Study on suppression of violent sloshing in a multiple tuned liquid column damper by porous media layers

Multiple tuned liquid multi-column damper (MTLCD) is a passive damping device that is used to suppress the motion of semisubmersible floating offshore wind turbine (FOWT). However, the violent sloshing within the MTLCD under extreme sea environments reduces the anti-pitch motion effect. Thus, this study employed an innovative MTLCD with porous media layers, which can increase the flow energy dissipation, suppress the violent sloshing and enhance the damping moment (Mdamping). The OpenFOAM is used to simulate the hydrody-namic force in MTLCD, where the numerical method is validated by the decay test of a scaled model. The numerical natural period of the MTLCD shows good agreement with the experimental and analytical results. The sloshing characteristics, including pressure, damping moment impulse and work of the MTLCD without porous media, are analyzed firstly under simple harmonic rotation amplitudes (theta(0)) of 2(degrees)-20(degrees). Subsequently, porous media layers are installed in the MTLCD with different heights (hp), including immersion and suspension, in order to improve the MTLCD's damping moment. Wavelet transform and wavelet-based bicoherence are utilized to analyze the nonlinear behaviors of the damping moment under violent sloshing. The breaking, surging and converging in the MTLCD are captured by the numerical method. The analyses show that the porous layers can suppress the violent sloshing by dissipating the fluid kinetic energy, and the damping moment work can be significantly improved when theta(0) >6(degrees). The liquid surging with bubbles leads to the nonlinear behaviors in the MTLCD, the sloshing energy transfer to the frequency doubling by three-wave interaction.