Hydrodynamic forces on a structure are the manifestation of fluid-structure interaction. Since this interaction is nonlinear, these\nforces consist of various frequencies: fundamental, harmonics, excitation, sum, and difference of these frequencies. To analyze this\nphenomenon, we performnumerical simulations of the flow past stationary and oscillating cylinders at low Reynolds numbers.We\ncompute the pressure, integrate it over the surface, and obtain the lift and drag coefficients for the two configurations: stationary and\ntransversely oscillating cylinders. Higher-order spectral analysis is performed to investigate the nonlinear interaction between the\nforces. We confirmed and investigated the quadratic coupling between the lift and drag coefficients and their phase relationship.\nWe identify additional frequencies and their corresponding energy present in the flow field that appear as the manifestation of\nquadratic nonlinear interaction.
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