OSCILLATING CHARACTERISTICS OF EXIT STREAMS FROM BUNDLES OF DIAMOND-SHAPED CYLINDERS IN STAGGERED ARRANGEMENT

摘要

Oscillation of exit streams, called flip-flop phenomenon, is seen in bundles of cylinders of various cross-sectional geometry. The oscillating frequency in dimensionless form, called Strouhal number, is found to be roughly constant with the Reynolds number. However, there is no thorough understanding of the flow characteristics which lead to flip-flop oscillation phenomenon. In the present work, flow visualization is performed inside diamond-shaped cylinder bundles and the oscillating characteristics of streams exiting downstream are investigated to determine the mechanisms that trigger flip-flop phenomenon. The apex angle of diamond-shaped cylinders staggeredly-arranged cylinder bundles are varied. A turbulence mesh is installed at the entrance to the first flow passage in each bundle to investigate turbulence effect on flow patterns. Flow visualization is conducted by injecting fluorescent dye into the flow stream illuminated by means of a laser light sheet. A CCD camera is used to record the oscillations of jet streams exiting from the downstream end of the cylinder bundles. The oscillating frequency (in term of the Strouhal number) is correlated with the Reynolds number. The mechanisms for triggering the flip-flop phenomenon are disclosed.