Three-dimensional nature of vortices in the near wake of a wavy cylinder

Abstract

The near wake of a wavy cylinder has been experimentally investigated using various techniques, including Laser Doppler Anemometer, Laser-Induced Fluorescence Visualization and Digital Particle Image Velocimetry. The work aims to provide understanding to the mechanism of the cross-flow around a wavy cylinder as well as to comprehend why the introduction of relatively small degrees of spanwise waviness could have a significant effect on drag reduction and a corresponding suppression of cylinder vibration. Distributions of the mean and fluctuating velocity components along the streamwise, spanwise and transverse direction are presented. The development of the flow patterns and the corresponding vortex interactions are obtained by flow visualization. The experimental results indicated that the average vortex formation length behind the wavy cylinder is longer than that behind the circular cylinder. It appears that there is a direct link between the long formation length and drag reduction and vibration suppression. For the wavy cylinder, the wake on the saddle plane has a longer vortex formation region and a more rapid reverse flow, as well as being wider than that on the nodal plane. Furthermore, the spanwise flow is from the saddle plane towards the nodal plane on both sides of the wavy cylinder. It was deduced that the free shear layers shed from the points near the saddles extend along the spanwise direction, while the shear layers near the nodes contract. The turbulence correlations show that the vortex streets in the circular cylinder wake are more regular, while the wavy cylinder wake shows more incoherent turbulence due to enhanced turbulent mixing; a consequence of three-dimensional (3-D) effects. The mean velocity and turbulence data also provide an important database for the validation of turbulence modelling and 3-D numerical simulations.