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Introduction: Hello, everyone I’m Jaime, and I go to talk about work I am investigating with the collaboration of David Cuñez from Rochester University and Prof. Erick Franklin as my advisor. We are working with a bi-disperse bed sheared by viscous flow, and analyzed the grain segregation and bed hardening. The video shows that granular materials are present in both industry and nature. In nature, we can find it such as sand, gravel, ripples, and pebbles, being driven by a fluid, for example: by the wind in the case of the deserts forms dunes or by the water within the bottom of the river bed. Wherever the ratio between the entrained forces due to the fluid shearing and resisting forces such as gravity, is within the moderate values bedload and creep can occur within the granular bed. This ratio is known as Shield Number which critical value represents the threshold for the motion. The bedload is a moving layer in which the grains roll, slide or effectuate small jumps keeping contact with the lower part of the bed. Let me show you similar previous works where the ratio between the density of grains and fluid is close to unity, The lower part of the bed has been shown that it may creep, with movements caused by very slow rearrangement of grains. Houssais et all., showed within the granular bed there is a continuous transition between the bedload and creep. Thus, when a granular bed is sheared by a fluid usually experiences hardening due to two contribution: The first one, is the percolation of grains migrating to vacancies becoming a geometric rearrangement of grains that leads to an increase in bed compactness (isotropic contribution). The second one is the forces chain aligned in a preferential direction due to the shear stresses applied by a fluid. (anisotropic contribution). In fact, bed hardening is one of the causes of the decrease in the mobility of the bedload, however, the existence of grains of different sizes within the granular bed (polydisperse) may reduce more the mobility by natural armoring due the grain segregation. The grain segregation is the upward motion of large particles occurring from lower regions in the bed, leading to a higher concentration of the large grain on the bed surface. Ferdowsi, showed two mechanisms of segregation. The first one close surface bed, with rapid segregation within the bedload layer associated with an advection mechanism, And another one below, where the creeping lead slow segregation toward the bedload layer, is associate with the diffusion mechanism. In summary, Being the ratio between the densities of the grain and fluid is close to the unit, the granular bed experience hardening while a Poli-disperse granular bed experience segregation in addition to hardening. Now, if the gravity effect is higher, close the natural flows of water and sand in rivers. How will be the behavior within the granular bed? Objective: Investigate the evolution of the beddisperse bed sheared by a viscous flow in order to analyze the grain segregation and bed hardening. Different from the previous experiments, the ratio between the grains and fluid was close to values found in rivers and oceans. Experimental setup: Our experimental device is basically an annular flume with a rotating lid, the lid imposed a shear-driven flow inside the flume. We made use of refractive index matching visualization, where a laser generated a vertical plane traversing the bed and forming a single laser sheet. The evolution of the bed is observed with a camera that recorder photos and videos from the laser sheet that traverses the bed, evolution 2D. About the fluid, its viscosity is seven hundred times more than the water, and the relative density is close to one. And the grains are of glass. Prior to each experiment, we setup the bed in an heterogeneous grain distribution. Rotating the lid in order to suspend the grains following the rest period for the grains to settle. We carried out experiments keeping the rotation of the lid constant for 140 hours. We film the evolution of the bedload layer and capture the rapid segregation during the first 40 to 80 minutes, and after, we record photos of each 20 seconds in order to sample the slow segregation and compaction in the creeping region. We carried out experiments for four different Shield numbers or rotations of the lid We developed scripts using processing images in order to, detect most of the grains, add an identification number in each one, and keep constant it for each frame. Thus, we can compute the packing fraction, velocity, and strain. Results: In the next two slides, we can see first: the granular bed as background for each experiment, the y-axis represents the height of the bed, and the x-axis represents the spatial-temporal average, going from left to right the first curve is the packing fraction profile, the second curve is the horizontal velocity profile, and the last curve is the strain of the bed. From these results, we can define the bedload region which is the area between the green and white line, and the creeping region, the area below the white line. Each figure shows how the white line or creeping’s height is going down and concur with the kinking point of the velocity, which represents the continuous transition between the bedload creeping region, while the green line or bedload’s height has a light displacement toward up due to dilatation of the bedload driven by the increase of the Shield Number. Here, we can see the all curves together, at the left. The packing fraction profile shows the dilatation in the surface of the bed defined by half of the saturated packing fraction. In the middle, with Respect to the velocity profile, we can see the kinking point of the velocity for each height, Moreover, the velocity values in the creeping region for all experiments have the magnitude order, and the increment of the velocity within the bedload layer. The pressure granular confinement shows the last figure, where the pressure increase linearly toward the bottom of the bed. Defined the boundary between the different regions, this slide shows the trajectories of the large grains segregation within the granular bed and for the different time scales. At the left, the segregation is fast within the bedload layer region most of the particle goes toward the surface of the bed due to the sheared fluid that lead a rapid motion between particles, segregation in a low timescale 20 min, While, at right, the large grains located near these two regions take more time by segregating, due to higher contacts between particles leading to a slow motion in this region to achieve the bedload, segregation in a high timescale 70 hrs. In both images, most of the large grains within the creeping region show a short displacement within segregation. In order to define the type of motion mechanisms for the large particles within the granular bed, we calculated the mean square displacement. As soon as the lid begins moving, the motion by diffusion appears in all large particles within the granular bed. Whereas in the bedload region, the diffusion becomes advection quickly and the segregation is fast during the first 20 minutes and most of the particles segregate. In the creeping region, the motion by diffusion keeps along time. However, at the top of creeping the segregation is slow until arriving at the bedload layer, and the diffusion becomes advection. While within the creeping layer, the diffusion becomes constrained motion and keeps along time. This slide shows; at the left, on the y-axis, the initial position of large grains with respect to the creeping’s height, whereas the x-axis represents the vertical displacement of them from beginning until the end of each experiment. To measure how much the particles segregated or sank with respect to the initial position. At right, the figure shows vertical displacement whit as function of the instant of time when they were last detected, normalized by the total time of the experiment. Here, we can see the vertical position of some large particles along time, Curves with different colors and line types represent a group of large grains originally in different regions, such as, close to the surface of the bed, or near the top of the creeping region, moreover, these groups of the particles are linked with the lapse of time that the segregation take place. This slide shows the displaced position of the large particles that segregated represent by symbols and the fittings (black lines) of the corresponding average as a function of position for each region where the segregation takes place and different shear stress. Here we can see a equation for the time of segregation as a function of the position of the large grain within the bed, it isn’t a function of the shear stress. The last compute to helped us to find the following characteristic time for the segregation which depends on the depth within the bed. While the segregation occurs within the bedload and the top of the creeping region, At the same time the hardening is present in most of the solid-like region creeping, and it affects both small and large grains until the end of the experiment. This video shows the evolution of hardening along time, at left, the figure represents a space-time diagram of the average strain for all experiments, we observed that the shear stress increases, the height of the creep layer decreases while the strain increases.