Scale-resolving Simulations of Multicomponent Nozzle Flows

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The project investigates in detail the injection and mixing processes of hydrogen in air, as they occur in direct injection systems. Due to the high inlet pressure, an underexpanded supersonic flow is established, in which a complex interaction of shock systems and turbulence occurs. This influences the mixing location and species distribution; likewise, a significant effect of thermodynamic parameters is to be expected. Uncertainties in inflow or nozzle geometry contribute to the complex, nonlinear behavior. This not only affects combustion but may also be part of the causal chain of intermittent cyclic variations. The characterization of these mixing processes as a function of the identified parameters is the goal of this subproject. For this purpose, high-resolution large eddy simulations of underexpanded hydrogen injections are performed. The resolution of the energy-carrying scales in space and time is absolutely necessary to capture the multiscale-multiphysics effects as well as the intermittent phenomena. Here, the coexistence of turbulent regions and shock systems pose great challenges to numerics. Additionally, the disparity of the occurring scales (e.g., more than 3 orders of magnitude lie between nozzle throat and diameter) cannot be represented with computational grids with globally constant grid spacing, even on high-performance computers. Therefore, state-of-the-art high-order methods with adaptation capabilities are used here, which can adapt the local grid resolution to the occurring scales. These methods are extended in this project for multi-component flows. With these methods, the nozzle flow as well as the subsequent mixing can be resolved with unprecedented accuracy and compared to the experimental results. Based on this methodology, the influence of the thermodynamic parameters (equation of state, diffusion approach) is further investigated in cooperation with numerical and experimental partner projects. The resulting high-resolution flow fields as well as the particle trajectories of passively advected tracer particles will be coupled to the combustion simulation code, taking into account the parallelization requirements. On the one hand, this will close the simulation chain from the nozzle to the combustion, and on the other hand, it will allow a joint investigation of the occurring variations (by correlation analysis as well as tracers). Multi-level and multi-fidelity methods are used to validate the results against changes in the nozzle flow. Overall, this project should contribute to a reliable and highly accurate mapping of hydrogen injection and mixing and thus to a contribution to the prediction of cyclic fluctuations and to a better understanding of their causes.