Nonlinear Flow Through Granular Media

Relative roles of steady and unsteady fluid motion in causing deviation from the linear Darcy resistance law are described. Air velocities and turbulence intensities were measured within the pores of a gravel bed by hot-wire anemometers. Results indicate that although the linear resistance relation ceases to be valid at a Reynolds number of about 2, velocity fluctuations do not begin until the Reynolds number is about 100, and turbulence is not fully established until it is at least 800. Studies of water flow through a coarse sand bed showed that convergence of the macro-streamlines significantly reduced the resistance at Reynolds numbers over about 10, the proportionate reduction increasing with flow. The observations are interpreted in terms of flow through coiled pipes or round immersed objects, and four regimes of flow through granular media are proposed.