[GTP] Joint GTP MMM seminar - James Brasseur

[Carolyn Mueller]

Transformation from Isotropic to Shear-dominated Turbulence
in 3-D Physical and Spectral Space
James G. Brasseur
Department of Mechanical Engineering
Pennsylvania State University
currently: ASP Fellow at NCAR

Thursday, January 12, 2012
Foothills Laboratory 2
Room 1001
Lecture 3:30 pm

The two primary mechanisms that generate turbulence are mean shear and 
buoyancy force, each of which act on covariances in underlying 
fluctuating velocity and temperature fields to alter turbulence 
structure. In the daytime atmospheric boundary layer, for example, shear 
generally dominates near the ground as convection dominates in the mixed 
layer above. Here we study the interaction between mean shear and the 
structure of fluctuating velocity and vorticity fields in homogeneous 
turbulent shear flow as a model for shear flows in general, by analyzing 
direct numerical simulations of the transition from initially isotropic 
to shear-dominated turbulence. I address two related issues: (1) the 
mechanisms that force the small-scale turbulent vorticity field from its 
initial tube/sheet-like structure in isotropic turbulence to one 
dominated by horseshoe/hairpin vortical structures in shear turbulence, 
and (2) the relationship in the velocity/vorticity fields between 3-D 
physical space and 3-D Fourier spectral space in the transition from 
unforced isotropic to shear-forced anisotropic turbulence. Study (1) 
evolves from a combined visual-quantitative analysis of individual 
turbulence “structures” algorithmically extracted from local 
concentrations of the fluctuating vorticity and strain-rate fields, 
while study (2) makes use of three-dimensional wavelet filtering to 
relate the evolving turbulence anisotropy of local physical space 
structure to global anisotropy in 3-D Fourier space. We conclude that 
the dynamics that generate hairpin vortical structures in shear flow 
lead to a global anisotropy in spectral space that is correlated to the 
average dimensions of the local streamwise-aligned vortices that pervade 
shear-dominated turbulence.

-- 
Carolyn Mueller
NCAR IMAGe
1850 Table Mesa Drive 
Boulder, CO 80305 
www.image.ucar.edu 
Tel: 303 497-2491
Fax: 303-497-2483