The UMIST-N Near-Wall Treatment Applied to Periodic Channel Flow

CHAPTER 1. INTRODUCTION & LITERATURE SURVEY 11
1.4 Relevance to Large Eddy Simulation
Large Eddy Simulation (LES) is a modelling approach that allows large-
scale turbulent fluctuations to remain represented within the Navier-Stokes
equations while small-scale turbulent fluctuations are filtered out statistically
and treated separately. Thus it can be thought of as offering a compromise
between DNS and RANS. The LES approach involves the complexities of
resolving large-scale turbulence, of modelling small-scale turbulence, and of
handling the interaction between these two scales of turbulence. However,
LES provides a potential for greater predictive accuracy than any RANS
method.
LES and RANS approaches are far from alien, and hybrid calculations have
been undertaken. Labourasse & Sagaut [32] have run LES within an overall
RANS calculation. This provided a solution that exhibited the robustness
of a RANS method with some additional accuracy derived from the use of
LES. Quéméré et al. [56] have run RANS and LES calculations alongside one
another in different zones within a flow domain. These hybrid investigations
highlight the complimentary strengths of RANS and LES in some flows.
LES and RANS face analogous tradeoffs in the treatment of flow near solid
boundaries. Performing a detailed LES calculation to resolve the turbulence
near a wall is very computationally expensive. In most LES calculations,
a wall function based on the logarithmic law of the wall is used to specify
boundary conditions at a finite distance away from the wall. Balaras et al. [5]
improved upon this by obtaining wall shear stress from a near-wall subgrid
result within an LES calculation. The subgrid employed an algebraic model
to obtain wall-parallel velocity and thus obtain a wall shear stress to act as