Validation of 3D multiphase pyroclastic dilute current model: Effects of confinement on the flows

The volcanology community started to systematically test numerical models for pyroclastic density currents (PDCs) against benchmarks that are defined through large-scale experiments. Here, we present the results of a three-dimensional LES model for the case of a partially channel-confined, fully dilute, fully turbulent experimental PDC. Our LES model is based on solving the three- dimensional Navier-Stokes equations using a finite-difference method, CIP-CUP scheme. The model results are compared to the experimental benchmark and to the results of a previous validation of a one-dimensional depth-averaged model. To better understand the effect of three dimensionalities of the flow and entrainment of ambient air on flow evolution, we contrast two scenarios: (1) a fully channel-confined simulation, limiting lateral entrainment of air, and representing a three-dimensional scenario of the two-dimensional case, and (2) a partially confined simulation following the set-up geometry of the experimental benchmark. For the first case, the modelling results are similar to those obtained in the depth-averaged simulation, with a close agreement of the position of the flow front against time but showing a more prominent head shape and body height than the physical experiment. In the second case, the front velocity is slightly overpredicting. However, the height of the gravity current nose, the body thickness, and the vertical velocity profiles closely mimic those measured in the experiment. The downstream dilution of the PDCs is significantly enhanced in the partially confined scenario due to the formation of vortexes that laterally spill over the sidewalls, which is also seen in the large-scale experiment.