A QUANTITATIVE STUDY OF THE FLOW AROUND AN IMPULSIVELY STARTED CIRCULAR-CYLINDER

The detailed flow structure behind an impulsively started circular cylinder has been investigated experimentally. The Reynolds number based on the steady state velocity and the diameter of the cylinder was 500 to 3,000. This work is unique in that unsteady spatial velocities were measured simultaneously by a quantitative visualization technique - Laser Induced Photochemical Anemometry (LIPA). The surface vorticity at theta = pi/2 and vorticity distribution behind the cylinder in the Lagrangian coordinates (i.e. coordinates fixed on the cylinder) were calculated from the measured velocities. The surface vorticity shows in the early stage of flow development a close agreement with the previous results obtained by analytical and numerical approaches. The large-field velocity and vorticity information provides an insight into the formation process of the vortices downstream of the cylinder. In addition to the quantitative information, the results of visualized flow pattern obtained by LIPA technique are also presented.