An investigation into the effect of the aft form and propeller location on the propulsive efficiency of a blunt ship

In the last century, the public has begun to consider global warming and climate change phenomena, more than ever, with the increasing awareness and major changes which could be seen as proof. Yet, on the background of these phenomena's there was only one human made reason which is emission of hazard gases. Onwards, several international agencies whom could take precautions on emissions of hazard gases or the countries who dealed on a agreement had stepped toward control and management of emissions. An international organization, which is IMO (International Maritime Organization) , has imposed EEDI (Energy Efficiency Design Index) to manage emissions derived from marine based sources. As a result, the marine vessels had to manage their fuel consumption, so that the EEDI parameter would be acceptable. Hence, it can be seen that the easiest way to decrease fuel consumption is either by decreasing service speed or achieving a better hydrodynamic performance, thus the ship-owners had requested in new designs to maintain service speed and improvision of hydrodynamic performance. To define hydrodynamic performance, it may be related with dimensionless propulsion efficiency. This parameter, on the other hand,has taken an important role in design circle of a ship since the 1st cycle which is the initial design. This parameter is evaluated through consideration of the interaction between the propeller and the ship's hull via experiments, or computational disciplines. Additionally, the interaction is also affected by the hull form, propeller geometry also the axial distance between propeller and hull. In this perspective, the studies have been conducted on effect of the changes in the hull form and position of the propeller. First of all, in the section 1 the studies on variated hull form and position of the geometry from the literature has been presented. In the thesis, all studies have been conducted via utilization of Computational Fluid Dynamics (CFD). In accordance with this objective, the section two presents the CFD methods, as well as the parameters have been used in these. Additionally, in this section, Reynolds' Averaged Navier Stokes based SST k-w turbulence model and the SIMPLE algorithm which is relating the velocity and pressure has been explained in details. In order to use CFD applications on this problem, the methods which has been used should be verified via experimental data. In Section three the verification cases of utilized CFD methods has been stated. The verification cases consist analysis of a U aft profiled KVLCC2 (KRISO- now MOERI – Very Large Crude Carrier 2) ship which has been developed by MOERI (Maritime and Ocean Engineering Research Institute), in addition to that, verification process of the case has been completed by comparison of the experimental data, that can be found on literature searches, to the analysis performed with assumptions of the double model hull and infinite depth. Moreover, during the analysis of verification the DOF motions were ignored, thus trim and sinkage of the hull were not accounted for. Following the verification study, the dimensionless axial velocity value, limit stream lines, turbulence kinetic energy and specific dissipation ratio contours have been compared with the experimental outputs. Finally, by evaluating the mesh independency for double model KVLCC2 analysis, uncertainty due to the mesh has been presented and GCI value was obtained. The propeller to be used in hull-propeller interaction analysis has been selected to be KP458 (MOERI). The corresponding propeller was developed by MOERI for using on KVLCC2 hull, moreover the experimental data for open water characteristics is available in literature. Similar to hull form, the verification of the propeller analysis has been achieved by considering dimensionless thrust and moment coefficients, and these values have been compared with experimental data in order to show the compliance. Finally, mesh independency for the propeller in the case of open water analysis have been presented via evaluation of the GCI. In the third subsection of the section three, in accordance the objective of the thesis the new variations of the KVLCC2 was created based on the existing hull form. These variations have been made in the 25% of the hull on the aft side with the modifications made by shifting the section lines toward or away from the aft. The amount of the shifting of selected section lines has been decided to be α=Αx/LBP= 1.812×10-3 units, the first variation which was named to be "variation 1" was created by shifting section lines toward the aft, also the second variation is named to be "variation 2" was created by shifting the section lines in the opposite direction. While forming the new hull forms as variation 1 and 2, the main dimensions of the ship was preserved yet due to the shifting of the sections wetted area of the hull was changed by -0.145% and +0.12% respectively. In addition to wetted area, position of the LCB is also affected as referenced in mid-point of the LBP and positive direction is toward fore, moving from 3.48% to 3.474% and 3.485% for variation 1 and 2 respectively. In the mentioned process, the continuity of the waterlines and the hull was carefully checked, but the changes in the hull form was also effected the total resistance coefficient in variation 1 by +0.8% and variation 2 by +1.6% based on the original hull. Finally, beside the numerical evaluation, the comparison between these three hulls has been made through pressure and limit streamlines and the reason why total resistance was increased is explained. In the third sub section of the section mentioned above, effect of the axial position of the propeller and combination of these position with the variation of the hull was covered. Within the study, 5 combinations of the propeller axial position -hull variation was created by 3 hull variations including one original hull form. Change in the propeller axial position was made apart from the variation of the hull which is α=Αx/LBP= 1.812×10-3, in order to find out the effect of the propeller axial position relative to each variated hull. In this perspective the analysis case was formed to be; the original hull and original propeller position as Case 1, original hull and propeller shifted by α to the fore as Case 2, variation 2 and propeller shifted by α to the aft as Case 3, variation 1 and the original propeller position as Case 4 and, the original hull and propeller shifted by α to the fore as Case 5. In the fourth subsection of the section three, the computational domain and parameters for hull-propeller interaction analysis was covered. Additionally, propulsive efficiency was compared for the five cases which is formed to analyze the interaction. Hull-propeller interaction analysis was performed by load varying method also known as constant speed method. The varying load method, can be defined as change in RPS (revolution per second) of the propeller in constant speed of hull. Propulsive efficiency was evaluated in the self-propulsion point where equilibrium between hull and propeller occurs, which can be expressed in ΑS=RS-TB=0 clearly. Due to this, searching the self-propulsion point is essential step of evaluation of the propulsive efficiency thus results in an interpolated search method. In this method, three independent cases with different advance coefficient (J) was analyzed and a second order polynomial was fitted on data, so that the equilibrium condition for which ΑS equals to zero can be found. Further the advance coefficient in this point was indicated as JS, showing the self-propulsion point. Even though JS was found to be insignificantly different in quantitative, in the other parameters of the analysis the difference was clear and striking. Through an examination, it can be said that the difference in propulsive coefficient remained to be in a diminutive range of 1.5% for the cases which are consisting of Case 1, Case 2 and Case 3 where relative distance of propeller has kept to be same but the hull was variated. In contrast, Case 1, Case 5 and Case 2 where the hull kept as original but the propeller was shifted the change in propulsive coefficient increased promisingly by 4%. As a result, it can be seen that the variation in the hulls was not effective in optimizing the propulsive coefficient, rather than that, the position of the propeller was the effective parameter for optimizing propulsive coefficient. In the end of all examination, it can be said that the Case 4 was the best combination so far from the 5 Cases.