SESAME Newsletter #2 (February 2017)
Direct Numerical Simulation of backward facing step
Within Work Package 1 of the SESAME project, the Jožef Stefan Institute is performing a DNS of a sodium flow over a backward facing step (BFS) geometry. To perform the simulations, the open source code NEK5000 (developed by Argonne National Laboratory) is selected. The spectral element method implemented in the code is a hybrid between the finite element method and the spectral method.
This method has some advantages over the two mentioned methods: it retains the high accuracy of spectral methods, but it is easily parallelisable and can be used on irregularly shaped domains. The cross-section of the domain along the stream and over the step is shown in Figure 1. The fluid is flowing from left to right and it is enclosed by the walls at each of the four sides. After the step, the flow separates and a vortex is formed behind the step. In the upper part of this region the fluid is flowing towards the outflow while in the lower region the fluid is flowing in the opposite direction. Oscillations of the flow separation vortex are the main source of thermal fatigue, when the vortex and the bulk flow temperatures are different.
The goal of this research is to understand the role of this flow separation in the behaviour of thermal fluctuations and penetration of thermal fluctuations into the walls, as well as to produce a database of various properties for validation and development of advanced turbulent heat flux models within the SESAME project.