1,721,773 research outputs found
A phenomenological model to describe turbulent friction in permeable-wall flows
No full textDescribing the canonical properties of turbulent flows over rough-permeable walls such as gravel beds, vegetatedor snow-covered surfaces have, to date, resisted complete theoretical treatment. The major complication in describing such geophysical flows is that the friction factor - Reynolds number relationships significantly deviate from their conventional Nikuradse curves or Moody diagrams derived over impermeable rough boundaries. A novel phenomenological model that describes such anomalous behavior is proposed. It expands the approach in Gioia and Chakraborty (2006) developed for rough-impermeable pipes to include finite velocity effects within the porous wall and canonical length scales governing the momentum exchanges between interstitial and superficial flows. Citation: Manes, C., L. Ridolfi, and G. Katul (2012), A phenomenological model to describe turbulent friction in permeable-wall flow
The effects of gentle topographic variation on dispersal kernels of inertial particles
No full textSeed dispersal kernels of wind-dispersed species imprint the initial spatial template over which later demographic processes such as establishment or re-colonization operate from. A major knowledge gap in seed dispersal modeling by wind is the role of complex topography in modifying the kernel shape when referenced to the flat-world case. How complex topography modifies the dispersal kernels of inertial particles such as seeds is explored here via novel flume experiments. A train of gentle cosine hills and a canopy composed of densely arrayed rods with a roughness density resembling the leaf area density profile of hardwood species at maximum leaf area were used to represent a canopy-hill system. Spherical inertial particles having a coefficient of variation in terminal velocity commensurate with values reported from field studies were employed as model seeds. It was demonstrated that the seed dispersal kernel maintained its canonical ‘Wald' form as derived from simplified turbulent dispersion theories for flat terrain and vertically uniform flow field. Seeds released near the canopy top and from the top of the hill have a dispersal kernel mean distance that was about 35% times larger than its counterpart for releases from the bottom of the hill. Moreover, the probability of occurrence of long-distance dispersal (LDD) events, defined here as seeds traversing a longitudinal distance exceeding 10 times the canopy height, were one order of magnitude larger for seeds released near the canopy top and from the hill top when compared to their counterpart released from the hill bottom. Citation: Katul, G. G., and D. Poggi (2012), The effects of gentle topographic variation on dispersal kernels of inertial particle
Cospectral budget of turbulence explains the bulk properties of smooth pipe flow
No full textConnections between the wall-normal turbulent velocity spectrum Eww (k) at wave number k and the mean velocity profile (MVP) are explored in pressure-driven flows confined within smooth walls at moderate to high bulk Reynolds numbers (Re). These connections are derived via a cospectral budget for the longitudinal (u') and wall-normal (w') velocity fluctuations, which include a production term due to mean shear interacting with Eww (k) , viscous effects, and a decorrelation between u' and w' by pressure-strain effects [=π(k)]. The π(k) is modeled using a conventional Rotta-like return-to-isotropy closure but adjusted to include the effects of isotropization of the production term. The resulting cospectral budget yields a generalization of a previously proposed “spectral link” between the MVP and the spectrum of turbulence. The proposed cospectral budget is also shown to reproduce the measured MVP across the pipe with changing Re including the MVP shapes in the buffer and wake regions. Because of the links between Eww (k) and the MVP, the effects of intermittency corrections to inertial subrange scales and the so-called spectral bottleneck reported as k approaches viscous dissipation eddy sizes (η) on the MVP shapes are investigated and shown to be of minor importance. Inclusion of a local Reynolds number correction to a parameter associated with the spectral exponential cutoff as kη → 1 appears to be more significant to the MVP shape in the buffer region. While the bulk shape of the MVP is reasonably reproduced in all regions of the pipe, the solution to the cospectral budget systematically underestimates the negative curvature of the MVP within the buffer layer
The Influence of Hilly Terrain on Aerosol-Sized Particle Deposition into Forested Canopies
No full textEvaluation of the turbulent kinetic energy dissipation rate inside canopies by zero- and level- crossing density methods
No full textHydraulic resistance of submerged rigid vegetation derived from first order closure models
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Turbulent Intensities and Velocity Spectra for Bare and Forested Gentle Hills: Flume Experiments
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