CERFACS CERFACS Scientific Activity Report Jan. 2010 – Dec. 2011

4 Advanced Methods and Multiphysics
The ”Advanced Aerodynamics and Multiphysics” group is a component of the CFD Team. Around
15 researchers (seniors, PhD and Post Doctoral students) are involved in aerodynamics activities. The
objective of the group is to develop, maintain and use efficient numerical solvers related to academic and
industrial CFD simulations. Most of our efforts focus on the elsA software (owned by Onera) but we
also use in-house solvers : AVBP for unstructured Large Eddy Simulation (LES) and NTMIX for Direct
Numerical Simulation.
During the past two years, LES activities have grown significantly, especially in unsteady simulations
(aeroacoustics or aerothermal) where RANS is not sufficient. To be able to deal with accurate LES in elsA,
several developments have been done (high order scheme, non reflexive boundary conditions, wall laws,
turbulence injection) leading to new applications : jet, airfoil and landing gear noise but also aerothermal
jet in cross flow.
In addition to advanced turbulence methods, the team (in collaboration with Onera) continues the extension
of elsA to the unstructured world. This will allow more flexibility to deal with complex geometries and
help us to answer the 2020 challenge ”PUMA” defined by CERFACS in 2011, which ambitions to compute
a fully unsteady aircraft.
To finish, the activity concerning design optimization is also growing in collaboration with the Algo team.
The work presented in the next sections has been done in close collaboration with our industrials partners
(Airbus, Snecma, Turboméca) but also with research centers among which Onera, Paris VI, KTH, ECL.
4.1 Numerical methods
4.1.1 Improve LES capabilities (S. Bocquet, J-C. Jouhaud)
Large-Eddy Simulation (LES) of industrial high-Reynolds number flows is still far from being practical on
a daily basis. Various methods have been proposed to reduce the computational cost of LES. These last
years, the team has investigated two approaches : Thin Boundary Layer models and embedded LES.
Thin boundary layer laws
The cost of LES is mainly due to the resolution of the small but dynamically important structures present
in the inner layer of the boundary layer. The LES with wall model approach is one technique to reduce the
computational cost of such computations. A coarse mesh is used close to the wall so that the inner layer
of the boundary layer is not captured. In the coarse cells adjacent to the wall, the wall fluxes need to be
approximated by an additional wall model. The wall model must contain the flow physics present in the
inner layer and can be either a quasi-analytical model composed of wall laws or a numerical model like the
Thin Boundary Layer (TBL) model described in Balaras (1996) where a simplified one dimensional model
is used between the wall and the first LES point in the flow.
Despite their cost, TBL models constitute an interesting framework for the derivation of wall models
156 Jan. 2010 – Dec. 2011