Experimental and numerical modeling of solitary wave loads on horizontal circular cylinders

Abstract

The present thesis deals with an experimental and numerical study on the horizontal and vertical hydrodynamic forces induced by solitary waves on submerged horizontal circular cylinders. Laboratory tests were performed in the wave flume of the University of Calabria. A battery of pressure transducers was mounted along the external contour of a cylinder while four wave gauges were located close to the cylinder. The correct displacement of the wavemaker was checked by an ultrasonic sensor located behind the paddle. A number of 134 experimental tests were conducted in the wave channel taking into account different wave attacks and five depths of the cylinder location ranging between half water depth and the bottom of the flume. From the numerical viewpoint, two different numerical models were adopted. The first one is the diffusive weakly-compressible Smoothed Particle Hydrodynamics (SPH) model. To improve the results and prevent spurious flows near the cylindrical contour, a packing algorithm has been applied to initialize the SPH fluid particles. The acoustic components occurring in the numerical pressure field were filtered through the application of Wavelet Transform. The numerical simulations provided to investigate in detail the flow field near the cylinder not modeled by the laboratory investigation. This Lagrangian model was used only in the case where the cylinder was placed at half water depth. The high time consuming of the SPH simulations led to adopt another numerical approach. In this context, the Eulerian OlaFlow model was used to investigate the other four depths of the cylinder. With respect to the experimental tests, additional numerical simulations were performed to extend the range of the analysis. Considering all the five positions of the cylinder, a total of 176 numerical simulations were carried out. The good agreement between experimental and numerical forces and kinematics at the cylinder has allowed the calibration of the hydrodynamic coefficients in the Morison and transverse semi-empirical equations by different time-domain methods. The present thesis has showed an alternative method (Gurnari and Filianoti, 2017) to assess the horizontal forces. Based on the concept that a solitary wave is subjected to a slowdown passing over the cylinder, this formulation was used after the experimental calibration of the speed drop factor. In this work, an extension of the transverse for-mulation which considered a new form of the lift force was also presented. For two specific depths, this formulation resulted necessary to model correctly the peaks and the phase shifts of the vertical forces. The experimental and numerical analysis were presented comparing the time variation of the experimental and numerical simulations for two test cases at each depth. The overall analysis of the peaks forces was evaluated as a function of the wave amplitude. In addition, the weight of the different force components, i.e. drag, lift and horizontal and vertical inertia, was evaluated and analyzed with respect to the maximum values of the horizontal and vertical force. The time variation of the horizontal forces calculated by the Gurnari and Filianoti (2017) solution was compared with the experimental ones for the two vertical extreme positions of the cylinder. The comparison between the experimental and the Gurnari and Filianoti (2017) equation was performed in relation to the horizontal force peaks.