The present paper aims to gain an understanding of the physics associated with the fluid-structure interaction behavior of a flexible filament interacting with the wake of an upstream rigid cylinder situated at a finite distance. The computational framework of the present high fidelity numerical simulations consist of an incompressible Navier-Stokes solver strongly coupled with a chord-wise flexible structural model based on a partitioned approach. The effect of body-wake interactions on the resulting vibrating modes of the chord-wise flexible foil due to its passive flapping is studied in detail. The gap between the cylinder and the flexible filament is seen to play a major role in the vortex impingements on the flexible flapper and the subsequent interactions. Insights are obtained into the dynamics through a parametric study for different values of this finite gap. Moreover, the effect of the length and thickness of the flapper on its dominant mode-shapes is also investigated. The present study shows that higher values of the gap and the length of the filament lead to the loss of periodicity in the body-wake interactions which is of current interest. © 2021, The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.