Fig. 7: Particle sizes of critical Stokes numbers and their sequential sedimentation.
From: Turbulent particle-gas feedback exacerbates the hazard impacts of pyroclastic density currents
a The frequency \({f}_{\varepsilon }\) associated with the condition of particle Stokes number St = 1 as a function of grainsize. The grey horizontal bar depicts the frequency band in \({f}_{\varepsilon }\) of c. 7–17 Hz of highest-pressure energies. The grey vertical bar delimit the experimentally determined corresponding critical grainsizes for St = 1 at \({f}_{\varepsilon }\) of c. 127–224 µm. The red horizontal bar and the red vertical bar mark the frequency associated with the most energetic coherent flow structure at a frequency f ~ 1.3 and the corresponding grainsize of c. 2 mm, respectively. b Histograms of time-integrated flow grainsize distributions captured in flow samplers at flow heights of 0.45 m above the flow base for various flow distances. As in ‘a’, the grey vertical bar highlights the grain-size range of c. 127–224 µm associated with \({f}_{\varepsilon }\), while the red vertical bar marks the critical grainsize of c. 2 mm supported by the most energetic coherent turbulence structures at f ~ 1.3. Note the downstream depletion of the flow in critical Stokes number particles associated with \({f}_{\varepsilon }\) (c. 127–224 µm). c The fractional time-integrated energy of particle impacts Pimpact relative to the entire pressure energy Pdyn as a function of flow distance. The black line is a best-fit power law to the data.
