Nonlinear coupling of fluctuating drag and lift on cylinders undergoing forced oscillations

Abstract

Experiments with cylinders undergoing forced oscillations show a coupling between the fluctuating lift and drag forces as a result of nonlinear wave interactions. The fluctuating lift and drag components are computed from the instantaneous pressure around the azimuth of a circular cylinder measured with an array of miniature microphones. Quadratic nonlinear interaction between the von Kármán vortex-shedding modes and the forcing field produces sum and difference modes, which appear in the spectrum of the surface pressure signals when the forcing frequency is different from the von Kármán vortex-shedding frequency, ($f_{o}$). The spatial symmetry of the interacting modes determines whether the combination modes appear in the lift spectrum or the drag spectrum. Furthermore, the spatial symmetry of the modes can be predicted from the symmetries of the two interacting parent modes. Crossflow excitation of the cylinder produces combination modes that appear in the drag spectrum. Consequently, attempts to enhance the fluctuating lift by crossflow excitation will necessarily affect the fluctuating drag through nonlinear interaction.