CONVECTION AND FLOW IN POROUS-MEDIA .2. VISUALIZATION BY SHADOWGRAPH

We present results for pattern formation at the onset of convection in fluid-saturated porous media obtained by a novel variation on the sha dowgraphic technique (modified shadowgraphic technique). Both ordered and disordered media are used, each exhibiting distinct behaviour. Ord ered porous media are constructed from grids of overlapping bars. Conv ective onset in this type of medium is characterized by a sharp, well- defined bifurcation to straight parallel rolls. The orientation of the convection rolls is determined by the number of bar layers, N-b; odd N-b leads to rolls with axes perpendicular to the direction of the top and bottom bars, and even N-b to rolls at 45 degrees to the bars. Dis ordered porous layers are produced by stacking randomly drilled disks separated by spacers. In this system, we observe a rounded bifurcation to convection with localized convection near convective onset. More s pecifically, the flow patterns take on one of several different three- dimensional cellular structures after each cycling through convective onset. These observations may be described by two different mechanisms : random spatial fluctuations in the Rayleigh number (Zimmermann et al . 1993), and/or spatial variation in the thermal conductivity on the l ength scale of the convection wavelength (Braester & Vadasz 1993).