Research On Moonpool Resonance Problem Based On Openfoam

Research on moonpool resonance problem is of relatively high impor-tance within ocean engineering application scenario.This research is based on the open source computational fluid dynamics platform-OpenFOAM-to investigate 3-dimensional circular moonpool resonance results from wave interaction and moonpool vertical oscillation.Wave induced moonpool resonance problem can be essentially classi-fied as wave-body interaction problem,which is investigated in this research by a numerical wave tank whose main body is established through combi-nation of InterIsoFoam and Waves2Foam solvers built in OpenFOAM be-forehand.Besides,three turbulent models are also incorporated in this new numerical wave tank,those turbulent models are k-ωSST turbulent model,buoyancy-modified k-ωSST turbulent model and stablized k-ω turbulent model.A newly adopted Geometrical Volume-of-fluid（VOF）free surface capturing method is contained in the InterIsoFoam solver,the 3-dimensional fifth-order Stokes wave is simulated by Geometrical VOF method in this research and its results are compared with results obtained from Algebraic VOF method,the comparison suggests that Geometrical VOF method could better simulate wave propagation with high accuracy.Waves2Foam solver includes Relaxation zone scheme for wave absorption,this research finds that the best practice for wave absorption using relaxation zone scheme is em-ploying exponential weighting distribution within the relaxation zone whose length should exceed 2 times of wave length.In order to further validate the numerical wave tank applying for the wave-body interaction problem,the cylinder run-up problem is simulated in this research.The CFD results are highly agreed with experimental data,which illustrates the newly adopted numerical wave tank could be employed in wave-body interaction problem.Furthermore,the research also reveals that the incorporation of relaxation zone scheme could effectively reduce the length of CFD zone,which leads to the decrease of wave damping when propagating in long distance.The wave induced moonpool resonance problem,or moonpool diffrac-tion problem,is evaluated in this paper,the results show that inner water region oscillates in piston mode,while wave elevations around the outer sur-face of moonpool distribute in a pattern of high surface elevation in xi direc-tion with low surface elevation in xj direction.The wave induced force in xi direction is largest,the second one is force in xk direction,while force in xj direction is negligible.It is observed that wave elevation within the moon-pool rises to its maximum value when the wave frequency（f≈0.615Hz）is coincident with the natural frequency of moonpool structure combined with water region surrounded by moonpool（inner water region）.Besides,the vor-ticity is generated due to fluid viscosity,which explains why the wave height gauged in the numerical wave tank is substantially lower than the one origi-nated from linear potential flow method.When investigating vertical motion induced moonpool resonance prob-lem,namely,moonpool radiation problem,the maximum value of wave el-evation within the moonpool is also found when the frequency of vertical oscillation（f≈0.615Hz）is close to the natural frequency of moonpool combined with inner water region.Different draught will influence the nat-ural frequency of moonpool because the natural frequency is determined by moonpool structure and the inner water region.Besides,the larger the am-plitude of moonpool vertical motion is,the lower the non-dimensional wave elevation η/Am in the central position of moonpool.