## Section: Research Program

### Numerical analysis

#### Non-hydrostatic scheme

The main challenge in the study of the non-hydrostatic model is to design a robust and efficient numerical scheme endowed with properties such as: positivity, wet/dry interfaces treatment, consistency. It must be noticed that even if the non-hydrostatic model looks like an extension of the Saint-Venant system, most of the known techniques used in the hydrostatic case are not efficient as we recover strong difficulties encountered in incompressible fluid mechanics due to the extra pressure term. These difficulties are reinforced by the absence of viscous/dissipative terms.

#### Space decomposition and adaptive scheme

In the quest for a better balance between accuracy and efficiency,
a strategy consists in the adaptation of models.
Indeed, the systems of partial differential equations we consider result from a hierarchy of simplifying
assumptions. However, some of these hypotheses may turn out to be irrelevant locally. The adaptation of
models thus consists in determining areas where a simplified model
(*e.g.* shallow water type) is valid and
where it is not. In the latter case, we may go back to the “parent” model
(*e.g.* Euler) in the corresponding area.
This implies to know how to handle the coupling between the aforementioned models from both theoretical
and numerical points of view. In particular, the numerical treatment of transmission conditions is a key point. It requires the estimation of characteristic values (Riemann invariant) which have to be determined according to the regime (torrential or fluvial).

#### Asymptotic-Preserving scheme for source terms

Hydrodynamic models comprise advection and sources terms. The conservation of the balance between source terms, typically viscosity and friction, has a significant impact since the overall flow is generally a perturbation around an equilibrium. The design of numerical schemes able to preserve such balances is a challenge from both theoretical and industrial points of view. The concept of Asymptotic-Preserving (AP) methods is of great interest in order to overcome these issues.

Another difficulty occurs when a term, typically related to the pressure,
becomes very large compared to the order of magnitude of the velocity.
At this regime, namely the so-called *low Froude* (shallow water)
or *low Mach* (Euler) regimes, the difference between the speed of the gravity waves
and the physical velocity makes classical numerical schemes inefficient:
firstly because of the error of truncation which is inversely proportional to the small parameters,
secondly because of the time step governed by the largest speed of the gravity wave.
AP methods made a breakthrough in the numerical resolution
of asymptotic perturbations of partial-differential equations concerning the first point.
The second one can be fixed using partially implicit scheme.

#### Multi-physics models

Coupling problems also arise within the fluid when it contains pollutants, density variations or biological species. For most situations, the interactions are small enough to use a splitting strategy and the classical numerical scheme for each sub-model, whether it be hydrodynamic or non-hydrodynamic.

The sediment transport raises interesting issues from a numerical aspect. This is an example of coupling between the flow and another phenomenon, namely the deformation of the bottom of the basin that can be carried out either by bed load where the sediment has its own velocity or suspended load in which the particles are mostly driven by the flow. This phenomenon involves different time scales and nonlinear retroactions; hence the need for accurate mechanical models and very robust numerical methods. In collaboration with industrial partners (EDF–LNHE), the team already works on the improvement of numerical methods for existing (mostly empirical) models but our aim is also to propose new (quite) simple models that contain important features and satisfy some basic mechanical requirements. The extension of our 3D models to the transport of weighted particles can also be here of great interest.

#### Optimisation

Numerical simulations are a very useful tool for the design of new processes, for instance in renewable energy or water decontamination. The optimisation of the process according to a well-defined objective such as the production of energy or the evaluation of a pollutant concentration is the logical upcoming challenge in order to propose competitive solutions in industrial context. First of all, the set of parameters that have a significant impact on the result and on which we can act in practice is identified. Then the optimal parameters can be obtained using the numerical codes produced by the team to estimate the performance for a given set of parameters with an additional loop such as gradient descent or Monte Carlo method. The optimisation is used in practice to determine the best profile for turbine pales, the best location for water turbine implantation, in particular for a farm.