Effect of pulsatile inflow on wake transition of surface-mounted circular cylinder in presence of upstream square cylinder

A three-dimensional numerical study has been performed on flow past a surface-mounted circular cylinder in the presence of an upstream square cylinder under pulsatile inflow. Computations are performed using Open Source Field Operation and Manipulation. While previous studies have extensively examined flow past isolated or tandem cylinders under steady inflow, the combined effects of pulsatile inflow and wake interference in surface-mounted configurations remain insufficiently understood. In order to address this, pulsatile inflow with a non-zero mean is prescribed at the inlet of the computational domain, where both amplitude (α) and frequency (f) of pulsation are varied. The center-to-center distance (S) between the cylinders takes three distinct values. The aspect ratio (H/D) and Reynolds number (Re) are kept fixed at 5.0 and 200, respectively. At smaller spacing, strong wake interference leads to suppressed and distorted vortex shedding, while increasing spacing promotes the recovery of shedding patterns. The introduction of pulsatile inflow significantly alters the wake structure, with increasing amplitude enhancing vortex strength and flow unsteadiness. Higher pulsation frequency further intensifies wake fluctuations and delays flow stabilization, particularly in the downstream cylinder wake. Hilbert spectra corresponding to the time series of the transverse component of velocity (v) have been employed to explore the characteristics of unsteady wakes of both the cylinders. The nature of complex wakes has been analyzed using snapshot-based dynamic mode decomposition to identify the dominant modes and associated frequency along with growth/decay rates. The present study provides new insights into the complex wake transition mechanisms under combined geometric and inflow unsteadiness, thereby contributing to the fundamental understanding of vortex dynamics in multi-body systems. From a practical perspective, the findings are useful for predicting flow-induced forces and designing engineering systems subjected to pulsating flows, such as marine structures, energy devices, and compact thermal systems.