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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 vs. “shearing stress velocity” or “frictional
! 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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- Fall '06