We perform experiments where air is injected at a constant overpressure P in , ranging from 5 to 250 kPa, into a dry granular medium confined within a horizontal linear Hele-Shaw cell. The setup allows us to explore compacted configurations by preventing decompaction at the outer boundary, i.e., the cell outlet has a semipermeable filter such that beads are stopped while air can pass. We study the emerging patterns and dynamic growth of channels in the granular media due to fluid flow, by analyzing images captured with a high speed camera (1000 images/s). We identify four qualitatively different flow regimes, depending on the imposed overpressure, ranging from no channel formation for P in below 10 kPa, to large thick channels formed by erosion and fingers merging for high P in around 200 kPa. The flow regimes where channels form are characterized by typical finger thickness, final depth into the medium, and growth dynamics. The shape of the finger tips during growth is studied by looking at the finger width w as function of distance d from the tip. The tip profile is found to follow w(d) ∝ d β , where β = 0.68 is a typical value for all experiments, also over time. This indicates a singularity in the curvaturee., more rounded tips rather than pointy cusps, as they would be for the case β > 1. For increasing P in , the channels generally grow faster and deeper into the medium. We show that the channel length along the flow direction has a linear growth with time initially, followed by a power-law decay of growth velocity with time as the channel approaches its final length. A closer look reveals that the initial growth velocity v 0 is found to scale with injection pressure as v 0 ∝ P 3 2 in , while at a critical time t c there is a cross-over to the behavior v(t) ∝ t −α , where α is close to 2.5 for all experiments. Finally, we explore the fractal dimension of the fully developed patterns. For example, for patterns resulting from intermediate P in around 100-150 kPa, we find that the box-counting dimensions lie within the range D B ∈ [1.53,1.62], similar to viscous fingering fractals in porous media.

By means of digital image correlation, we experimentally characterize the deformation of a dry granular medium confined inside a Hele-Shaw cell due to air injection at a constant overpressure high enough to deform it (from 50 to 250 kPa). Air injection at these overpressures leads to the formation of so-called pneumatic fractures, i.e., channels empty of beads, and we discuss the typical deformations of the medium surrounding these structures. In addition we simulate the diffusion of the fluid overpressure into the medium, comparing it with the Laplacian solution over time and relating pressure gradients with corresponding granular displacements. In the compacting medium we show that the diffusing pressure field becomes similar to the Laplace solution on the order of a characteristic time given by the properties of the pore fluid, the granular medium, and the system size. However, before the diffusing pressure approaches the Laplace solution on the system scale, we find that it resembles the Laplacian field near the channels, with the highest pressure gradients on the most advanced channel tips and a screened pressure gradient behind them. We show that the granular displacements more or less always move in the direction against the local pressure gradients, and when comparing granular velocities with pressure gradients in the zone ahead of channels, we observe a Bingham type of rheology for the granular paste (the mix of air and beads), with an effective viscosity μ_{B} and displacement thresholds ∇[over ⃗]P_{c} evolving during mobilization and compaction of the medium. Such a rheology, with disorder in the displacement thresholds, could be responsible for placing the pattern growth at moderate injection pressures in a universality class like the dielectric breakdown model with η=2, where fractal dimensions are found between 1.5 and 1.6 for the patterns.

<p>Recent studies on infrasonic signatures related to atmospheric tides are mostly focused on stratospherically ducted infrasound or on tidal signatures in recorded infrasound signal power.</p><p>In the current work, we address microbarom infrasound ducted by mesosphere-lower thermosphere (MLT) waveguides and the associated infrasound apparent velocity (trace velocity) of arrivals at a ground-based array station in northern Norway.</p><p>A hypothesis is that the infrasound apparent velocity – which is related to the incidence angle of the wavefront impinging the station – is linked to the altitude of the final refraction of the infrasound waves. This altitude would be affected by the regional MLT tidal pattern.</p><p>We apply specialized beamforming and filtering recipes to highlight the MLT-ducted microbarom arrivals and we find semidiurnal patterns in the infrasound apparent velocity measurements.</p>