Section: Overall Objectives

Turbulent flows with complex interactions

This interdisciplinary project brings together researchers coming from different horizons and backgrounds (applied mathematics and fluid mechanics) who progressively elaborated a common vision of what should be the simulation tool of fluid dynamics of tomorrow. Our team focuses on wall bounded turbulent flows featuring complex phenomena such as aeroacoustics, hydrodynamic instabilities, wall roughness, buoyancy. Because such flows are exhibiting a multiplicity of time and scale fluctuations resulting from complex interactions, their simulation is extremely challenging. Even if various methods of simulation (DNS (Direct numerical simulation)) and turbulence modeling (RANS (Reynolds averaged Navier-Stokes), LES (Large-eddy simulation), hybrid RANS-LES) are available and have been significantly improved over time, none of them does satisfy all the needs encountered in industrial and environmental configurations. We consider that all these methods will be useful in the future in different situations or regions of the flow if combined in the same simulation in order to benefit from their respective advantages wherever relevant, while mutually compensating their known limitations. It will thus lead to a description of turbulence at widely varying scales in the computational domain, hence the name multi-scale simulations. For example, the RANS mode may extend throughout regions where turbulence is sufficiently close to equilibrium leaving to LES or DNS the handling of regions where large scale coherent structures are present. However, a considerable body of work is required to:

• Establish the behavior of the different types of turbulence modeling approaches when combined with high order discretization methods.

• Elaborate relevant and robust switching criteria between models, similar to error assessments used in automatic mesh refinement, but based on the physics of the flow in order to adapt on the fly the scale of resolution from one extreme of the spectrum to another (say from the Kolmogorov scale to the geometrical large scale, i.e., from DNS to RANS).

• Ensure a high level of accuracy and robustness of the resulting simulation tool to address a large range of flow configurations, i.e., from a generic lab scale geometry for validation to practical systems of interest of our industrial partners.

But the best multi-scale modeling and high order discretization methods are useless without the recourse to high performance computing (HPC) to bring the simulation time down to values compatible with the requirement of the end users. So, a significant part of our activity is devoted to the proper handling of the constantly evolving supercomputer architectures. The long-term objective of this project is to develop, validate, promote and transfer an original and effective approach for modeling and simulating generic flows representative of flow configurations encountered in the field of energy production and aeronautical propulsion. Our approach will be combining mesh (h) + turbulence model (m) + discretization order (p) adaptivity. This will be achieved by:

• Contributing to the development of new turbulence models.

• Improving high order numerical methods, and increasing their efficiency in the current High Performance Computing context.

• Developing experimental tools.

In that framework, in 2015, the team members developed their activity around the following axes:

• The development of the AeroSol library and the direct numerical simulations of single jets in crossflow including that experimentally studied on the team test facility MAVERIC.

• The development of low Mach schemes.

• The development of advanced turbulence RANS and hybrid RANS-LES turbulence models adapted to zero Mach flows with a specific emphasis on the wall-flow interaction.