The talk presents results of two ongoing studies by the author and collaborators that focuses on mixing and momentum transport in turbulent boundary layer flows over complex topographies. The first study pertains to fully developed atmospheric surface layer flows over aeolian sand dune fields from the White Sands National Monument in southern New Mexico. An aspect of particular importance in these flows is the downwind migration of coherent motions associated with Kelvin-Helmholtz instabilities that originate at the dune crests. In the dune field sublayer, the flow statistics resemble a mixing layer: at approximately the dune crest height, vertical profiles of streamwise velocity exhibit an inflection and turbulent Reynolds stresses are maximum; below this, the streamwise and vertical velocity fluctuations are positively and negatively skewed, respectively. Moreover, we found that the macroscale of Kelvin-Helmholtz eddies in the dune field sublayer – shear length and vortex core spacing – exhibit a linear proportionality for flows over significantly different dune fields. The second study focuses on the role of transverse mixing in flows over roughness composed of streamwise elongations of high roughness. Control cases featuring strips of adjacent “high” and “low” roughness are considered; the transverse/spanwise variation of imposed drag sustains transverse turbulent mixing in the immediate vicinity of the roughness change, and leads to delta-scale secondary flows, or the so-called low momentum pathway (Mejia-Alvarez, et al., 2013: Structural attributes of turbulent flow over a complex topography, in Coherent Flow Structures at the Earth’s Surface (Wiley-Blackwell), Chapter 3). The low momentum pathways are spatially stationary and flanked by counter-rotating vortices that serve to pump fluid up, away from the wall. Parametric variation has been used to study the role of parameters responsible for inducing the low momentum pathway. In both studies, the large-eddy simulation method has been used to model the flow.

Host: Ricardo Mejia Alvarez