Finite element and machine learning modeling of vortex-induced vibrations of the upstream cylinder in tandem configurations: Effects of wake interference and geometry

Wake interference influence on the transverse vibrations of an elastically mounted upstream cylinder in tandem configurations is investigated using finite-element simulations and a data-driven surrogate model at Reynolds number Re=150 over a reduced-velocity range U*=3–20. Three cross sections are considered: square, circular, and diamond (square rotated by 45°), with streamwise spacing varied from Lx=3B–10B (or 3D–10D for the circular case). For the square cylinder, gaps Lx≤5B confine the interstitial shear layers, intensifying vortex shedding, strengthening fluid–structure energy transfer, and producing large-amplitude lock-in. Circular cylinders, with smoother separation, exhibit weaker sensitivity to spacing, whereas the diamond geometry shows intermediate behavior: synchronization dominates the lock-in response, but wake interference controls vibrations once synchronization is lost. To assess three-dimensional effects, simulations of the square-cylinder configuration at Re=1000 and U*=10 reveal that small gaps suppress spanwise instabilities, preserve wake coherence, and amplify oscillations. The results demonstrate that streamwise spacing systematically modulates upstream wake coherence, providing a mechanism to either enhance or mitigate vibrations in tandem arrangements.