LAUSR

Geophysical granular flows occur at the surface of the Earth and other planets with reduced atmospheric pressure. In this paper, we investigate the run-out of dam-break flows of particle-air mixtures with fine ( $$d=75\,\upmu {\hbox {m}}$$ ) or coarse ( $$d=150\,\upmu {\hbox {m}}$$ ) grains in a flume with different bottom roughness ( $$\delta$$ ) and vacuum degrees ( $$P^*$$ ). Our results reveal an increase of the flow run-out as $$d/\delta$$ decreases for fine $$d=75 \,\upmu {\hbox {m}}$$ -particles, and run-out decreases with the dimensionless ambient pressure ( $$P^*$$ ) for a given $$d/\delta$$ . In contrast, the run-out for coarser $$d=150\,\upmu {\hbox {m}}$$ -particles, is almost invariant respect to $$P^*$$ and $$d/\delta$$ . These results show that autofluidization of fine-grained flows demonstrated by earlier works at ambient pressure also occurs at reduced pressure though being less efficient. Hence, autofluidization is a mechanism, among others, to explain long run-out of geophysical flows in different environments.