Page 244 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

CFD—Introduction 215

the primary obstacle to practical use of LES for industrial flows, which involve
attached wall boundary layers at high Reynolds numbers. Indeed, the scales of motion
responsible for turbulence production impose severe demands on the grid resolution
near solid walls.
To overcome this, a distinction can be made between wall-modeled and wall-
resolved LES. A wall-modeled LES uses wall functions in the near-wall region and
is therefore computationally much less costly than a wall-resolved LES. Obviously,
this comes at the cost of accuracy. A more detailed description of LES with a focus
on flow and heat transfer for liquid-metal applications is provided in Section 6.1.2.

6.3 Reynolds-averaged Navier Stokes equations

Reynolds-averaged Navier-Stokes (RANS) methods aim for statistical description of
the flow. Time averaging is employed in Reynolds-averaged modeling to reduce the
range of scales present in turbulent flows. The averaging time is much larger than the
largest timescale of the turbulent fluctuations, and as a result, one ends up with con-
servation equations that describe the evolution of the mean flow quantities only. Flow
quantities such as velocity and pressure are split into an average and fluctuation com-
ponents, based on the Reynolds decomposition. The influence of the removed turbu-
lence fluctuations on the mean flow is incorporated into the Reynolds tress tensor.
One of the differences between LES and RANS models is the range of turbulent
scales embedded in the stress components. In RANS models, the Reynold stress tensor
describes the influence of all scales of turbulent motion (including the anisotropic
large scales) on the mean flow, whereas in LES, the subgrid scale stress tensor only
reflects the influence of small-scale turbulence on large-scale (grid-scale) flow quan-
tities. It should be mentioned, however, that a highly resolved URANS simulation and
a wall-modeled LES may be able to resolve similar scales.

Hybrid RANS-LES methods
Hybrid RANS-LES methods have been developed to alleviate the high mesh resolution constraint
(for LES) in the near-wall region, another strategy being the use of local increase in the grid res-
olution as reported by Qu em er e et al. (2001) and Terracol et al. (2001). Hybrid methods can be
classified into two major classes, namely, the global and the zonal hybrid methods as explained by
Sagaut (2006). In a similar way, Fr€ ohlich and von Terzi (2008) propose to classify the hybrid
approaches as unified models and segregated models. These methods can also be distinguished
based on the way of coupling between RANS and LES, that is, weak and strong RANS-LES coupling
methods.
The zonal hybrid methods are based on a discontinuous treatment of the RANS-LES interface.
In practice, information must be exchanged at the RANS-LES interface between two solutions
with different spectral content. The global hybrid methods are based on a continuous treatment
of the flow variables at the interface between RANS and LES. These methods, like detached eddy
simulation (DES) as reported by Spalart et al. (1997), introduce a “gray area” in which the solu-
tion is neither pure RANS nor pure LES since the switch from RANS to LES does not imply an
instantaneous change in the resolution level. These methods can be considered as weak