🚀 go-pugleaf

>
>Understanding why coherent structures (usually long-lived monopolar or
>dipolar vortices) emerge from turbulence and survive it is interesting
>and important. However, the practical importance is overblown. Most of
>the results are specific to homogeneous and isotropic situations, which
>do not describe the important natural ones. In plasmas, inhomogeneity
>usually kills these things. Claims that coherent solitons can explain
>turbulent transport of heat, particles, or momentum is certainly going
>too far.

Well, some folks here at Stanford might have a thing or to say about that.
Gary Coleman's DNS of the Ekman Layer in 1990 showed how a balance between
convective heating and shear can produce large scale roll cells in the
planetary boundary layer, and these modes alone (1 spike in the midrange
of the energy spectra) transfer much more momentum and scalars than in
plain old shear driven boundary layers.  I don't know much about Langmuir
cells (in the ocean) but I believe they operate under a somewhat similar
principle.  Also, knowing what coherent structures can do to a typical
boundary layer allows for better means of turbulence control (i.e. vortex
generators on wings, as well as vortex `suppressors', such as riblets, that
reduce drag).

Perhaps I misunderstood what you meant by `overblown', or `going too far',
but to suggest that coherent structure study is somehow meaningless and/or
a waste of time would be foolhardy, IMHO.

Matt Bohnert