The Role of Bluff-Body Roughness Elements on Turbulent Evaporative Fluxes from Porous Surfaces: Momentum and Energy Partitioning

Abstract:

Bluff body obstacles in turbulent flows are common in many natural and engineering applications (from desert pavement and boulders to shrubs). The deceivingly simple geometry of a surface-mounted obstacle in a boundary layer induces a complex and unsteady flow field that affect the surface drag partitioning and transport of scalars form surrounding evaporating surfaces. The readily observable spatio-temporal distribution of localized evaporation rates from infrared measured thermal signatures provide the impetus for a simple zonation scheme that enables estimates of evaporative fluxes around an obstacle. Results from evaporation experiments from sand surfaces with isolated cylindrical elements (bluff bodies) subjected to constant turbulent airflows were in good agreement with model predictions for localized exchange rates. The results show consistent enhancement of evaporating fluxes relative to a smooth surface to formation of vortices that induce thinner boundary layers. We also considered effects of obstacles on (solar) short wave radiation and energy partitioning over surfaces and successfully represented zonation of surface exchange due to momentum (drag partitioning) and variable surface energy balance on localized evaporation. A scheme for upscaling the results to evaporation from surfaces with multiple obstacles will be presented.