Turbulent mixing of a hydrogen jet in crossflow: direct numerical simulation and model assessment

A hydrogen (H Subscript 2 2 $_2$ ) jet in crossflow (JICF) was studied using direct numerical simulations (DNS), large-eddy simulations (LES) and Reynolds-averaged Navier–Stokes (RANS) approaches, based on a geometric representation of port fuel injection (PFI) in an H Subscript 2 2 $_2$ -fuelled heavy-duty internal combustion engine. The focus of this work was on evaluating the turbulent species flux models used in LES and RANS approaches using DNS of an H Subscript 2 2 $_2$ JICF with realistic PFI flow conditions and mixture properties. Results showed that LES perform very well in predicting both the mean velocity and the Reynolds stresses. In contrast, RANS significantly under-predicts all Reynolds stress components, while predicting the mean flow field relatively well. Regarding the prediction in H Subscript 2 2 $_2$ mixing, LES show excellent agreement with DNS, while RANS significantly under-predicts the mixing process. The underlying reasons for the poor performance of RANS were systematically investigated by extracting relevant turbulent transport properties used in the RANS approach from the DNS data. It was shown that the turbulent diffusivity used in RANS is much smaller than that derived from DNS. This under-prediction was found to be attributed to the over-prediction on the turbulent Schmidt number ( italic Sc Subscript t Sc t $ \textit{Sc}_t$ ), as well as the under-prediction on the turbulent viscosity. By further analysing the anisotropy of italic Sc Subscript t Sc t $ \textit{Sc}_t$ , the commonly used assumption of isotropic turbulent diffusivity in RANS was demonstrated to be invalid for the present configuration. This study provided a unique DNS dataset for H Subscript 2 2 $_2$ JICF relevant to H