The present work is devoted to the assessment of the energy fluxes physics in the space of scales and
physical space of wall-turbulent flows. The generalized Kolmogorov equation will be applied to
DNS data of a turbulent channel flow in order to describe the energy fluxes
paths from production to dissipation in the augmented space of wall-turbulent flows.
This multidimensional description will be shown to be crucial
to understand the formation and sustainment of the turbulent fluctuations fed by the energy fluxes
coming from the near-wall production region.
An unexpected behavior of the energy fluxes comes out from
this analysis consisting of spiral-like paths in the combined physical/scale space where the controversial
reverse energy cascade plays a central role. The observed behavior conflicts with the classical notion of the
Richardson/Kolmogorov energy cascade and may have strong repercussions on both theoretical and modeling
approaches to wall-turbulence. To this aim a new relation stating the leading physical processes governing
the energy transfer in wall-turbulence is suggested and shown able to capture most of the rich dynamics of
the shear dominated region of the flow. Two dynamical processes are identified as driving mechanisms
for the fluxes, one in the near wall region and a second one further away from the wall. The former,
stronger one is related to the dynamics involved in the near-wall turbulence regeneration cycle. The second
suggests an outer self-sustaining mechanism which is asymptotically expected to take place in the log-layer and
could explain the debated mixed inner/outer scaling of the near-wall statistics.
The same approach is applied for the first time to a filtered velocity field. A generalized Kolmogorov
equation specialized for filtered velocity field is derived and discussed.
The results will show what effects the subgrid scales have on the resolved motion in both physical
and scale space, singling out the prominent role of the filter length compared to the cross-over scale
between production dominated scales and inertial range, lc, and the reverse energy cascade region lb. The systematic characterization of the resolved and subgrid physics as function of the filter
scale and of the wall-distance will be shown instrumental for a correct use of LES models in the
simulation of wall turbulent flows. Taking inspiration from the new relation for the energy transfer in
wall turbulence, a new class of LES models will be also proposed. Finally, the generalized Kolmogorov equation
specialized for filtered velocity fields will be shown
to be an helpful statistical tool for the assessment of LES models and for the development of new ones.
As example, some classical purely dissipative eddy viscosity models are analyzed via an a priori procedure.
| Identifer | oai:union.ndltd.org:unibo.it/oai:amsdottorato.cib.unibo.it:3821 |
| Date | 02 May 2011 |
| Creators | Cimarelli, Andrea <1983> |
| Contributors | De Angelis, Elisabetta |
| Publisher | Alma Mater Studiorum - Università di Bologna |
| Source Sets | Università di Bologna |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, PeerReviewed |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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