10 research outputs found
Boundary-Conforming Free-Surface Flow Computations: Interface Tracking for Linear, Higher-Order and Isogeometric Finite Elements
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. 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. Standard displacement techniques are full velocity, velocity in a
specific coordinate direction, and velocity in normal direction. In this work,
we investigate how the interface-tracking approach can be combined with
isogeometric analysis for the spatial discretization. If NURBS basis functions
of sufficient order are used for both the geometry and the solution, both a
continuous 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 an alternative that relies on
strong imposition at discrete points. Furthermore, we examine several coupling
methods between the fluid equations, boundary conditions, and equations for the
adjustment of interior control point positions.Comment: 20 pages, 16 figure
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