Philipp Schlatter
Department of Mechanics
Royal Institute of Technology
Stockholm
Conventional numerical predictions of engineering or geophysical turbulent
flows are based on the Reynolds-averaged Navier-Stokes (RANS) equations for
which statistical turbulence models are employed. Such computations can only
give statistical information about turbulence, and severe limitations of
existing turbulence models in non-standard situations represent a major
obstacle to reliable predictions. Alternative turbulent flow simulation
approaches are direct numerical simulations (DNS), in which all relevant
length and time scales down to the Kolmogorov scales have to be resolved,
and large-eddy simulations (LES), in which only the large-scale,
energy-carrying vortices of the flow are accurately resolved on the
numerical grid whereas the small-scale fluctations are taken care of by a
subgrid-scale (SGS) model. The application of LES to flows of technical
interest is promising as LES provide reasonable accuracy at significantly
reduced computational cost compared to DNS (order of a few percent).
The difficulty of a successful LES of a given flow problem is the choice of
an appropriate SGS model and also the underlying numerical method. Nowadays,
LES of turbulent flows are quite well established with a variety models
proposed in the literature. However, the application of LES to transitional
flows has obtained increased attention only recently. Laminar-turbulent
transition encompasses the evolution of a flow with initially ordered
laminar motion into the chaotic turbulent state. Transition is important in
a variety of technical applications, however its accurate prediction and the
involved physical mechanisms are still a matter of active research.
In the talk, an overview is given of recent advances with large-eddy
simulations of transitional and turbulent incompressible wall-bounded shear
flows. Different modelling strategies for LES are introduced and discussed,
in particular recent SGS models including the approximate deconvolution
model (ADM) and related approaches, classical and high-pass filtered (HPF)
eddy-viscosity models, and dynamic models. Results based on a-posteriori
simulations are given for the prototype problem of laminar-turbulent
transition in plane channel flow at comparably low grid resolutions. The
applicability of LES to the faithful prediction of vortical structures
during turbulent breakdown is discussed.