Retaining primary wall roughness for flow in rock fractures and implications on heat transfer and solute transport

Abstract Wall roughness is often found to have evident influences on flow-related processes in rock fractures. In this work, we first define the primary wall roughness to have dominating effects on the overall flow, whereas the secondary roughness only generates additional flow resistance and have insignificant impacts on the bulk flow. Accordingly, an effective procedure is presented to quantitatively identify primary and secondary roughness, and the validity of the procedure is tested with flow simulations in a two-dimensional rough fracture. The results show that the reconstructed fracture with only primary roughness can still well resemble the general flow behavior in the original fracture with a steady transmissivity increment of 5% for the Reynolds number from 0.01 to 100. We further examine the effects of both primary and secondary roughness on processes associated with heat transfer and solute transport in fracture flow. It is found that the heat transfer and solute transport behaviors are closely related to the flow behavior under the effect of wall roughness. As the flow velocity increases to cause strong flow inertia, larger eddies can occur to affect the heat transfer and solute transport processes. Although the presence of secondary roughness shows limited effects on the overall transport behaviors, secondary roughness is found to give rise to more irregularly-shaped eddies, which result in different local behaviors. The findings in this study can provide new insights into the roughness effect on fracture flow and associated processes. In addition, the roughness quantification presents a straight-forward evaluation of potential deviations when neglecting small-scale secondary roughness, as is often needed when analyzing larger-scale problems.