Fully Determined Fluid Velocity Fields for 2D Materials with Complex Geometries.

The movement of fluid in geological structures is crucial to many geologic processes, yet we remain unable to accurately model transport in realistically complex media in either 2 or 3 dimensions. The essence of the problem is the scale invariance of natural materials, whose geometry directly governs fluid movement and makes the prediction of fluid velocities in realistic situations difficult. At present, discrete modelling techniques such as the Lattice Boltzmann scheme, offer the greatest potential for numerically simulating fluid flow in such geometries. A lack of accurate empirical data for flow through complex geometries, however, means that existing models can only be tested for simple scenarios and, as a consequence, are reliant on the assumption that the models accurately predict flow in the geometries of interest. In response to this, an experimental technique has been developed to measure high-resolution velocity fields in complex media. The media are created by numerically constructing a fractally correlated porous matrices and superimposing them with fractal fracture sets. A modified Lattice Boltzmann scheme can then be used to simulate the fluid velocity field through this numeric model. The experimental technique is based on the translation of this complex medium geometry into a physical model, or flow cell, capable of conducting fluid. To produce the flow cell, the Boolean digital map of the solid and void space in the medium is rendered in plastic using stereolithography. This technique produces a precise copy of the medium in a layer 200x200x2mm, held between 2 transparent plates. The flow cell is then enclosed in a purpose built rig that permits controlled fluid flow and direct observation of the fluid behaviour. The fluid is seeded with 0.01mm neutrally buoyant particles and the velocity of the fluid in 0.5mm interrogation areas is determined using high-resolution digital particle imaging velocimetry. Systematic analysis of all porous subareas using both cross-correlation and streak analysis techniques allows a complete, high-resolution velocity field to be mapped. This measured velocity field is then compared directly with the predicted velocity field, and the accuracy of the numeric scheme evaluated.