42 research outputs found
Biofilm growth in porous media: derivation of a macroscopic model from the physics at the pore scale via homogenization
3-D image-based numerical computations of snow permeability: links to specific surface area, density, and microstructural anisotropy
We used three-dimensional (3-D) images of snow microstructure to carry out numerical estimations of the full tensor of the intrinsic permeability of snow (<b>K</b>). This study was performed on 35 snow samples, spanning a wide range of seasonal snow types. For several snow samples, a significant anisotropy of permeability was detected and is consistent with that observed for the effective thermal conductivity obtained from the same samples. The anisotropy coefficient, defined as the ratio of the vertical over the horizontal components of <b>K</b>, ranges from 0.74 for a sample of decomposing precipitation particles collected in the field to 1.66 for a depth hoar specimen. Because the permeability is related to a characteristic length, we introduced a dimensionless tensor <b>K</b>*=<b>K</b>/<i>r</i><sub>es</sub><sup>2</sup>, where the equivalent sphere radius of ice grains (<i>r</i><sub>es</sub>) is computed from the specific surface area of snow (SSA) and the ice density (&rho;<sub>i</sub>) as follows: <i>r</i><sub>es</sub>=3/(SSA&times;&rho;<sub>i</sub>. We define <i>K</i> and <i>K</i>* as the average of the diagonal components of <b>K</b> and <b>K</b>*, respectively. The 35 values of <i>K</i>* were fitted to snow density (&rho;<sub>s</sub>) and provide the following regression: <i>K</i> = (3.0 &pm; 0.3) <i>r</i><sub>es</sub><sup>2</sup> exp((&minus;0.0130 &pm; 0.0003)&rho;<sub>s</sub>). We noted that the anisotropy of permeability does not affect significantly the proposed equation. This regression curve was applied to several independent datasets from the literature and compared to other existing regression curves or analytical models. The results show that it is probably the best currently available simple relationship linking the average value of permeability, <i>K</i>, to snow density and specific surface area
Heterogeneous grain growth and vertical mass transfer within a snow layer under a temperature gradient
Inside a snow cover, metamorphism plays a key role in snow evolution at different scales. This study focuses on the impact of temperature gradient metamorphism on a snow layer in its vertical extent. To this end, two cold-laboratory experiments were conducted to monitor a snow layer evolving under a temperature gradient of 100 K m−1 using X-ray tomography and environmental sensors. The first experiment shows that snow evolves differently in the vertical: in the end, coarser depth hoar is found in the center part of the layer, with covariance lengths about 50 % higher compared to the top and bottom areas. We show that this heterogeneous grain growth could be related to the temperature profile, to the associated crystal growth regimes, and to the local vapor supersaturation. In the second experiment, a non-disturbing sampling method was applied to enable a precise observation of the basal mass transfer in the case of dry boundary conditions. An air gap, characterized by a sharp drop in density, developed at the base and reached more than 3 mm after a month. The two reported phenomena, heterogeneous grain growth and basal mass loss, create heterogeneities in snow – in terms of density, grain and pore size, and ice morphology – from an initial homogeneous layer. Finally, we report the formation of hard depth hoar associated with an increase in specific surface area (SSA) observed in the second experiment with higher initial density. These microscale effects may strongly impact the snowpack behavior, e.g., for snow transport processes or snow mechanics.</p
Towards the macroscopic modeling of dry snow metamorphism using an upscaling method.
International audienc
Macroscopic modeling of heat and water vapor transfer with phase change in dry snow based on an upscaling method: Influence of air convection
International audienc
Simulation numérique du largage d’endoprothèse : vers une application clinique
International audienc
Linking snow microstructure to its macroscopic elastic stiffness tensor: A numerical homogenization method and its application to 3-D images from X-ray tomography
International audienc
Effect of packing cerebral aneurysms with hydrogel-coated coils on the wall stresses
International audienc