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

Above-surface mean wind speed profiles and corresponding below-surface horizontal pore air velocity profiles were measured for two porous media with different particle size and gas permeability under six different wind speed conditions. Experiments followed a 2(k) factorial design to minimize the number of required experiments. Below-surface pore air velocity profiles were measured using a newly developed gas tracer tracking method applied at different elevations below the surface. Results suggested that pore air velocity decreases exponentially with decreasing elevation. A simple empirical exponential model was therefore proposed to approximate the pore air velocity-elevation relationship. The pore-velocity model was chosen such that it, together with an existing, well-known, semi-empirical wind-speed profile model commonly used for describing wind speed in the atmospheric boundary layer, is able to approximate the entire air velocity profile above and below the surface. The pore air velocity profile is described by two empirical parameters that depend on the air velocity at the surface and the shape of the pore velocity profile. The links between the velocity profile shape (via the two empirical parameters), porous medium gas permeability and above-surface wind speed conditions were analyzed based on the experimental data. Results indicate that below-surface horizontal pore velocity profiles are strongly related to both above-surface wind conditions and porous medium characteristics. Highlights Horizontal air velocity profiles above and below porous medium surfaces were measured A novel approach for approximating wind-induced, horizontal pore velocity was proposed Medium and wind speed characteristics strongly affected pore velocity profiles Key parameters were: gas permeability > mean wind speed > wind gust frequency