Forced oscillation experiments on time-averaged drag and lift forces of two closely spaced circular cylinders in steady flow

Forced oscillation experiments were conducted to systematically investigate the time-averaged drag and lift forces acting on two closely spaced circular cylinders in steady flow. The rigid cylinders were driven by an external actuation mechanism according to prescribed motions and were not allowed to undergo flow-induced vibration. Three modes representative of closely spaced bundled risers were examined: translational, rotational, and separation motions. Drag and lift coefficients were measured over controlled ranges of oscillation amplitude, excitation frequency, flow orientation angle, and cylinder spacing. Single-cylinder tests were first performed to validate the experimental setup against existing forced-oscillation data. For two-cylinder configurations, translational excitation produced substantial downstream drag reduction in near-tandem arrangements because of wake shielding. Reorganization of the results by flow orientation also revealed trends consistent with classical stationary two-cylinder experiments. Mean drag was governed primarily by oscillation amplitude, while excitation frequency had a secondary but non-negligible effect. Empirical drag amplification formulations developed for oscillating single cylinders therefore provide a first-order approximation for translationally oscillating two-cylinder systems. Rotational and separation excitations, however, produced force characteristics not represented by existing drag models. Rotational excitation showed lower drag amplification than translational excitation, whereas separation excitation generated strong proximity effects, including drag amplification and lift reversal near contact. Together with previously reported added-mass and damping data obtained under identical conditions, these results provide a consistent experimental force database for modeling hydrodynamic interaction in offshore multi-pipe systems.