METHRIC-Modelling of Incompressible Turbulent Flows at High Reynolds numbers and Couplings

The objective of the METHRIC group is the numerical modeling of turbulent flows of incompressible viscous fluids around complex geometries under real conditions.This concerns both hydrodynamics and low-speed unsteady aerodynamics within the limit of incompressible fluid assumptions. The group's research provides an understanding of vessel-propulsion interactions under real conditions in order to optimize the energy performance of ships.
LHEEA-METHRIC
In the context of hydrodynamics, the group is interested in any type of propulsion system, ie engine / propeller, sail / wind and human strength/ muscle. Its aim is to use numerical modeling to carry out simulations taking into account the different interactions.

The group's research is organized around 3 topics:
  • Physical modeling: turbulence, cavitation, interface captures
  • Numerical modeling: precision control and high precision order, automatic mesh adaptation, relative motion
  • Couplings: Flow / movement coupling, fluid / structure interaction

Research areas

The scientific objectives of the team are divided into two parts:
  • "Existing developments", in the natural continuity of existing developments without changing the context and which must be pursued
  • "Towards more fundamental CFD developments", more in breach, on the observation of the current limitations with the desire of the team to focus on the need for more fundamental Computational Fluid Dynamics (CFD) developments.

It is also to better mark this desire that the DSPM team decided to change its name to 1 July 2015 in "Modeling of Turbulent flows top Reynolds incompressible and Coupling" under the acronym METHRIC.

"Existing developments"

In the context of hydrodynamics, it is a matter of increasing the existing capacities of the digital basin to free itself from the effects of scales.
Physical modeling

Statistical modeling of turbulence for strong (106) or very high (109) Reynolds numbers for the viscous hydrodynamics to the real and for the unsteady aerodynamics within the limit of the incompressibility hypothesis. Non-isotropic model EARSM with and without wall functions and hybrid RANS-LES. Modeling of cavitation by a liquid / vapor interface capture model.            
                                             

Numerical modeling

In the context of hydrodynamics, it is a question of increasing the existing capacities of the digital basin to get rid of the effects of scales but always with a fine evaluation of the modeling compared to experimental data carefully Chosen and whose measurement uncertainty is available.
                                                                  

Couplages

The scope of application aims, among other things, to continue the analysis and the objectification of performance for water sports such as rowing and canoeing, in collaboration with the CREPS de Nantes and the federations With the use of the coupling already implemented with the multi-body dynamics code MBDyn to model the complete systems. In the continuity of the previous works, it is the coupling of the resolution of the flow with deformable structures that feeds this topic. The coupling with structural models reduces dimensionally beam (1D) or membranes / shell (2D) is now acquired. The increasingly common use of complex structures with non-homogeneous materials such as composites in hydrodynamic applications makes coupling with dedicated 3D structure codes more and more prominent. These digital tools require revisiting the coupling algorithms so as not to lose effectiveness.
                                                                                      

"Towards more fundamental CFD developments"

The current context of our developments with the two-dimensional finite volume approach on unstructured meshes applied to complex geometries with a high number of Reynolds is sufficiently mastered to point out its limitations, such as the modeling of Turbulence with a RANS approach, or numerical errors related to unsteady simulations. In addition to the theme of coupling that must be pursued, the axes considered relate mainly to turbulence and the research and the control of a high precision of numerical simulation. For turbulence, it is necessary to consider the impact of numerical modeling on a modeling model rather than modeling the turbulence itself. Our will is to focus on the following fundamentals:
Turbulence

Although RANS modeling remains widely used, these approaches seem to be slowly supplanted by the LES approaches, but they can not yet approach the real situations of the hydrodynamics of viscous fluids. The LES approach remains prohibitive for high-reynolds simulations. Consequently the hybrid modeling technique RANS-LES, whose DES (Detached Eddy Simulation) approach and its variants, is an unavoidable alternative. We have already implemented it on bodies of low elongation but it remains to be evaluated and validated in the field of hydrodynamics. As far as the contribution of calculated turbulence (instationalities) is concerned, it is then essential to be able to develop high order methods of precision which are stable and which remain usable on geometries of great complexity. With the non-structured finite volume approach and implicit method, the main difficulty then lies in the discretization of the second derivatives and in the formation of a pressure operator on collocative meshing. The automatic mesh refinement implemented is a fundamental asset which now possesses a high degree of reliability and maturity and is essential to access the fine details of the flow. In the framework of hybrid modeling, it will be necessary to develop a criterion of automatic choice of the scales of local length of the turbulent model. Indeed, in LES hybrid modeling, the choice between RANS and LES as well as the filtration length in LES depends directly on the size of the cells.

Towards high order simulations of precision

The pointed limitations of the two-order modeling developed in the finite volume method and which will have to be lifted are mainly (1) numerical dissipation and dispersion which, too important, degrade the Monitoring and transport of flow structures, and (2) access to small scales requires fine meshes and time steps with high computing times. In the field of aeronautics high order methods as alternatives to methods of order two already exist. With regard to the calculation means and the expected CPU costs, in the use of computers in parallel mode, it will be necessary to exceed the limits of the conventional MPI approach used in the exchange of information between the processors and MPI / OPENMP hybrid parallelization by exploitation of co-processors.

Control accuracy

This objective is related to the notion of targeted calculation, which consists in automatically making a simulation aimed at, for example, calculating a specific quantity related to the solution (such as the drag of a body). Every effort of calculation must aim at the control of the precision of this quantity. The simulation tool must then automatically adjust the parameters of the calculation, globally or locally (mesh, etc.) to produce the best approximation of this quantity and the error must be estimated in order to guarantee the accuracy of the result. The local contributions to the error can be used as a criterion of refinement in the method of automatic refinement that we have in order to construct an optimal mesh. A strategy for estimating the discretization error consists in forming and solving a linearized transport equation of the error whose source term corresponds to the differential residue of the problem considered but evaluated to a higher order.

Immersed boundaries

It is a question of adding, and not of substituting, to the existing finite volume method the possibility of being able to represent the modeling of the interactions between an incompressible viscous fluid and deformable or dislocated structures. The typical example is the boat / ice interaction. The implementation should take advantage of the automatic mesh adaptation and the developments made on the generation of iso-surfaces as part of the modeling of the surface tension.

Its research tool is numerical simulation, in particular thanks to calculations carried out on national (IDRIS and CINES) and local machines (ECN Intensive Computing Institute).

On a daily basis, the group studies and verifies physical models, develops suitable numerical models and validates these results on the basis of experimental data.  METHRIC research provides results that respond to societal challenges, such as the requirement to reduce pollutant emissions (specifically in maritime and inland waterway transport) or design and optimization in the marine field. Applications also stem from its research in water sports (mainly rowing and sailing) with regard to the requirements of precision and relevance of sports results.
A special development action has also been carried out since 2006 between Centrale Nantes, CNRS and NUMECA International around the FINE / Marine software of the ISIS-CFD integrated calculation code.Applications have resulted from the use of this software, in particular during the last 34th America's Cup and for the 35th (challenges Oracle Team USA, Emirates Team New Zealand and Groupama Team France), Seven King boats Ship of the year 2013) or the Galactica Star (World Super Yacht Award 2014). This action is included in the "Top Ten" of CNRS development projects since 2012.
Published on March 27, 2017 Updated on January 25, 2021