5% - eTheses Repository - University of Birmingham

grains are not spherical and so must have defined separation
points depending on their instantaneous orientations to the local
flows. This is not the case for spherical particles which are
often used as model substitutes for such grains, so we might
argue that it is a reasonable compromise to use cylinders falling
somewhere between these two extremes, perhaps even that our model
particles may be more appropriate than spheres?
The methods employed in the trajectory experiments of section 3
were described in chapter 2; recall that the landing position and
flight time of individual particles was recorded from video
records of the trajectories of particles released on the upstream
stoss slope.
Their contribution to the particle concentration
C in the lee fluid was measured by following individual
particles. The time averaged relative concentration C ± /C 0
in any
reference mesh is then related to the number of particles N ± that
have passed through at mean velocity Vpi by the expression (Sene,
1985) .
Vpi
(1)
where N 0 is the total number of particles tracked, C 0 is total
particle concentration in the measurement grid and Vp0
is mean
velocity of the particles travelling through the grid. Vpi is
related to the number of times that any single particle is
counted in a square, here termed the number of footprints F ±
that
it leaves.
For a square grid of side L(m) and time step t(sec)
3-5