Thermal Equilibrium in Gravitational Mass Flows - a Natural Compensation of Frictional Energy Dissipation

Montreal

2019

1 S.

Sonderdruck

12639S

205080

Snow avalanches, landslides, and ice-rock avalanches produce heat and subsequently encounter different flow regimes as they flow down the mountain. Snow avalanches, for instance, may transition from cold powder-snow avalanches to warm, dense-flow avalanches, depending on snow temperature. Recently, laboratory flow-experiments in a rotating drum revealed more details of the mechanisms driving the temperature evolution in moving snow. Alongside measuring the detailed temperature evolution the experiments allowed to reproduce associated flow regime transitions. Additionally to the known and expected transitions it was possible to discover a new phenomenon, namely the thermal equilibrium of the flow. At this steady flow state, the particle-gas interaction governs the temperature evolution. At sufficiently low ambient temperatures, ambient air-cooling compensates frictional heating and thus prevents flow regime transitions on a laboratory scale. Furthermore it turns out that the thermal energy balance of the gravity-driven mixture can be described by a simple analytical model, only taking into account frictional energy dissipation and heat exchange with the ambient medium. The model accurately captures the measured temperature evolution and predicts the observed thermal equilibrium. The setup additionally allows for a novel approach to measure and determine heat transfer coefficients and total shear stresses of the flowing material solely based on measured temperatures. Moreover, the temperature and with it flow regime evolution could be incorporated into new avalanche models calculating run-outs and impact pressure - an important step for improving hazard assessment and mitigation measures.