Fluid-structure interaction during hydraulic transients in pressurized pipes

The aim of the present research is to identify, describe and quantify the principal mechanic-hydraulic relationships during hydraulic transients in pressurized pipe flows in view of improving pipe design and reduce pipe and system failure. Phenomena affecting the transient wave, such as fluid-structure interaction, unsteady skin friction, dry friction or pipe-wall viscoelasticity are analysed from both the experimental and numerical standpoints. The main goal is the improvement of one-dimensional (1D) waterhammer modelling in the time-domain by means of the well-known method of characteristics approach. Experimental work is presented for three different experimental facilities: a straight copper pipe, a coil copper pipe and a coil polyethylene pipe. The analysis of the experimental data highlights differences in the response of each system in terms of wave shape, damping, and dispersion. The straight copper pipe behaviour is highly dependent on the pipe supports and anchoring; the coil copper pipe to the deformation in the radial direction; while the polyethylene facility to the pipe-wall viscoelasticity. In a second stage, the research focuses on the numerical modelling of hydraulic transients in pipe coils. The analysis is based on the experimental data collected in the coil copper pipe facility. First, a structural analysis is carried out for static conditions and then for dynamic. A four-equation model is implemented incorporating the main interacting mechanisms: Poisson, friction and junction coupling. The model is successfully validated for different flow rates showing a good performance of the dynamics of the coil behaviour during hydraulic transients. Finally, the research focuses on the straight copper pipe facility, for which the simplicity of the set-up allows deepening on the basic modelling assumptions in fluid-structure interaction. First, friction coupling is assessed using the basic four-equation model and unsteady skin friction and dry friction are incorporated in the solver. The analysis shows the dissipative effect of dry friction phenomenon, which complements that of skin friction. In a second approach junction coupling is tackled and the resistance to movement due to inertia and dry friction of the pipe anchor blocks is analysed. Numerical results successfully reproduce laboratory measurements for realistic values of calibration parameters. The work successfully identifies, describes and quantifies different physical phenomena related with FSI by means of experimental modelling and valid numerical reproduction of experimental results. Experimental modelling approaches are developed and data is made available for benchmark testing of numerical tools considering facilities with different set-up geometries and materials. A new standpoint based on pipe-degrees-of-freedom is suggested for facing FSI problems, the structural behaviour of pipe coils is successfully described and FSI in straight pipelines is analysed focusing on both junction and friction coupling. A new set of numerical solvers are developed, presented and thoroughly discussed, which can be readily used for the design of new industrial piping systems or the safety assessment of existing piping facilities.