COMPUTING This section was written by Associate Editor Jean Thilmany.
From far away, flowing sand resembles a liquid, streaming down the center of an hourglass like water from a faucet. But up close, you can see the individual grains that slide against each other, forming a mound at the base that holds its shape, much like a solid. But granular materials are tricky to model. This curious behavior—part fluid, part solid—
has made it difficult for researchers to predict how sand and
other granular materials flow under various
conditions. So why is a model needed?
A precise model for granular flow would be
particularly useful in optimizing processes such
as pharmaceutical manufacturing and grain
production, where tiny pills and grains pour
through industrial chutes and silos in mass
quantities, said Ken Kamrin, professor in the
Massachusetts Institute of Technology’s department of mechanical engineering.
When they aren’t well controlled, such large-scale flows can cause blockages that are costly
and sometimes dangerous to clear, he said.
“Granular material is the second-most-han-dled material in industry, second only to water,”
Kamrin has come up with a model that predicts
the flow of granular materials under a variety of
conditions. The model improves on existing ones
by taking into account one important factor: how
the size of a grain affects the entire flow, he said.
Developing such a flow model, which is also known as a
continuum model, means blurring out individual grains or
molecules, he said. While a computer can be programmed to
predict the behavior of every single molecule in, say, a cup of
flowing water, this exercise would take years, he said.
Instead, researchers have developed continuum models.
They imagine dividing the cup into a patchwork of tiny cubes of
water, each cube small compared to the size of the entire flow
environment, yet large enough to contain many molecules and
The water flow model works because molecules are so small
their effects stay within their respective cubes, Kamrin said.
However, in granular flow, much larger grains such as sand
can bleed over into neighboring cubes, creating
cascade effects that are not accounted for in
To get around that problem, Kamrin modified
equations for an existing continuum model to
factor in grain size, and tested his model on
several configurations, including sand flowing through a chute and rotating in a circular
The new model not only predicted areas of
fast-flowing grains, but also where grains
would be slow moving, at the very edges of
each configuration—areas traditional models
assumed would be completely static.
The model, run on a computer, can produce
accurate flow fields in minutes and could allow
engineers to test various shapes of chutes and
troughs to find a geometry that maximizes
flow, or mitigates potentially dangerous wall
pressure, before ever actually designing or
building equipment to process granular materials, he said.
Understanding how granular materials flow could also
help predict geological phenomena such as landslides and
avalanches, and help engineers come up with new ways to
generate better traction in sand he added.
Sands of Time
m Sand: looks like liquid from
afar, granular up close.
Small CAVE A computer-aided virtual environment,
or CAVE, requires projectors and other
equipment. So the total structure can
be as much as four to five times larger
than the actual workspace where the
immersive visualization experience
In March, Mechdyne Corp. of Mar-
shalltown, Iowa, announced its small
form-factor CAVE for commercial and
educational visualization, said Doug
Betts, manager of research engineer-
ing for the company. The smaller
CAVE design compacts the overall
system footprint significantly, he said.