Flume experiments on turbulent flows across gaps of permeable and impermeable boundaries

Fontan, Stefano, Katul, Gabriel, Poggi, Davide, Manes, Costantino and Ridolfi, Luca (2013) Flume experiments on turbulent flows across gaps of permeable and impermeable boundaries. Boundary-Layer Meteorology, 147 (1), 21-39. (doi:10.1007/s10546-012-9772-z).

Abstract

Laser Doppler anemometery and laser-induced fluorescence techniques were used to explore the spatial structure of the flow within and above finite cavities created within porous and solid media. The cavities within these two configurations were identical in size and were intended to mimic flow disturbances created by finite gaps and forest clearing. Because flows over permeable boundaries differ from their solid counterparts, the study here addresses how these differences in boundary conditions produce differences in, (i) bulk flow properties including the mean vorticity within and adjacent to the gaps, (ii) second-order statistics such as the standard deviations and turbulent stresses, (iii) the relative importance of advective to turbulent stress terms across various regions within and above the gaps, and (iv) the local imbalance between ejections and sweeps and momentum transport efficiencies of updrafts and downdrafts. Both configurations exhibited a primary recirculation zone of comparable dimensions inside the gap. The mean vorticity spawned at the upstream corner of the gap was more intense for the solid configuration when compared to its porous counterpart. The free-shear layer spawned from the upstream corner-edge deeper into the gap for the porous configuration. The momentum flux at the interface within and above the gap was enhanced by a factor of 1.5–2.0 over its upstream value, and this enhancement zone was much broader in size for the porous configuration. For the turbulent transport terms in the longitudinal and vertical mean momentum balances, these transport terms were significant inside the gap for both boundary configurations when compared to their upstream counterpart. The effectiveness of using incomplete cumulant expansion methods to describe the momentum transport efficiencies, and the relative contributions of ejections and sweeps to turbulent stresses, especially in this zone, were also demonstrated. The flatness factor for both velocity components, often used as a measure of intermittency, was highest in the vicinity of the upstream corner in both configurations. However, immediately following the downstream corner, the flatness factor remained large for the porous configuration, in contrast to its solid configuration counterpart.

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