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Person# Marco Giovanni Giometto

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Large eddy simulation

Large eddy simulation (LES) is a mathematical model for turbulence used in computational fluid dynamics. It was initially proposed in 1963 by Joseph Smagorinsky to simulate atmospheric air currents,

Turbulence

In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows

Boundary layer

In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction wi

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Jiannong Fang, Marco Giovanni Giometto, Gabriel George Katul, Marc Parlange

Stably stratified turbulent flows over an unbounded, smooth, planar sloping surface at high Grashof numbers are examined using direct numerical simulations ( DNS). Four sloping angles ( alpha = 15 degrees; 30 degrees; 60 degrees and 90 degrees) and three Grashof numbers (Gr = 5 X 10(10); 1 X 10(11) and 2 : 1 X 10(11)) are considered. Variations in mean flow, second- order statistics and budgets of mean- ( MKE) and turbulent- kinetic energy ( TKE) are evaluated as a function of ff and Gr at fixed molecular Prandtl number. Pr = 1 . Dynamic and energy identities are highlighted, which diagnose the convergence of the averaging operation applied to the DNS results. Turbulent anabatic ( upward moving warm fluid along the slope) and katabatic ( downward moving cold fluid along the slope) regimes are identical for the vertical wall set- up ( up to the sign of the along- slope velocity), but undergo a different transition in the mechanisms sustaining turbulence as the sloping angle decreases, resulting in stark differences at low ff. In addition, budget equations show how MKE is fed into the system through the imposed surface buoyancy, and turbulent fluctuations redistribute it from the low- level jet ( LLJ) nose towards the boundary and outer flow regions. Analysis of the TKE budget equation suggests a subdivision of the boundary layer of anabatic and katabatic flows into four distinct thermodynamical regions: ( i) an outer layer, corresponding approximately to the return flow region, where turbulent transport is the main source of TKE and balances dissipation; ( ii) an intermediate layer, bounded below by the LLJ and capped above by the outer layer, where the sum of shear and buoyant production overcomes dissipation, and where turbulent and pressure transport terms are a sink of TKE; ( iii) a buffer layer, located at 5 / z C / 30, where TKE is provided by turbulent and pressure transport terms, to balance viscous diffusion and dissipation; and ( iv) a laminar sublayer, corresponding to z C / 5, where the influence of viscosity is significant.. / C denotes a quantity rescaled in inner units. Interestingly, a zone of global backscatter ( energy transfer from the turbulent eddies to the mean flow) is consistently found in a thin layer below the LLJ in both anabatic and katabatic regimes.

Francesco Comola, Marco Giovanni Giometto, Michael Lehning, Marc Parlange

Preferential deposition of snow and dust over complex terrain is responsible for a wide range of environmental processes and accounts for a significant source of uncertainty in the surface mass balances of cold and arid regions. Despite the growing body of literature on the subject, previous studies reported contradictory results on the location and magnitude of deposition maxima and minima. This study aims at unraveling the governing processes of preferential deposition in a neutrally stable atmosphere and to reconcile seemingly inconsistent results of previous works. For this purpose, a comprehensive modeling approach is developed, based on large eddy simulations of the turbulent airflow, Lagrangian stochastic model of particle trajectories, and immersed boundary method to represent the underlying topography. The model is tested against wind tunnel measurements of dust deposition around isolated and sequential hills. A scale analysis is then performed to investigate the dependence of snowfall deposition on the particle Froude and Stokes numbers, which fully account for the governing processes of inertia, flow advection, and gravity. Model results suggest that different deposition patterns emerge from different combinations of dimensionless parameters, with deposition maxima located either on the windward or the leeward slope of the hill. Additional simulations are performed, to test whether the often used assumption of inertialess particles yields accurate deposition patterns. Results indicate that this assumption can be justified when snowflakes present dendritic shape but may generate unrealistic results for rounded particles. We finally show that our scale analysis provides qualitatively similar results for hills with different aspect ratios.

2019Jiannong Fang, Marco Giovanni Giometto, Peter Monkewitz, Marc Parlange

The Nieuwstadt closed-form solution for the stationary Ekman layer is generalized for katabatic flows within the conceptual framework of the Prandtl model. The proposed solution is valid for spatially-varying eddy viscosity and diffusivity (O'Brien type) and constant Prandtl number (Pr). Variations in the velocity and buoyancy profiles are discussed as a function of the dimensionless model parameters and , where is the hydrodynamic roughness length, is the Brunt-Vaisala frequency, is the surface sloping angle, is the imposed surface buoyancy, and is a reference velocity scale used to define eddy diffusivities. Velocity and buoyancy profiles show significant variations in both phase and amplitude of extrema with respect to the classic constant model and with respect to a recent approximate analytic solution based on the Wentzel-Kramers-Brillouin theory. Near-wall regions are characterized by relatively stronger surface momentum and buoyancy gradients, whose magnitude is proportional to and to . In addition, slope-parallel momentum and buoyancy fluxes are reduced, the low-level jet is further displaced toward the wall, and its peak velocity depends on both and .