Comparison between blade element momentum theory and computational fluid dynamics methods for performance prediction of marine propellers

This thesis illuminates the comparison of blade element momentum theory and computational fluid dynamics method on marine propellers. For this purpose, on the first part of the thesis, blade element momentum theory algorithm is implemented using MATLAB and XFOIL softwares. In order to fulfill verification and validation of blade element momentum theory algorithm, the practical analysis technique is applied to DTMB 4119 propeller, for which hydrodynamic characteristics and open water results are given in the literature. First, propeller blade geometry is subdivided into finite blade elements using piecewise cubic Hermitian interpolation method. According to Benini's studies on blade element momentum theroy, a tentative value for axial inflow factor is specified for the first iteration based on innermost or outermost section. Second, angular inflow factor (swirl factor) can be computed using ideal blade element efficiency. Slipstream and axial velocity components are computed using these factors. Pitch angles are obtained using pitch distribution along the blade. Angle of attack values are calculated using the difference between geometric and hydrodynamic pitch angles. Lift and drag coefficients per blade element could be computed using angle of attack, local chord distribution and local Reynolds number values. Third, thrust and torque effects per blade element are defined and corrected by Goldstein-Tachmindji correlation factors. Then, new axial inflow factor value is computed and this loop is kept on the difference between old and new axial inflow factors are reach up to better tolerance. Finally, obtained thrust and torque values for each radial strip are radially integrated over the blade. According to McCormick's linear, small angle approximation, a tentative value for induced angle of attack is specified for the first iteration. After the normalized induced velocity is computed, the new value of local induced angle of attack is calculated. This algorithm is iterated until the changes in the computed induced angle of attack become sufficiently small. Finally, thrust and power coefficients are computed when the convergence is reached. On the second part of the thesis, incompressible and viscous fluid flow around DTMB 4119 propeller is discretized using computational fluid dynamics method. On this study, open water simulations are implemented using OpenFOAM which is an open source, object oriented library for numerical computations in continuum mechanics, written in the C++ programming language. In order to perform open water computations using multiple reference frame technique, physical domain is constructed by rotating and static subdomains according to ITTC recommendations. Both subdomains are imported into Numeca Hexpress/Hybrid software and grid generation is done seperately. First, an isotropic Cartesian block grid is generated around the geometries. Second, the grid is refined in refinement regions by splitting the initial cell volumes. To avoid numerical errors arising from interpolation between non-conforming arbitrary mesh interfaces, the refinement cell sizes on interfaces are obtained approximately equal. Then, the volumic grid is snapped onto the model. Finally, so as to capture viscous effects, viscous layers are inserted according to the foreseen y+ value. SST k-$\omega$ turbulence model is chosen for near-wall treatment and effects of various turbulence models are neglected. Grid dependency is taken into account in order to satisy open-source CFD code algorithm. Coarse, medium and fine mesh levels are specified according to the refinement levels by terms of propeller diameter. Owing to the fact that steady time discretization scheme is chosen, SIMPLE (Semi-Implicit Pressure Linked Equation) formulation with consistent algorithm is used to improve convergence, accuracy and solution time. Finally, pressure coefficient distributions on specified sections from blade element momentum theory and OpenFOAM are compared with experimental results. Besides, numerically computed open water performance values are compared with PVL and experimental results. However, in this study wake adaption, nonlinear blade element momentum theory solutions and cavitation effects are neglected.