Nota il moto del livello dell'acqua, il quale
fa a uso de' capelli, che ànno due
moti, de' quali l'uno attende al peso del vello, l'altro al liniamento delle volte; coś
l'acqua à le sue volte revertiginose, delle quali una parte attende al impeto del
corso principale, l'altra attende al moto incidente e riflesso.


Observe the motion of the water surface, which resembles
that of hair, that has two motions: one due to the weight of the shaft,
the other to the shape of the curls;
thus, water has eddying motions, one part of which is due to the principal current, the
other to the random and reverse motion.
Leonardo da Vinci, ca. 1510


The prediction and control of fluid flows over solid bodies
is very important from a technological point of view: both
the performance and observability of aircraft, surface or
submerged vessels, or automobiles, for instance, are very
much affected by the flow patterns around the body
itself. In many instances, in fact, the aerodynamic (or
hydrodynamic) loads are the main source of noise, drag or
unsteadiness.
The main obstacle in the prediction of flows is the presence of turbulent motions. These motions can be
calculated quite accurately by a numerical solution of
the complete set of equations of motion, the NavierStokes
equations, albeit at costs that are
prohibitively high, except for very simple
configurations in conditions quite different from
those encountered in realistic applications. Any
simplified model developed so far, although applicable
in realistic cases, requires ad hoc adjustments
that are casedependent. This is due to the fact that
turbulent motions in general are strongly affected by
the flow configuration, and cannot be reliably
described by universal models.
It has been observed, however, that the smallest turbulent
motions are more universal than the large ones. If one
could develop reliable models for the small turbulent
eddies, the numerical solution of the NavierStokes
equations would be greatly simplified, and its cost
decreased by several orders of magnitude.
These considerations form the foundations of, and the
motivation for, the technique known as largeeddy simulation
(LES). In LES the small turbulent eddies are modeled, and
only the motion of the large ones is computed
numerically. LES may very well be the only technique capable
of predicting some particularly complex flows, especially if
threedimensional effects or unsteadiness are present in the
mean.
