Hydraulic characteristics of blow-back and blow-out phenomena in bottom outlets based on model experimentation Fatemeh habibi khoozani f.habibi@cv.iut.ac.ir Date of Submission: Department of Civil Engineering Isfahan University of Technology, Isfahan 84156-83111, Iran Degree: M.Sc. Language: Farsi Supervisors: Dr. Abdorreza Kabiri Samani ( akabiri@iut.ac.ir ) Flow through the bottom outlets is accompanied by the formation of free-surface vortices at the intake. Generation of vortices results in air suction into the pipeline, creating small to large air pockets in the conduit of the bottom outlets, thereby an air-water two-phase flow generates inside the pipelines and bottom outlets. This phenomenon has caused severe and sometimes irreparable damages in the hydraulic structures. One of these problems with air entrainment into the bottom outlets is the occurrence of blow-back and blow-out phenomena. These two phenomena occur as a result of the movement of air pockets up- and downward, exiting from the inlet and outlet of the pipe, respectively and reducing the pipeline flow discharge capacity. Due to the severity and problematic behavior of these phenomena, in hydraulic structures and sewage networks, more researches are essential. In the present experimental study, by creating the necessary conditions for the generation of free-surface vortex at the inlet of the bottom outlets, the conditions for the establishment of air-water two-phase flow as well as the occurrence of blow-back and blow-out in the bottom outlet are provided. The tests were performed in a cylindrical reservoir made of galvanized iron sheet with a diameter of 2 and a height of 1 meter, in which a vertical shaft spillway is installed at its center to the reservoir floor. By changing the flow hydraulic conditions, such as flow discharge, water depth at up- and downstream reservoirs, the hydraulic characteristics of flow inside the bottom outlet were investigated. The velocity and volume of air pockets, the internal pressure inside the pipeline and the swirling flow velocity profiles and water free-surface level in the upper reservoir during the generation of blow-back and blow-out phenomena were measured. The blow-back phenomenon occurs at flow discharges of 8.5 ? Q ? 18.5 and a range of Froude numbers between 0.2 and 0.8. While the blow-back creates, by increasing the swirling parameter by about 1.43, dimensionless air pocket velocity increases. Increasing the upper reservoir water head tends to increase the volume of air pockets with an increased velocity in both blow-back and blow-out flow regimes. By increasing the lower reservoir submergence up to 3.78, the trend of dimensionless air bubble velocity is ascending, however, by increasing the downstream submergence depth up to 3.97 the air pocket velocity decreases. By generating the blow-back phenomenon, the internal pressure inside the bottom outlet decreases, whereas it increases while the blow-out generates. Furthermore, by increasing the water depth at the downstream reservoir, the average internal pressure applied to the pipeline increases by about 20%. Applying the dimensional analysis and examining different relationships, the best correlations for the estimation of air bubble velocities in both blow-back and blow-out flow regimes were proposed using the statistical software. Key words: Air bubble velocity, Air-water flow, Blow-back, Blow-out, Bottom outlet, Pressure fluctuations, Two phase flow.