Efficiency and accuracy of the immersed boundary method in offshore structures: Effects of patterns of cross-flow and in-line finite protrusions

In this paper, a rigorous examination of the immersed boundary method (IBM) and some of its offshore implications are presented. Marine risers with helical strakes and marine-fouled structures are modeled with finite surface protrusions on a circular cylinder. It is shown that a slight difference in the implementation of the cut-cell IBM can make it much more efficient. A series of simulations are conducted with and without IBM to show its accuracy and efficiency. Effects of mesh metrics on performance of conformal grids is investigated. Proximity factor (PrF) is introduced that divides the hydrodynamic characteristics of patterns into reinforcing and mitigating. It has been found that the IBM simulates hydrodynamic coefficients and Strouhal numbers with an average accuracy of 94.1% with the least preprocessing time. The IBM needs 73% less computation time than hexagonal-only body-fitted meshes made with O-grids. The IBM reaches vortex shedding 7.2% sooner than combined-cell body-fitted meshes generated by snappyHexMesh. The IBM's performance is less dependent on geometries. The proximity factor shows that a finite protrusion in a cross-flow direction can be utilized to enhance the performance of energy harvesters. Besides, with the least force amplification in unwanted directions, it should be placed near a free surface.