Numerical Simulation of Atmospheric Boundary Layer Flow over Battle Eld-Scale Complex Terrain: Surface Fluxes from Resolved
Abstract
The objective of this research is to improve understanding and prognostic characterization of surface fluxes associated with complex topography. Such prognostic capabilities address the challenge inherent to the broad spectrum of spatial scales present in the atmospheric boundary layer. Under the fully rough, inertial-dominated conditions typical of the atmospheric boundary layer, turbulent mixing dominates transport of momentum and scalars (humidity, temperature, or other quantities of concern to the Department of Defense (DoD) in battlefield environments). At the largest scale, large- and very-large-scale coherent motions with spatial extent approaching twenty times the boundary layer depth are present and are responsible for turbulence production and modulation of flow within the complex topography canopy. Moreover, unsteady vortex shedding from individual topographic elements that compose the collective complex topography induces vigorous mixing that is structurally decoupled from the aloft atmospheric boundary layer. For a topography of aggregate elevation, h, mixing within the lowest 3h to 5h of the atmospheric surface layer Ð the so-called roughness sublayer Ð is dominated by h-scale vortices due (in a mean sense) to the mean streamwise velocity profile inflection at elevation, h. These dynamics are responsible for the largest streamwise-vertical Reynolds shear stresses present and, thus, set the aerodynamic signature of the topography. And though the complete spectrum of spatial scales instantaneously present in the atmospheric boundary layer is physically important, the range of scales renders complete resolution intractable. Large-eddy simulation is a well-established numerical procedure that addresses this challenge through parameterization of spatially unresolved turbulent fluctuations. But, the method itself is (generally) dependent on models for surface effects. Realistic complex topographies of relevance of the DoD (for example, realistic urban-like terrains or undulating landscapes) are composed of a broad spectrum of topographic modes. For example, realistic urban topographies may be composed of an extensive range of obstacles from large rectangular prisms to arrays of smaller obstacles. Within this, high- or low-frequency details such as vegetation or underlying landscape undulations are also present, and will influence the aggregate aerodynamic properties of the landscape. The present research effort is composed of two stages: (1) in the first stage, the PI is working to develop appropriate prognostic wall models for aerodynamic properties of urban-like topographies. This involves development of models for the aerodynamic roughness length Ð the elevation at which mean streamwise velocity is zero in fully rough flow over a complex topography Ð and displacement height. Since roughness length is set by spatial distribution of aerodynamic drag, each landscape has a unique value and this value can only be determined retroactively once simulation or experimental data is available. Many correlations leverage area idices (plan and frontal area per unit volume), although recently findings from the engineering roughness literature suggest that statistical moments of the topography can also be used to parameterize roughness length. By using an immersed boundary method to fully resolve a wide array of urban-like topographies, the PI are developing roughness correlations ideally suited to a priori roughness specification. In the second stage, the PI will consider topographies composed of a broad spectrum of topographic modes, with non-zero amplitude at frequencies exceeding the LES grid-filter width. That is, subgrid-scale topographic details. The PI anticipates use of a self-similarity argument in which the aerodynamic drag imposed by unresolved buildings is informed by the resolved scale dynamics (that is, insprired by the Germano identity: Germano, 1992: J. Fluid Mech. 238, 325Ð336).
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 12, 2017
- Source ID
- W911NF1510231
Entities
People
- William E Anderson
Organizations
- Army Contracting Command
- United States Army
- University of Texas at Dallas