parameter in the correction is taken to be an effective airfoil thickness.