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Large Eddy Simulations Of The Atmospheric Boundary Layer

Speaker: Stimit Shah, Graduate Student
Series: EEWR Brown Bag Seminars
Location: Engineering Quad E219
Date/Time: Friday, April 15, 2011, 12:00 p.m. - 1:00 p.m.

Abstract:

Atmospheric boundary layer consists of a wide range of scales of motion. Nonlinear interactions between these scales leads to seemingly random velocity and pressure fields. If one decides to use brute force to generate and study these fields, the amount of computational power required to resolve all the scales would be immense. The atmospheric boundary layer is further complicated by heterogeneous terrain and responds differently to different surface forcings. The question then is can we understand these complex, seemingly random fields and the effect of boundary conditions on them at low cost? Is it possible to separate scales and model the effect of ones which cannot be computed explicitly?

In Large Eddy Simulations only the highly energetic large scales are computed and the influence of smaller scales is modeled. Large scales and small scales are separated by a filtering operation. This operation results in a residual term appearing in the filtered equations. Our task then is only to find a means of characterizing this residual term. Investigations of turbulence under neutral, unstable and stable conditions carried out using scale dependent Lagrangian dynamic model for unresolved scales will be presented. We have extended this model to dynamically compute the subgrid-scale Prandtl number. We examine the resolved flow field by looking at the mean profiles of wind speed, potential temperature, and momentum and buoyancy flux within the boundary layer, in addition to the the streamwise velocity and potential temperature spectra. All these will help us understand real world boundary layers and improve parameterizations in weather and climate models.