Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and funda- mental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers.. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabil- ities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs

Large eddy simulation of turbulent flows using finite volume methods with structured, unstructured, and zonal embedded grids

Structured, unstructured, and zonal embedded grid finite volume formulations have been developed to solve the Favre filtered Navier-Stokes equations for performing large eddy simulation of turbulent flows. These compressible formulations were developed using a dual time stepping approach with low Mach number preconditioning. For the structured and zonal embedded grid formulations, time marching was done with either an explicit Runge-Kutta scheme or an implicit lower-upper symmetric-Gauss-Seidel scheme. For the unstructured formulation, time marching used either an explicit Runge-Kutta scheme or traditional Gauss-Seidel scheme. All codes were parallelized to reduce the overall time required for simulations;These schemes were second-order accurate in space and time. Validations were performed using laminar and turbulent incompressible benchmark flows. The results were compared to experimental data, direct numerical simulation results, and other large eddy simulation results;The large eddy simulations yielded excellent agreement with the experimental results for isotropic decaying turbulence. Excellent agreement was also found with the direct numerical simulation data and experimental results for the structured and zonal embedded grid formulations in turbulent channel flow at a low Reynolds number of Re[tau] = 180. Good agreement was found with the unstructured hexahedral grid formulation. The zonal embedded grid formulation achieved greater near wall grid resolution with a fraction of the computational resources required for a single zone simulation;The zonal embedded grid formulation was applied to a high Reynolds number turbulent channel. The results at Re[tau] = 1,050 agreed well with experimental and other large eddy simulation results. The results indicated that the dynamic model required slightly greater grid resolution than Smagorinsky model for accurately modeling wall bounded flows

Computational aerodynamics : advances and challenges

Computational aerodynamics, which complement more expensive empirical approaches, are critical for developing aerospace vehicles. During the past three decades, computational aerodynamics capability has improved remarkably, following advances in computer hardware and algorithm development. However, most of the fundamental computational capability realised in recent applications is derived from earlier advances, where specific gaps in solution procedures have been addressed only incrementally. The present article presents our view of the state of the art in computational aerodynamics and assessment of the issues that drive future aerodynamics and aerospace vehicle development. Requisite capabilities for perceived future needs are discussed, and associated grand challenge problems are presented

Implicit large eddy simulation of weakly-compressible turbulent channel flow

This paper concerns the accuracy of several high-resolution and high-order finite volume schemes in Implicit Large Eddy Simulation of weakly-compressible turbulent channel flow. The main objective is to investigate the properties of numerical schemes, originally designed for compressible flows, in low Mach compressible, near-wall turbulent flows. Variants of the Monotone Upstream-centred Scheme for Conservation Laws and Weighted Essentially Non-Oscillatory schemes for orders of accuracy ranging from second to ninth order, as well as with and without low Mach corrections, have been investigated. The performance of the schemes has been assessed against incompressible Direct Numerical Simulations. Detailed comparisons of the velocity profiles, turbulent shear stresses and higher-order turbulent statistics reveal that the low Mach correction can significantly reduce the numerical dissipation of the methods in low Mach boundary layer flows. The effects of the low Mach correction have more profound impact on second and third-order schemes, but they also improve the accuracy of fifth order schemes. The ninth-order Weighted Essentially Non-Oscillatory scheme is the least dissipative scheme and it is shown that the implementation of the low Mach correction in conjunction with this scheme has a significant anti-dissipative effect that adversely affects the accuracy. Finally, the computational cost required for obtaining the improved accuracy using increasingly higher order schemes is also discussed

Institute for Computational Mechanics in Propulsion (ICOMP)

The Institute for Computational Mechanics in Propulsion (ICOMP) is a combined activity of Case Western Reserve University, Ohio Aerospace Institute (OAI) and NASA Lewis. The purpose of ICOMP is to develop techniques to improve problem solving capabilities in all aspects of computational mechanics related to propulsion. The activities at ICOMP during 1991 are described

Development of Numerical Methods for Accurate and Efficient Scale-Resolving Simulations

Hybrid RANS (Reynolds-Averaged Navier-Stokes)-LES (Large-Eddy Simulation) techniques are considered to be sufficiently accurate and computationally affordable for the aeronautical industry. Scale-resolving simulations is a powerful tool that can accurately predict complex unsteady compressible high-Reynolds-number turbulent flows, as often encountered aeronautical applications. However, since the turbulent scales are resolved instead of modeled, higher demand is placed on the underlying numerical methods used in the simulations.This thesis explores and develops numerical methods suitable for hybrid RANS-LES. The methods are implemented in the Computational Fluid Dynamics (CFD) solver M-Edge, a compressible unstructured node-centered edge-based solver. A low-dissipative, low-dispersive numerical scheme was calibrated and verified in LES of turbulent channel flow and Decaying Homogeneous Isotropic Turbulent (DHIT). It was shown that numerical dissipation and dispersion needs to be carefully tuned, in order to accurately predict resolved turbulent stresses and the correct decay of turbulent kinetic energy. The reported results are in good agreement with reference DNS and experimental data.The optimized numerical scheme was then applied to simulate developing hybrid RANS-LES turbulent channel flow. In order to mitigate the grey area region in the LES zone, a Synthetic Turbulence Generator (STG) was applied at the RANS-LES interface. It was shown that using upstream turbulent statistics from a precursor LES or RANS, the recovery length of the skin friction coefficient could be reduced to just a few boundary layer thicknesses.A new implicit gradient reconstruction scheme suitable for node-centered solvers was proposed. It was shown that the reconstruction scheme achieves fourth-order scaling on regular grids and third-order scaling on irregular grid for an analytical academic case. The Navier-Stokes Characteristic Boundary Condition (NSCBC) was implemented and verified for transport of an analytical vortex. It was shown that special boundary treatment is needed for transporting turbulent structures through the boundary with minimal reflections

On the performance of a high-order multiscale DG approach to LES at increasing Reynolds number

The variational multiscale (VMS) approach based on a high-order discontinuous Galerkin (DG) method is used to perform LES of the sub-critical flow past a circular cylinder at Reynolds 3 900, 20 000 and 140 000. The effect of the numerical flux function on the quality of the LES solutions is also studied in the context of very coarse discretizations of the TGV configuration at Re = 20 000. The potential of using p-adaption in combination with DG-VMS is illustrated for the cylinder flow at Re = 140 000 by considering a non-uniform distribution of the polynomial degree based on a recently developed error estimation strategy. The results from these tests demonstrate the robustness of the DG-VMS approach with increasing Reynolds number on a highly curved geometrical configuration

A matrix-free high-order discontinuous Galerkin compressible Navier-Stokes solver: A performance comparison of compressible and incompressible formulations for turbulent incompressible flows

Both compressible and incompressible Navier-Stokes solvers can be used and are used to solve incompressible turbulent flow problems. In the compressible case, the Mach number is then considered as a solver parameter that is set to a small value, $\mathrm{M}\approx 0.1$, in order to mimic incompressible flows. This strategy is widely used for high-order discontinuous Galerkin discretizations of the compressible Navier-Stokes equations. The present work raises the question regarding the computational efficiency of compressible DG solvers as compared to a genuinely incompressible formulation. Our contributions to the state-of-the-art are twofold: Firstly, we present a high-performance discontinuous Galerkin solver for the compressible Navier-Stokes equations based on a highly efficient matrix-free implementation that targets modern cache-based multicore architectures. The performance results presented in this work focus on the node-level performance and our results suggest that there is great potential for further performance improvements for current state-of-the-art discontinuous Galerkin implementations of the compressible Navier-Stokes equations. Secondly, this compressible Navier-Stokes solver is put into perspective by comparing it to an incompressible DG solver that uses the same matrix-free implementation. We discuss algorithmic differences between both solution strategies and present an in-depth numerical investigation of the performance. The considered benchmark test cases are the three-dimensional Taylor-Green vortex problem as a representative of transitional flows and the turbulent channel flow problem as a representative of wall-bounded turbulent flows