We probe the dynamics of intermittent avalanches caused by steady addition of grains to a quasi-two dimensional heap. To characterize the time-dependent average avalanche flow speed v(t), we image the top free surface. To characterize the grain fluctuation speed dv(t), we use Speckle-Visibility Spectroscopy. During an avalanche, we find that the fluctuation speed is approximately one-tenth the average flow speed, and that these speeds are largest near the beginning of an event. We also find that the distribution of event durations is peaked, and that event sizes are correlated with the time interval since the end of the previous event. At high rates of grain addition, where successive avalanches merge into smooth continuous flow, the relationship between average and fluctuation speeds changes to dv Sqrt[v].
If a granular material is poured from above on a horizontal surface between two parallel, vertical plates, a sand heap grows in time. For small piles, the grains flow smoothly downhill, but after a critical pile size $X_c$, the flow becomes intermittent: sudden avalanches slide downhill from the apex to the base, followed by an "uphill front" that slowly climbs up, until a new downhill avalanche interrupts the process. By means of experiments controlling the distance between the apex of the sandpile and the container feeding it from above, we show that $X_c$ grows linearly with the input flux,…
Role of transverse displacements in the formation of subaqueous barchan dunes
新月形沙丘，又称新月形沙丘，是由流体作用于颗粒床而形成的。这些底形在许多环境中是常见的，存在于水下或空中，并且由以不同初始排列组织的颗粒形成。虽然它们在自然界和工业中经常被发现，但关于它们发展的细节仍有待了解。在最近的一篇论文[C。A. Alvarez和E. M. Franklin，Phys. Rev. E 96，062906（2017）]，我们提出了一个基于角生长的单个barchans发展和平衡的时间尺度。在这封信中，我们报告了在颗粒尺度上角的生长测量。在我们的实验中，圆锥形堆被放置在一个封闭的管道和单个颗粒跟踪每个堆，在水流的作用下，演变成新月形沙丘。我们确定了迁移到生长角的颗粒的轨迹，并发现它们中的大多数来自初始堆外围的上游区域，平均位移为堆大小的顺序。此外，我们表明，个别晶粒有横向位移滚动和滑动，是不可忽略的，其中许多人去周围的堆。我们的实验揭示的角形成的机制与一般的图片，新月形角形成的沙丘侧面的前进，由于迁移速度与沙丘大小的倒数缩放。我们的研究结果改变了水下新月形沙丘的生长方式。
An X-ray imaging technique is used to probe the stability of 3-dimensional granular packs in a slowly rotating drum. Well before the surface reaches the avalanche angle, we observe intermittent plastic events associated with collective rearrangements of the grains located in the vicinity of the free surface. The energy released by these discrete events grows as the system approaches the avalanche threshold. By testing various preparation methods, we show that the pre-avalanche dynamics is not solely controlled by the difference between the free surface inclination and the avalanche angle. As a…
We study the flow of elongated grains (wooden pegs of length $L$=20 mm with circular cross section of diameter $d_c$=6 and 8 mm) from a silo with a rotating bottom and a circular orifice of diameter $D$. In the small orifice range ($D/d<5$) clogs are mostly broken by the rotating base, and the flow is intermittent with avalanches and temporary clogs. Here $d\equiv(\frac{3}{2}d_c^2L)^{1/3}$ is the effective grain diameter. Unlike for spherical grains, for rods the flow rate $W$ clearly deviates from the power law dependence $W\propto (D-kd)^{2.5}$ at lower orifice sizes in the intermittent regi…
Gravity-driven collapses involving large amounts of dense granular material, such as landslides, avalanches, or rockfalls, in a geophysical context, represent significant natural hazards. Understanding their complex dynamics is hence a key concern for risk assessment. In the present work, we report experiments on the collapse of quasi-two-dimensional dry granular columns under the effect of gravity, where both the velocity at which the grains are released and the aspect ratio of the column are varied to investigate the dynamics of the falling grains. At high release velocity, classical power l…
During the past sixty minutes only, oil companies have extracted six trillions liters of oil from the ground, i.e. the volume of about two hundreds Olympic swimming pools. This phenomenal number gives a striking illustration of the impact of multiphase flows on the world economy and environment. From a fundamental perspective, we now clearly understand the large-scale patterns formed when liquid interfaces are driven through heterogeneous environments. In stark contrast, the displacement of fragmented fluids through disordered media remains limited to isolated droplets and bubbles. Here, we el…
