Transient Dynamics in Particle‐laden Density Currents: Insights Into Dilute Pyroclastic Density Current Runout

Pyroclastic density currents (PDCs) are complex, multiphase systems where density strati cation controls the ows' mixing, deposition, and runout, presenting signi cant challenges in hazard forecasting. In dilute PDCs, a decrease in density leads to buoyancy reversal as the mixture becomes lighter than the surrounding uid and transforms into a thermal. Many numerical and analytical models make the simplifying assumption of vertically well-mixed currents. To investigate the in uence of density strati cation, we performed 3D multiphase ow simulations that reproduce laboratory experiments. All currents develop self-similar velocity pro les. However, buoyant currents exhibit minimum density at the nose, whereas nonbuoyant currents display a typical Rousean density strati cation. This distinction is determined by the dynamic interplay between density strati cation and thermal buoyancy, which consequently governs the ow runout. Our ndings indicate that uid buoyancy prolongs particle settling, enhances vertical mixing, and minimizes lateral spreading, consistent with experimental ndings. The bulk Richardson number, however, fails to accurately describe the entrainment coef cient of these ows through the well-known scaling derived from turbidity currents, thereby underscoring a need to reassess the parameterization applied in depth-averaged models. These models, used for simulating dilute PDCs, need to account for their highly transient nature to reliably assess their hazards.