Experimental study of the flow characteristics on the top surface region of a finite circular cylinder

Abstract

An experimental study is carried out to investigate the three-dimensional separated flow characteristics on and over the free-end zone of a finite circular cylinder (aspect ratios of 1.25 and 4.25) mounted on a flat plate. Firstly, a flow visualization using the oil streak-line method and the smoke-laser light sheet arrangement is carried out to grasp the three-dimensional separated flow pattern limited to over the free end surface region. The experiment is performed for the cases of two Reynolds numbers: $5.92 × 10^3$ and $1.48 × 10^5$. An owl-faced configuration of various kinds of singular points (two focal points and a saddle point in the fore-half portion on the free-end surface and two nodes and saddle point in the rear-half portion) is disclosed from the oil surface flow visualization. Critical points on the free-end surface conjectured for the visualization are topologically characterized. The smoke-laser light sheet visualization, to aid in understanding the oil streak-line pattern, clearly demonstrates that a pair of tornado-like vortices, evolving from the eyes of the owl face, march along the downstream distance over the free end surface together with a pair of side tip vortices. The owl-faced configuration on the free-end surface, the pair of tornado-like vortices, and the pair of side tip vortices are also visualized through a numerical study using a commercial CFD code (CFD-ACE). Secondly, relation between the surface flow pattern and the local mass transfer characteristics on the free end surface is investigated using the naphthalene sublimation technique for the flow of aspect ratio of 1.25 and the Reynolds number of $1.48 × 10^5$. Several kinds of critical points on the free-end surface are also identified by the mass transfer measurement. It is found that the rate of mass transfer is largest in the vicinity of the nodes in the rear-half of the free-end surface. The Sherwood number near the node points in the rear-half portion is about twice a...