Effect of CFRP on vortex-induced vibration of a brimmed-diffuser shroud for a wind turbine

A brimmed-diffuser shroud for a wind turbine is a ring-like short duct installed around a rotor. It is also called a wind-lens because its shape is designed to gather and accelerate wind passing through the rotor to enhance the power generation efficiency of the wind turbine. This effect results from vortex shedding that is intentionally generated by the brim. However, the periodical vortex shedding can induce a vibration in the thin structure of the wind-lens mostly made of aluminum alloy. Besides, this vortex-induced vibration possibly harms the wind-lens structure in the case where resonance occurs due to the coincidence between the vortex frequency and the natural frequency. In order to prevent the wind-lens from the resonance, the modal characteristics of the wind-lens need to be improved, which are related to the stiffness and mass distribution of its structure. This study conducted a numerical aeroelastic analysis using both three-dimensional FEM (Finite Element Method) and two-dimensional CFD (Computational Fluid Dynamics) for the wind-lens structure to simulate the vortex-induced vibration. The analytic models are the wind-lens whose material is replaced with a CFRP (Carbon Fiber Reinforced Plastic) laminate, and they were compared to the original model made of aluminum alloy. The result showed that the CFRP laminate used for the wind-lens structure is valid for the change of the vibrational modes and an increase in the natural frequencies. Nevertheless, the overall displacement amplitude of the vortex-induced vibration did not reduce. It is because the replacement of CFRP on the diffuser is not much effective on the vibrational characteristics of the most susceptible vibrational mode to the vortex shedding, i.e., rotation of the cross-section even though it improves the vibrational characteristics of the other modes.