Investigation of Air Fluidization during Intruder Penetration in Sand
This study combines CFD-DEM simulations and experimental cone penetration tests to reveal how airflow-assisted fluidization reduces penetration resistance in sand, identifying a nonlinear relationship with depth and characterizing four distinct meso- and micro-scale penetration stages to advance the design of self-burrowing robots and excavation technologies.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Pushing a solid object into loose sand is a stubborn business. The grains lock together, creating a rigid skeleton that fights back with every inch of progress. This resistance is a fundamental challenge in geotechnical engineering, the field that studies how soil behaves under the weight of buildings, bridges, and tunnels. When engineers need to measure the strength of the ground or install deep foundations, they often use a cone-shaped probe that is driven into the earth. In dense, dry sand, this task requires massive machinery and immense force, limiting where and how these tests can be performed. For decades, scientists have looked for ways to make this intrusion easier, drawing inspiration from nature. Some animals, like certain worms and beetles, burrow by vibrating or injecting fluids into the soil to loosen the grains around them. One promising strategy involves blowing air into the sand to temporarily turn the solid ground into a fluid-like state, a process known as fluidization. If the air pressure is high enough, it can lift the grains apart, reducing the friction and weight that the probe must push against. However, while the idea is simple, the exact mechanics of how air moves through sand and how the grains rearrange themselves in real-time have remained a mystery.
A team of researchers at RWTH Aachen University in Germany has now peeled back the layers of this complex interaction. They combined high-speed computer simulations with physical experiments to watch exactly what happens when a cone penetrates sand while air is pumped out of its tip. In their laboratory, they built a transparent box filled with a specific type of sand and used a 3D-printed probe equipped with tiny holes to inject air at controlled rates. They pushed the probe down at a steady speed while measuring the force required to move it. To see what was happening inside the sand, where the human eye cannot reach, they created a detailed digital twin of the experiment. This computer model tracked the movement of nearly a million individual grains and the flow of air around them, allowing the team to observe forces and pressures that are impossible to measure directly in a physical test.
The results revealed that the process is not a single, smooth event but rather a sequence of four distinct stages. At the very beginning, when the probe first enters the sand and air is released, the force required to push it down drops to almost zero. The air blasts out of the probe's tip, creating a vertical channel or tunnel between the metal and the surrounding sand. In this phase, the air moves fast enough to lift the grains and throw them outward, clearing a path for the probe to slide through without touching the soil. The researchers found that this "channel" remains open only for a short distance. As the probe goes deeper, the weight of the sand above increases, and the air can no longer keep the grains suspended. The channel seals shut, and the sand begins to press against the probe again.
The study identified a critical threshold that determines whether the air can keep the ground fluidized. The simulations showed that for the penetration resistance to disappear completely, the air pressure inside the sand must rise to roughly twice the weight of the soil column pressing down from above. When this pressure is reached, the effective weight of the sand grains becomes negligible, and they float apart. However, this state is temporary. As the probe moves deeper, the overlying weight of the sand grows, eventually overpowering the air pressure. Once the air pressure can no longer support the weight of the soil above, the fluidization stops, the grains settle back together, and the resistance force climbs back up to normal levels. The researchers observed that higher airflow rates could extend the depth at which this fluidization works, but there is a limit to how deep a fixed amount of air can keep the sand loose.
By breaking the process down into these stages, the team mapped out exactly how the sand grains move, how they touch each other, and how the air pressure changes at every moment. They found that the reduction in resistance is directly linked to the air pressure lifting the grains, which breaks the network of forces holding the sand together. This insight provides a clear rule for engineers: to significantly reduce the effort needed to penetrate dense sand, the injected air must generate a pressure high enough to counteract the weight of the soil. While the study focused on a specific type of sand, the findings offer a quantitative framework for designing better excavation tools and self-burrowing robots. It suggests that by carefully controlling the airflow, machines could penetrate the ground with far less energy, opening up new possibilities for construction in difficult terrain or for exploring other planets where drilling is a major challenge. The work confirms that while air can turn sand into a fluid, it is a delicate balance that depends entirely on the depth and the strength of the air being pumped.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.