The effect of a piggyback cylinder on the flow characteristics in oscillatory flow

Two-dimensional numerical simulations of oscillatory flow around a pair of cylinders of unequal diameters are conducted in order to investigate how the presence of the smaller cylinder affects the hydrodynamic forces acting on the main cylinder. A series of calculations are undertaken in which the frequency of the oscillation is held fixed but its amplitude is varied. Computations are carried out for a range of distances of the secondary cylinder from the primary. In addition, the line joining the two centres is allowed to vary across the whole range from the case when the two centres are aligned with the oncoming oscillatory flow right through till the point when the two cylinders sit side-by-side perpendicular to the far-stream flow. The lift and in-line force coefficients are analysed and reasons for the observed dependence on the relative positions of the two cylinders are discussed in terms of the associated flow structures.