## Seminar in Numerical Analysis: Théophile Chaumont-Frelet (INRIA, Nice/Sophia Antipolis)

**High order finite element discretizations of Helmholtz problems
**

The Helmholtz equation models the propagation of a time-harmonic wave.

It has received much attention since it is widely employed in applications,

but still challenging to numerically simulate in the high-frequency regime.

In this seminar, we focus on acoustic waves for the sake of simplicity

and consider finite element discretizations. The main goal of the

presentation is to highlight the improved performance of high order

methods (as compared to linear finite elements) when the frequency is large.

We will very briefly cover the zero-frequency case, that corresponds to the well-studied Poisson equation. We take advantage of this classical setting to recall central concepts of the finite element theory such as quasi-optimality and interpolation error.

The second part of the seminar is devoted to the high-frequency case.

We show that without restrictive assumptions on the mesh size,

the finite element method is unstable, and quasi-optimality is lost.

We provide a detailed analysis, as well as numerical examples, which

show that higher order methods are less affected by this phenomena,

and thus more suited to discretize high-frequency problems.

Before drawing our main conclusions, we briefly discuss advanced topics,

such as the use of unfitted meshes in highly heterogeneous media

and mesh refinements around re-entrant corners.

For further information about the seminar, please visit this webpage.

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