Scale-Adaptive Simulation in the Context of
Unsteady Flow Simulations
Florian Menter
ANSYS/Fluent/CFX, Germany
florian.menter@ansys.com
Abstract In the last decade, the spectrum of methodologies for the modeling of
unsteady flows has significantly increased. Historically, the two choices were Unsteady Reynolds Averaged Navier-Stokes (URANS) and Large Eddy Simulation
(LES) methods. Both have severe limitations with respect to their application to
engineering flows. URANS, typically produces no turbulent spectral information,
even in regions, where time and space resolution would be sufficient to do so,
whereas LES requires very high grid counts for modeling of wall-bounded flows
at moderate to high Reynolds numbers. CFD users therefore had little choice than
to concentrate on steady state RANS solutions, even for applications, where it was
clear that these models are not adequate. Detached Eddy Simulation (DES) as
proposed by Spalart in 1997, lead to a shift in paradigm, as it combined the elements of RANS and LES in a way that allowed the simulation of unsteady detached flows with available computing power. More recent studies by Menter and
Egorov, have however shown that the inability of classical URANS models to resolve turbulent structures is not an inherent shortcoming of the URANS approach,
but only related to the way the scale equations have historically been derived. After re-visiting an exact scale equation that Rotta had developed in the 1950s, it became clear that an important term was omitted in all scale equations, which resulted in the inability of classical RANS models to resolve unsteady structures.
Closer inspection of Rotta’s equation shows that the second derivative of the velocity field should be included in the length scale equation. This so-called ScaleAdaptive Simulation (SAS) methodology behaves in many ways and for many applications similar to the DES formulation, but avoids some of the dangers of DES.
On the other hand, it is not the goal of SAS to replace DES and it will be shown
that both methods have their place in the spectrum of unsteady turbulence models.
The paper will discuss the practical implications, but also the limitations of the
SAS technology. Many results produced during the European research project
DESIDER on external aerodynamic flow will be shown.