Experiments with non-Darcy flow in joints with large scale roughness

Joints are normally in contact at roughness asperities. Consequently, the relative roughness, the ratio of the asperity protrusion to the mean flow aperture, is approximately one, which is orders of magnitude greater than that encountered in more usual situations such as pipe and channel flows. In these experiments the joint roughness is idealized with sawtooth corrugations. It is found that Darcy's Law becomes invalid for Reynolds numbers greater than about 100. Hence, for most of these studies the flow is non-Darcian, and so the usual convention of representing pressure losses in terms of an apparent friction factor is adopted. When the roughness peaks are aligned, resulting in maximum flow disturbance, the friction factor increases when the relative roughness is increased, simulating the closure of a joint due to normal-direction loading. However, when the roughness peaks are 180 deg out of phase, so that peaks are opposite valleys, the friction factor initially increases with greater relative roughness, but then decreases.