Page 243 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

214 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Fig. 6.1 Hierarchy of CFD
methods.

determinesthescalesthatarerepresented,whereastheaccuracywithwhichthesescales
are resolved is determined by the numerical method. This in turn is responsible for the
two major drawbacks of DNS—its extreme computational cost and its severe limitation
on the maximum Reynolds number that can be considered.
DNS has become a method of outmost importance, and its relevance is constantly
growing with the increasing popularity of high-performance computing techniques.
However, it is not expected to be used for practical nuclear reactor engineering pur-
poses in the coming decades. DNS with a special focus on liquid-metal applications is
described in more detail in Section 6.1.1.

6.2 Large-eddy simulation

A remedy to overcome the limitations of DNS with increasing Reynolds number is to
reduce the range of scales that are resolved on the numerical grid. A possible concept
is to remove the small-scale eddies by a spatial filtering procedure and to resolve the
large-scale eddies only. This approach is called large-eddy simulation (LES). The sep-
aration of small and large scales is inspired by the observations that the large eddies of
macrostructure are anisotropic and depend on the geometry of the flow considered. On
the other hand, small eddies of the microstructure can be considered to be closer to
isotropic and much less dependent on the flow geometry. Therefore, the microstruc-
ture eddies may be regarded as universal. Mathematically, this scale separation is
carried out by applying a spatial filter to the velocity field, splitting it in a resolved
(large-scale eddies) and an unresolved (small-scale eddies) part.
LES has become one of the most promising and successful methodologies that con-
cerns turbulent flows. It is reaching a level of maturity that brings this approach to the
mainstream of engineering and industrial computations, while it opens new opportu-
nities and brings new challenges for further progress. Computational resources remain