Design of submarine outfall systems under internal and external flow

In this age and time, elimination of the urban and industrial liquid waste has become important by paying attention to environmental and human health. Discharging of waste water to the sea after pretreatment has been a current issue due to high cost of the systems after pretreatment. This method which is also widely used in our day is based on giving the waste water by diluting in receiving environment and using sea discharge systems from one point that is deep and far from the seaside to the sea environment. In the scope of this study, main pipe of the diffuser, risers and outlet ports, inner and outer environments forming the discharge system which is used in application are modeled by ABAQUS finite elements program. This solution which is performed by finite elements method is named as integrated analysis. The last two spans of the system (20 m) and four outlet ports on these spans are investigated. While the diameter and geometry of the risers and outlet ports of the diffuser remain constant, the diffuser pipe is modeled in three different ways. These are constant, stepped narrowing and gradually narrowing sectioned models. While diameter of the two-span main diffuser pipe is 0,50 m in constant sectioned model, diameter of the main pipe is 0,50 m in the first span and 0,40 m in the second span in stepped narrowing sectioned model and beginning diameter of the first span in 0,50 m and ending diameter of the second span is 0,40 m in gradually narrowing sectioned model. Diameters of the risers and outlet ports are taken as 0,13 m in all models. Outlet ports are circular (bell-mouth). Pipe thicknesses of the models are 0,01 m. Provision of the diffuser flow conditions according to geometric properties are verified by DIFDES code. A rectangular prism geometry which has 20 m height, 25 m length and 1,50 m wide is created to model the outer flow. The solutions are performed according to wavy environment as well as stationary environment assumption which is prevalent in the literature in outer flow model. Change in outer velocities of the diffuser according to stationary and wavy outer environment and effect of outer flow on inner one are investigated and the results are compared. Linear Wave Theory is used in wavy outer model. While fluid has some effects on the pipe, the pipe has also some effects on fluid in the solution of the fluid-structure interaction problems. Fluid-structure interaction shall be analyzed together. While fluid problem is analyzed in one environment, the pipe problem can be analyzed in another one. However, behavior of the structural system and movement of the fluid is not evaluated together in this case. Rigidity of the pipe is not effected against change in inner pressure values of the fluid. Two ways as separated and integrated analyses are performed in this thesis study. In separated analysis, rigidity of the pipe is not effected by the fluid and fluid forces are calculated by a computer code (HYDRODIF) and transferred to finite elements program (SAP 2000). In separated analysis, outer velocities of the diffuser pipe and inner-outer pressures are obtained by HYDRODIF code, diffuser displacements and Von Mises stresses are obtained by SAP 2000 program. In integrated analysis, fluid and diffuser are modeled by ABAQUS finite elements program in one single interface, fluid-structure integration is provided and the results which are obtained after several solutions in separated analysis are obtained in the same analysis. The relationship between the separated and integrated analyses is stated and suggestions are proposed. Keywords: ABAQUS (Finite Elements Method), external flow, fluid structure interaction, internal flow, submarine outfall diffusers.