Flow-induced oscillations of three tandem rotating cylinders

The oscillation responses and the associated wake structures of three rotating tandem cylinders, mounted on elastic supports in both the streamwise (x-) and transverse (y-) directions, are numerically studied. Finite element computations are carried out at Reynolds number Re = 150, where the flow is two-dimensional (2-D), and at Re = 2000 (3-D flow), varying the reduced velocity (U*) in the range U* = 2-14. Each cylinder is placed at a distance of 5 diameters (5D) from its immediate neighbor. In the two-dimensional flow, the upstream and the second downstream cylinders are rotated at the rotation rate (alpha) of 1, while for the first downstream cylinder, alpha = 0, 0.5, and 1 are employed. In the 3-D computations, alpha = 1 is considered for the three bodies. It is observed that at Re = 150, all the cylinders exhibit three distinct lock-in modes, namely, modes I, II, and III. Depending on the rotation configuration, 2S, P + S, and 2P shedding patterns appear in the wake of upstream cylinder, while the downstream cylinders shed 2S, P, P + S, 2P, and T + P modes of primary vortices. In the 3-D flow (at Re = 2000), the upstream cylinder exhibits a bell-shaped profile in the variation of amplitude response as a function of U*. Oscillation responses of the two downstream bodies appear in three distinct regions. During high amplitude oscillations, the upstream cylinder sheds the 2P mode of primary vortices, while in the near wakes of the two downstream bodies, small and incoherent primary vortical structures form due to the presence of three-dimensional instability. Low and high amplitude responses are associated with weak and strong 3-D flow instabilities, respectively. Published by AIP Publishing.