The impact and penetration of a projectile in a particle-laden space, which are expected to have frequently occurred during the formation of the solar system and will occur in the case of an impact probe for future planetary exploration, were experimentally simulated by using the ballistic range. A two-dimensional sheet made from small glass beads or emery powder was formed by the free-falling device through a long slit in the test chamber evacuated down to about 35 Pa. A polycarbonate projectile of a hemi-sphere-cylinder or sphere shape with the mass and diameter about 4 g and 25 mm, respectively, was launched at the velocity up to 430 m/s, and the phenomena were observed by the high-speed camera at 20,000 fps. From a series of images, the bow-shock-wave-like laterally facing U-shaped pattern over the projectile and the absence of particles in the trail behind it were clearly seen. At the impact of the particles on the projectile surface, fine grains were formed due to the destructive collision and injected outward from the projectile. The images obtained by different lighting methods including the laser light sheet were compared. The effects of the particle diameter, its material and the impact velocity were also investigated.
Impact in a particle-laden space in a vacuum is not an unusual event. For example, it is well known that the destruction and aggregation of objects at the impact played an important role during the formation of the solar system [
When the particles are packed in the space, the dynamics of their motions has been intensively and extensively investigated in the field of the terra-dynamics. The numerical method using virtual particles called DEM (Discrete Element Method) [
When the particles at the undisturbed state are separately located at some distance, the situation seems similar to the rarefied gas dynamics with relatively large mean free path. In the presence of the atmosphere, various studies have been numerically and experimentally conducted in the framework of the two-phase flow. For example, the combination of the Eulerian description of the dynamics of the fluid and the Lagrangian description of the motion of the floating solid particles is known to reasonably simulate the dusty flow around a body at a supersonic speed [
In the present experimental study, a body was shot into a particle-laden space as seen in [
The major objectives in the present study are 1) to reveal the characteristic features of the granular flow around a projectile penetrating on the sheet of particles, 2) to clarify the presence of the destructive collision of particles at the projectile surface and its role in the granular flow field, and 3) to investigate the effect of the impact velocity, the diameter and the material of particles.
The ballistic range in the authors’ laboratory [
projectile was launched and penetrating on the particle sheet in this period. The projectile was finally caught in a semi-hard-landing manner by the projectile catcher made from the sponge layer and the oil clay.
As discussed later, the obtained image strongly depends on the lighting method and the viewing area of the camera. We used three types of arrangement of the lighting and viewing area of the camera as shown in
image. After the penetration proceeded for some distance, the granular flow field around a projectile is expected to be in a steady state. The arrangement (b) was used for the observation after long penetration. We have to be careful to judge the presence of matter from a visualized image, because the dark image can be produced by not only the absence of matter but also the absence of light. To reduce the risk of misunderstanding, the arrangement (c) with the front-lighting was tested and compared with the arrangement (a). For the lighting, the high-intensity metal halide lamp MID-25 FC (Lighterrace Inc.) with the maximum power 250 W was used. To capture the images of the impact phenomena, the high-speed monochrome camera Phantom Miro 310 (Nobby Tech. Ltd.) was used. The sensitivity is 12 bit. The frame rate, exposure time and the spatial resolution were set to be 20,000 fps, 1 μs and 512 by 256 pixels, respectively, in the present experiment.
Three types of the glass beads (Types #40, 60 and 80) with different size were used for the particle sheet. They are originally supplied as the grinding powder with the particle size controlled under the industrial standard, that is, 355 - 500 μm for Type #40, 250 - 355 μm for #60 and 180 - 250 μm for #80. The magnified image of the particles by the microscope showed that the shape of a glass bead was not a smooth sphere but a rugged irregular ball [
Two types of the projectile shapes were tested, that is, the hemisphere cylinder and the sphere, as shown in
The projectile velocity was estimated from the frame rate and the difference in the projectile position between two continuous snapshots. The uncertainty in the estimated velocity was mainly caused by the blur of the projectile image in the exposure time [
Thanks to the launching of the projectile without the sabot, the uncertainty in the flight condition was relatively small. The free flight of the projectile before reaching the edge of the particle sheet was stable. The fluctuation in the velocity and the path angle during the free flight was smaller than ±10 m/s and ±1.5 degrees, respectively [
because of its simple mechanism of the free falling. The variation in initial pressure at the test chamber was smaller than 1 Pa. The test chamber was carefully cleaned before each shot, because the presence of the residual particles and fine grains may affect the experimental pictures and becomes the serious source of the uncertainty. In the present study, qualitatively the same pictures were obtained in the same experimental condition and the same arrangement of the light source and the camera.
The present experimental setup was designed under the assumption that the motion of the particles mainly occurs on the plane of the particle sheet, because the plane of symmetry of a projectile coincides with that of the sheet. To confirm that, the setup of the lighting and the camera shown in
To check the two-dimensionality of the phenomena, the visualization using the laser light sheet was also conducted. Two types of the laser light sheets, that is, the horizontal sheet and the vertical sheet, were tested as shown in
contrast. In both pictures, the projectile moved from left to right, and the cross section curve of the projectile surface was clearly seen. Though the projectile velocity could not be estimated because of unclear images, it was expected to be about 400 m/s from the experiments in the similar condition. This fact indicated that the out-of-plane granular flow was not so significant to fully cover the projectile surface.
Consequently, the two-dimensional behavior of the particles and fine grains around a projectile was expected to be visualized in the present experimental setup as illustrated in
The typical pattern of a snapshot taken by the back-lighting arrangement (a) in
projectile. The hazy pattern of the bow-shock-wave-like structure implies that this zone was composed of much finer grains than the glass beads. Such fine grains are expected to be produced by the destructive collision of the glass beads at the projectile surface. The fine grains spread away from the body and formed the laterally facing U-shaped structure as seen in
The image of the granular flow field around the projectile varies with the time from the impact at the edge of the particle sheet.
used for the projectile and the particle sheet, respectively. The velocity was estimated as about 380 m/s from the snapshots. The images were taken by the camera and lighting arrangement (b) in
Consequently, the change in the pattern of the above pictures can be explained by the destructive collision of the glass beads into fine grains at the projectile surface, the removal of the particles or fine grains behind the projectile by the sweeping effect, the shock-wave-like propagation of the laterally facing U-shaped zone of the fine grains in front of the projectile, and the diffusion of the fine grains into the trail of the projectile.
The above features were commonly observed in both cases of the sphere model and the hemisphere cylinder model. However, the diffusion of the fine grains into the trail was much weaker in the case of the hemisphere cylinder model as shown in
filled with the diffusive fine grains in the case of the sheet of coarser particles. This fact indicates that the properties of the fine grains produced from the glass beads at the collision with the projectile surface depend on the initial particle size.
The effect of the material of the particle sheet on the granular flow field around a hemisphere cylinder model is shown in
Finally, the effect of the impact velocity was shown in
particle sheet, respectively. The images were taken at 0.15 ms after the impact. At the velocity 410 m/s, stronger emission of the light scattered by the laterally facing U-shaped layer of the fine grains was seen in front of the projectile than at the velocity 330 m/s. In addition, the luminous zone of the fine grain layer in front of the projectile becomes wider in the case of higher impact velocity. The mass flux of the colliding glass beads increases with the impact velocity and the destruction of the colliding glass beads becomes more significant at higher impact velocity. The velocity of the fine grains injected in the forward direction at the projectile surface is expected to increase with the impact velocity. As a result, higher production rate and higher injection velocity of the fine grains are obtained at higher impact velocity. Consequently, the zone of the fine grains spreads more quickly in front of the projectile at higher velocity.
The impact and penetration of a projectile in a particle-laden space were experimentally investigated by using the ballistic range. A thin sheet made from small glass particles or emery powder was formed by the free-falling device through a long slit over the trajectory of a projectile in the test chamber. A polycarbonate projectile of a hemisphere cylinder or sphere shape with the mass and diameter about 4 g and 25 mm, respectively, was launched at the velocity from 310 m/s to 430 m/s. The phenomena were observed by the high-speed camera at 20,000 fps. To reduce the effect of the flow of the residual air in the test chamber, it was evacuated beforehand. From the obtained images using various lighting and camera arrangement including the laser light sheet, the two-dimensionality of the phenomena was discussed. The bow-shock-wave-like laterally facing U-shaped pattern in front of the projectile and the zone of absence of the particles or fine grains in the trail behind it were clearly observed in the pictures. The process of the formation of the granular flow field around a projectile was characterized by the destructive collision of the glass beads into fine grains at the projectile surface, the removal of the particles or fine grains behind the projectile by the sweeping effect, the shock-wave-like propagation of the laterally facing U-shaped zone of the fine grains in front of the projectile, and the diffusion of the fine grains into the trail of the projectile. The similar pattern was observed irrespectively to the projectile velocity, the size and material of the particles. However, the production rate and the spread speed of the fine grains depend on these conditions.
The above results suggest that the present phenomena can be numerically simulated by the model including the appropriate description for the motion of the original particles, the formation of the fine grains at the projectile surface and the flow of the fine grains. The numerical analysis using such model is expected to be quite useful for understanding of the formation of the celestial objects in the solar system, designing the impact probe for planetary exploration in the future and so on.
This work is supported by Grant-in-Aid for Scientific Research (B) No. 16H04585 of Japan Society for the Promotion of Science.
Masaki, C., Suzuki, K. and Watanabe, Y. (2018) Visualization of High-Speed Impact of Projectile in Granular Sheet with Destructive Collision of Particles. Journal of Flow Control, Measurement & Visualization, 6, 136-151. https://doi.org/10.4236/jfcmv.2018.63012