Experimental Investigation on Translational Velocity and Air Pockets Volumes Associated with Pressure Transients in Rectangular Conduits of Stormwater Systems

Severe structural damages in stormwater systems are related to biphasic unsteady flows and interactions between air and water phases. The entrapment of air pockets in stormwater systems may be triggered during the transition between free surface to pressurized flow regimes, as the inflow rate exceeds the conduct’s conveyance capacity in intense rain events. As indicated in the literature, pressure transients may arise in the system as the entrapped air pockets are released to the atmosphere.

The structural failure of a stormwater system in the city of Porto Alegre (Brazil) in February 2013 encouraged the implementation of a physical model at the Institute of Hydraulic Research of the Federal University of Rio Grande do Sul. The hydraulic physical model reproduces a long section of the stormwater system, including detailed representations of its geometric singularities, e.g. sharp bends, drop sections, decrease cross sectional area in the downstream direction. Recent experiments accomplished in the model concerning air-water interactions corroborated the influence of entrapped air pockets on the hydraulic behavior of the system during the rapid filling process. Moreover, it was observed that the passage of air pockets through a geometric singularity located at the model ceiling causes pressure spikes.

In this context, this paper brings experimental results of entrapped air pockets volumes and its translational velocity. Experimental runs will be held at the facility located at the Federal University of Rio Grande do Sul. The instrumentation used at the experimental procedure comprehends flowmeters, a high-speed camera and 4 high resolution video cameras, pressure transmitters for air and water phase and 3 ADV probes (Acoustic Doppler Velocimeter). Image processing techniques will be applied in order to estimate air pockets volumes and translational velocity. A correlation between the measured entrapped air pockets features and pressure transients is expected to be recognized as an additional outcome of this experimental investigation.