Direct Correlation of Fluctuating Lift with Radiated Sound for an Airfoil in Turbulent Flow

Abstract

An airfoil in turbulent flow is thought to radiate sound in consequence of its fluctuating lift. The instantaneous sound and lift should be connected by Curle's equation, which describes sound radiation from a rigid surface in contact with a turbulent flow. It follows that the cross correlation of sound and lift should be related in a corresponding way to the lift autocorrelation. Such correlations were measured in the present investigation of an airfoil in a jet flow, and the Curle relationship was verified semiquantitatively. The predicted comparative shapes of the correlation curves were found; the comparative magnitudes showed a 27% (2.7-dB) discrepancy. This small discrepancy is attributed to model vibration that falsely enhanced the lift signal via inertial loading. The work answers in part questions that have been raised as to the applicability of Curle's equation. Further, it confirms that a rigid surface in contact with a turbulent flow need not be inherently passive—a reflector of sound—but can be an active generator of sound.