For intermediate Q, the granular bed deforms and develops instabilities and microchannels, but the macroscopic flow-rate still obeys Darcy's law. For low flow-rate, the grains remain at rest and the fluid percolates the medium-as a rigid porous medium. For unconstrained beds, where the grains are likely to move and their free surface to deform, different patterns have been thoroughly described when changing the fluid injection flow-rate, Q, in the granular matrix. Many studies have been performed in two-phase systems to analyze the mechanisms at play in the fluid venting process, either for dry (gas/grains) or wet (liquid/grains) beds. Indeed, quick fluidization due to fluid venting is often associated with catastrophic events such as soil liquefaction, volcaniclastic events or, at a larger scale, global warming. Understanding these processes is important not only for economy and ecology, but also for the mitigation of natural hazards. It consists of injecting gases into the saturated zone in the subsurface, in order to remove water contamination by volatile compounds. Since the 90's, the technique of air sparging has also been developed. Depending on the experimental conditions, regular bubbling, continuous degassing, intermittent regime or even spontaneous flow-to-fracture transition are observed.Ī fluid (liquid or gas) crossing an immersed granular medium is a process widely encountered in nature, and often referred to as “venting.” It includes a large variety of phenomena such as methane venting, hydraulic fractures, pockmarks, hydrothermal complexes, kimberlite pipes or mud volcanoes. Finally, we describe the degassing dynamics inside the fluidized zone, in the stationary regime. A method based on image analysis makes it possible to detect not only the fluidized zone profile in the stationary regime, but also to follow the transient dynamics of its formation. We report original results showing a dependence of the fluidized zone shape, at long times, on the injection flow rate and grain size. After describing the invasion pattern at short and long time, a tentative regime-diagram is proposed. We propose a brief review of the experimental investigation of the dynamics of air rising through a water-saturated, unconstrained granular bed, in both two and three dimensions. With a free upper boundary, however, the grains can be entrained by the ascending gas or fluid motion, and the competition between the upward motion of grains and sedimentation leads to new patterns. Many studies have focused on constrained porous media, in which the grains are fixed in the bed and only the interstitial fluid flows when the gas invades the system. The injection of gas in a liquid-saturated granular bed gives rise to a wide variety of invasion patterns. 2Laboratoire de Physique, École Normale Supérieure - Centre National de la Recherche Scientifique, Université de Lyon, Lyon, France.1Instituto de Física, Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile.Germán Varas 1 Gabriel Ramos 1 Jean-Christophe Géminard 2 Valérie Vidal 2 *
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