Unformatted text preview: # Vorticity is stretched only. Hence inadequate representation of
turbulent field by a collection of purely stretched (or other ‘simple’)
# Concentrated vorticity dominates the flow and is quasi-twodimensional.
# Enhanced dissipation in turbulent flows is due to vortex stretching. # Strain rate in turbulent flows is irrotational.
# Turbulent energy production is due predominant (vortex and/or
material elements) stretching
# The difference between quasi-two-dimensional and pure-twoThe
dimensional turbulent flows is always small.
# Q2D and even P2D is always low dissipative.
# For the very smallest eddies the motion is entirely laminar.
# Efficient mixing requires random velocity field.
# Well established phenomenological parallels between the statistical
description of mixing and fluid turbulence itself and similar analogies
between ‘passive’ and genuine turbulence.
# The primary mechanism for production of scalar dissipation is the
nonlinear amplification of scalar gradients by strain rate SELECTED PROBLEMATIC
Mentioned and not mentioned in previous
lectures. Some to be discussed in more
detail in January – March 2008 Eddy viscosity. One of the greatest misconceptions is the
Eddy one on eddy viscosity (and similar as mixing length, etc.) in the sense
that it ‘explains’ the enhanced transfer rates (it is just an empirical way
of accounting for such rates but not at all an explanation).
Gradient transport ideas (which have been around
since the beginning) are understood to be wrong in
principle, yet they are used daily with moderate
success by industry. Understanding how this can be
(it is thoroughly explained by Tennekes and Lumley,
1972, p. 57) sheds light on turbulence. KRAICHNAN
1976 Models. A widespread assumption (and a great variety and
Models huge number of papers) that models represent physical processes in
real turbulence. Most of them are attempts to mimicking turbulence
which is not the sa...
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This note was uploaded on 09/16/2011 for the course ME 563 taught by Professor Staff during the Spring '11 term at Auburn University.
- Spring '11
- The Land