Daniel Mira

Alya: Multiphysics engineering simulation toward exascale

Alya is a multi-physics simulation code developed at Barcelona Supercomputing Center (BSC). From its inception Alya code is designed using advanced High Performance Computing programming techniques to solve coupled problems on supercomputers efficiently. The target domain is engineering, with all its particular features: complex geometries and unstructured meshes, coupled multi-physics with exotic coupling schemes and physical models, ill-posed problems, flexibility needs for rapidly including new models, etc. Since its beginnings in 2004, Alya has scaled well in an increasing number of processors when solving single-physics problems such as fluid mechanics, solid mechanics, acoustics, etc. Over time, we have made a concerted effort to maintain and even improve scalability for multi-physics problems. This poses challenges on multiple fronts, including: numerical models, parallel implementation, physical coupling models, algorithms and solution schemes, meshing process, etc. In this paper, we introduce Alyas main features and focus particularly on its solvers. We present Alyas performance up to 100.000 processors in Blue Waters, the NCSA supercomputer with selected multi-physics tests that are representative of the engineering world. The tests are incompressible flow in a human respiratory system, low Mach combustion problem in a kiln furnace, and coupled electro-mechanical contraction of the heart. We show scalability plots for all cases and discuss all aspects of such simulations, including solver convergence.

The Effect of Partial Premixing and Heat Loss on the Reacting Flow Field Prediction of a Swirl Stabilized Gas Turbine Model Combustor

This work addresses the prediction of the reacting flow field in a swirl stabilized gas turbine model combustor using large-eddy simulation. The modeling of the combustion chemistry is based on laminar premixed flamelets and the effect of turbulence-chemistry interaction is considered by a presumed shape probability density function. The prediction capabilities of the presented combustion model for perfectly premixed and partially premixed conditions are demonstrated. The effect of partial premixing for the prediction of the reacting flow field is assessed by comparison of a perfectly premixed and partially premixed simulation. Even though significant mixture fraction fluctuations are observed, only small impact of the non-perfect premixing is found on the flow field and flame dynamics. Subsequently, the effect of heat loss to the walls is assessed assuming perfectly premixing. The adiabatic baseline case is compared to heat loss simulations with adiabatic and non-adiabatic chemistry tabulation. The results highlight the importance of considering the effect of heat loss on the chemical kinetics for an accurate prediction of the flow features. Both heat loss simulations significantly improve the temperature prediction, but the non-adiabatic chemistry tabulation is required to accurately capture the chemical composition in the reacting layers.

A Large-Eddy Simulation-Linear-Eddy Model Study of Preferential Diffusion Processes in a Partially Premixed Swirling Combustor With Synthesis Gases

A lean partially premixed swirling combustor operated with synthesis gases is studied using large-eddy simulation (LES). The linear-eddy model (LEM) is employed to close the unresolved scalar fluxes with the nonunity Lewis number assumption. Several terms resulting from the LES filtering operation are not modeled but directly resolved considering their unique length and time scales, such as molecular diffusion, scalar mixing, and chemical reactions. First, the validation results on a well-established jet flame indicate a good level of correlation with the experimental data and allow a further analysis of syngas combustion on a practical combustor. Second, the effects of preferential diffusion on the characteristics of flow and combustion dynamics on a lean partially premixed swirling combustor are investigated. The obtained results are expected to provide useful information for the design and operation of gas turbine combustion systems using syngas fuels.

Saiph: Towards a DSL for High-Performance Computational Fluid Dynamics

Nowadays high-performance computing is taking an increasingly central role in scientific research while computer architectures are becoming more heterogeneous and complex with different parallel programming models and techniques. Under this scenario, the only way to successfully exploit a high-performance computing system requires that computer and domain scientists work closely towards producing applications to solve domain problems, ensuring productivity and performance at the same time. Facing such purpose, Saiph is a domain specific language designed to ease the task of simulating complex systems characterized by partial differential equations, focused on computational fluid dynamics. Saiph allows to model real physical systems providing numerical method and high-performance computing expertise in a transparent and automated fashion.

A Partitioned Methodology for Conjugate Heat Transfer on Dynamic Structures

A partitioned coupling approach for conjugate heat transfer applications is presented. The coupling scheme is based on the extension of the parallel algebraic domain composition method already validated in fluid-structure interactions problems for thermal coupling. The method alters the original Dirichlet-Neumann approach enforcing the boundary conditions over the subdomains through matrix operations. The algorithm is tested on two benchmark cases with conjugate heat transfer: flow over a heated cylinder and flow over a flat-plate. The results indicate good agreement with previous research and encourages its application for large-scale problems.

Study of the Wall Thermal Condition Effect in a Lean-Premixed Downscaled Can Combustor Using Large-Eddy Simulation

The primary purpose of this study is to evaluate the ability of LES, with a turbulent combustion model based on steady flamelets, to predict the flame stabilization mechanisms in an industrial can combustor at full load conditions. The test case corresponds to the downscaled Siemens can combustor tested in the high pressure rig at the DLR. The effects of the wall temperature on the prediction capabilities of the codes is investigated by imposing several heat transfer conditions at the pilot and chamber walls. The codes used for this work are Alya and OpenFOAM, which are well established CFD codes in the fluid mechanics community. Prior to the simulation, results for 1-D laminar flames at the operating conditions of the combustor are compared with the detailed solutions. Subsequently, results from both codes at the mid-plane are compared against the experimental data available. Acceptable results are obtained for the axial velocity, while discrepancies are more evident for the mixture fraction and the temperature, particularly with Alya. However, both codes showed that the heat losses influence the size and length of the pilot and main flame.