1 Foundation of Fluid Mechanics
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The study of turbulence has been an unsolved problem for 136 years since
Reynolds’ transition test in 1880. At present, there are many branches in
the basic theory of turbulence, such as turbulence stability theory, turbulence
statistical theory, turbulence model theory, turbulence experiment, coherent
structure of shear turbulence, large eddy simulation, and direct numerical
simulation. Among them, the turbulence model is the most representative
one, but its time-averaged operation erases all the details of the fluctuating
motion, losing a lot of important information contained in the fluctuating
motion, and all kinds of turbulence models have certain limitations. The
rapid development of modern computer technology has provided people with
a new way to solve turbulence problems. It is recognized that large eddy
simulation and direct numerical simulation are promising. However, due
to the limitation of computer speed and capacity, direct numerical simulation is still limited to low Reynolds number flow, and it is impossible to
complete numerical simulation for high Reynolds number at present. Large
eddy simulation (LES) is a compromise between direct numerical simulation
and turbulence model theory. Because of its low computational cost and high
computational accuracy, LES has attracted much attention. Chinese scholars
have made remarkable achievements in turbulence research in recent decades,
which are not listed here because of the limitation of space.
1.12 Multi-scale Discussions of Turbulent Eddies
1. Multi-scale Structure of Turbulent Eddies
Turbulence is a kind of complex flow phenomenon in nature. Since the
British scientist Renault completed the laminar flow transition test in 1880,
many basic problems of turbulence have not been solved. However, after
136 years of exploration, considerable achievements have been made in the
study of turbulence. Especially with the rapid development of computer and
modern measurement technology, the recognition of turbulence has gone
from simple time-averaged level to turbulent structure level of different scales.
Since the 1950s, with the discovery of turbulent coherent structure, it is
generally believed that turbulence is not determined by the randomness of
small-scale eddies, but by the existence of large-scale eddies. Turbulence is
a complex flow phenomenon consisting of eddies of different scales and
frequencies. The scale of the largest eddies is the same order of magnitude
as the characteristic size of the flow region, and the scale of the smallest
69
The study of turbulence has been an unsolved problem for 136 years since
Reynolds’ transition test in 1880. At present, there are many branches in
the basic theory of turbulence, such as turbulence stability theory, turbulence
statistical theory, turbulence model theory, turbulence experiment, coherent
structure of shear turbulence, large eddy simulation, and direct numerical
simulation. Among them, the turbulence model is the most representative
one, but its time-averaged operation erases all the details of the fluctuating
motion, losing a lot of important information contained in the fluctuating
motion, and all kinds of turbulence models have certain limitations. The
rapid development of modern computer technology has provided people with
a new way to solve turbulence problems. It is recognized that large eddy
simulation and direct numerical simulation are promising. However, due
to the limitation of computer speed and capacity, direct numerical simulation is still limited to low Reynolds number flow, and it is impossible to
complete numerical simulation for high Reynolds number at present. Large
eddy simulation (LES) is a compromise between direct numerical simulation
and turbulence model theory. Because of its low computational cost and high
computational accuracy, LES has attracted much attention. Chinese scholars
have made remarkable achievements in turbulence research in recent decades,
which are not listed here because of the limitation of space.
1.12 Multi-scale Discussions of Turbulent Eddies
1. Multi-scale Structure of Turbulent Eddies
Turbulence is a kind of complex flow phenomenon in nature. Since the
British scientist Renault completed the laminar flow transition test in 1880,
many basic problems of turbulence have not been solved. However, after
136 years of exploration, considerable achievements have been made in the
study of turbulence. Especially with the rapid development of computer and
modern measurement technology, the recognition of turbulence has gone
from simple time-averaged level to turbulent structure level of different scales.
Since the 1950s, with the discovery of turbulent coherent structure, it is
generally believed that turbulence is not determined by the randomness of
small-scale eddies, but by the existence of large-scale eddies. Turbulence is
a complex flow phenomenon consisting of eddies of different scales and
frequencies. The scale of the largest eddies is the same order of magnitude
as the characteristic size of the flow region, and the scale of the smallest