Numerical investigations of Piston mode resonance in a moonpool using OpenFOAM
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- Institutt for marin teknikk 
Moonpool dynamics is a complex phenomenon, and is part of a series of problems referred to as gap problems involving entrapped fluid in a small gap. The fluid in these gaps has an infinite number of resonance modes; a piston mode resonance in heave, and an infinite number of sloshing modes. The piston mode is often the most troublesome, and must be considered when designing a moonpool. A numerical verification is done on an experiment of a 2D section of a moonpool in shallow water. The numerical tool used will be OpenFOAM (http://www.openfoam.com/) by OpenCFD. The solver library interDyMFoam with dynamic meshing and free surface modeling using a Volume-Of-Fluid method will be used. Four different configurations of the moonpool will be assessed numerically, but only two will be compared to experimental data. Of these four design alternatives, it will be seen if OpenFoam accurately can describe the physics involved. Before assessing the moonpool case, a verification of a simpler case is done with the NACA0012 free surface experiment by Duncan (1983). This is done with interFoam. Good quantitative agreements were found in all cases; the resonance frequency was found in all cases with 3 digit accuracy. Qualitative agreements were only acceptable in some cases, and 10-20% deficiency was experienced. A boundary conforming grid made 0-2% errors at medium mesh, but was unphysical when refined and was not used. A huge problem was encountered, and not figured out: solutions were often worse instead of better when refining the grid. Possible explanations include numerical diffusion of the indicator variable , insufficient numerically dissipation in the damping zone, reflection energy into the system and poor boundary layer conformity, with maximum of 8 elements within the boundary.