VIV of Six-Cylinder Array with Partial Biofouling in Oscillatory Flow

This paper presents a numerical investigation of vortex-induced vibration (VIV) of six elastically mounted circular cylinders in oscillatory flow, three smooth and three biofouled with triangular surface roughness elements. The study aims to characterise the influence of the longitudinal spacing ratio (L/D=3,4, and 5) on the two-degree-of-freedom (2DOF) vibration response at a constant Keulegan–Carpenter number of KC=10. Simulations are performed using the transient RANS equations with the SST k–ω turbulence model, and structural motion is resolved using a dynamic mesh approach. Lock-in behaviour is observed over the reduced velocity range 5≤Ur≤10. Biofouled cylinders generally exhibit higher in-line displacement amplitudes than smooth cylinders in the initial and lower lock-in branches, whereas smooth cylinders tend to attain higher in-line amplitudes in the upper lock-in branch. The spacing ratio L/D is found to significantly influence the response, with peak vibration amplitudes varying non-uniformly across the array and no single spacing configuration being optimal for all cylinders. This behaviour is further supported by analyses of trajectories, frequency content, and vorticity fields. Among the smooth cylinders, the middle cylinder exhibits the largest in-line displacement amplitude of 3.28D at L/D=5 and the largest cross-flow displacement of 1.34D at L/D=3. For the biofouled configurations, the middle and upstream cylinders show the highest in-line displacement amplitude of 2.69D at L/D=4, while the maximum cross-flow displacement of 1.27D is observed for the upstream cylinder at L/D=5.