Impact on Granular Material
What do meteor impacts and quick sand in the dessert have in common? Both phenomena can be studied in the lab by "fluidizing" very fine sand. Fluidizing is a method of letting air bubble through sand to obtain a very loose packing, down to 41 % volume packing. If a solid object is now impacted on this fluidized sand a cavity (void) is created as the object penetrates the sand. This void subsequently collapses due to the hydrostatic pressure, causing a jet of sand to shoot upwards into the air and downwards into the void. At the end a "granular eruption" occurs under certain conditions as air enclosed by the void collapse rises up to the surface and is released.
A crucial parameter is the ambient air pressure: At lower ambient pressure, the jet height turns out to be dramatically decreased. We were able to trace this effect back to the drag the sphere experiences in the sand bed, which increases with decreasing pressure. An active line of research is why this surprising behavior occurs.
The different aspects such as void collapse, jet creation, and drag on penetrating objects are studied by experiments in sand as well as in water, numerical calculations, and analytical models based on continuum descriptions and hydrodynamic principles.
Info: Devaraj van der Meer