Graduation of Giovanni Doni

11 December 2018 13:30 till 15:00 - Location: Lecture Hall A , Faculty of Civil Engineering and Geosciences - By: Webmaster Hydraulic Engineering

"Force characterization for a submerged velocity cap in unsteady flows"| Professor of graduation: Prof. dr. ir. W.S.J. (Wim) Uijttewaal, supervisors: Dr. ir. J.D. (Jeremy) Bricker (TU Delft), Dr. N.G. (Niels) Jacobsen (Deltares).

Coastal facilities such a desalination plant or a nuclear power plant need a continuous discharge of salty water to carry out a range of activities. The velocity cap is an intake structure that can be used to take in water from the sea. These intakes are open seafloor founded constructions mounted at a sufficient water depth. The action of waves and in general of unsteady flows is of primary importance when defining the design loads on such a submerged intake cap.

This thesis has the aim to investigate the nature of the forces and turning moment acting on the structure and to provide tools, in the form of hydrodynamic coefficients, that can be used to compute the design loads. These coefficients are only known from literature for simple geometries and defining them for the specific shape of the velocity cap can optimize the design.

The analysis was first based on experimental records collected during a previous campaign which included forces and surface elevation measurements as well as Particle Image Velocimetry (PIV). The measurements were then used to validate a CFD model in OpenFOAM. The focus was mainly on the loads generated by the passage of solitary waves which were simulated both experimentally and numerically.

The characterization of the flow field was based on the analysis of the inline velocity which was used to estimate the Keulegan-Carpenter number (KC) and the frequency number (????). The availability of PIV measurements allowed studying the development of the turbulent patterns around the cap during the passage of a solitary wave. Structure-induced turbulence is then shown to be fundamental to define the total loads on the structure in the numerical model. The use of a turbulence closure was in fact observed to be needed in order to come to a validation of the CFD model.

Once the CFD tool was validated, it was used to generate additional test cases on solitary waves and regular waves in order to expand the research. The velocity and load estimates of the numerical model were used to define hydrodynamic coefficients. The inline force characterization follows the theory of the Morison equation which is shown to provide a good fit in all analyzed cases. Vertical force and overturning moment signals were originally fitted with the formulas mainly used in literature which relate the variation of these two loads to the square of the horizontal velocity only. The mismatch between the fit obtained and the original signals suggested that the classical formulas needed to be expanded with one or more extra terms. The best fits for the vertical force are found with an equation that includes the effect of the horizontal velocity squared and of the vertical acceleration, while in the case of the turning moment the best fits are obtained with a combination of horizontal velocity squared, vertical velocity squared and horizontal acceleration.

The hydrodynamic coefficeints are obtained by means of weighted least squares method fitting and they are in many cases shown to have a dependency of the two definition of KC number used in this thesis for solitary waves and regular waves .