14:15 〜 14:30
[MIS16-03] 粉体層上への気流衝突で形成されるクレーターのスケーリング則
キーワード：粉体、エアージェット、衝突
When an air jet impacts on a granular surface, a crater is formed. The understanding of the excavation of grains by air jet is important to suppress the ejecta splashing. Particularly, if we want to land safely on the surface of various planetary bodies, high-speed exhaust thrust is frequently used to reduce the landing velocity of the spacecraft. In this process, a lot of grains must be splashed. The splashed grains might cause the malfunction of the spacecraft. Besides, these grains might also cause trouble for already existing instruments. To avoid such incidents, we have to understand the grain splashing behavior properly. Recently, this type of plume–surface interaction (PSI) process has been one of the most crucial issues in the field of spacecraft engineering. Although the problem is not very easy, the model can be reduced to a simple physical problem of the air-jet impact cratering on the surface of the granular bed because most of the planetary surfaces are covered by regolith (granular matter).
Therefore, we built a simple experimental setup and performed a set of systematic experiments. Air jet of velocity 19-375 m/s and diameter 2-6 mm (at the nozzle tip) was impacted onto the surface of a granular bed consisting of glass beads, SUS cut wires, or Toyoura sand. The range of grain size is 0.2 to 2 mm. Using a half-space experimental setup, which has been frequently used to directly visualize the crater shape, the resultant crater shape was recorded by a USB camera. Using the acquired data, the aspect ratio (crater width/crater depth) was analyzed. We found that the conventionally used dimensionless numbers cannot explain the data behaviors. Instead, we found a new type of scaling variables and corresponding scaling function which properly explain the data behaviors.
According to the obtained scaling, the crater’s aspect ratio is simply determined by the air-jet speed, nozzle tip diameter, and the distance between the nozzle tip and granular surface when the distance is long enough. In this regime, any granular property does not affect the scaling. However, when the nozzle tip approaches the surface of the granular target, the relevant length scale switches from the nozzle tip diameter to the grain size. In this regime, grain size plays a key role in understanding the aspect ratio scaling.
By combining all these results, we developed the unified scaling function [1]. The details of the experiments and analyses will be shown in this presentation.
Reference:
[1] P. Sonar and H. Katsuragi, Air-jet impact craters on granular surfaces: a universal scaling, J. Fluid Mech. 998, A29 (2024).
Therefore, we built a simple experimental setup and performed a set of systematic experiments. Air jet of velocity 19-375 m/s and diameter 2-6 mm (at the nozzle tip) was impacted onto the surface of a granular bed consisting of glass beads, SUS cut wires, or Toyoura sand. The range of grain size is 0.2 to 2 mm. Using a half-space experimental setup, which has been frequently used to directly visualize the crater shape, the resultant crater shape was recorded by a USB camera. Using the acquired data, the aspect ratio (crater width/crater depth) was analyzed. We found that the conventionally used dimensionless numbers cannot explain the data behaviors. Instead, we found a new type of scaling variables and corresponding scaling function which properly explain the data behaviors.
According to the obtained scaling, the crater’s aspect ratio is simply determined by the air-jet speed, nozzle tip diameter, and the distance between the nozzle tip and granular surface when the distance is long enough. In this regime, any granular property does not affect the scaling. However, when the nozzle tip approaches the surface of the granular target, the relevant length scale switches from the nozzle tip diameter to the grain size. In this regime, grain size plays a key role in understanding the aspect ratio scaling.
By combining all these results, we developed the unified scaling function [1]. The details of the experiments and analyses will be shown in this presentation.
Reference:
[1] P. Sonar and H. Katsuragi, Air-jet impact craters on granular surfaces: a universal scaling, J. Fluid Mech. 998, A29 (2024).