Structure of Streamwise Vortices Developping around a Rotating Circular Cylinder

Abstract

The three-dimensional flow structure around a circular cylinder is investigated by visualization experiment using aluminium flakes as tracers when it impulsively starts to rotate at a constant speed in the range of the Reynolds number, 500 <__- Re <__- 4000. The flow in the boundary layer along the cylinder surface with the thickness larger than the critical value is subject to the three-dimensional instability, and then it forms streamwise vortices around the cylinder. The flow structure in the early stage is a raw of these streamwise vortices of almost same size and of opposite rotations alternately along the cylinder axis and it is extended to the whole depth of the boundary layer. Among these vortices adjacent ones interact with each other so as to coalesce or absorb another repeatedly at times and places, and consequently the vortices become greater in size and fewer in number. The boundary layer develops along the cylinder surface with the thickness increasing with time to the power of order 1/2, while the flow structure of streamwise vortices with time to the power of order 1. The development of the former boundary layer is scaled with viscous unit, while that of the latter structure depends on the outer scales as well. In addition the number of vortices decreases exponentially through the interaction with each other. At the higher Reynolds numbers, the local interaction much stronger, but not synchronously in the azimuthal direction, causes to break the axisymmetry of the flow.