Computational Fluid Dynamics Investigation of the Hydraulic Behaviour of the Roman Inverted Siphon System at Aspendos, Turkey

The hydraulic characteristics of the 3rd century ce Roman inverted siphon system at Aspendos is subject to analysis using modern computational fluid dynamics methods. The 1·67 km long siphon consists of a header basin, two separate elevated tower open basins and a receiving basin all interconnected by long piping segments. The receiving basin distributes water to the fountains, baths and housing structures of the ancient city of Aspendos. Based upon the Roman text of Vitruvius, transient pressure wave phenomena occurring during siphon start-up, if not counteracted by hydraulic and structural defensive design measures, can lead to pipeline breakage. The present study examines the hydraulic behaviour of the Aspendos siphon during start-up and steady state conditions for both impulsive and slow-filling water supply conditions. By revealing the underlying hydraulic behaviour of the siphon, the meaning of Vitruvius' text statements concerning siphon operation, albeit in prescientific terminology, can be interpreted in light of the computed physical behaviour of the system. Analysis results indicate that impulsive siphon start-up is characterized by fluid column oscillations in all siphon branches downstream of the header basin. Depending upon the internal wall roughness of the piping and the filling rate, oscillations can vary in periodicity and amplitude before being damped by fluid friction effects. Slowing the siphon infilling rate (as Vitruvius advises) is shown to reduce start-up oscillatory behaviour and reduce forces on the piping. The presence of the elevated tower open basins is shown to limit oscillations from propagating into the downstream branches of the siphon, i.e., the tower basins act as accumulators to dampen the transmission of fluid column oscillations and pressure waves past the first leg of the siphon. Several suggestions as to the enigmatic meaning of the term colliquiaria with respect to siphon operation are investigated to attempt to recover Vitruvius' original meaning.