COUPLING FREE-SURFACE FLOW AND MESH DEFORMATION IN AN ISOGEOMETRIC SETTING

The simulation of certain flow problems requires a means for modeling a free fluid surface; examples being viscoelastic die swell or fluid sloshing in tanks. In a finite-element context, this type of problem can, among many other options, be dealt with using an interface-tracking approach with the Deforming-Spatial-Domain/Stabilized-Space-Time (DSD/SST) formulation [1]. A difficult issue that is connected with this type of approach is the determination of a suitable coupling mechanism between the fluid velocity at the boundary and the displacement of the boundary mesh nodes. In order to avoid large mesh distortions, one goal is to keep the nodal movements as small as possible; but of course still compliant with the no-penetration boundary condition. One common choice of displacement that fulfills both requirements is the displacement with the normal component of the fluid velocity. However, when using finite-element basis functions of Lagrange type for the spatial discretization, the normal vector is not uniquely defined at the mesh nodes. This can create problems for the coupling, e.g., making it difficult to ensure mass conservation. In contrast, NURBS basis functions of quadratic or higher order are not subject to this limitation. These types of basis functions have already been used in the context of free-surface boundaries, in connection with the NURBS-enhanced finite-element method (NEFEM) [2]. However, this method presents some difficulties due to the fact that it does not adhere to the isoparametric concept. As an alternative, we investigate the suitability of using the method of isogeometric analysis for the spatial discretization. If NURBS basis functions of sufficient order are used for both the geometry and the solution, both a well-defined normal vector as well as the velocity are available on the entire boundary. This circumstance allows the weak imposition of the no-penetration boundary condition. We compare this option with a number of alternatives. Furthermore, we examine several coupling methods between the fluid equations, boundary conditions, and equations for the adjustment of interior control point positions.