Influence of fluid dynamics on flow and transport in natural fracture networks

Abstract The flow velocity in metre-scale natural fracture networks readily exceeds centimetres per second, the threshold for non-stationary flow. However, despite widespread evidence of such dynamics, these are rarely considered in subsurface engineering applications, where steady-state simulation approaches dominate. Here, we compare Reynolds-averaged Navier-Stokes (RANS) and Detached-Eddy Simulation (DES) methods for the transient Navier-Stokes equation applied to fracture flow. These models are validated with experimental data of flow through fracture intersections. DES is then applied to a metre-scale fracture pattern with hundreds of discrete fractures, examining flow dynamics at velocities up to metres per second (m/s). DES accurately captures the temporal flow fluctuation and multiscale eddy formation, especially when a fine computational mesh is used in wake regions. By contrast, unsteady RANS fails to capture flow-field variations and produces results similar to steady RANS. DES reveals significant network flow periodicity ( $$\sim $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math> 40 Hz) at m/s velocities, unlike the low-frequency results ( $$\sim $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math> 0.4 Hz) from RANS. We also explore the impact of unsteady flow on particle transport by integrating mixture-multiphase and rheological models into DES. Corresponding results indicate that inertia alters the concentration of transported solids, mixture viscosity, and particle dynamics such as clustering.