Page 278 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

248 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Fig. 6.1.2.2 Simulation of a channel flow: different levels of detail according the type of
simulation.

cutoff k c ¼ π/h is related to the local grid size, which should be carefully chosen. As it
is shown by Fig. 6.1.2.1, the LES cutoff should be in the inertial range in order to cap-
ture the correct dissipation rate; for a challenging LES this requires h ≳30η which is
roughly the transition between the inertial range and the dissipation range. The differ-
ence between RANS and LES is huge as LES requires 3D unsteady computations and
high-order numerical schemes like DNS with low numerical diffusion and dispersion
to ensure a correct representation of interscale transfers. Fig. 6.1.2.2 illustrates the dif-
ference of achievable results (resolution) between an RANS, an LES, and a DNS of a
channel flow. This chapter describes the LES technique and its application to compu-
tational heat transfer in a liquid metal flow. General numerical aspects of LES, as well
as implementation and postprocessing considerations, are not covered.

6.1.2.2 LES equations

As it was introduced in the previous section, the LES approach is based on a scale
separation between the large-scale eddies, containing most of the energy, and the
smallest eddies responsible for the final energy dissipation. This scale separation
could be envisaged according to two filtering options. The first consists in projecting
the different fields (velocity, temperature, etc.) on an existing grid of size h. From the
Nyquist theorem, this means that the smallest scale which could be captured has a size