Momentum and Mass Transfer from Atmosphere to Rough Surfaces: Improvement on Drag Partition Theory and Dry Deposition Model

The transfers of momentum and mass from the atmosphere to rough surfaces are fundamental scientific problems for meteorology, environment and industry. The transfer of momentum is crucial for the transfer of mass, heat, etc., in the boundary layer. The mass transfer, e.g., dust dry deposition, is a key process of the dust cycle. Both processes are closely related, but not well understood, particularly on rough surfaces or in unsteady conditions. Momentum and mass flux have been found to be associated with the geometric dimensions of the wake behind roughness elements. The dimensions of these wakes can be determined by the geometry of the obstacles on rough surfaces and wind speed. The objective of this thesis is to improve the theories of momentum transfer and drag partition between roughness elements and the exposed underlying surface, as well as the parameterization of particle deposition by means of numerical simulations of the air flow and dust flux over rough surfaces. To investigate the transfer of momentum and mass to rough surfaces, both two-dimensional Reynold Stress Model (2D RSM) simulations and three-dimensional Large Eddy Simulations (3D LES) are carried out. A rough surface in the simulation refers to a flat surface with regularly distributed identical roughness elements. The wind profile, surface drag, and the geometric dimensions of the wake are determined from the simulation results. Friction velocity (u*) and a friction coefficient (u*/uh) are estimated as functions of roughness density (λ), threshold roughness density (λa), and wind speed (ur). The dimensions of the wake, which influence the drag and are controlled by wind speed, are subject to the roughness density and the dimensions of the elements. Hence, it is important to repeat the numerical experiments for various element heights (h), roughness densities and wind speeds. To study dust dry deposition, particle injections are included in the 3D simulation. It allows for particle tracking in the turbulent flow; thereby, the deposition velocity can be determined under different wind speeds, particle diameters and roughness densities. The 2D simulation consists of 260 runs on 13 distinctive surfaces (1/30 < λ < 2/3, h = 5, 7.5 and 10 mm) at 20 different wind speeds (1 - 20 ms-1). The purpose of the 2D simulation is to analyze the geometric dimensions of the wake in the absence of spanwise disturbance. 2D simulations limit the possible mutual sheltering of elements in the streamwise direction. Without these disturbances, the length of the wake behind isolated element is presented as a function of wind speed. The height-to-length ratio of the wake (λw = hw/Lw) is analyzed and found to be independent of element height. When the wake is a full wake, λw is also independent from wind speed, and λw = λa. The relation among the dimensions of roughness elements, the wake and the drag are estimated. A physical model of drag and drag partition is proposed, based on a resistance method. The drag and drag partitions are expressed as functions of λ, and λa, without empirical parameters. The estimation of the new model are analyzed and compared to classical experimental results and a 3D simulations results. The 3D simulation for air flow over rough surface are conducted for 11 distinctive surfaces (1/30 < λ < 1/2) with identical elements of 10 mm height, at 6 different wind speeds (1-25 ms-1). In the resistance method for the momentum flux, the resistances of the element (Rr), and the underlying surface (Rs) in the canyon layer are respectively determined. The threshold roughness density (λa) is introduced in the expressions of resistances. This threshold is defined as the roughness density of the surface which has equal momentum flux on the element and on the underlying surface. This threshold can be determined by the length of the wake (Lw) on rough surfaces and helps to distinguish elements of different length-to-height ratios (b/h). New expressions of friction velocity and drag partitions (τr + τs) are derived without any empirical parameter. The friction coefficient is determined empirically. Classical wind tunnel data of drag for rough surfaces with various roughness densities, and results from the 3D simulation are successfully reproduced, and in response to different length-to-height ratios of roughness elements. Thus, the new expressions of drag and drag partition on rough surfaces are validated. The discrepancy between the estimation of existing dry deposition model and field measurements reaches 2 orders of magnitude. In the existing models of dust dry deposition, the rough surfaces are treated as a single cylinder. Sensitivity tests show that the possible uncertainty on the deposition velocity generated by this method can reach 337%. To investigate dust dry deposition in more details, 3D simulations of deposition on rough surfaces are conducted and 15 groups of particles with different diameters (0.1 μm < dp < 10 μm) are injected into the simulation domain. Deposition velocity is deduced by counting trapped particle on the surfaces, in fully developed flow. Regression analysis is applied to fit the deposition velocity as functions of wind speed, roughness density or particle size from simulation results. The resulting prediction of the deposition velocity is consistent with field measurements and explains the discrepancies among existing field measurements and previous model estimations. The measurement of deposition process in the natural flow is also studied. The aim of this part is to examine the influence of unsteady dust flux on the measurement of deposition velocity and errors caused by field measuring method. An existing vertical dispersion framework is introduced to simulate the one-dimensional deposition velocity. Intermittent dust flux data from a well-known field measurement during a dust event, as input data. The resulting estimations of deposition velocity are consistent with field measurements. The understanding of momentum and mass transfer on rough surfaces could thereby be improved.