Symmetric and antisymmetric periodic disturbances introduced directly into the boundary layer on a circular cylinder at low Reynolds number are shown by experiment to be capable of modifying the vortex formation process and changing the vortex shedding frequency. Spectral measurements have shown that the antisymmetric vortex shedding mode is strongly coupled to the symmetric first harmonic mode. When symmetric excitation is applied, three different shapes of the mean velocity profiles can be identified as the forcing amplitude is increased. At low forcing amplitudes nonlinear interaction between the forcing field and the natural wake oscillator produces sum and difference modes. Symmetric forcing with intermediate-amplitude disturbances suppresses the natural shedding frequency, and the dominant vortex shedding energy appears as a sinuous mode at half the excitation frequency. At high symmetric forcing amplitudes a threshold is reached, above which the large-scale vortices do not form. The symmetries of the combination modes follow two simple rules based on the symmetries of the interacting modes. The symmetry rules provide an explanation for the fundamental difference in wake structure that occurs between symmetric forcing and antisymmetric forcing.

No abstract

No abstract