10 ignimbrite flow model initial velocities may

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Unformatted text preview: ial velocities may approach sonic High velocities maintained 10’s of km from source. Limitations Flows topographically channeled, so they don’t loose velocity as fast as predicted. Reynolds Number (ratio of inertial to visous forces) likely drops during flow, meaning viscous forces become more important. Particle Settling During Flow Pyroclastic Flows are laterally zoned Clast size decreases with distance Indicates coarse particles are settling out. Particle suspension depends on settling velocity, vs, of Particle particle vs. “shearing stress velocity” or “frictional particle velocity”, vf: 1/ 2 ! cf $ vf = # & " 2% Particles will settle if: u vs > ! v f Where β is 0.12 to 0.65 Sparks et al. found that even in fast flows, only particles finer than 1 mm can be truly suspended; 20 cm pumice in large flow should settle in 3 km. This rapid settling is not observed. 11 Segregation in Flows S p a r k s e t a l. c o n c lu d e d flo w s s e g r e g a te in to d e n s e lo w e r la y e r ( 1 0 ’ s o f m e te r s th ic k ) a n d d ilu te u p p e r p a r t. L o w e r p a r t u...
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