Visualization of convective fluid flow in a porous medium

WHEN a horizontal layer of fluid is heated from below, it may undergo Rayleigh-Bénard convection (RBC), leading to the spontaneous appearance of regular patterns of fluid flow¹. The shadowgraph technique², which allows visualization of the convection patterns, has assisted in developing an understanding of RBC. Related to RBC is convection in a fluid permeating a porous medium (called Horton-Rodgers-Lapwood convection or HRLC) when it is heated from below^3-7. HRLC is relevant to geothermal applications and to flow in soils. Pattern formation in HRLC is less easily visualized by shadowgraph techniques because of the difficulties of transmitting light through the porous medium. Here we show how these difficulties can be overcome by constructing porous media in which the interfaces between solid and liquid are either parallel or perpendicular to the confining boundaries of the experimental system. Convection in such a medium can be visualized using conventional shadowgraph methods, and we compare the stationary flow patterns observed against measurements of heat transport.