S U M M A R YA finite element formulation of the level set method, a technique to trace flow fronts and interfaces without element distortion, is presented to model the evolution of the free surface of a spreading flow for a highly viscous medium on a horizontal surface. As an example for this class of problem we consider the evolution of an axisymmetric lava dome. Equilibrium configurations of lava domes have been modelled analytically as brittle shells enclosing pressurized magma. The existence of the brittle shell may be viewed as a direct consequence of the strong temperature dependence of the viscosity. The temperature dependence leads to the formation of a thin predominantly elastic-plastic boundary layer along the free surface and acts as a constraint for the shape and flow of the lava dome. In our model, we adopt Iverson's assumption that the thin boundary layer behaves like an ideal plastic membrane shell enclosing the ductile interior of the lava dome. The effect of the membrane shell is then formally identical to a surface tension-like boundary condition for the normal stress at the free surface. The interior of the dome is modelled as a Newtonian fluid and the axisymmetry equations of motion are formulated in a Eulerian framework. We show that the level set is an effective tool to trace and model deforming interfaces for the example of the free surface of a lava dome. We demonstrate that Iverson's equilibrium dome shapes are indeed steady states of a transient model. We also show how interface conditions in the form of surface tension involving higher order spatial derivative (curvature) can be considered within a standard finite element framework.
The level set method has been implemented in a computational volcanology context. New techniques are presented to solve the advection equation and the reinitialisation equation. These techniques are based upon an algorithm developed in the finite difference context, but are modified to take advantage of the robustness of the finite element method. The resulting algorithm is tested on a well documented Rayleigh-Taylor instability benchmark [19], and on an axisymmetric problem where the analytical solution is known. Finally, the algorithm is applied to a basic study of lava dome growth.
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