Hervé Jeanmart

Explicit-filtering large-eddy simulation using the tensor-diffusivity model supplemented by a dynamic Smagorinsky term

Large-eddy simulation (LES) with regular explicit filtering is investigated. The filtered-scale stress due to the explicit filtering is here partially reconstructed using the tensor-diffusivity model: It provides for backscatter along the stretching direction(s), and for global dissipation, both also attributes of the exact filtered-scale stress. The necessary LES truncations (grid and numerical method) are responsible for an additional subgrid-scale stress. A natural mixed model is then the tensor-diffusivity model supplemented by a dynamic Smagorinsky term. This model is reviewed, together with useful connections to other models, and is tested against direct numerical simulation (DNS) of turbulent isotropic decay starting with Re-lambda=90 (thus moderate Reynolds number): LES started from a 256(3) DNS truncated to 64(3) and Gaussian filtered. The tensor-diffusivity part is first tested alone; the mixed model is tested next. Diagnostics include energy decay, enstrophy decay, and energy spectra. After an initial transient of the dynamic procedure (observed with all models), the mixed model is found to produce good results. However, despite expectations based on favorable a priori tests, the results are similar to those obtained when using the dynamic Smagorinsky model alone in LES without explicit filtering. Nevertheless, the dynamic mixed model appears as a good compromise between partial reconstruction of the filtered-scale stress and modeling of the truncations effects (incomplete reconstruction and subgrid-scale effects). More challenging 48(3) LES are also done: Again, the results of both approaches are found to be similar. The dynamic mixed model is also tested on the turbulent channel flow at Re-tau=395. The tensor-diffusivity part must be damped close to the wall in order to avoid instabilities. Diagnostics are mean profiles of velocity, stress, dissipation, and reconstructed Reynolds stresses. The velocity profile obtained using the damped dynamic mixed model is slightly better than that obtained using the dynamic Smagorinsky model without explicit filtering. The damping used so far is however crude, and this calls for further work

On the modelling of the subgrid-scale and filtered-scale stress tensors in large-eddy simulation

The large-eddy simulation (LES) equations are obtained from the application of two operators to the Navier{Stokes equations: a smooth lter and a discretization operator. The introduction ab initio of the discretization influences the structure of the unknown stress in the LES equations, which now contain a subgrid-scale stress tensor mainly due to discretization, and a filtered-scale stress tensor mainly due to filtering. Theoretical arguments are proposed supporting eddy viscosity models for the subgrid-scale stress tensor. However, no exact result can be derived for this term because the discretization is responsible for a loss of information and because its exact nature is usually unknown. The situation is different for the filtered-scale stress tensor for which an exact expansion in terms of the large-scale velocity and its derivatives is derived for a wide class of filters including the Gaussian, the tophat and all discrete filters. As a consequence of this generalized result, the filtered-scale stress tensor is shown to be invariant under the change of sign of the large-scale velocity. This implies that the filtered-scale stress tensor should lead to reversible dynamics in the limit of zero molecular viscosity when the discretization effects are neglected. Numerical results that illustrate this effect are presented together with a discussion on other approaches leading to reversible dynamics like the scale similarity based models and, surprisingly, the dynamic procedure.

On the comparison of turbulence intensities from large-eddy simulation with those from experiment or direct numerical simulation

The relation between the Reynolds stresses from experiment or direct numerical simulation (DNS) and large-eddy simulation (LES) is reviewed. As is well known, the Reynolds stresses can only be reconstructed from a LES when the average contribution from the subgrid-scale model is taken into account. However, in the case of LES using traceless models (e.g., effective viscosity models: Smagorinsky model, dynamic Smagorinsky model, etc.), or even partially traceless models (e.g., mixed models), only the deviatoric Reynolds stresses can be reconstructed. This obvious point is often overlooked in the literature. It has important consequences in all flows with at least one inhomogeneous direction (channels, boundary layers, wakes, shear layers, jets, etc.): as far as the rms turbulence intensities are concerned, one can only properly reconstruct, and thus directly compare with experimental or DNS data, their deviation from isotropy.

Multiphysic modelling of railway vehicles equipped with pneumatic suspensions

This article presents a multidisciplinary approach of railway pneumatic suspension modelling: both multibody and pneumatic aspects are taken into account. The work aims at obtaining a realistic model of the secondary suspension and coupling it with a multibody model of a train. Various components of the pneumatic circuit such as bellows, tanks, pipes and valves are taken into account. The article focuses on the bellow–pipe–tank subsystem for which several modelling approaches are presented and compared. Differences between differential and algebraic models are highlighted, and an application-dependent choice between them is suggested. A complete model of the pneumatic circuit is then obtained and coupled with a multibody model of the train. As a result, the behaviour of a suburban train equipped with a pneumatic secondary suspension is analysed, in particular undesired oscillating motions which affect the comfort. Topological modifications and improvements of the suspension are also investigated and discussed.

Hyper viscosity and vorticity-based models for subgrid-scale modeling

Large-eddy simulations corresponding to the decaying isotropic turbulence experiment of Comte-Bellot and Corrsin are performed, using a pseudo-spectral code that incorporates four models: viscosity and hyperviscosity types, each implemented for both the subgrid scale stress tensor and the subgrid scale force. Two 1/T scalings are also considered for the viscosity amplitude. The dynamic procedure is extended to the four models and is tested. Results are obtained with and without this procedure and for both scalings. The main conclusions are: (a) the two viscosity models perform equally well; (b) the Kolmogorov scaling performs as well as the Smagorinsky scaling, yet it is computationally more efficient; (c) in the dynamic procedure, there is a fairly wide range of test to grid filter ratios which produces results insensitive to this ratio; and (d) the hyperviscosity models lead to energy decay curves that follow the experimental data as well as the usual viscosity models.

Simulations of advanced combustion modes using detailed chemistry combined with tabulation and mechanism reduction techniques

Multi-dimensional models represent today consolidated tools to simulate the combustion process in HCCI and Diesel engines. Various approaches are available for this purpose, it is however widely accepted that detailed chemistry represents a fundamental prerequisite to obtain satisfactory results when the engine runs with complex injection strategies or advanced combustion modes. Yet, integrating such mechanisms generally results in prohibitive computational cost. This paper presents a comprehensive methodology for fast and efficient simulations of combustion in internal combustion engines using detailed chemistry. For this purpose, techniques to tabulate the species reaction rates and to reduce the chemical mechanisms on the fly have been coupled. In this way, the computational overheads related to the use of these mechanisms are significantly reduced since tabulated reaction rates are re-used for cells with similar compositions and, when it becomes necessary to perform direct integration, only the relevant set of species and reactions is taken into account. The proposed approach named tabulation of dynamic adaptive chemistry (TDAC) has been implemented in the Lib-ICE code, which is a set of libraries and applications for IC engine modeling developed using the OpenFOAM® technology. In particular, a modified version of the in-situ adaptive tabulation (ISAT) algorithm has been developed for systems with variable temperature and pressure, and the directed relation graph (DRG) method has been used to reduce the mechanism at run-time. The validation has been carried out with HCCI and Diesel cases both using a simplified case to compare the results obtained with and without TDAC, and a detailed case that is validated with experimental data. For each tested condition, a detailed comparison between computed and experimental data is provided along with the achieved speed-up factors compared to the use of direct-integration.

Exact Expansions for Filtered-Scales Modelling with a Wide Class of LES Filters

An exact expansion for the filtered-scale stress tensor in terms of the large-scale velocity and its derivatives is presented for large-eddy simulation (LES) that are based on filters with a C ∞ kernel (e.g., most filters defined in physical space, such as the Gaussian, the top hat, all discrete filters, etc.). This result constitutes a generalisation of the exact expansion proposed independently by Yeo and Leonard in the case of the Gaussian filter. An important consequence of this expansion is that the filtered-scale stress tensor does not introduce any irreversibility into the large-scale dynamics. An approximation of this expansion is also derived for filters with a compact support in Fourier space. In this case, the filtered-scale can be decomposed into two parts corresponding respectively to reversible and irreversible effects.

Model-based evaluation of railway pneumatic suspensions

On classical passenger trains, the secondary suspension is commonly ensured by airsprings which are integrated in a pneumatic circuit. In addition to the pneumatic bellows, the circuit contains various pneumatic components such as tanks, pipes, orifices, levelling valves, etc., the purpose of the latter being to maintain a constant height between the carbody and the bogie. These elements can be connected in several ways leading to many existing suspension topologies. This paper deals with the multidisciplinary modelling of a vehicle equipped with such a suspension taking into account both multibody and pneumatic aspects. First, it depicts in more detail the various elements of a pneumatic suspension. Then, many criteria that influence the suspension morphology are presented: the kind of bogie, the use of an auxiliary tank, the position of this tank relative to the bellows, the kind of levelling system, the use of an additional anti-roll bar, the use of an additional hydraulic damper, etc. The description of the complete pneumatic circuit implies the use of thermodynamic models which take into account the airflow through the pipes and the valves, the pressure in the bellows, etc. Several pipe models are presented, the choice depending on the length of the pipe: an algebraic model is sufficient for short pipes, while a differential model is needed to take into account the dynamics of longer pipes. The pneumatic model is then coupled with a SIMPACK multibody model in order to analyse the behaviour of the complete vehicle. Two-point and four-point configurations are compared for Δ Q/Q and curve entry tests.

KINETICS IN AMMONIA-CONTAINING PREMIXED FLAMES AND A PRELIMINARY INVESTIGATION OF THEIR USE AS FUEL IN SPARK IGNITION ENGINES

By using molecular beam sampling mass spectrometry, the structure of a NH3/H2/O2/Ar flat premixed flame burning at 50 mbar and with an equivalence ratio equals to one has been determined. Simulated mole fraction profiles have been obtained by using the kinetic mechanisms of Konnov (2000), Bian et al. (1990), Lindstedt et al. (1994), and Smith (1999), but only the first two give results in agreement with the experimental ones. Furthermore, an initial study on a spark ignition engine has been performed to define which parameters must be applied to reach the best possible efficiency and reduce the formation of pollutants when ammonia is used as fuel in engines.

Assessment of some models for LES without/with explicit filtering

The practical approach in LES is concerned with modelling the effective “subgrid-scale” stress due to the projection from the complete u i field to the incomplete ũ i field: a non-regular operation, the effect of which must be modelled. On the other hand, the mathematical approach usually assumes a regular explicit filter: a regular convolution acting on u i to produce ū i , leading to an effective “filtered-scale” stress. On can also consider practical LES with regular filtering added to the projection, thus solving for \({\bar \tilde u_i}\) instead of ũ i . The effective stress is then the sum of a filtered-scale stress (that can be reconstructed) and a subgrid-scale stress (that must be modelled). A view that reconciles both practical approaches is reviewed, together with some models. Of particular interest are models that behave as viscosity at low k and higher order viscosity at high k. The spectral behavior of the models is investigated numerically, in 483 LES of decaying isotropic turbulence (started from 2563 DNS). Two diagnostics are used: model dissipation spectrum and obtained energy spectrum.