Measuring horizontal pore gas velocity profiles in porous media in response to near‐surface wind speed and gustiness

Abstract

Summary A simple method for experimental determination of horizontal wind‐induced, near‐surface pore gas velocities in porous media is presented. This method uses traditional tracer gas tracking methodology, but is designed for applications where mass loss of tracer gas from the experimental domain occurs at an unknown rate (as is the case in near‐surface, wind‐exposed porous media), making traditional inverse transport modelling inapplicable. The method was applied to a dry, granular porous medium consisting of crushed basalt (2–4 mm grain size), using CO2 as the tracer gas. Experiments were conducted under controlled (wind tunnel) conditions, at average near‐surface wind speeds ranging from 0 to 5.6 ms−1, for both gusty and non‐gusty winds. Velocity profiles of near‐surface horizontal pore gas in the top 15 cm of the porous medium were determined for 10 different combinations of average wind speed and wind gust frequency. Pore gas velocities of up to 0.5 cms−1 were observed that depended on depth and near‐surface wind conditions. Results indicate that horizontal gas flow can occur in wind‐exposed porous media under both non‐gusty and gusty wind conditions, and that wind gustiness has a significant effect on the velocity profiles of horizontal pore gas inside porous media. Results show further that pore gas velocities were almost linearly proportional to near‐surface average wind speed, regardless of porous medium depth. Highlights Developed a novel method for measuring wind‐induced pore gas velocity in porous media. Near‐surface wind action induced horizontal pore gas velocities. Pore gas velocity profiles depended strongly on wind gustiness (gust frequency). Pore gas velocities were directly proportional to average near‐surface wind speed.

Linking below‐surface horizontal pore velocity profiles in porous media with near‐surface wind conditions and porous medium gas permeability

“…where k is porous medium gas permeability, P is gas pressure, ϕ is porous medium active porosity, μ is gas kinematic viscosity and x is horizontal distance. In cases where advective pore gas velocity is affected by multiple horizontal driving forces, U may alternatively be estimated experimentally based on tracer tracking (Poulsen, 2018). An instantaneously injected mass of conservative gaseous tracer at location x 0 , y 0 , z 0 , at time t = t 0 , will move horizontally with velocity U.…”

“…Results of both experimental and theoretical modelling work indicate that wind action can significantly affect subsurface gas transport and soil-atmosphere gas exchange (Haghighi & Or, 2015b;Poulsen & Moldrup, 2006;Redecker, Baird, & Teh, 2015). Wind action impacts subsurface gas transport via two different mechanisms: (i) high-frequency, subsurface gas velocity fluctuations created by above-surface air pressure fluctuations associated with turbulent wind conditions near the surface (Laemmel, Mohr, Schack-Kirchner, Schindler, & Maier, 2017;Maier et al, 2012;Poulsen, Furman, & Liberzon, 2017;Poulsen & Moldrup, 2006), and (ii) more stable subsurface gas flow patterns caused by transfer of momentum from above the soil-atmosphere interface down into the soil gas phase (Manes, Pokrajac, Nikora, Ridolfi, & Poggi, 2011;Poulsen, 2018;Poulsen, Furman, & Liberzon, 2018;Suga & Kuwata, 2014). Although subsurface gas movement associated with the first mechanism seems to occur mainly in the vertical direction (Massmann & Frank, 2006;Poulsen, Pouber, Furman, & Papadikis, 2017;Pourbakhtiar, Poulsen, Wilkinson, & Bridge, 2017), subsurface gas movement associated with the second mechanism is mainly horizontal (Poulsen, 2018;Rosti, Cortelezzi, & Quadrio, 2015).…”

“…Pressure pumping generally does not give rise to any consistent subsurface, advective pore gas velocities in either the horizontal or vertical directions but acts instead as a dispersive process (Pourbakhtiar et al, 2017). In contrast, continuous momentum transfer across the surface of a porous medium as a result of wind action near the surface can result in the generation of stable advective pore velocity profiles below the surface (Dong, Gao, & Fryrear, 2001;Poulsen, 2018;Rosti et al, 2015). A schematic of a typical air velocity profile from above the surface to the inside of the porous medium is illustrated in Figure 1.…”

“…In comparison, much less is known about the shape of the velocity profile in the shear flow region within the porous medium, and its dependence on wind speed characteristics and porous medium physical properties. Measurements of pore gas flow in and below the shear flow region in response to near-surface wind conditions were carried out by Poulsen (2018). However, these measurements were conducted only on a single porous medium (2-4-mm crushed basalt) with relatively small sample dimensions (40 by 40 by 40 cm).…”

Editorial for Soil chemical, physical and biological interfacial reactions (Commission 2.5, WCSS) special section

“…The paper by Poulsen (Poulsen, 2020) follows on from an earlier paper also published in the European Journal of Soil Science (Poulsen, 2018) and is a theoretical study of the soil–air interface, supported by experimental data. It shows the importance of horizontal air (wind) velocity for the gaseous exchange in soil air‐filled pores.…”