69 research outputs found
The FDF or LES/PDF method for turbulent two-phase flows
In this paper, a new formalism for the filtered density function (FDF)
approach is developed for the treatment of turbulent polydispersed two-phase
flows in LES simulations. Contrary to the FDF used for turbulent reactive
single-phase flows, the present formalislm is based on Lagrangian quantities
and, in particular, on the Lagrangian filtered mass density function (LFMDF) as
the central concept. This framework allows modeling and simulation of particle
flows for LES to be set in a rigorous context and various links with other
approaches to be made. In particular, the relation between LES for particle
simulations of single-phase flows and Smoothed Particle Hydrodynamics (SPH) is
put forward. Then, the discussion and derivation of possible subgrid stochastic
models used for Lagrangian models in two-phase flows can set in a clear
probabilistic equivalence with the corresponding LFMDF.Comment: 11 pages, proceedings of the 13 europena turbulence conference,
submitted to JPC
A note on the consistency of Hybrid Eulerian/Lagrangian approach to multiphase flows
The aim of the present paper is to introduce and to discuss inconsistencies
errors that may arise when Eulerian and Lagrangian models are coupled for the
simulations of turbulent poly-dispersed two-phase flows. In these hydrid
models, two turbulence models are in fact implicitely used at the same time and
it is essential to check that they are consistent, in spite of their apparent
different formulations. This issue appears in particular in the case of
very-small particles, or tracer-limit particles, and it is shown that coupling
inconsistent turbulence models (Eulerian and Lagrangian) can result in
non-physical results, notably for second-order fluid velocity moments. This
problem is illustrated by some computations for fluid particles in a turbulent
channel flow using several coupling strategies.Comment: 14 pages, 3 figure
A two-dimensional relaxation scheme for the hybrid modelling of two-phase flows
International audienceRecently, a new relaxation scheme for hybrid modelling of two-phase flows has been proposed. This one allows to obtain stable unsteady approximations for a system of partial differential equations containing non-smooth data. This paper is concerned with a two-dimensional extension of the present method, in which two alternative relaxation schemes are compared. A partial analysis of continuous stability is given
A relaxation scheme for hybrid modelling of gas-particle flows
International audienceThis paper aims at proposing a relaxation scheme that allows to obtain stable approximations for a system of partial differential equations which governs the evolution of the void fraction and the mean velocity in the particle phase of two-phase flows. This system involves the divergence of a particle kinetic tensor, which is provided by a Lagrangian code and whose components are not smooth. The simulation algorithm is based on the combined use of upwinding and relaxation techniques. The main properties of the method are given, together with the Finite Volume Godunov scheme and this approach is compared to an analogous one that was developed earlier. Some measured rates of convergence in L 1-norm are provided, for a particular choice of the kinetic tensor. To complete the picture, we present some numerical results obtained when non-smooth external data are provided to the system
Langevin PDF simulation of particle deposition in a turbulent pipe flow
The paper deals with the description of particle deposition on walls from a
turbulent flow over a large range of particle diameter, using a Langevin PDF
model. The first aim of the work is to test how the present Langevin model is
able to describe this phenomenon and to outline the physical as- pects which
play a major role in particle deposition. The general features and
characteristics of the present stochastic model are first recalled. Then,
results obtained with the standard form of the model are presented along with
an analysis which has been carried out to check the sensitivity of the
predictions on different mean fluid quantities. These results show that the
physical repre- sentation of the near-wall physics has to be improved and that,
in particular, one possible route is to introduce specific features related to
the near-wall coherent structures. In the following, we propose a simple
phenomenological model that introduces some of the effects due to the presence
of turbulent coherent structures on particles in a thin layer close to the
wall. The results obtained with this phenomenological model are in good
agreement with experimental evidence and this suggests to pursue in that
direction, towards the development of more general and rigorous stochastic
models that provide a link between a geometrical description of turbulent flow
and a statistical one.Comment: 40 pages, 8 figure
PDF model based on Langevin equation for polydispersed two-phase flows applied to a bluff-body gas-solid flow,
The aim of the paper is to discuss the main characteristics of a complete
theoretical and numerical model for turbulent polydispersed two-phase flows,
pointing out some specific issues. The theoretical details of the model have
already been presented [Minier and Peirano, Physics Reports, Vol. 352/1-3, 2001
]. Consequently, the present work is mainly focused on complementary aspects,
that are often overlooked and that require particular attention. In particular,
the following points are analysed : the necessity to add an extra term in the
equation for the velocity of the fluid seen in the case of twoway coupling, the
theoretical and numerical evaluations of particle averages and the fulfilment
of the particle mass-continuity constraint. The theoretical model is developed
within the PDF formalism. The important-physical choice of the state vector
variables is first discussed and the model is then expressed as a stochastic
differential equation (SDE) written in continuous time (Langevin equations) for
the velocity of the fluid seen. The interests and limitations of Langevin
equations, compared to the single-phase case, are reviewed. From the numerical
point of view, the model corresponds to an hybrid Eulerian/Lagrangian approach
where the fluid and particle phases are simulated by different methods.
Important aspects of the Monte Carlo particle/mesh numerical method are
emphasised. Finally, the complete model is validated and its performance is
assessed by simulating a bluff-body case with an important recirculation zone
and in which two-way coupling is noticeable.Comment: 23 pages, 10 figure
A time-step-robust algorithm to compute particle trajectories in 3-D unstructured meshes for Lagrangian stochastic methods
The purpose of this paper is to propose a time-step-robust cell-to-cell
integration of particle trajectories in 3-D unstructured meshes in
particle/mesh Lagrangian stochastic methods. The main idea is to dynamically
update the mean fields used in the time integration by splitting, for each
particle, the time step into sub-steps such that each of these sub-steps
corresponds to particle cell residence times. This reduces the spatial
discretization error. Given the stochastic nature of the models, a key aspect
is to derive estimations of the residence times that do not anticipate the
future of the Wiener process. To that effect, the new algorithm relies on a
virtual particle, attached to each stochastic one, whose mean conditional
behavior provides free-of-statistical-bias predictions of residence times.
After consistency checks, this new algorithm is validated on two representative
test cases: particle dispersion in a statistically uniform flow and particle
dynamics in a non-uniform flow
- …