Extended Abstract (744K)9.10
A model for the large-scale ramp structures observed in the atmospheric surface layer
K. G. McNaughton, University of Edinburgh, Edinburgh, Scotland; and R. E. Blundell
This paper presents a structural account of the large-scale ramp-like structures that are observed in the lower parts of boundary layers. Such structures were first reported by Head and Bandyopadhyay in 1981 in a wind tunnel using light sheets in the x-z plane to illuminate smoke tracers released at the wall. They described them as packets of hairpin vortices. Similar structures have been identified in the atmospheric surface layer. These ramp-like structures appear to be isolated disturbances that grow autonomously under the action of the mean shear. If the background flow is plane-parallel with a logarithmic velocity profile then scale analysis shows that such disturbances must be bounded by a linear envelope originating at a point on the ground. The observed ramps are x-z sections cut through such envelopes. Within this envelope the disturbance grows in a cyclic fashion as an upscale cascade of self-similar coherent structures. The individual stages of this cascade begin when an upward ejection of air initiates the roll-up of a horseshoe vortex, and ends when that vortex, by combination of its growth and rotation, produces a second, larger ejection from within its arc. This structure can be called a 'Theodorsen ejection amplifier' (TEA) structure, after the scientist whose work 50 years ago so nearly predicted it. Preliminary simulation results support this model. These TEA cascades explain, for example, the mechanism of backscatter of TKE from subgrid to resolved scales: a process that must be accommodated when parameterizing subgrid-scale processes for LES models.
Session 9, Fundamental Studies of Turbulence, including advances in LES and DNS, Laboratory Studies, and Observations
Wednesday, 17 July 2002, 8:30 AM-12:30 PM
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