Unsteady Characteristics of Longitudinal Vortices behind a Stepped Circular Cylinder and Nonlinier Interaction

Since a stepped circular cylinder is composed of two cylinders of different diameters, two different vortex shedding frequencies are presented in the wake. In the wake of a stepped circular cylinder, a pair of longitudinal vortices is formed behind the step. From spectral analysis, the longitudinal vortices are found to fluctuate with time at the frequency equal to the difference of the two vortex shedding frequencies. In this experiment, time variation of the flow field behind the stepped circular cylinder was investigated by phase ensemble averaging technique by means of X type hot wire anemometer. Pressure transducers are installed in the each cylinder part of different diameter, and their signals are digitally mixed and digitally filtered to produce an signal fluctuates at difference of the two vortex shedding frequencies, and it is used as reference signal of the phase ensemble averaging. From results of the experiments, velocity vector distributions in the plane perpendicular to the mean stream direction at the downstream of the stepped circular cylinder were found to vary drastically with time, and the strength of the longitudinal vortices was also found to vary periodically with time at the difference of the two vortex shedding frequencies. Furthermore, from TSC analysis of velocity signal obtained by means of I type hotwire anemometer, nonlinear interaction between two vortex shedding frequencies was detected around the longitudinal vortices.