Effect of bed proximity on the in-line forces acting on a cylinder oscillating in still water

• The Morison equation underestimates the peak values of forces acting on near bed pipes.
• A modification to the Morison equation for near bed cylinders is proposed.
• The forces are obtained from direct force measurements and pressure measurement.
• The terms added to the Morison equation are determined based on spectral analyses results.

In this paper, measured forces acting on a circular cylinder oscillating over a plane bed in still water are used to propose a modification to the Morison equation for near bed cylinders. Emphasis is placed on the effect of gap between the cylinder and the bed on the forces. The range of Keulegan–Carpenter number of the oscillatory cylinder in the present study is between 15 and 40. The flow is in the subcritical regime with Reynolds number (Re) between 9500 and 26,000. Different gap ratios between 0.1 and 2 are considered to study the effect of bed proximity on the hydrodynamic forces exerted on the cylinder. The cylinder and the plane bed are smooth. The forces are obtained from direct force measurements and from pressure measurements around the cylinder. The forces obtained from the experiments and from the Morison equation compare well for the case of an isolated cylinder but they differ for the cylinder close to the bed with respect to the amplitude and phase angle. Spectral analysis is performed and an additional term according to the frequency content of the forces is added to the Morison equation to account for the effect of bed proximity.