Deriving and Comparing Tunnel Friction Factors from Surface Roughness and CFD Simulations

This research analyses how a useful surface roughness characteristic can be extracted from laser scan point cloud data with relative ease, and how computer simulations can be used as a substitute for conventional ventilation surveys. To investigate, a laser scan survey of a network of tunnels at an underground talc mine was conducted and the resulting point cloud data representing the tunnel was loaded onto the software CloudCompare, from which roughness and friction factor values were obtained. Furthermore, the same point cloud data was used to construct 3D models of the mine tunnel and with Computational Fluid Dynamics software ANSYS Fluent, airflow at various speeds was simulated to obtain pressure differential readings from which friction factor values could also be extracted. Results showed the friction factors derived from the roughness were in total agreement with documented values and that for a good range of acceptable air velocities, the friction factors derived from the CFD readings were also within acceptable ranges, albeit with higher values than their corresponding values derived from the surface roughness. This can be attributed to the highly uneven nature of the tunnel surfaces and dimensions which in simulated airflow is captured fully. The friction factors derived from simulated pressure drops agreeing with documented values for a good range of low to medium air speeds indicates that the simulated air pressure results are comparable to those that might have been observed during a ventilation survey, and that this indicates the viability of CFD methods in providing a good substitute to ventilation surveys for low to medium air speeds.