Presentation pdf 5.7MB

On the evaluation of mesh resolution for large-eddy simulation of internal flows using OpenFOAM

The central aim of this research is to assess mesh resolution requirements for large-eddy simulation (LES) of flows similar to the ones which occur inside the draft-tube of hydraulic turbines at off-design operating condition using OpenFOAM. The importance of this study is related to the fact that hydraulic turbines often need to be operated over an extended range of operating conditions, which makes the investigation of fluctuating stresses crucial. Scale-resolving simulation (SRS) approaches such as LES and detached-eddy simulation (DES) have received more interests in the recent decade for understanding and mitigating unsteady operational behavior of hydro turbines. This interest is due to their ability to resolve a larger part of turbulent flows. However, for some industrial flows, where there is no adequate experimental data to support a deep understanding of the flow physics, such as the ones which happen at part load, deep part load and speed no-load operations of hydraulic turbines, the reliability of numerical simulations in terms of their grid-dependency is still an open question. Verification studies in LES are also very challenging, since errors in numerical discretization, but also subgrid-scale (SGS) models, are both influenced by grid resolution. A comprehensive examination of the literature shows that SRS for different operating conditions of hydraulic turbines is still quite limited and that there is no consensus on mesh resolution requirement for SRS studies. Therefore, the goal of this research is to develop a reliable framework for the validation and verification of SRS, especially LES, so that it can be applied for the investigation of flow phenomena inside hydraulic turbine draft-tube and runner at their off-design operating conditions. Two academic test cases are considered in this research, a turbulent channel flow and a case of sudden expansion. The sudden expansion test case resembles the flow inside the draft-tube of hydraulic turbines at part load. Several resolution criteria for LES analysis have been identified in the literature and their applicability and the level of insight which they put into our analysis are scrutinized. In this study we concentrate on these academic test cases but it is expected that hydraulic turbine flow simulations will eventually benefit from the results of the current research.