Low-gravity Environment Research Articles

Numerical Study of Cone Penetration Tests in Lunar Regolith for Strength Index

The cohesive properties of lunar regolith, combined with a low-gravity environment, result in it having a distinct mechanical behavior from sandy soil on Earth. Consequently, empirical formulas derived from cone penetration tests (CPTs) for calculating the shear strength parameters of Earth’s sand cannot be directly applied to lunar regolith. This study utilized the three-dimensional discrete element method (DEM) to numerically simulate triaxial shear tests and cone penetration tests in a lunar environment. The particle contact model for lunar regolith in the discrete element method (DEM) simulation incorporated the hysteresis effect of van der Waals forces, thereby simulating the cohesive properties of lunar regolith in a lunar environment. We proposed a relationship for calculating the shear strength index of lunar regolith based on normalized cone tip resistance using the results from triaxial and CPT simulations and referencing empirical formulas derived from ground-based CPT data. The results of this study provide a valuable reference for future lunar CPTs.

Lunar Leap Robot: 3M Architecture–Enhanced Deep Reinforcement Learning Method for Quadruped Robot Jumping in Low-Gravity Environment

Lunar Leap Robot: 3M Architecture–Enhanced Deep Reinforcement Learning Method for Quadruped Robot Jumping in Low-Gravity Environment

LBM-DEM modeling of particle-fluid interactions on active small solar bodies

Aeolian-like surface features observed on small Solar System bodies have piqued interest in their underlying formation mechanisms. Understanding the evolution of fluid-solid interactions is crucial for elucidating the nature of cometary activity. We established a resolved fluid-particle simulation approach and implemented it into our self-developed DEMBody and LBMCoupler codes to simulate the wind erosion process on comet 67P. We developed this novel framework by applying the lattice Boltzmann method-discrete element method (LBM-DEM) in a low-gravity and rarefied atmosphere environment. The inter-particle forces were modeled using the Hertz contact model, friction, and cohesion. The fluid field was calculated by solving the lattice Boltzmann equations, which use the distribution function as the variable. The fluid-particle forces were modeled using the partially saturated cells method, in which the force is calculated based on the populations of the fluid cells occupied by the solid phase. We conducted 2D and 3D validation simulations and a series of simulations of a regolith layer as a preliminary application to validate the framework. The validation results of the drag coefficient under 2D and 3D conditions are in good agreement with previous theoretical and numerical estimates. Additionally, the wind erosion process on the surface of comet 67P is reproduced using the presented approach. This preliminary application show that the threshold velocity to initiate grain motion on comet 67P is about $25$ $ m/s$, which is consistent with the observations that sediment transport driven by winds frequently occurs near the perihelion of comet 67P. The proposed LBM-DEM framework can be successively applied to simulate the fluid-solid interaction on small solar bodies that have extremely low-gravity and rarefied atmosphere environments. Future works based on this tool and focused on aeolian geologic landforms, such as sand dunes, can help us understand the dynamics of cometary activity.

Torque about electrostatically charged spheres makes them more attractive.

The strength of interparticle interactions in a granular system controls how a collection of insulating particles flow, cohere and fragment. Forces due to electrostatic charging, particularly in free-fall or low gravity environments, can dominate the static and dynamic interactions with important implications for understanding natural and industrial processes. Here we show that shaking of homogeneous, spherical particles can result in a non-uniform surface charge distribution. The measured dipole moment and torque for each particle are found to be strongly correlated. However, our model shows that to predict the torque and force requires one to consider the full surface charge distribution. This overlooked torque is not only significant, but would amplify attractive interactions through particle reorientation.

Role of gravity magnitude on flowability and powder spreading in the powder bed fusion additive manufacturing process: Towards additive manufacturing in space

Understanding powder spreading under low gravity conditions is essential for optimizing final products using additive manufacturing in space. In this study, we investigated the role of gravity on flowability and spreading mechanisms through combined experimental and discrete element method (DEM) studies. Three powders with different theoretical densities were used to reenact low compressive conditions resembling those in a low-gravity environment. The influence of low compressive conditions on flowability and spreading behavior was examined using the Hall flowmeter, rotating drum, and spreading experiments. In the experimental result, the static flowability was primarily affected by the presence of elongated particles rather than the compressive conditions. The dynamic AoR of TD_4 powder increased compared to that of TD_8 powder, despite the presence of spherical particles with a smooth surface finish. A DEM simulation study was conducted using TD_8 powder to investigate the impact of different gravity levels on dynamic flowability. The DEM studies revealed that the dynamic flowability under rotation was decreased under low gravity owing to the promoted cohesive interactions. The powder spreading experiment was performed using the three powders with different theoretical densities. The in-situ observation with particle image velocimetry analysis revealed that kinetic energy dissipation in the spreading process was accelerated in the TD_8 powder pile, despite its high interparticle friction and cohesive force. The powder spreading simulation was conducted using TD_8 powder to clarify the effect of low gravity on the powder spreading process. In TD_8 powder under 1 G, particle supply was facilitated by a synergistic effect of free-falling and deposited particles. However, increased cohesive interactions under 0.5 G and 0.16 G restricted particle supply via free-falling, consequently reducing the powder bed density by about 2 % and 6.2 %, respectively. These findings prove that the cohesive force predominantly controls dynamic flowability and powder bed quality in the spreading process under low gravity conditions.

The bearing capacity of asteroid (65803) Didymos estimated from boulder tracks

The bearing capacity - the ability of a surface to support applied loads - is an important parameter for understanding and predicting the response of a surface. Previous work has inferred the bearing capacity and trafficability of specific regions of the Moon using orbital imagery and measurements of the boulder tracks visible on its surface. Here, we estimate the bearing capacity of the surface of an asteroid for the first time using DART/DRACO images of suspected boulder tracks on the surface of asteroid (65803) Didymos. Given the extremely low surface gravity environment, special attention is paid to the underlying assumptions of the geotechnical approach. The detailed analysis of the boulder tracks indicates that the boulders move from high to low gravitational potential, and provides constraints on whether the boulders may have ended their surface motion by entering a ballistic phase. From the 9 tracks identified with sufficient resolution to estimate their dimensions, we find an average boulder track width and length of 8.9 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document} 1.5 m and 51.6 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document} 13.3 m, respectively. From the track widths, the mean bearing capacity of Didymos is estimated to be 70 N/m2, implying that every 1 m2 of Didymos’ surface at the track location can support only ~70 N of force before experiencing general shear failure. This value is at least 3 orders of magnitude less than the bearing capacity of dry sand on Earth, or lunar regolith.

Hydroponics for plant cultivation in space – a white paper

The microgravity conditions experienced in space prevent the proper distribution of water throughout root modules of plant growth hardware, and the lack of convective mixing and buoyancy reduces gas exchange. To overcome this problem, cultivation technologies should be designed that take advantage of the unique traits of the spaceflight environment instead of attempting to recreate Earth-like conditions. Such technologies should be adaptable to both the microgravity of spaceflight and the low gravity environments of the lunar and Martian surface. Current space plant cultivation relies on traditional terrestrial practices and uses porous substrates that are nutrient poor and difficult to regenerate, and does not consider the dominance of surface- or thermal gradient-controlled rather than gravity-controlled water flow in space as a potential beneficial property. We propose systems that control water dispensation and removal by parallel but independent means in a soil-free cultivation system that is adaptable and expandable to crops of varying sizes and shallow or deep rooting plants. Water dispensation and removal in a substrate-free hydroponic system can be achieved through the misting of nutrient solutions combined with special root module geometry and temperature gradients. The use of hydrogels as substrate, and a means of providing required nutrients and water for plant cultivation in space, can aid in the transition to low-gravity systems by eventual incorporation of on-site regolith to establish Earth-like soil.

Receptors Implicated in Microgravity-Induced Bone Loss

For humans to explore and colonize the universe, both engineering and physiological obstacles must be successfully addressed. A major physiological problem is that humans lose bone rapidly in microgravity. Understanding the underlying mechanisms for this bone loss is crucial for designing strategies to ameliorate these effects. Because bone physiology is entangled with other organ systems, and bone loss is a component of human adaptation to microgravity, strategies to reduce bone loss must also account for potential effects on other systems. Here, we consider the receptors involved in normal bone remodeling and how this regulation is altered in low-gravity environments. We examine how single cells, tissues and organs, and humans as a whole are affected by low gravity, and the role of receptors that have been implicated in responses leading to bone loss. These include receptors linking cells to the extracellular matrix and to each other, alterations in the extracellular matrix associated with changes in gravity, and changes in fluid distribution and fluid behavior due to lack of gravity that may have effects on receptor-based signaling shared by bone and other regulatory systems. Inflammatory responses associated with the environment in space, which include microgravity and radiation, can also potentially trigger bone loss.

Comparative Studies on Effects of Simulated Microgravity on Growth and Photosynthetic Parameters in Rice and Mungbean

Microgravity or low gravity environment of space can be simulated on Earth by using clinorotation with appropriate speed of rotation. Previous reports show that microgravity environment (real or simulated) affects growth, chlorophyll content and photosynthetic performance in different plants. However, most of the reports showed variation in results. Earlier studies carried out on simulated microgravity (SMG) using clinorotation in our laboratory showed increase in growth, chlorophyll content and various yield attributes in case of laboratory and field grown rice. Subsequent studies were carried out on mungbean seedlings. In the present work, comparative aspects of these studies are reported. Results of the present study showed increase in growth, chlorophyll content and chlorophyll fluorescence parameters in SMG samples as compared to control samples in rice and mungbean seedlings. Interestingly, percentage of enhancement in growth, chlorophyll content and chlorophyll fluorescence parameters of SMG samples is significantly higher in rice (monocotyledonous) seedlings as compared to mungbean (dicotyledonous).

Ultraviolet and thermal dual-curing assisted extrusion-based additive manufacturing of lunar regolith simulant for in-site construction on the Moon

The establishment of a lunar base is crucial for lunar studies, resource extraction, and advancement in deep space exploration. Considering the limitations of rocket capacity and Earth-Moon transportation costs, lunar regolith is regarded as the most practical material for lunar base construction. Consequently, the in-situ additive manufacturing technology for simulated lunar regolith has been widely studied in recent years. This paper proposes a novel additive manufacturing approach for lunar regolith simulants. The simulants and several resins were mixed into a paste with high simulant content of 86 wt%, which was suitable for extrusion-based additive manufacturing process. To fully leverage the lunar environment, the feasibility of the ultraviolet (UV) and thermal curing of the resins was first verified, which was followed by the validation of the lunar regolith paste with the solid content of 86 wt%. After the characterization of the ultraviolet and heat curing properties, the paste was further 3D printed via the extrusion-based device. The extruded paste underwent preliminary curing in a UV environment and was then heated to approximate the daytime lunar conditions, achieving complete curing of the printed component. The cured parts exhibited a compressive strength of 9.83 ± 1.07 MPa, meeting the requirements for infrastructure construction in the low-gravity lunar environment. This work offers significant implications for future in-situ construction on the Moon using extrusion-based additive manufacturing of lunar regolith.

Direct ink writing of viscous inks in variable gravity regimes using parabolic flights

Additive manufacturing (AM) has significant utility for off-planet fabrication where dedicated infrastructure is severely limited, weight reduction and in situ resource utilization is desirable, and demands for complex systems are high. Direct ink writing (DIW) is a useful AM technique since it enables the deposition of a broad set of materials and the co-printing of multiple materials simultaneously. This allows for the fabrication of complex functional devices and systems in addition to structural objects. To evaluate this technique for space applications, this study characterized DIW in low gravity environments. Parabolic flights were used to simulate Martian, Lunar, and Micro gravity, and the effects that these 3 gravity regimes have on two critical print performance parameters, drooping and slumping, was evaluated using viscous paste inks deposited with an auger-driven extrusion head. In the drooping case, bridging structures were printed across gaps without support material, and the deformation was monitored. In the slumping case, a wall was printed through sequential layer deposition, and the vertical displacement of each layer under reduced gravity was explored. As expected, we found that a reduction in apparent gravity led to a decrease in the droop of a printed line, and as apparent gravitational acceleration is decreased its impact on the magnitude of drooping becomes less significant. For wall structures printed in simulated Lunar or Martian gravity regimes, the total structure height was found to be similar to that of a structure printed under Earth's gravitational conditions. In contrast, for prints performed in microgravity, it was found that slumping was significantly reduced and structure height was larger. These results provide experimental data to enable the design and optimization of appropriate structures and tool paths for printing objects using DIW for off-planet manufacturing.

Study on the mechanical model of footpad-terrain for walking robot moving in low gravity environment

Due to the low gravity environment and the influence of complex terrain condition in deep space exploration, wheeled mobile systems are prone to meet motion abnormalities. The excellent motion performance of walking robot is more suitable for the future deep space exploration, but the robots are prone to occur large sinkage in soft terrain. A mechanical model is built to describe a gait cycle of a walking robot under soft terrain and low gravity environment. The force on the footpad during actual movement in a gait cycle is obtained through a single-legged test bench under the simulated planet terrain. The effects of sizes of footpads, sinkage and other factors are explored. The results indicate that the larger the size of the footpad, the greater the horizontal force on the footpad, the better the motion performance is. But as the size of footpad increase, the vertical force decreases which indicates poor support performance. By comparing and analyzing the model values with the experimental values, for the horizontal force FT, the average errors for the average force and peak force are 10.05% and 7.76%. The average errors for average force and peak force are 5.19% and 5.86% for vertical force FN. The values are not significantly different from the model values and experimental values which indicates that the mechanical model has high accuracy. The obtained mechanical model can provide a reference for the motion of walking robots in complex low gravity environment.

Effects of vibration on granular chute flow under low-gravity conditions

The essence of most geological hazards and industrial processes is gravity-driven flows of dense particles. In this research, we use the discrete element method to simulate the particle chute flow, and discuss the effect of vibration acceleration on the granular flow under different gravity conditions. The results indicate that vibration has a greater impact on the particle flow state under low-gravity environments and low inclination angles. The granular temperature (T) and inertia number (I) increase with increasing vibration acceleration. Vibration enhances the flowability of particles by changing the stress state and reducing the volume fraction of the granular system. This work reveals the rheological state of vibration-triggered particle flow on an inclined surface under different gravity environments.

Dust emission evaluation of the flexible metal wheel over lunar terrain and fender effect

The lunar rover’s wheels produce substantial dust emission during movement, presenting safety concerns for astronauts and potential equipment wear. Replicating the dust emission behavior in the low-gravity environment on Earth is challenging. This study proposes a numerical approach to analyze the dust emission distribution of an elastic wire mesh wheel over lunar terrain. The analysis considers adhesion and electrostatic forces within a Discrete Element Method (DEM) model of lunar dust. Then a Multi-Flexible Body Dynamics (MFBD) model of a flexible wire mesh wheel was developed. A virtual single-wheel soil bin experiment was conducted to examine dust emission features under various motion conditions and on rough terrain. Additionally, alternative structures of dust fender were designed to effectively suppress dust, and the effects of the structural features on dust emission were further explored. This comprehensive study provides a valuable reference for simulation and dust-control technologies related to elastic lunar wheels, offering insights that contribute to the effective management of dust emissions from lunar rover wheels.

Intimate Outer Space: Towards a Politics of Gravity, Waste, and the Spatial Orientation of Bodies

Examining the feat of maintaining life in orbit draws a sharp focus to the relationship between the human body and its environment, the porous and circulatory matter that blurs any boundaries between habitat and habitant. These intimate, engineered spaces evoke a microcosm of urgent planetary concerns surrounding air and water resources, and waste capture, storage, and elimination. This paper explores NASA’s management of biological operations and discharge wastes in low gravity environments. Without strong gravitational fields, liquids coalesce at the location they are created, instead of flowing down and away. Such excesses disrupt the orderly engineered environments and minutely monitored bodies of these techno-scientific endeavors. Analyzing astronaut tears, space gynecology, zero-g surgery, and NASA’s “Space Poop Challenge” through feminist queer and disability theory, new materialist, and discard studies lenses, this paper seeks to refigure the deeply entangled relationships between fleshy bodies and planetary bodies, biomass and geomass, and prompt new discussions of gravity politics.

Lunar Mare Lava Flow Dynamics and Emplacement: Predictions of Non-Newtonian Flow Dynamics, Syn- and Post-emplacement Cooling and Volatile Release Patterns, and Vertical and Lateral Flow Structure Development

We apply basic principles of magma ascent from deep source regions and its eruption into a low-gravity vacuum environment to develop a theoretical treatment of the fluid dynamics and thermodynamics of mare basalt lava flow emplacement and evolution on the Moon. The vacuum conditions influenced the release of volatiles in magma passing through lava fountains, thus controlling the syn- and post-emplacement vesicularity of the resulting deposits. To explain observed lengths and volumes of Mare Imbrium–type flows, high (106–105 m3 s−1) initial magma eruption rates were needed. Combined with low lunar magma viscosity, these caused flows to be initially turbulent. Resulting high radiative heat loss and consequent high crystallization rates caused rapid non-Newtonian rheological evolution and suppression of turbulence at tens of kilometers from vents. Slower cooling rates in the subsequent laminar parts of flows imply distinctive crystal growth rate histories. In a four-phase sequence, (i) initial transient dike-tip gas release followed by (ii) Hawaiian fire fountain activity with efficient volatile loss (iii) transitioned to (iv) Strombolian explosions in a lava lake. Late-stage lava now able to retain volatiles intruded and inflated existing flow deposits after flow front advance ceased. Volatiles forced out of solution by second boiling as lava cooled caused additional inflation. Low gravity and lack of atmospheric pressure commonly produced very vesicular lava. Escape of such lava through cracks in flow crusts is a possible source of ring-moat dome structures; collapse of such lava may explain irregular mare patches.

Development of 3D Printable Highly Flexible Siloxane Elastomers for Direct Ink Writing (DIW) of Soft Robotic Actuators for Space Applications

There is an interest in soft robotics for various space applications within low gravity and tight space environments to navigate, observe and even service various components within space assets, spacecraft and planetary stations. As an example, in an emergency, such as a pipe system failure occurring in a spacecraft, a soft robot may be able to assist in navigating the structure to detect the location and means of failure. The manufacturing of soft robotic actuators in space can be crucial. In addition, the fabrication of soft robotic tooling in a single step plays a critical role in space. 3D printing systems allow the manufacturing of soft robotic actuator parts in a single step. Stereolithography (SLA) and digital light processing (DLP) systems are not convenient because of the space environment. However, direct ink writing (DIW) which is the most commonly available and affordable system compared to SLA or DLP can be utilized to produce 3D objects, in addition, the DIW process permits the ability to in situ produce composite structures during the printing process. Various inorganic particles can be inserted within the elastomer print to control the mechanical, electrical and magnetic properties on-demand to produce smart or multi-functional soft robotic actuators and structures.The current work focuses on the development of a highly flexible and low (or nearly no) cytotoxicity silicone-based elastomer via thiol-ene click reaction that allows rapid polymerization of an elastomer to prevent the collapse of the structure during the printing. The work also demonstrates the use of this novel UV-durable elastomer to produce structures with embedded electrical interconnects and sensors for in-situ measurement of temperature and strain. In this study Poly(mercaptopropylmethylsiloxane-co-dimethylsiloxane) (M-MPS) was first prepared through the cohydrolysis-condensation reaction of 3-Mercaptopropylmethyltrimethoxysilane (MPS) and dimethyldimethoxylsilane (DMDES) with water and hydrochloric acid. Then M-MPS was compounded with various molecular weight vinyl-terminated polydimethylsiloxane (VPS) (MW~800-25000) and different dosage (0.1- 4.0 wt%) of a photoinitiator (1:1 weight ratio of 2-Hydroxy-2-methyl-phenyl-propane-1-one (PI 1173) and phenyl bis (2,4,6-trimethylbenzoyl) phosphine oxide (PI819)). For example, Figure 1 depicts the change in the viscosity of (M-MPS)-VPS 6000 2 wt% when it exposes to UV light irradiation. When (M-MPS)-VPS 6000 2 wt% was exposed to UV light irradiation, it polymerized at ~2 sec which is enough time to retain the structure during printing. To elucidate the properties of synthesized elastomer, FTIR, NMR, SEM, and tensile test measurements were performed. Furthermore, the synthesized elastomer was loaded with conductive inorganic particles so as to utilize the printed material as a temperature and tactile sensor (and sensor array). In addition, other printing variables were evaluated to produce 2D/3D structures of the pure and composite elastomers, such as the effect of the power and wavelength of UV light source, loading different types of conductive powders, and thiol-ene ratio. Acknowledgments: Work supported by a subcontract under the NASA-EPSCOR program (project number is 80NSSC20M0218). The authors would like to thank our NASA technical monitor Mr. Curtis Hill and project manager Dr. Melanie Page due to their contributions. Material characterization and imaging work were made possible with the support of the West Virginia University Shared Research Facilities (SRF). Figure 1

Droplet capture in a fiber array

An experiment is conducted investigating the dynamics of a droplet as it impacts a fiber array. To remove gravitational effects, experiments are conducted in the unique low gravity environment of a drop tower. The droplet dynamics are described with a model showing the dominating role of contact line dissipation. The model shows good agreement with experiments for important metrics such as the droplet penetration length.

Spatiotemporal characteristics of near-earth object monitoring from a moon-based station: case from 1962 to 2020 in CNEOS

ABSTRACT Numerous discovered and undiscovered near-Earth objects (NEOs), celestial bodies from outer space with diverse physical properties, have ensued major potential impact threats to the Earth. For a timely and accurate response in monitoring NEOs entering Earth’s orbit, the future applied Moon-based monitoring platform (MBMP) will fully utilize the vast flat Moon’s surface, along with low thermal noise in the prospective permanent shadow area, low gravity and vacuum environment, and special space relation, among other advantages over the established ground-based monitoring platform (GBMP) and space-based monitoring platform (SBMP). Future single or clustered platforms might indeed survey space with limited solar influence involved. Through downloaded data from the Center for Near Earth Object Studies, this study analysed MBMP performance and studied the commonly visible ratio (VR) based on the overall geometry of Sun–Moon–Earth and NEOs. Results from the average near-side MBMP VR outperformed the far side GBMP, and other satellites even by 0.4. This provides locations with a high probability for monitoring platforms among the downloaded NEOs, while the rest of the Moon’s surface can serve as crucial underlying support for timely NEO monitoring and a valuable complement for GBMP and SBMP.

CFD–DEM simulation of gas-driven sampling on asteroid regolith: Dependence of collected mass on gas ejection

Asteroids are the remnants of the process by which planets formed from the protoplanetary disk. Laboratory analyses of materials originating from asteroids give us the unique opportunity to reveal the formation history of our Solar System about 4.6 billion years ago. To obtain surface regolith in the low-gravity and vacuum environment, a gas-driven sampling strategy has been proposed, but the relation between the collected mass and the gas ejection remains to be explored, which is important for the development of efficient sampling mechanism designs. In this paper, we simulate the gas-particle two-phase flow in the gas-driven sampling process by using the coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) method. The results show that the sampling process can be divided into three stages according to the mass of particles that are sampled. Initially, the impact ejection of the pressurized gas on the regolith layer dominates, during which the sampling speed increases to a maximum value rapidly. After that, the gas flow transports the ejected particles to the collection chamber. In this stage, the conveyance effect of gas plays a primary role. As grains accumulate near the entrance of the collecting tube, jamming state happens and obstructs the particle flow, making the sampling rate drop to a stable value. By varying the inlet gas velocity, we show that higher gas velocity increases the collected mass rate greatly at the first stage while no significant differences are found after the development of jamming state inside the chamber. The position of the inlets affects the flow field and thus could be used to prevent the unfavorable aggregation of particles near the collecting tube, which facilitates the sampling. The influence of the inlet gas angle is also explored. We find two inward tilting gas inlets could increase the sampling efficiency considerably through striking a balance between ejected mass and ejecting velocity. The results presented in this article are expected to provide important information on the optimal designs of gas-driven sampling devices for future asteroid space mission. • CFD-DEM method is used to study the gas-driven sampling on asteroid regolith. • The impact ejection of pressurized gas and its conveyance effect dominate at different stages. • The gas inlet velocity affects the sampling efficiency only at the initial stage. • The inlet position and angle can be used to prevent the unfavorable jamming of particles.