Test of large eddy simulation sub-grid-scale models for flows in annular channels

Traditionally two main approaches are employed to study unsteady flows. These are the direct numerical solution of the Navier-Stokes equations (DNS) and Large Eddy Simulation (LES) . LES has gained popularity in the last years because of the availability of increasing computing power, and its lower computational cost compared with DNS². The basic idea of LES, is that only the larger scales of motion are resolved while the smaller scales have to be modelled. The small scales of motion, correspondent to the higher part of the Kolgomorov spectra, have in fact a nearly universal behaviour and can therefore be modeled. In this paper we will present a priori and a posteriori analysis of flows in concentric annular and eccentric annular channels (a geometries that share similarities with bare fuel bundles), from a turbulence database in development at Tokyo Institute of Technology. The interest of this work relies on the lack of this kind of studies in relatively complex geometries like the eccentric annular flow. In fact, the application of filtered equations in complex geometries (through boundary fitted coordinates transformation, for example) raises some doubts about the validity and applicability of models usually derived and tested on parallel plates flow or decay of isotropic turbulence. The models tested in this work are the traditional Smagorinsky model³, the dynamic model of Germano⁴, and variants of the dynamic model⁵. Different filtering approaches are discussed as well as the necessary modifications a sub-grid scale model has to undergo in order to be used in a boundary fitted coordinate environment⁹.