Journal of Hydraulic Engineering

The stability of the tank-level controller of hydraulic infrastructures is a complex issue. Past studies have focused on run-of-river hydropower plants with a short penstock, and where the upstream tank level is regulated by a variable-speed turbine. The short penstock entails that the flow regulation performed by the turbine can be considered instantaneously effective in the upstream tank. This is not true in water supply systems in mountain areas, where the length of the water main easily exceeds 10 km. This work is devoted to assessing the stability of the water-level controller of hydraulic systems made up of very long conduits. To do this, we numerically studied the stability of a large number of ideal case studies representative of real hydraulic infrastructures. The key results are (1) the understanding of the causes of the water-level controller instability; and (2) a set of charts that predicts whether the control system is unstable. These charts should be used during the design stage of this type of systems to select values of the tank area that avoid tank-level-controller instabilities.

Laboratory studies of tidal vortex formation and propagation through an idealized inlet configuration are presented for various water depths over flat bathymetry. Using surface particle image velocimetry, starting-jet vortices were tracked and vortex diameter was calculated over the course of four consecutive tidal cycles. It was determined that the ratio of tidal excursion to frictional length scale is a critical parameter for determining whether the flow is advection or friction dominated, and plays a role in characterizing the vortex structure. To investigate tidal vortex stability, the tendency of instabilities to stop growing, the authors applied a variation of the shallow wake parameter equation using the vortex diameter as the length scale. Flow characterizations were further analyzed using the aspect ratio $\delta$ and vortex Reynolds number ${\mathsf{R}}_v$ to evaluate ${\delta }^2{\mathsf{R}}_v$, giving the degree of three-dimensional structure and two-dimensional confinement in the vortices.

Blockages in urban water supply systems are commonly formed from various complicated physical, chemical, and biological processes; thus, they usually constrict randomly and nonuniformly along their lengths. Although the transient-based method has been developed for blockage detection, the applications of this method are mainly limited to blockages of uniform constriction along their lengths (termed as uniform blockages). The influence of blockages with linearly varying diameters (termed as linear nonuniform blockages) on transient frequency responses was studied by the authors in a previous study. It was found that the resonant frequency shifts induced by linear nonuniform blockages have totally different patterns from that of uniform blockages. Specifically, the resonant frequency shifts induced by linear nonuniform blockages become less evident for higher harmonics. But the physical mechanism of this pattern is still unclear. This study intends to clarify this phenomenon from an energy perspective. For this purpose, the energy transmission coefficient of an unbounded pipeline containing various blockages is analytically derived, which is numerically validated by the method of characteristics. Afterward, the influence of nonuniform blockage properties on the energy transmission is investigated systematically based on the validated result. The results indicate that the impedance of nonuniform blockages is frequency dependent, which becomes smaller for higher frequency waves. This means that nonuniform blockages have a less blocking effect on the propagation of higher frequency waves; thus, the resonant frequency shifts induced by nonuniform blockages become less evident.