Configuration and Frequency of Longitudinal Vortices Shedding from Two Circular Cylinders in Cruciform Arrangement.

Abstract

A new vortex excitation of a circular cylinder placed in an otherwise uniform flow was reported in earlier papers by the present authors. This excitation is induced by longitudinal vortices which are originated by setting another cylinder downstream of the first one in cruciform arrangement with a gap less than one half of the cylinder diameter. In this work, the configuration and shedding frequency of the longitudinal vortices causing the excitation were investigated through flow visualization experiments in a water tunnel and quantitative measurements in a wind tunnel. The visualized configurations of the two types of longitudinal vortices, the trailing vortex and the necklace vortex, correlated well with their corresponding contours obtained from pressure-velocity cross-spectrum measurements in the wind tunnel. Strouhal numbers of the longitudinal vortices were also investigated over a wide range of the Reynolds number Re, by using both a wind tunnel and a water tunnel. While the Strouhal numbers for the two longitudinal vortices are practically independent of Re for Re>10000, they increase appreciably with an increasing Reynolds number in the range of Re<10000.