A Computationally Efficient Swapping Based Dynamic Mesh Algorithm for Three-Dimensional Fluid-Structure Interactions Involving Large Displacement Towards Free-Surface

Computational fluid dynamic (CFD) simulation of an aircraft or a rocket flying is traditionally performed by fixing the object in the computational domain and assuming fluid is flowing over the body. However, there are many applications such as any object entering/leaving from the free-surface where there is a relative motion between the object and the free-surface where the traditional CFD approach can not be applied directly. One of the approaches considered for performing such simulations is a dynamic mesh approach which involves mesh motion, creation, collapsing, merging and splitting of elements. However, this approach is computationally very expensive, and is very challenging to retain the mesh quality for performing Reynolds averaged Navier Stokes (RANS) or Large Eddy Simulations (LES). In this work, a swapping based dynamic mesh algorithm is presented that can handle very large displacements of the solid objects from one side of the domain to the other end that is computationally efficient and retains the mesh quality around the object. The algorithm is implemented in a parallel three-dimensional second-order accurate in time finite element based variational multi scale (VMS) framework. The proposed methodology splits the entire computational domain into three sub-domains: (i) inner, (ii) outer, and (iii) sliding gap regions. The inner region will hold the solid object that is undergoing large displacements, and the sliding gap region is a thin single layer of elements that connect the inner-region fluid elements with the outer-region fluid elements. The connections between the inner-region with outer-region elements will be updated depending on the element quality. Since the mesh quality check is purely limited to a very small number of elements present in the sliding-gap, the computational cost of the proposed methodology will be significantly lower. As part of the demonstration, a circular cylindrical object with a hemispherical head is considered moving towards a free surface with hydrostatic effects acting against the direction of motion. The work also presents a systematic investigation on the effect of object speed, length to radius and its gap from the free surface on the pressure distribution over the body and the drag experienced. The current methodology has direct application in ocean exploration and defense.