The figure shows an oil tunnel and associated equipment used in experiments, using the full-flow-field tracking (FFFT) technique, to characterize flows in beds packed with poly (methyl methacrylate) spheres. The indices of refraction of the oil and the spheres were matched to make the spheres invisible to the eye and cameras. The oil was seeded with magnesium oxide particles as flow tracers. The packed bed and the seeded oil in its interstices were illuminated with laser light in a plane aligned along the direction of bulk flow, thereby making visible some aspects of the flow dynamics. The image of the illuminated plane was recorded by a television camera aimed perpendicularly to the flow. The light sheet was traversed from one side of the tunnel to the other to acquire image data in different planes for use in synthesizing a three-dimensional image of the entire flow field.
The optical nature of the boundary interface between the working fluid and the spheres rendered the spheres black, permitting visualization of the exact locations of the circular oil/sphere interfaces in both the axial and transverse directions, with direct visualization of the complex interstitial spaces between the spheres within the bed. Strobing the laser provided a means to estimate the velocities of the flows within the bed of spheres and facilitated tracking the flow. Flows were observed near the planar tunnel walls and sets of spheres as well as near minor circles that appeared with great circles at various transverse positions and were not always uniformly ordered. The recorded images revealed very complex flow fields, and it was observed that flow channeling in the direction of bulk flow occurs between sets of adjacent spheres.
The flow was found to be fully three-dimensional and complex to describe. The most prominent finding involved conclusive experimental demonstration of flow threads as computed for hyper-cluster spheres in NASA Technical Memorandum 107361, "Numerical Flow Visualization in Basic- and Hyper-Cluster Spheres." More specifically, it was found that the bulk of the flow field has a natural tendency to establish the flow paths of least resistance (the above-mentioned threads) through the packed bed that are parallel and distinct, and that for a regular array of spheres, the number of threads is related to the number of open areas in a cross section in a plane perpendicular to the direction of bulk flow.
Beds of spheres used in the experiments were constructed, variously, with regularly or randomly packed spheres of 12.7-mm or 19.05-mm diameter and were used to obtain various flow patterns. The effects of bed voids were also characterized and tended to disrupt flow threads and create vortices. Still photographs and video recordings that illustrate the flow phenomena are available.
This work was done by R. C. Hendricks of John H. Glenn Research Center, S. Lattime of B & C Engineering, M. J. Braun of the University of Akron, and M. M. Athavale of CFD Research Corp.
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