Speaker: James G. Brasseur
Department of Mechanical Engineering
Pennsylvania State University
Date: August 5, 2013
Place: FL 2 – Room 1022
The structure of the energy-dominant turbulent eddies in the atmospheric boundary layer (ABL) is known to depend strongly on the global stability state of the inversion-capped lower troposphere. In the purely shear-driven neutral limit, the ABL is characterized by (Coriolis-oriented) streamwise-elongated coherent eddies of negative fluctuating horizontal velocity, weakly correlated with vertical fluctuations. As the surface is heated and surface heat flux increases, buoyancy drives vertical fluctuations that are strongly correlated with the low-speed shear-driven motions. With sufficient surface heating, the interaction between buoyancy-driven vertical and shear-driven horizontal fluctuations leads to the creation of strong streamwise-coherent rolls with helical turbulent motions that couple the upper with lower boundary layer. We have studied this transitional process with a series of carefully designed spectral large-eddy simulations of the ABL with fixed geostrophic wind above the capping inversion and systematically increased surface heating. A goal was to identify the transition between "near neutral" and "moderately convective" by quantifying correlations and integral scales as a function of the global stability parameter -zi/L; we expected to find a rapid but systematic change in boundary layer structure below a well-defined –zi/L value of order 1. We were surprised to find a complex and rich transitional process that includes a critical transition in horizontal turbulence structure, a special stability state that is characterized by exceptionally coherent streamwise rolls, and an unexpected sensitivity in horizontal fluctuations to surface heating.
*This work is in close collaboration with Dr. Balaji Jayaraman, Research Associate in the "Cyber Wind Facility" program at Penn State, supported by the Department of Energy.
James (Jim) Brasseur is Professor of Mechanical Engineering, Bioengineering and Mathematics at the Pennsylvania State University. Jim did his graduate work at Stanford University in Aeronautical and Astronautical Sciences and has carried out postdoctoral fellowships at NASA-Ames Research Center (computational fluid dynamics), the University of Southampton England (aerodynamics), and The Johns Hopkins University (turbulence physics). Jim is currently Chair of the APS Topical Group on the Physics of Climate (GPC). Currently, Jim leads a research team developing a HPC "Cyber Wind Facility" for wind turbine research.