Behringer's Nonlinear Flow Group Page

Shearing and Photoelastic Experiments

A hallmark of our group's work is the study of forces in granular materials using. In granular materials, force is rarely transmitted uniformly, but rather preferentially along a network forming force chains -- often right next to regions where there is little or no force.

When viewed through circular polarizers, high forces show up as bright regions. When the force is very large, the polarization is rotated through multiple phases of Pi, showing fringes. The picture to the right is a closeup of these disks under a large load where the high-force areas are highlighted in red (false color). Note that some of the disks in the photo are not colored at all (no force), while the disk right next to it is highlighted in the color red (high force). This is the nature of granular material and partly why study of the fluctuations yield such interesting and surprising results.

Meteor Impacts

The morphology of meteor craters has historically been studied via static analysis, after the fact, of what are highly dynamic impact events. As such, there are long-standing questions about the means through which a meteor comes to rest and forms a crater. Using high speed video analysis on a 2D lab-scale system, we characterize the dynamics of a "meteor" impacting on a granular bed.  In this case, the particles are made of the photoelastic material described above, so that it is possible to measure the instantaneous elastic energy stored in the bed. To understand the energy dissipation mechanisms involved in slowing the meteor, we track the kinetic, potential, and elastic energies associated with individual grains.  Two initial findings from this work are: 1) Damped oscillations occur as the energy is dissipated within the granular material; and 2) The angle of impact strongly influences the dynamics and final state.

Movies available here

Gravity and Granular Materials

Phase transitions in 3D granular materials are greatly influenced by gravity, particularly in the compaction gas- and fluid-like phases. We are presently working with the NASA microgravity program to prepare low-G experiments for the space station, and concurrently conducting earth-bound laboratory experiments.

Our setup consists of an annular shear cell which is vibrationally fluidized from below. This provides two separate means of injecting energy into the system, and we are able to study the interaction of the two effects in the formation of shear bands. We are presently investigating the statistical properties of this flow by quantifying velocities, granular temperatures, local density, and clustering (shown below).

Movies available here

In addition, simulations of this system are being performed by Lou Kondic and Oleh Baran at NJIT.

Contact Forces in 2D Granular Systems

Extracting contact forces from photoelastic images is a difficult nonlinear inverse problem. We have developed a fully automated method to obtain contact forces from photoelastic data. The main idea is to characterize the stress pattern for each disc and fit it to plane elasticity solution which contains contact forces as parameters. An example is shown below in which contact forces are calculated from an experimental image and plugged back in the elastic solution to obtain a computed image.

With the help of this technique, we can characterize granular materials at a microscopic level. Ongoing work involves studying small shear system where we have obtained normal and tangential force distributions. We are also studying the response of a 2D granular system to biaxial tests.

Force chains in (a) experiment and (b) numerical reconstruction.

Secondary Circulation in Granular Hopper Flows

We are testing numerical predictions that indicate flow in non-axisymmetric hoppers (such as a hopper tilted with respect to gravity) exhibits a secondary, swirling motion in addition to the previously known radial downward flow.

We image flowing sand through a plexiglass window in a hopper made of brass wedges, and track the positions of dyed tracer sand grains to determine how the velocity depends upon tilt angle.

Force Propagation in Granular Materials

We are using photo-elastic discs to study the force networks within vertically confined granular materials (for example, a grain silo). Piles of granular materials exhibit the phenomenon of pressure saturation -- unlike water, where the pressure increases with depth, the pressure within granular materials eventually levels off with depth and increases no more (this is called the Janssen effect).

As theoretical models that look more and more like experimental observations of force networks continue to be developed, we are trying to enhance our experimental understanding. By applying loads to the top of netwowk, we are examining how pressure saturation works and whether there are cases where it does not occur.

q-Model	Socolar's a-Model	Experiment

Left two images modified from J.E.S. Socolar, PRE 57, 3204-3215 (1997)

Thin-Film Experiments

Photo-elastic Shearing Experiments

Advisor:

• Prof. Robert P. Behringer

PostDocs:

• Matthias Sperl
• Jie Zhang

Graduate Students:

• Kenneth McKenzie
• Shomeek Mukhopadhyay
• Trushant Majmudar
• Jun Uehara
• John Wambaugh
• Peidong Yu

• Karen Daniels (2005)
• Jean-Phillipe Matas (2004)
• Jeanman Sur (2004)
• Brian Utter (2004)

Recent Graduates:

• Junfei Geng (2003)
• Bob Hartley (2003)
• Meenakshi Dutt (2002)
• Mike Carey (2000)
• Ben Painter (2000)
• Mark Steen (2000)
• Dan Howell (1999)
• Brian Miller (1997)
• Eric VanDoorn (1996)
• Jason Fiering (1996)
• Mark Shattuck (1995)
• Karen Anderson (1994)
• Jeff Olafsen (1994)
• Richard Leone (1991)
• Guy Metcalfe (1991)
• G. Bill Baxter (1990)
• Hong Gao (1987)