Force characterization for a submerged velocity cap in unsteady flows

Abstract

Coastal facilities such as desalination plant or a nuclear power plant need a continuous discharge of salty water to carry out their functions. Hereby an intake structure, such as a velocity cap, can be used to take in water from the sea. These intakes are open seafloor founded constructions mounted at a water depth ranging between 10 and 20 meters. In order to design such an intake cap in an offshore environment, the action of waves and in general of unsteady flows is the most important design load to be accounted for. This thesis has the aim to investigate the nature of the forces and turning moment acting on the structure in presence of waves and to provide tools, in the form of hydrodynamic coefficients, that can be used to compute the design loads. The analysis is based at first on experimental records collected during a previous campaign including force measurements and PIV recordings. The measurements are then used to validate a CFD model in OpenFOAM. Structure-induced turbulence is shown to be fundamental to define the total loads on the structure in the numerical model. The use of a turbulence closure is in fact observed to be needed in order to come to a validation of the CFD model. Even if the flow separation around the cap is not always accurately predicted, the peak of the inline force is estimated by the model with an error of 8%. In the case of the vertical load the error observed reaches up to 15% but part of the mismatch is attributed to a bias in the experimental records. The CFD tool is then used to generate additional test cases on solitary waves and regular waves in order to expand the scope of this research. The hydrodynamic force coefficients are defined fitting both the experimental records and the load estimates of the numerical model by means of the weighted least squares method. The inline force characterization follows the theory of the Morison equation which is shown to provide a good fit in all analyzed cases. Good agreement is found between numerical and experimental results with regards to the estimate of the inertia coefficient while the numerical estimate of the drag coefficient is up to 19 % lower than the experimental estimate. Vertical force and overturning moment signals are originally fitted with the most common formulas used in literature which however produced poor fits to the force and moment signals. New equations are suggested therefore to come to a better characterization of these loads. The best fits for the vertical force are found with an equation that includes the effect of the horizontal drag, the vertical drag and of the vertical inertia, while in the case of the turning moment the best fits are obtained with a combination of horizontal drag, vertical drag, horizontal inertia and vertical inertia.