The flow of granular materials is of fundamental interest because of the prevalence of these materials in the pharmaceutical, agricultural, food service, and power industries, as well as others. The transport and handling operations of these materials is performed using hoppers, chutes, and belt and pneumatic conveyers. It has been estimated that related industries total one trillion dollars a year in gross sales in the United States and that they operate at only 63 percent of capacity. By comparison, industries that rely on fluid transport processes operate at 84 percent of design capacity (ref. 1).
Of particular interest to NASA, the exploration and colonization of extraterrestrial bodies will require us to address the effects of reduced gravity on granular materials, and their transport and handling operations. Specifically, granular materials found in Lunar and Martian soils can be utilized for foundation and construction purposes, and in materials processing for the extraction of valuable resources.
Granular materials exhibit very complex behavior, behaving at times like solids and at other times like fluids (liquids and gases). This dual nature leads to very complex and rich behavior, which is presently not well understood. When in a solidlike state, the load bearing is taken up heterogeneously by a finite number of stress chains, which can lead to local high-stress concentrations in the interior and walls of the system. The dynamics are such that any generated stress fluctuations, because of disturbances at the boundaries and the internal rearrangement of particles or flow conditions, are propagated as acoustic and solitary (soliton) waves along the stress chains. A prime example of a granular flow is an avalanche. Such a flow phenomenon consists of several flow regimes, ranging from the fast (dilute) flows of the particles near the top of the falling particle layer which undergo many collisions with different particles, to the slow (dense) flow near the bottom of this layer, where there is significant contact time between neighboring particles. All these stress-related and flow dynamics are very sensitive to gravitational loading conditions. It is clear that there is a great need to better understand the dynamic properties of these materials.
At the NASA Glenn Research Center, we have several research programs underway to study these related phenomena. They include (1) the impaction of two- and three-dimensional granular systems, (2) two-dimensional dense Couette flow, (3) shock absorption using granular particles, (4) detection of buried objects in granular beds, and (5) investigation of granular slug flows with electrostatic effects.
Acceleration signals from the granular bed impaction experiment. The propagation of the
acoustic wave through the granular bed produces a delayed response on the second accelerometer.
Data reduction of these signals will provide valuable correlation and spectral information on the acoustic wave.
For the impaction studies, acoustic waves are produced and then analyzed using time and spatial correlation measurements. For the two-dimensional granular Couette system, employing a roughened inner moving surface, photoelastic disks are used to create birefringence images to investigate the force chain fluctuations. The particles are levitated to isolate them from the frictional surface.
Granular particles also have the potential to serve as shock absorbers. We are testing a prototype shock absorber made of granular particles that can attenuate an impact load by up to 80 percent. In addition, we are using the backscattering of acoustic waves to probe granular systems for inhomogeneities such as minerals in granular soils. This method could be extended to detect different types of buried objects.
The pneumatic conveyance of granular particles, which suffers significantly from inefficient transport modes and jamming is very important to industry. As an alternative transport mode, we have started studying the slug flow mode in granular materials and the effects of electrostatics on the flow.
In support of these research efforts, we have set up and developed a few diagnostic techniques for probing the dynamics of granular systems. The first is the capability to perform high-resolution dynamic measurements of acceleration/vibrations, pressure, and forces. This technique provides valuable dynamic information on the propagation of stresses and waves, and system response. We are also using birefringence-imaging techniques to visualize force chains, and high-speed video to look at the dynamics of certain systems. Lastly, we are developing Magnetic Resonance Imaging (MRI) capabilities and hope to take advantage of this powerful technique to probe the dynamics of the interior of granular systems. To address gravitational effects, we will probably run future experiments in reduced-gravity facilities.
Glenn contacts: Dr. Juan H. Agui, 216-433-5409,
Juan.H.Agui@grc.nasa.gov; and Dr. R. Allen Wilkinson, 216-433-2075, R.A.Wilkinson@grc.nasa.gov
Author: Dr. Juan H. Agui
Headquarters program office: OBPR
Programs/Projects: Microgravity Science
Last updated: June 2002
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
