# Arbitrary Lagrangian-Eulerian implementation#

The question of how to handle the motion of the mesh with a free surface is challenging. Eulerian meshes are well behaved, but they do not move with the fluid motions, which makes them difficult for use with free surfaces. Lagrangian meshes do move with the fluid, but they quickly become so distorted that remeshing is required. ASPECT implements an Arbitrary Lagrangian-Eulerian (ALE) framework for handling motion of the mesh. The ALE approach tries to retain the benefits of both the Lagrangian and the Eulerian approaches by allowing the mesh motion $$\textbf{u}_m$$ to be largely independent of the fluid. The mass conservation condition requires that $$\textbf{u}_m \cdot \textbf{n} = \textbf{u} \cdot \textbf{n}$$ on the free surface, but otherwise the mesh motion is unconstrained, and should be chosen to keep the mesh as well behaved as possible.

ASPECT uses a Laplacian scheme for calculating the mesh velocity. The mesh velocity is calculated by solving

\begin{split}\begin{aligned} -\Delta \textbf{u}_m &= 0 & \qquad & \textrm{in } \Omega, \\ \textbf{u}_m &= \left( \textbf{u} \cdot \textbf{n} \right) \textbf{n} & \qquad & \textrm{on } \partial \Omega_{\textrm{free surface}}, \\ \textbf{u}_m \cdot \textbf{n} &= 0 & \qquad & \textrm{on } \partial \Omega_{\textrm{free slip}}, \\ \textbf{u}_m &= 0 & \qquad & \textrm{on } \partial \Omega_{\textrm{Dirichlet}}. \end{aligned}\end{split}

After this mesh velocity is calculated, the mesh vertices are time-stepped explicitly. This scheme has the effect of choosing a minimally distorting perturbation to the mesh. Because the mesh velocity is no longer zero in the ALE approach, we must then correct the Eulerian advection terms in the advection system with the mesh velocity (see, e.g., ). For instance, the temperature equation (26) becomes

$\rho C_p \left(\frac{\partial T}{\partial t} + \left(\mathbf u - \mathbf u_m \right) \cdot\nabla T\right) - \nabla\cdot k\nabla T = \rho H \quad \textrm{in \Omega}.$