2022, Mostafa Koushki Graduated

Numerical and experimental modeling of forced hydraulic jump by a vertical sill

Abstract

Scouring is defined as the excavation and movement of bed particles and riverbank by water flow. This phenomenon may lead to a collapse or damage the bridge piers. The 3-D complex flow around piers, simultaneous transfer of sediment and water particles, and continuous change of bed elevation are the factors that make the scouring a complicated task to study. The water flow level may rise above the bridge deck's lower level in any flood, and the flow regime changes from free-surface to pressure-flow. Most previous studies are focused on free-surface flow, and the pressure-flow flow patterns are not deeply investigated. 

In the present study, the flow patterns of pressure-flow scour around cylindrical, semi-conical, and completely-conical piers are investigated. Physical models of bridge piers and deck are constructed in the Hydraulics Laboratory of the Civil Engineering Faculty, Isfahan University of Technology. Experiments have been performed in a rectangular section flume with 0.6 m width, 0.7 m height, and 9 m length. The bridge deck's width is equal to the flume's width, and its height and length (in flow direction) are equal to 0.2 and 0.25 m, respectively. Two cylindrical, semi-conical and conical piers of 50 mm and 70 mm diameters have been built. The median diameter of uniform sand grain used in the bed is 0.76 mm, with a specific gravity of 2.65. The scour equilibrium time criterion is set when the scour depth rate in 8 hours is less than twice of grain size. All of the tests were performed in a clear-water condition with flow intensities of 0.5, 0.75, and 0.9. Different parameter effects on scour depth in pressure flow, including the upstream flow depth, the upstream flow velocity, opening height of the bridge deck, and pier diameter have been investigated.

As the upstream flow impacts the bridge deck, a separated circulated flow is formed under the deck. The separated flow zone reduces the flow section and increases the flow velocity under the deck. The present study's results reveal that at low deck submergence ratios, the maximum scour depth of pressure-flow and dimensions of the scour hole around semi-conical and conical piers are less than those of cylindrical piers in the first 60 minutes. The equilibrium maximum scour depths of cylindrical and semi-conical piers are equals, and they are less than the conical piers. Increasing upstream flow depth, upstream flow velocity, and pier diameter increase the equilibrium maximum scour depth. The equilibrium maximum scour depth reduces when the opening height of the bridge deck reduces. Equations are proposed for estimating the equilibrium maximum scour depth in cylindrical, semi-conical, and conical piers.