Turbulence models with adaptive meshing for industrial CFD
Creators: name: Bull, Jonathan R.
Publisher: name: Imperial College London
Year: 2013

description: Computational fluid dynamics (CFD) and affordable computing power have advanced considerably in recent years, bringing full 3D simulation of complex high Reynolds number flows within reach of industry. However, providing accurate and trustworthy results in diverse flows with constraints on computational resources is still a considerable challenge. Owing to the complexity of commonly-encountered turbulent flows, robust turbulence models are required which do not have to be manually tuned to specific flow conditions to ensure their accuracy. In this regard, a highly effective approach is unstructured mesh adaptivity which automatically refines or coarsens the mesh locally in order to achieve a desired accuracy with minimum computational effort. However, the use of such adaptive meshes with turbulence models raises questions about the origins and interactions of various errors. This thesis describes the development, verification and validation of robust turbulence models suited to high Reynolds number single-phase turbulent flow using unstructured adaptive meshes. The main focus of this thesis is a new tensorial dynamic large eddy simulation (LES) model. The novel combination of the dynamic LES method with a tensorial definition of filter width is ideal for capturing the anisotropy and inhomogeneity of turbulence. This model is designed for use with unstructured mesh adaptivity, which enables the simulation of turbulent flow with high efficiency in terms of mesh resolution. Furthermore, the model is robust since both the resolution and the sub-filter-scale (SFS) stresses adapt to local flow conditions so that little a priori knowledge of the flow is required. Verification tests of the filtering method and validation of the new LES model in the 3D backward-facing step are presented. To provide context for the research, the contribution made by CFD simulations to the analysis of nuclear reactor safety and performance is discussed. The practicalities of performing simulations on high performance computing (HPC) facilities are also discussed. Background theory necessary to understand the research is presented, including a mathematical description of turbulent flow and the classes of CFD methods used to approximate it. A review of turbulence models, discretisation methods, boundary conditions and adaptive meshing methods is included. The construction and testing of a Reynolds-averaged Navier-Stokes (RANS) k - ε turbulence model and a scale-adaptive very large eddy simulation (VLES) model in the open-source CFD code Fluidity are also described. The development of a law-of-the-wall boundary condition for turbulent flow in variational (weak) form is also presented. Verification tests are performed to establish that the k - ε model has been coded correctly. Validation of the RANS model and the wall function using fixed and adaptive meshes is carried out in the 2D backward-facing step. Finally, results of simulations of a vortex diode device using various turbulence models are presented and compared to results from the commercial CFD code CFX and experimental results. This study was carried out during the industrial component of the Engineering Doctorate, which was intended to further the development and understanding of CFD at Rolls-Royce Nuclear. The device presents a challenging test case for CFD but some useful conclusions are reached about how to model it. The thesis concludes with a summary of findings and proposals for further research.