Vincent Moureau

An accurate conservative level set/ghost fluid method for simulating turbulent atomization

This paper presents a novel methodology for simulating incompressible two-phase flows by combining an improved version of the conservative level set technique introduced in [E. Olsson, G. Kreiss, A conservative level set method for two phase flow, J. Comput. Phys. 210 (2005) 225-246] with a ghost fluid approach. By employing a hyperbolic tangent level set function that is transported and re-initialized using fully conservative numerical schemes, mass conservation issues that are known to affect level set methods are greatly reduced. In order to improve the accuracy of the conservative level set method, high order numerical schemes are used. The overall robustness of the numerical approach is increased by computing the interface normals from a signed distance function reconstructed from the hyperbolic tangent level set by a fast marching method. The convergence of the curvature calculation is ensured by using a least squares reconstruction. The ghost fluid technique provides a way of handling the interfacial forces and large density jumps associated with two-phase flows with good accuracy, while avoiding artificial spreading of the interface. Since the proposed approach relies on partial differential equations, its implementation is straightforward in all coordinate systems, and it benefits from high parallel efficiency. The robustness and efficiency of the approach is further improved by using implicit schemes for the interface transport and re-initialization equations, as well as for the momentum solver. The performance of the method is assessed through both classical level set transport tests and simple two-phase flow examples including topology changes. It is then applied to simulate turbulent atomization of a liquid Diesel jet at Re=3000. The conservation errors associated with the accurate conservative level set technique are shown to remain small even for this complex case.

Evaluation of numerical strategies for large eddy simulation of particulate two-phase recirculating flows

Predicting particle dispersion in recirculating two-phase flows is a key issue for reacting flows and a potential application of large eddy simulation (LES) methods. In this study, Euler/Euler and Euler/Lagrange LES approaches are compared in the bluff body configuration from Boree et al. [J. Boree, T. Ishima, I. Flour, The effect of mass loading and inter-particle collisions on the development of the polydispersed two-phase flow downstream of a confined bluff body, J. Fluid Mech. 443 (2001) 129-165] where glass beads are injected into a complex recirculating flow. These tests are performed for non-reacting, non-evaporating sprays but are mandatory validations before computing realistic combustion chambers. Two different codes (one explicit and compressible and the other implicit and incompressible) are also tested on the same configuration. Results show that the gas flow is well predicted by both codes. The dispersed phase is also well predicted by both codes but the Lagrangian approach predicts root-mean-square values more accurately than the Eulerian approach. The effects of mesh, solvers and numerical schemes are discussed for each method.

Towards Large Eddy Simulation in Internal-Combustion Engines: simulation of a compressed tumble flow.

The development of the Large Eddy Simulation (LES) 3D CFD code AVBP to yield a CFD tool able to predict cyclic variability in Internal Combustion (IC) engines is reported. In a first step the implementation of an Arbitrary Lagrangian Eulerian (ALE) method into AVBP is described, allowing to move solid boundaries. Then the principles and implementation of the Conditioned Temporal Interpolation (CTI) mesh management technique is described, and some specific adaptations for LES simulations are discussed. Finally a first validation of the so obtained LES IC engine code is presented by comparing predictions with findings on the square piston experiment.

A ghost-fluid method for large-eddy simulations of premixed combustion in complex geometries

In this paper, a new ghost-fluid method for interfaces of finite thickness is described. It allows to compute efficiently turbulent premixed flames with a finite thickness in low-Mach flows. A level set algorithm is used to track accurately the flame and to define the overlapping region where the burned and unburned gases satisfy the jump conditions. These algorithms are combined with a fractional-step method to alleviate the acoustic CFL constraint. The full algorithm is verified for simple flame-vortex interactions and it is validated by computing a turbulent flame anchored by a triangular flame-holder. Finally, the algorithm is applied in the LES of an industrial lean-premixed swirl-burner.

Optimization of the deflated Conjugate Gradient algorithm for the solving of elliptic equations on massively parallel machines

The discretization of Partial Differential Equations often leads to the need of solving large symmetric linear systems. In the case of the Navier-Stokes equations for incompressible flows, solving the elliptic pressure Poisson equation can represent the most important part of the computational time required for the massively parallel simulation of the flow. The need for efficiency that this issue induces is completed with a need for stability, in particular when dealing with unstructured meshes. Here, a stable and efficient variant of the Deflated Preconditioned Conjugate Gradient (DPCG) solver is first presented. This two-level method uses an arbitrary coarse grid to reduce the computational cost of the solving. However, in the massively parallel implementation of this technique for very large linear systems, the coarse grids generated can count up to millions of cells, which makes direct solvings on the coarse level impossible. The solving on the coarse grid, performed with a Preconditioned Conjugate Gradient (PCG) solver for this reason, may involve a large number of communications, which reduces dramatically the performances on massively parallel machines. To this effect, two methods developed in order to reduce the number of iterations on the coarse level are introduced, that is the creation of improved initial guesses and the adaptation of the convergence criterion. The design of these methods make them easy to implement in any already existing DPCG solver. The structural requirements for an efficient massively parallel unstructured solver and the implementation of this solver are described. The novel DPCG method is assessed for applications involving turbulence, heat transfers and two-phase flows, with grids up to 17.8billion elements. Numerical results show a two- to 12-fold reduction of the number of iterations on the coarse level, which implies a reduction of the computational time of the Poisson solver up to 71% and a global reduction of the proportion of communication times up to 53%. As a result, the weak scaling of the LES solver is shown to be clearly improved for massively parallel uses.

About the numerical robustness of biomedical benchmark cases: Interlaboratory FDA's idealized medical device

The need for reliable approaches in numerical simulations stands out as a critical issue for the development and optimization of cardiovascular biomedical devices. This led the US Food and Drug Administration to undertake a programme of validation of computational fluid dynamics methods for transitional and turbulent flows. In the current investigation, large-eddy simulation is used to simulate the flow in the first benchmark medical device, and results are confronted to the existing laboratory experiments. This idealized medical device has the particularity to feature transition to turbulence after a sudden expansion. The effects of numerical parameters and low-level inlet perturbations are investigated. Results indicate a considerable impact of numerical aspects on the prediction of the location of the transition to turbulence. The study also demonstrates that injecting small perturbations at the inflow greatly improves the streamwise velocity estimation in the transition region and substantially contributes to the robustness of the flow statistical data. Copyright

Design of implicit high-order filters on unstructured grids for the identification of large-scale features in large-eddy simulation and application to a swirl burner

The analysis of large-scale structures from highly refined unsteady simulations becomes challenging as the mesh resolution increases, and some new tools must be developed in order to perform their identification and extraction. A solution is to use filters to remove the smallest flow motions. High-order filters, characterized by their good selectivity properties, were implemented in an unstructured finite-volume solver for large-eddy simulation, and their ability to extract structures of a given scale was tested on canonical flows. Then, these filters were applied on an aeronautical swirl burner with a complex geometry. The results show that novel high-order filters are able to extract the precessing vortex core from this realistic turbulent flow. High-order filtering enables to study in detail this large-scale structure and to gain insight into the dynamic of swirl flows.

Large-Eddy Simulation of Flow and Heat Transfer Around a Low-Mach Number Turbine Blade

If flowines has been In the ideal Brayton cycle, an increase of the pressure ratio directly leads to an increase of the thermodynamic efficiency and subsequently to a decrease of the specific fuel consumption. Unfortunately, this pressure ratio growth causes a direct increase of the temperature ratio through the turbine stages, which may impact the design of turbine blades.

A multi-grid framework for the extraction of large-scale vortices in Large-Eddy Simulation

Abstract The analysis of large-scale vortices from highly refined unsteady simulations becomes challenging as the mesh resolution increases. Beyond the large amount of data that needs to be processed, classical vortex visualization techniques based on invariants of the velocity gradient tensor fail in extracting the large-scale vortices as the velocity gradient tensor magnitude is greater for small turbulent eddies than for energy-containing vortices. This problem is even more important in highly-resolved simulations with a broad range of eddies. The methodology presented here is a geometric multi-grid high-order filtering (MGHOF) framework for on-line analysis of high-fidelity simulations. This approach relies on high-order implicit filters and enables the extraction of large-scale features from Large-Eddy Simulations (LES) on massive and distributed unstructured grids at a reduced cost. The MGHOF framework is first described and validated, then the methodology is applied to a 3D turbulent plane jet and to the LES of a 3D low-Mach number turbine blade with various mesh sizes, ranging from a few million to a few billion tetrahedra. In the latter case, the MGHOF enables to perform the dynamic mode decomposition of the velocity and temperature fields for the finer grid resolution.

Modeling and Analysis of the Interactions of Coherent Structures with a Spray Flame in a Swirl Burner

With the constant increase in super-computing power, Large-Eddy Simulation (LES) has become an important tool for the modeling and the understanding of flame dynamics in complex burners. A fine description of the reaction layers in such devices requires fine meshes and the resolution of a broad range of turbulent scales. Unfortunately, extracting the large-scale features is not trivial. To this aim, implicit high-order filters that are based on simple low-order finite-volume operators have been proposed. These filters are applied in the LES of the MERCATO burner in order to study the complex interactions of the Precessing-Vortex Core, a large vortex typical of swirl burners, and a spray flame. High-order filters conveniently enable the analysis of the flame anchoring and its dynamics in the wake of the PVC.