Scognamiglio, Raffaele (2017) Prediction of the propulsion performances of planing stepped hulls: CFD in support of experimental towing tank tests. [Tesi di dottorato]

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As it is clear, nowadays, the Computer Fluid Dynamics (CFD) has become a fundamental support for the hydrodynamic investigations in order to perform detailed analysis and to reduce the number of more expensive towing tank tests, which are, however, always necessary alongside for validation of numerical results. Indeed, for CFD applications in ship hydrodynamic field it is well known that the numerical simulations of high speed planing hulls are significantly less reliable those regarding displacement hulls. In particular for the stepped hulls, the physics of the hydrodynamic field is quite complex, more than for planing hulls; This is why it is important to perform a comprehensive approach for verification and validation (V&V) methodologies and procedures in order to obtain high-quality results of CFD simulations. In the first part of this thesis, a new systematic series of eight hull models derived by one stepped hull is developed. In detail, the factors considered crucial for the experimental tests and for the physics of the hydrodynamic field are changed for each of the above models. The experimental tests are carried out in towing tank, through the “down thrust” methodology, to investigate the hull performances i.e., total resistance, longitudinal trim angle, sinkage, and wetted surface. Moreover, a detailed investigation of the flow phenomena in the separated region behind the step is carried out on one of the eight hull models, which has a single-step. In the second part, for the same single-step hull model, an assessment of the accuracy and effectiveness of different simulations setups and techniques is performed, with particular attention to the different techniques of moving mesh, such as the overset/chimera grid and morphing mesh. Afterwards, the V&V study is performed for one hull model belonging to the systematic series, and the Unsteady Reynolds Average Navier Stokes (URANS) code results are validated by using benchmark experimental data. The analysis of grid independence, iteration, time-step, and statistical convergence analysis for measured variables, performances i.e., total resistance, longitudinal trim angle, sinkage, and wetted surface, is performed by using the deterministic methods available in the literature for uncertainty estimation. Results of this work show that the numerical results are in good accordance with the experimental data, and the overset/chimera grid is found to be the best approach between the analyzed ones. In addition, another contribution of this work is the detailed reading of the vortex structures in the unwetted aft body area behind the step (within the air region), and their development into the downstream water flow. The flow patterns observed in numerical test through Large Eddy Simulations on a very refined grid, appear similar to the ones observed in towing tank investigations through photographic acquisitions.

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