Water Vapor and Heat Exchanges over Lakes

Quantifying the interaction of the atmosphere and water surfaces is of great importance for water resources management, climate studies of ocean-atmosphere exchange and regional climate in coastal areas. Atmospheric dynamics over water surfaces have generally received less attention than land-atmosphere interactions due to difficulties in operating field studies. In this research we are trying to improve the physical parameterizations of lake-atmosphere processes by integrating measurements and modeling studies. The Lake-Atmosphere Turbulent EXchanges (LATEX) field measurement campaign is analyzed to understand air-water interactions over Lac Léman (Geneva). Large eddy simulations are used to study sensible and latent heat fluxes over heterogeneous wet surfaces. We present parameters of interest for land-surface modeling, i.e. the surface energy budget and the roughness lengths for momentum, heat and water vapor that are embedded in the Monin-Obukhov similarity theory used in atmospheric models. The storage of energy in the lake is a very important term, yet methods used to quantify it, relying on temperature profile measurements from a fiber optic and the theory of conduction of heat, are not successful. We revisit classical wet surface evaporation estimation methods that include this challenging term and derive an evaporation formulation based on sensible heat flux measurements. We then focus on numerical (large eddy) simulations of the flow above a water surface. Small-scale turbulence (the so-called subgrid scales, SGS) over the lake that cannot be captured in large eddy simulations is investigated. The measurements of LATEX allowed, for the first time, the study of subgrid-scale turbulent transport of water vapor over a lake, which reveals itself well correlated with the transport of heat. Results from an a priori analysis of subgrid-scale fluxes and dissipations indicate that the observed subgrid-scale statistics are very similar to those observed over land surfaces. We use the EPFL-LES code, with its scale-dependent Lagrangian dynamic SGS model, to simulate flows over transition from dry land to wet surfaces. We observe the fetch requirement for evaporation formulations and compare to results from simplified Lagrangian footprint models. We explore the limits of applicability of Monin-Obukhov similarity theory that are due to the finite fetch.

Turbulence and flows in the plasma boundary of snowflake magnetic configurations

Maurizio Giacomin, Paolo Ricci, Louis Nicolas Stenger

The power exhaust through the scrape-off layer (SOL) in fusion reactors is expected to be significantly higher than in ITER, thus questioning the extrapolation of the ITER exhaust solution to these devices. The snowflake (SF) magnetic configuration is one of the alternative exhaust configurations being considered to mitigate the heat vessel loads in fusion reactors. The SF configuration features a second-order null of the poloidal magnetic field, i.e. a point where the poloidal magnetic field and its spatial derivatives vanish. As a consequence, the null-point is connected to the wall through four legs, which define four strike points. The presence of the four strike points allows for a heat flux distribution on a larger area compared to standard divertor configurations that feature two strike points. SF configurations are obtained experimentally by generating two first-order X-points close to each other. When the two X-points coincide, a second-order null point is obtained. However, in practice, the two X-points never coincide perfectly and, according to their relative position, we distinguish between the snowflake plus (SF+) and snowflake minus (SF-). The configuration with the two X-points coinciding is usually referred to as ideal SF. All these configurations have been experimentally investigated in the TCV, NSTX, EAST, and DIII-D tokamaks and are considered for the DTT tokamak. Numerical simulations of SF configurations, carried out by means of the EMC3-Eirene and the SOLPS codes, are unable to reproduce the heat flux distribution observed experimentally, calling for detailed investigations of the turbulence and flows in the SF plasma boundary. We present the first global turbulent simulations of the plasma dynamics in SF configurations, including the ideal SF, the SF+ and SF- configurations [1]. These simulations carried out by using the GBS code [2], evolve self-consistently the fluctuating and equilibrium quantities, as they result from the interplay of a heat and particle source in the core, turbulent transport, and parallel losses to the vessel wall. The simulations allow us to disentangle the mechanisms behind the heat flux distribution among the different strike points. As pointed out by our simulations, the heat flux can be reduced by a factor of two in the SF configurations, with respect to single-null configurations. The activation of the unconnected strike points in the ideal SF and in the SF+ configurations, also observed in the experiments, is due to the presence of a steady ExB equilibrium flow in the null region that provides a cross-field transport mechanism towards the private flux region. The origin, the properties and the effects of this steady ExB equilibrium flow are carefully analyzed and described as well as its dependence on the direction of the toroidal magnetic field and on the distance between the two X-points.

2020

A scale-dependent Lagrangian dynamic model for large eddy simulation of complex turbulent flows

Charles Vivant Ignacio Meneveau, Marc Parlange

A scale-dependent dynamic subgrid model based on Lagrangian time averaging is proposed and tested in large eddy simulations sLESd of high-Reynolds number boundary layer flows over homogeneous and heterogeneous rough surfaces. The model is based on the Lagrangian dynamic Smagorinsky model in which required averages are accumulated in time, following fluid trajectories of the resolved velocity field. The model allows for scale dependence of the coefficient by including a second test-filtering operation to determine how the coefficient changes as a function of scale. The model also uses the empirical observation that when scale dependence occurs ssuch as when the filter scale approaches the limits of the inertial ranged, the classic dynamic model yields the coefficient value appropriate for the test-filter scale. Validation tests in LES of high Reynolds number, rough wall, boundary layer flow are performed at various resolutions. Results are compared with other eddy-viscosity subgrid-scale models. Unlike the Smagorinsky–Lilly model with wall-damping swhich is overdissipatived or the scale-invariant dynamic model swhich is underdissipatived, the scale-dependent Lagrangian dynamic model is shown to have good dissipation characteristics. The model is also tested against detailed atmospheric boundary layer data that include measurements of the response of the flow to abrupt transitions in wall roughness. For such flows over variable surfaces, the plane-averaged version of the dynamic model is not appropriate and the Lagrangian averaging is desirable. The simulated wall stress overshoot and relaxation after a jump in surface roughness and the velocity profiles at several downstream distances from the jump are compared to the experimental data. Results show that the dynamic Smagorinsky coefficient close to the wall is very sensitive to the underlying local surface roughness, thus justifying the use of the Lagrangian formulation. In addition, the Lagrangian formulation reproduces experimental data more accurately than the planar-averaged formulation in simulations over heterogeneous rough walls.

2005

Global fluid simulation of plasma turbulence in a stellarator with an island divertor

António João Caeiro Heitor Coelho, Maurizio Giacomin, Joaquim Loizu Cisquella, Paolo Ricci

Results of a three-dimensional, flux-driven, electrostatic, global, two-fluid turbulence simulation for a five-field period stellarator with an island divertor are presented. The numerical simulation is carried out with the GBS code, recently extended to simulate plasma turbulence in non-axisymmetric magnetic equilibria. The vacuum magnetic field used in the simulation is generated with the theory of Dommaschk potentials, and describes a configuration with a central region of nested flux surfaces, surrounded by a chain of magnetic islands, similar to the diverted configurations of W7-X. The heat outflowing from the core reaches the island region and is transported along the magnetic islands, striking the vessel walls, which correspond to the boundary of the simulation domain. The radial transport of particles and heat is found to be mainly driven by a field-aligned coherent mode with poloidal number m = 4. The analysis of this mode, based on non-local linear theory considerations, shows its ballooning nature. In contrast to tokamak simulations and experiments, where blobs often contribute to transport, we do not observe the presence of intermittent transport events.