Hydrodynamic In-Line Force Coefficients of Oscillating Bluff Cylinders (Circular and Square) at Low Reynolds Numbers

An attempt to measure indirectly the hydrodynamic drag $(cD)$ and inertia $(cM)$ coefficients on oscillating bluff cylinders (circular and square) in quiescent fluid at low Reynolds numbers (low Stokes number) is presented. The Keulegan–Carpenter number was below 15. The experimental approach is based on performing free-decay tests of a spring-mounted cylinder submerged in a water tank. The identification of the instantaneous modal parameters (damping and frequency), via Hilbert transform, of the decaying oscillations allows the determination of $(cD)$ and $(cM)$ by direct comparison with the damping and natural frequency of the system in still air (tank without water). Advantages and shortcomings of this novel experimental approach are presented along the paper.