Surging in Combined Free Surface‐Pressurized Systems

Abstract

Unsteady flow conditions are often encountered in hydraulic systems wherein open channel flow and submerged or pressurized flow conditions may exist simultaneously being separated by a moving interface. Examples of combined free surface‐pressurized flows include power plant tailrace tunnels following turbine load acceptance or rejection, storm water‐sewerage systems, irrigation systems, and pipelines with air vents or undersized surge tanks. A mathematical model is presented which utilizes a method of characteristics solution for the open channel flow. The pressurized flow region is described by the rigid‐column surge equation. Depending on the direction of movement, the surge front that separates the free surface and pressurized flow regions is assumed analogous to a hydraulic bore or a negative surge wave. Results of an experimental laboratory study verified the ability of the numerical model to simulate the magnitude and timing of the surge phenomenon. Discrepancies concerning the predicted shape and celerity of the negative surge front are attributed to the difficulty of mathematically modeling the actual flow conditions at the front. As a practical application, surge in a power plant once‐through circulating water system surge chamber and discharge pipe was analyzed. The results agreed favorably with the field measurements.