Vortex induced vibration response and energy harvesting of a marine riser attached by a free-to-rotate impeller

The vortex induced vibration response of a riser attached by a free-to-rotate impeller and the rotation response of the impeller are investigated numerically by computational fluid dynamics models in order to examine the feasibility of energy harvesting. Unsteady Reynolds- Averaged-Navier-Stokes equations coupling with a shear stress transport k-ω turbulence model are employed to calculate the flow field with Reynolds number ranging from 30512 to 488567, while an improved fourth order Runge-Kutta method is adopted to capture the motion of the system. The results indicate that the boundary layer separation points move from the riser surface to the tips of the impeller. The classical three branches of amplitude response are found in the vibration response of a riser with an impeller. However, the rotation of impeller delays the appearance of the lock-in phenomenon to a larger incoming flow rate. Although the attachment of impeller does not enhance the cross-flow vibration, the enhancement of the in-line vibration is apparent. The energy harvesting should be based on the premise of the riser's security service. Therefore, the two goals, vibration suppression and high energy extraction, are achieved simultaneously when the reduced velocity is larger than 9.85 for a riser with a free-to-rotate impeller.