Dynamic Response of a Turbulent Cylinder Wake to Forced Excitation

Three-dimensional large-eddy simulations were carried out to determine the dynamic response of a turbulent cylinder wake to forced excitation at a subcritical Reynolds number of 2580. The excitation frequency was varied across the primary lock-on range while the amplitude of sinusoidal velocity perturbation and the mean velocity imposed at the inflow boundary were kept constant. The velocity fluctuations in the wake and the fluctuating forces on the cylinder are analyzed employing spectral, time-domain and phase-portrait methods. The results show that the dynamic response of the inline force is different than that of the transverse one on the border of the lock-on range; while the inline force exhibits a phase-locked response, the transverse force indicates an intermittent response. This behavior is linked to the wake dynamics which is similar to that of the transverse force. This result is explained on the basis of the Morison equation which shows that the inline force is biased by the inertial components associated with the added mass and pressure waves in unsteady flows. It is further shown that the existence of a mean velocity component alters radically the dynamics of the inline force and appropriate ranges of a dimensionless parameter are proposed to describe the response.