Unformatted text preview: me as reflecting the real physical processes. The
list of models is a very long one indeed. It starts with the ‘eddy
viscosity’ as the simplest version of the ‘solution’ of the ‘problem of
closure’, low dimensional representation and ends with most
sophisticated versions of LES and similar (so far). Many of these
models are based on qualitatively different (and even contradictory)
premises/assumptions. Nevertheless, most of the numerous models are in good
agreement with the experimental and numerical evidence.
This is not surprising since agreement with limited (by
necessity) experimental evidence is not very much significant
when one deals with such a highly dimensional system as
turbulence. The only exception seems to be the Navier-Stokes
equations: Perhaps the biggest fallacy about
turbulence is that it can be reliably described
(statistically) by a system of equations which
is far easier to solve than the full timedependent three-dimensional Navier-Stokes
equations BRADSHAW 1994 Ambiguity of language. What is ‘scale’ or
What is meant by ‘scale’ is hard to define precisely, CHORIN,
1994 Fully turbulent flows consist of a wide range of scales,
which are classified somewhat loosely as either large or
small scales. SREENIVASAN AND ANTONIA, 1997
An eddy eludes a precise definition, but it is conceived to be
turbulent motion, localized within a region of size l .
POPE, 2000 ...to a large extent we have failed. The key point is that we
have not yet agreed on what we mean by an eddy.
DAVIDSON, 2004 Ambiguity of language.
Ambiguity …there exists a range of eddy (what it is?) sizes which are too
large to be influenced by viscosity yet too small to
`feel' directly the time dependence of the large eddies
Another example is when it is said that very large spatial
scales of motion convect very small scales without
directly causing significant internal distortion of the
small scales, see below
see In many cases it is s...
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