EP012-0030
Understanding particle suspension by a single water droplet impacting an immersed granular bed
Tuesday, 8 December 2020
Poster
Hadis Matinpour1, Alban Sauret2, Douglas J Jerolmack3, Eckart Heinz Meiburg1 and Thomas Dunne4, (1)University of California Santa Barbara, Santa Barbara, CA, United States, (2)University of California- Santa Barbara, Mechanical Engineering, Goleta, United States, (3)University of Pennsylvania, Department of Earth & Environmental Science, Philadelphia, PA, United States, (4)Univ California Santa Barbara, Bren School of Environmental Science & Management, Santa Barbara, CA, United States
Abstract:
Surface water erosion during rainstorms begins when raindrops penetrate and disturb a thin sheet of runoff and mobilize particles from the substrate into the overrunning flow. The intensity of this disturbance, and the associated suspension of particles, is known to vary with the ratio of drop diameter to flow depth. We perform laboratory experiments that record the effects of a single water drop impacting an immersed granular bed covered by a thin film of still water, in order to quantify the grain erosion mechanism. We have two objectives: (1) to characterize the erosion threshold for different impact velocities, thin-film thickness and grain properties; and (2) to quantify the grain suspension mechanism for different impact regimes.
The water droplet is formed at the tip of a needle and falls onto submerged granular bed of glass beads. A high-speed camera is used to capture the drop impact dynamics, the resulting flow generated, and the response of granular bed. We characterize the erosion threshold for varying fall distance, droplet diameter, thickness of the liquid film, and particle size.
Our experiments quantify how the thickness of the liquid film and the drop impact velocity affect the erosion of the granular bed. For liquid film depths smaller than a few drop diameters, the erosion is independent of the falling distance. We also report how the erosion threshold and re-suspension mechanism vary with grain size and Weber number (the ratio of the inertia effect and the surface tension force). Coupling the flow generated by the impacting drop with the properties of the granular bed allows us to develop a scaling relation that constitutes a first step in describing the formation of a suspension during intense rainstorms.
