Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session E18: Granular Flows |
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Chair: Gregory Bewley, Cornell Room: North 131 C |
Sunday, November 21, 2021 2:45PM - 2:58PM |
E18.00001: The cratering dynamics of granular soil induced by an impinging jet Matt Gorman, Juan Sebastian Rubio, Miguel X Diaz-Lopez, Rui Ni The interactions between an impinging jet and a regolith bed, often referred to as Plume-Surface Interactions (PSI), occur during takeoffs and the entry, descent, and landing sequences of spaceflights. Here, we present the results of an experimental campaign to characterize regolith erosion and cratering dynamics from impinging jets under a range of environmental and flow conditions. For these experiments, a nozzle was placed over the transparent front edge of a regolith bin to create a half-crater and expose the crater profile. A high-speed camera was used to image the evolution of the exposed crater profile and the complex interactions between regolith particles and the impinging flow throughout the jet impingement process. The role of the jet flow Mach number, the normalized nozzle height, and the ambient pressure on the measured cratering and erosion dynamics were investigated in this work. In helping to unveil the underlying physics governing Plume-Surface Interaction erosion processes, this study constitutes a valuable resource for future spaceflight missions to the Moon and Mars. |
Sunday, November 21, 2021 2:58PM - 3:11PM |
E18.00002: Granular scaling laws for helically driven dynamics Hamid Marvi, Andrew Thoesen, Teresa McBryan, Darwin Mick, Marko Green, Justin Martia We develop scaling laws for the mobility of screw-propelled vehicles on granular media and use experimental and computational approaches to verify those. A modular screw propelled vehicle is, therefore, designed for testing the accuracy of this helical granular scaling law in predicting vehicle translational velocity and power. Experimental agreement for prediction of power (3–9% error) and translational velocity (2–12% error) are observed. A similar set of geometries is subjected to multibody dynamics and discrete element method cosimulations of Earth and lunar gravity to verify a gravity-dependent subset of the scaling laws. These simulations show agreement (under 5% error for all sets) and support law validity for gravity between Earth and lunar magnitude. These results support further expansion of granular scaling models to enable prediction for vehicle-terrain dynamics for a variety of environments and geometries. |
Sunday, November 21, 2021 3:11PM - 3:24PM |
E18.00003: Excavation performance of a screw-propelled vehicle Hamid Marvi, Marko Green, Teresa McBryan, Darwin Mick, David Nelson The effectiveness of excavation robots for in-situ resource utilization (ISRU) on planetary bodies such as the moon or Mars is contingent upon their ability to generate high traction forces while minimizing the amount of force required to excavate regolith. Also, these excavation robots must be light and use minimal power since they must be launched from Earth. This study explores the performance of CASPER, a Counter-rotating Archimedes Screw-Propelled Excavation Rover that is capable of meeting these requirements and reaching similar levels of performance as other proposed excavation platforms at a much lower total mass. It does so by combining a screw-propelled mobility system with a simple ramp excavation system, which has never been attempted before. In particular, the effect of mobility system geometry and other system parameters on the robot's excavation capability and mobility performance were studied. Additionally, a novel parameter, the excavation transport rate (ETR) is defined to obtain greater insight on how the performance of this robot compares to other proposed architectures on the combined basis of mobility and excavation capabilities. The results indicate that this architecture shows promise as a planetary excavation system because it provides significant excavation capability with low mass and power requirements. |
Sunday, November 21, 2021 3:24PM - 3:37PM |
E18.00004: Pinned Bubble Dynamics in Locally Fluidized Granular Media Andras Karsai, Daniel I Goldman Recent studies of soft robot movement in sand [Naclerio & Karsai et al., Science Robotics, 2021] have shown how airflows impinging into granular media can aid in intrusion by creating local flows that transport grains. To investigate the physics of such multiphase flows, we experimentally study emergent flow phenomena in single air-driven cavities in a bed of granular media (1 mm poppy seeds) at Reynold's numbers of up to 3000. A downward impinging buried air jet creates locally pinned bubbles as grains circulate within the bubble cavity. We observe emergent oscillatory behavior in these bubbles for both sidewall and mid-bed experiments for certain combinations of depth, airflow, and effective grain strength. These ‘bubblators’ are cavities oscillating at characteristic frequencies of approximately 1 Hz as damped travelling waves along the length of the bubble. These frequencies chiefly depend on the cavity’s overall intrusion depth. The oscillation arises from the periodic vorticity switching of granular flow along the bubble boundaries. Bubblators form when a pinned bubble overexpands and collapses into a pinned oscillating state. We also show that these pinned structures can also be transported vertically within the granular medium without significant reductions in volume. |
Sunday, November 21, 2021 3:37PM - 3:50PM |
E18.00005: Bedforms Produced on a Particle Bed by Vertical Oscillations of a Plate Kasey M Laurent, Luigi La Ragione, James T Jenkins, Gregory P Bewley Natural flows are nominally turbulent and predicting the onset of sediment motion in these environments is an ongoing challenge. Traditional sediment transport models utilize the mean shear at the bed to predict the onset of motion. However, these models fail in cases where mean shearing is minimal and turbulence is dominant, such as in a swash zone. We conducted an experiment to observe the formation of a heap in a subaqueous granular bed induced by vertical oscillations of a square plate above the bed. We performed the experiment for oscillation frequencies between 10 and 40 Hz and amplitudes between 0.02 and 0.14 cm to determine how the intensity of these fluctuations influences bed deformation. We observed that the cross-section of the heap is self-similar in time. Its surface contracted and its height grew approximately with the square root of time, in agreement with dimensional arguments predicated on an incompressible viscous coarse-grained analysis of the grain flow in the bed. |
Sunday, November 21, 2021 3:50PM - 4:03PM |
E18.00006: Grain-resolving simulations of submerged cohesive granular collapse Rui Zhu, Kunpeng Zhao, Bernhard Vowinckel, Zhiguo He, Eckart H Meiburg We investigate submerged cohesive granular collapses via fully coupled, grain-resolving direct numerical simulations, for weakly polydisperse, loosely packed columns. We quantify the time-dependent spreading velocity of the collapsing front, as well as the final runout length and deposit thickness, as functions of the aspect ratio of the initial particle column and a cohesive number formulated as the ratio of cohesive to gravitational forces. We find that the dependence of these properties on the initial aspect ratio and the cohesive number can be accurately captured by piecewise power-law relationships. By means of computational particle tracking we obtain insight into the Lagrangian dynamics of the granular collapse, and specifically into which regions of the initial particle column travel farthest or end up at the surface of the final deposit. In this regard, we observe fundamentally different behaviors for short and tall columns. The simulations furthermore enable us to identify aggregates of particles held together by cohesive forces that undergo very little deformation during the collapse process. Finally, the simulations provide information on the emergence of cohesive and contact force chains, as well as on their preferred spatial orientation. |
Sunday, November 21, 2021 4:03PM - 4:16PM |
E18.00007: Segregation forces in dense granular flows: From single intruders to high concentrations Richard M Lueptow, Yifei Duan, Lu Jing, Julio M Ottino, Paul B Umbanhowar Particle size segregation in dense granular flows has been studied extensively in the context of mixtures and, more recently, single intruder particles. However, the connection between the single intruder limit (i.e., a vanishing concentration) and a mixture with a finite concentration remains unclear. Here we extend the virtual-spring-based force meter approach for single intruder particles to bidisperse mixtures of arbitrary species concentrations to characterize the segregation force as a function of concentration for various particle size ratios in controlled, constant-shear-rate flow simulations. The segregation force on particles exhibits a plateau at low concentrations and varies monotonically above a critical concentration, indicating a transition from non-interacting intruders to cooperative phenomena in mixtures. The segregation force is then used to develop an accurate model of stress partitioning as a function of species concentration, thus bridging the gap between single intruder migration and mixture segregation in dense granular flows. |
Sunday, November 21, 2021 4:16PM - 4:29PM Not Participating |
E18.00008: Attraction and repulsion between objects in a granular flow. Carlos Malaga, Gabriel Caballero The objective of this work is the understanding of lift forces exerted on two side-by-side intruders within a granular flow that is uniform upstream. To this end, we have performed experiments that reveal long-range attraction and repulsion depending on flow velocity and the distance between intruders. Additionally, a hydrodynamic model based on kinetic theory for inelastic particles was solved numerically. The model reproduces remarkably well the general qualitative features of the experiments which shows that the system behaves similarly to a pair of hot intruders immersed in a liquid of temperature-dependent viscosity. Our results confirm what we had found previously through numerical simulations in a similar geometry; namely, that attraction or repulsion exists between the intruders depending on the granular flow velocity and the distance separating the obstacles. In particular, an empirical equation relating the difference in mean flow velocity at both sides of the obstacles. |
Sunday, November 21, 2021 4:29PM - 4:42PM Not Participating |
E18.00009: Study of granular flow in a wedge-shaped hopper using DEM simulations Afroz F Momin, Devang V Khakhar Granular materials are made up of particles or discrete solids that flow like liquids. The flow of material through a hopper is a fundamental industrial unit operation and a granular flow problem in which material flows under gravity and leaves the storage bin through the outlet at the bottom of the bin. Using a discrete element method (DEM), the interaction between particles is evaluated using Newton’s laws of motion. It is important to understand and model such granular flows in terms of parameters such as grain size, solid fraction, wall roughness, particle-particle interactions, and others that affect them. The continuity equations and radial momentum balance are solved in a wedge-shaped hopper for a smooth wall and radial gravity flow. Our computational results fit the theory developed by Savage (1965) and Prakash and Rao (1988). The present study also includes parametric analyses to investigate wedge-shaped hopper rheology in depth. |
Sunday, November 21, 2021 4:42PM - 4:55PM |
E18.00010: Drag forces in granular segragtion Robbie S Bancroft, Chris G Johnson Granular materials that contain particles of a range of sizes frequently segregate when sheared. The rate at this segregation occurs is strongly dependent on flow parameters such as the shear rate and pressure. Here we show that this dependence is not due to changes in the forces which drive segregation, but instead occurs primarily due to changes in the strength of the inter-species drag force, which opposes the driving segregation forces and acts to slow segregation. We study the drag force in detail, using a novel configuration of discrete element simulations to study drag in a uniform environment. Measuring the speed at which two species of grains percolate through each other then allows us to accurately measure the drag force, which we show follows a power law in the granular inertial number with exponent -7/4. We explain this scaling with a simple model, and show that it explains much of the previously observed dependence of segregation rate on shear rate and pressure in free-surface flows. |
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