<p>Reynolds Averaged Navier-Stokes (RANS) models still represent the most common turbulence modeling technique used in Computational Fluid Dynamics (CFD) today. RANS models are preferred primarily due to their relatively low computational demand and ease of use. Despite their popularity, RANS models have been found to perform poorly in flows with separated shear layers, unsteady wakes, and temporally evolving flows. There has been ongoing progress towards high-fidelity methods such as Large Eddy Simulation (LES) to more accurately represent these flow features. &nbsp;LES models apply filters to the equations of fluid motion to resolve the large turbulent structures that are responsible for energy transfer. The smaller scales however, are represented using a sub-grid scale (SGS) model. LES models perform well in separated shear layers where large eddies dictate the energy and momentum transfer, due to the small time and length scales associated with near wall flow. The costs associated with LES are a major limiting factor in their adoption in industrial and academic research. This has led to the development of Hybrid RANS-LES (HRL) models which offer improved performance over RANS models while being relatively inexpensive compared to LES models. The hybrid modeling approach aims to provide the best of both worlds. In hybrid models, LES models are used far away from the wall to resolve large scale structures primarily responsible for the transfer of momentum and energy, while the wall bounded turbulence is treated using a RANS model. However, HRL models suffer from inherent drawbacks associated with their handling of RANS to LES transition in addition to a high degree of grid sensitivity. This study proposes a new Dynamic Hybrid RANS-LES (DHRL) model for complex turbulent flows. Since the baseline DHRL model uses the ratio of resolved to modeled turbulent stress in RANS-to-LES blending, it was observed that for some types of flows the RANS component of the DHRL model can significantly overpredict turbulent stress in regions where appreciable levels of LES fluctuation could be sustained. A new statistical variable is introduced in this study and implemented into a modified blending function to facilitate model transition from RANS to LES based on the presence of appreciable levels of resolved turbulence. Numerical simulations are carried out using the density-based finite-volume solver Loci-CHEM for canonical test cases such as fully developed channel flow, flow over a backward facing step, and flow over a three-dimensional axisymmetric hill. A comprehensive analysis of results indicate that the new model successfully addresses some of the shortcomings of the baseline DHRL model and it is concluded that new DHRL model variant is a useful alternative to traditional HRL models.</p>

A New Variant of the Dynamic Hybrid Rans-Les Model for Complex Turbulent Flows

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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