Flow-induced vibration of the filleted (rounded corner) oscillating cylinder is numerically investigated at Re = 150 and Pr = 0.7. The upstream cylinder (UC) is static while the downstream cylinder (DC) is elastically mounted which is allowed to vibrate in the transverse direction with a spacing ratio (L/D) = 5 and staggered angle (α) of 45°. Square cylinder is gradually filleted into the circular cylinder by changing r* (filleted radius) = 0 (square cylinder), 0.5, 0.75 and 1 (circular cylinder). Computations were carried out for reduced mass m* = 10 and varying reduced velocity (Ur) = 2, 4, 6, 8 and 10, structural damping constant is set to be zero which gives rise to high vibrational amplitude. Both cylinders were maintained at a constant temperature (T* = 1) while the upcoming flow is set to be at T* = 0. Vibrational characteristics are scrutinized with help of frequency characteristics of vibrating cylinder, vibrational amplitude, drag coefficient, lift coefficient and instantaneous z-vorticity contours. When natural frequency (fn) and vortex shedding frequency (fs) overlap, it causes synchronization or lock-in. The lock-in phenomenon usually occurred at Ur = 6 leading to higher vibrational amplitude for all geometries and dropped at Ur = 8 but persisted at Ur = 8 and 10 for r* = 0. Due to higher vibrational amplitude, the flow structure is more complicated at Ur ≥ 6. The maximum value of average Nusselt number (Nuavg) is at Ur = 6 for circular DC, whereas the minimum lies at Ur = 2 for square upstream cylinder. Generated results envisage results of flow interferences and forced convection of tube arrays in heat exchangers and marine structures. Understanding the mechanisms of flow-induced vibration in staggered cylinders is crucial for designing and optimizing these systems, as well as for ensuring their safe and reliable operation.

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