Numerical investigation of the flow past two transversely forced oscillating cylinders in a tandem arrangement

A numerical investigation is conducted of the flow past two transversely oscillating cylinders in a tandem arrangement for Reynolds number 100, gap L = 2.0D to 7.0D, and oscillation amplitude A = 0.1D to 0.7D, where D is the width of each cylinder. The moving reference frame method combined with the spectral/hp element method is employed to simulate the flow in the lock-in region. The results reveal that the fluid forces and flow structures are sensitive to the oscillation amplitude and gap. The competition of the lift forces was observed: the lift forces of the up-cylinder are higher than the down-cylinder at small gaps (L/D = 2–3) while they gradually show inverse relations at wide gaps (L/D = 6–7). Moreover, all the flow structures behind the down-cylinder show weak 2S or 2S mode for the stationary cases, whereas the type of vortex mode changed into 2S (L/D = 2.0), 2P (L/D = 5.0), and subsequently to C(2S) (L/D = 6.0) with oscillating amplitude increased to A/D = 0.7. The relations between the fluid forces and statistical flow structures as well as the vortex modes transition were discussed in detail for various oscillation amplitudes and gaps. This research is expected to enhance the fundamental understanding of flow-induced forces and the interaction between a flow and tandem square cylinders.