9.3 Large Eddy Simulation (LES)
277
anomalous results in an experiment on the effects of rotation on isotropic
turbulence.
DNS makes it possible to investigate certain effects much more accurately
than would be otherwise possible. It is also possible to try methods of control
that cannot be realized experimentally. The point of doing so is to provide
insight into the physics of the flow and thus to indicate possibilities that may
be realizable (and to point the direction toward realizable approaches). An
example is the study of drag reduction and control on a flat plate conducted
by Choi et al. (1994). They showed that, by using controlled blowing and
suction through the wall (or a pulsating wall surface), the turbulent drag of
a flat plate could be reduced by 30%. Bewley (1994) used optimal control
methods to demonstrate the possibility that the flow could be forced to relaminarize at low Reynolds number and that reduction in the skin friction is
possible at high Reynolds numbers.
9.3 Large Eddy Simulation (LES)
As we have noted, turbulent flows contain a wide range of length and time
scales; the range of eddy sizes that might be found in a flow is shown schematically on the left-hand side of Fig. 9.3. The right-hand side of this figure shows
the time history of a typical velocity component at a point in the flow; the
range of scales on which fluctuations occur is obvious.
LES
DNS
I u
Fig. 9.3. Schematic representation of turbulent motion (left) and the time dependence of a velocity component at a point (right)
The large scale motions are generally much more energetic than the small
scale ones; their size and strength make them by far the most effective transporters of the conserved properties. The small scales are usually much weaker
and provide little transport of these properties. A simulation which treats the
large eddies more exactly than the small ones may make sense; large eddy
simulation is just such an approach. Large eddy simulations are three dimensional, time dependent and expensive but much less costly than DNS
277
anomalous results in an experiment on the effects of rotation on isotropic
turbulence.
DNS makes it possible to investigate certain effects much more accurately
than would be otherwise possible. It is also possible to try methods of control
that cannot be realized experimentally. The point of doing so is to provide
insight into the physics of the flow and thus to indicate possibilities that may
be realizable (and to point the direction toward realizable approaches). An
example is the study of drag reduction and control on a flat plate conducted
by Choi et al. (1994). They showed that, by using controlled blowing and
suction through the wall (or a pulsating wall surface), the turbulent drag of
a flat plate could be reduced by 30%. Bewley (1994) used optimal control
methods to demonstrate the possibility that the flow could be forced to relaminarize at low Reynolds number and that reduction in the skin friction is
possible at high Reynolds numbers.
9.3 Large Eddy Simulation (LES)
As we have noted, turbulent flows contain a wide range of length and time
scales; the range of eddy sizes that might be found in a flow is shown schematically on the left-hand side of Fig. 9.3. The right-hand side of this figure shows
the time history of a typical velocity component at a point in the flow; the
range of scales on which fluctuations occur is obvious.
LES
DNS
I u
Fig. 9.3. Schematic representation of turbulent motion (left) and the time dependence of a velocity component at a point (right)
The large scale motions are generally much more energetic than the small
scale ones; their size and strength make them by far the most effective transporters of the conserved properties. The small scales are usually much weaker
and provide little transport of these properties. A simulation which treats the
large eddies more exactly than the small ones may make sense; large eddy
simulation is just such an approach. Large eddy simulations are three dimensional, time dependent and expensive but much less costly than DNS