Evaporation from Wavy Porous Surfaces into Turbulent Airflows

Abstract

The relief and roughness of natural surfaces interacting with airflows and with radiation affect rates and distributions of heat and vapor fluxes into the atmosphere. The study quantifies interactions of regular sinusoidal wavy porous surfaces (with different geometrical characteristics) affecting heat and vapor transport into prescribed turbulent airflows. A model for turbulent eddies interacting with an undulating evaporating surface with mean boundary layer that varies across sinusoidal wavy surfaces was developed and experimentally evaluated in a wind tunnel. The surface of a 1 m 2 shallow (0.3 m deep) sand-filled basin was imprinted with regular sinusoidal ridges and troughs; water content and temperature sensors were embedded in the sand, and the instrumented basin was placed on a balance in the wind tunnel. Detailed thermal signatures of the evaporating surface for different wind speeds and surface patterns were obtained using high-resolution infrared thermography. The evaporative mass loss measurements and observed thermal patterns were in good agreement with model predictions for turbulent exchange over various wavy sand surface geometries. Results suggest that evaporative fluxes can be either enhanced or suppressed (relative to a flat surface) due to complex interplay between local boundary layer thickness and internal limitations to water flow to the evaporating surface. For a practical range of air velocities (0.5-4.0 m/s), and for sinusoidal configurations studied (amplitudes of 50-100 mm), the evaporative mass loss (relative to the flat surface) was reduced by up to 60 % for low surface aspect ratio and high wind velocity, and enhanced by up to 80 % for high aspect ratio and low wind velocity. The study offers a framework for interpreting and upscaling evaporative fluxes from rough terrestrial surfaces. Ongoing work considers shortwave radiation and geometrical interactions for a more complete account of surface energy balance and fluxes from natural rough surfaces. A b Surface area of wavy building block (m 2 ) C a Vapor concentration in air (kg/m 3 ) C s Saturated vapor concentration (kg/m 3 ) c 1 Coefficient in Eq. (1) (-) c 2 Coefficient in Eq. (4) (-) c 3 Coefficient in Eq. (4) (-) c 4 Coefficient in Eq. (8) (-) c p Air-specific heat capacity (J/kg K) D Water vapor diffusion coefficient in air (m 2 /s) E Evaporation flux (kg/m 2 s) E o Potential evaporation flux (kg/m 2 s) e Total evaporation rate (kg/s) e b Evaporation rate from wavy building block (kg/s) g h Aerodynamic conductance (m/s) H Drying front depth (m) H C Evaporative characteristic length (m) H G Gravity characteristic length (m) H wt Water table depth measured from ridges (m) K a Air thermal conductivity (W/mK) K eff Effective hydraulic conductivity (m/s) K s Saturated hydraulic conductivity (m/s) Length of evaporating system (m) M w Molar mass of water (kg/mol) m Largest integer smaller than α (-) N b Number of wavy building blocks (-) n Pore size distribution index (-) P sat Saturated vapor pressure (Pa) R BL Boundary layer resi...

Effect of near‐surface wind speed and gustiness on horizontal and vertical porous medium gas transport and gas exchange with the atmosphere

“…These authors, however, focused on a single fracture and did not include the effect of gusts. Haghighi & Or () and Haghighi & Or (,b) studied the distribution of wind‐induced water evaporation near obstacles on the soil surface. Investigations, however, included only surface evaporation and not vapour transport inside the soil.…”

Near‐surface turbulence as a missing link in modeling evapotranspiration‐soil moisture relationships

“…The resulting turbulent structures near soil and leaf surfaces modify the local wind stress [ Raupach , ; Shao and Yang , ] and alter the dynamics of the viscous sublayer accordingly [ Haghighi and Or , ] (see also supporting information Text S1 and Figure S1). Note that viscous sublayer is the region close to the surface that underlies a turbulent airflow boundary layer and sets the boundary conditions for heat and vapor transfer by thermal conduction and molecular diffusion, respectively (Figure ) [ Gaikovich , ; Shahraeeni et al ., ; Haghighi and Or , ; Haghighi , ]. Recent studies of heat and mass exchange from surfaces covered by isolated cylindrical obstacles [e.g., Giordano et al ., ; Haghighi and Or , ] reveal the important role of obstacle‐induced turbulence (i.e., horseshoe and wake vortices) [ Sumner , ; Zhang et al ., ] in thinning the viscous sublayer and enhancing the resulting localized heat and mass exchange rates (see also supporting information Figure S2).…”

“…The viscous sublayer sets the upper boundary for diffusive water vapor fluxes and is influenced by turbulence generation adjacent to the soil and leaf surfaces due to interactions between vegetation and the boundary layer (see Figure a) [ Raupach and Thom , ; Finnigan , ; Poggi et al ., ; Villagarcía et al ., ]. These important (but until now empirically treated) diffusion‐based evaporative resistances link soil/leaf hydraulic properties, surface wetness status (varying by soil drying and stomatal closing/opening) and boundary layer characteristics (affected by the turbulent flow), and thus control their highly dynamic evaporative fluxes and associated surface energy partitioning [ Collatz et al ., ; Shahraeeni et al ., ; Schymanski et al ., ; Aminzadeh and Or , ; Haghighi , ; Lehmann and Or , ].…”

“…Given the success of recently developed soil boundary layer resistance parametrizations [ Haghighi et al ., 2013; Haghighi , ] in advancing the predictive capabilities of atmospheric and land surface models [ Haghighi and Or , ; Decker et al ., ], here we invoke similar concepts and establish a physically based parametric ET model that explicitly incorporates how vegetation modifies airflow turbulence near partially/sparsely vegetated soil surfaces. The proposed ET model is expected to (1) deepen our understanding of physical mechanisms governing turbulent energy flux exchange in the most sensitive region for surface fluxes, (2) shed light on hidden dynamics not captured (or quantified) by direct measurements of ET, such as the nonlinearity of turbulence‐surface cover relations influencing ET‐soil moisture relationships and their thermal inferences by remote sensing, and (3) ultimately provide a physical basis for improving near‐surface boundary conditions in dual‐source ET models so that they become independent of empirical resistance terms.…”

Drying of Porous Media

“…They concluded that a pore network model with inclusion of capillary ring formation is a step toward the modeling of secondary capillary effects in drying of porous media in more complex geometries such as a random packing of particles. Haghighi and Or (2015) investigated theoretically and experimentally the interactions between wavy drying surfaces and turbulent airflow above the surface to predict evaporation rates in the presence of surface roughness. Evaporation rates from wavy surfaces could be enhanced or suppressed relative to values from smooth surfaces, depending on characteristics of surface roughness (waviness) and turbulence.…”