WER-18: Further investigations of near-field wakes of circular and 12-sided cylinders and effects of shrouds and strakes

Abstract

Near-field physical characteristics of wakes downstream of cylinders were determined in a wind tunnel . Cylinders included a 12 - sided cylinder , a plane circular cylinder, circular cylinders surrounded by shrouds of 46% porosity and of 79% porosity, and the basic circular cylinder with plane strakes and with perforated strakes added . Characteristics measured included velocity deficit of the air in the wake, the dominant frequency of the periodic component of the wake vibration, and the level of intensity of the general turbulence of the wake. Conclusions revealed include the following : Both circular cylinders and 12-sided cylinders have low velocity wakes which have strong periodic motion . The chief factor determining the nature of the wake of a circular cylinder is the Reynolds number; supercritic~l Reynolds numbers produce wakes which are smaller and less turbulent . Geometry and orientation of the 12- sided cylinder strongly affect the character of the wake with little sensitivity to Reynolds number. The wake is marked by lower veloci ty , higher turbulence and stronger periodic motion than the wake of a round cylinder . The periodic motion may be attenuated by the use of porous shrouds and strakes. These devices produce, however, broader wakes with lower velocities and with more extensive regions of reversed flow. The shrouds are superior to the strakes; they are not sensitive to wind direction as are the strakes.