Numerical Simulations of a Fluidized Granular Flow Entry Into Water: Insights Into Modeling Tsunami Generation by Pyroclastic Density Currents

• Published in: Journal of geophysical research. Solid earth 2021-11, Vol.126 (11), p.n/a
• Main Authors: Battershill, L., Whittaker, C. N., Lane, E. M., Popinet, S., White, J. D. L., Power, W. L., Nomikou, P.
• Format: Article
• Language: English
• Subjects: Aquatic reptiles; Boundary conditions; Continental interfaces, environment; Debris flow; Density; Density currents; Dynamics; Energy transfer; Fluid mechanics; fluidized granular flow; Fluidizing; Fluids; Geophysics; Laboratories; Laboratory experiments; Landslides; Lava; Mathematical models; Mechanics; Modelling; Newtonian fluids; numerical modeling; Numerical models; Numerical simulations; Partial differential equations; Physics; pyroclastic density current; Sciences of the Universe; Sea currents; Shearing; Slopes; Stratification; tsunami; Tsunami generation; Tsunamis; Vertical distribution; Viscosity; Viscous fluids; Volcanic activity; Volcanic eruptions; Volcanic gases; Volcanic rocks; Volcanoes; Water; Wave breaking; Wave generation
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2021JB022855

The tsunami generation potential of pyroclastic density currents (PDCs) entering the sea is poorly understood, due to limited data and observations. Thus far, tsunami generation by PDCs has been modeled in a similar manner to tsunami generation associated with landslides or debris flows, using two‐layer depth‐averaged approaches. Using the adaptive partial differential equation solver Basilisk and benchmarking with published laboratory experiments, this work explores some of the important parameters not yet accounted for in numerical models of PDC‐generated tsunamis. We use assumptions derived from experimental literature to approximate the granular, basal flow component of a PDC as a dense Newtonian fluid flowing down an inclined plane. This modeling provides insight into how the boundary condition of the slope and the viscosity of the dense granular‐fluid influence the characteristics of the waves generated. It is shown that the boundary condition of the slope has a first‐order impact on the interaction dynamics between the fluidized granular flow and water, as well as the energy transfer from the flow to the generated wave. The experimental physics is captured well in the numerical model, which confirms the underlying assumption of Newtonian fluid‐like behavior in the context of wave generation. The results from this study suggest the importance of considering vertical density and velocity stratification in wave generation models. Furthermore, we demonstrate that granular‐fluids denser than water are capable of shearing the water surface and generating significant amplitude waves, despite vigorous overturning. Plain Language Summary When a volcano erupts, it ejects large quantities of volcanic rock, gas, ash, and debris. These ejected materials can flow very rapidly down the side slopes of the volcano; these flows are called pyroclastic density currents (PDCs). When PDCs enter the sea, they displace water and can generate tsunami waves with enormous destructive potential. One method of understanding this potential is by mathematically modeling the flow and its interactions with water, and confirming these model results against laboratory data. The present study compares numerical model results with published laboratory experiments of PDC generated tsunamis, to understand how our assumptions about the flow and its motion along the boundary can affect the amount of energy transferred to the generated waves. We approximate a PDC generated tsunami as a dens