Dynamic responses of asymmetric vortices over slender bodies to a rotating tip perturbation

The dynamic responses of asymmetric vortices over a slender body to a rotating tip perturbation were investigated experimentally in a wind tunnel. A small rotating nose with an artificial micro-perturbation on the nose tip was driven by a servomotor with various rates to change azimuthal locations of tip perturbation. Wall pressures and spatial velocity fields were measured using pressure scanner and particle image velocimetry based on a phase-locked method. The results show that the spinning tip perturbation enables asymmetric vortices to exhibit significantly dynamic characteristics different from a case with a static perturbation. The orientations of asymmetric vortices and associated side forces show apparent phase delay that are enlarged with increasing rotational rates of the nose. The switching rates of asymmetric vortices among various orientations also increase with the rotational rates increasing, but asymmetry level of vortices is lowered, which reveals that the asymmetric vortices change requires an amount of time to switch from one orientation to another. The phase delays of vortices, however, are determined by the amount of time required for the propagation of disturbance waves along a body axis. As the rotational frequencies are sufficiently high, the orientations of vortices almost hold to be unchanged. The unchanged orientation of vortices is asymmetric, depending on the directions of rotation. The asymmetric vortices arising from high-frequency rotation of the nose are attributed to wall effects induced by the rotating nose with a finite length. In addition, there exist small intrinsic vortex oscillations which are superimposed on the average vortex structures with symmetric and asymmetric orientations for the cases of static and rotational tip perturbations.