Tandem cylinders in impulsively started flow

The impulsively started flow held for circular cylinders of equal diameter arranged in tandem was investigated using flow visualization and particle i mage velocimetry (PIV), over a longitudinal pitch ratio range of L/D = 1.0- 3.0, and for Reynolds numbers from Re = 1200-3800. The PIV technique was us ed to obtain a time history of the instantaneous in-plane vorticity field f rom the moment of impulsive start, from which the spatial and temporal deve lopment of the flow was studied. Measurements of vortex strength and vortex position relative to the cylinders were obtained from these data. Three ty pes of fluid behaviour were identified based on L/D: single bluff-body beha viour when the cylinders are in contact, constrained streamwise growth and lateral expansion of the gap recirculation zones at small and intermediate L/D, and independent formation of recirculation zones similar to a single i mpulsively started circular cylinder at larger L/D. (C) 1999 Academic Press .