Numerical resistance calculation of Darpa Suboff submarine and new submarine models by using computational fluid dynamics (CFD)

Lately, most researchers prefer using computer softwares instead of doing experiments of models. For this reason, software technology has been increasingly improving and researchers use these softwares to make necessary calculations about their work models. In other words, the more the use of software becomes popular, the more the technology advances. These advanced softwares are useful for academic researches particularly due to relatively longer time required for model experiments. Numerical modeling on the other hand enables researchers to make faster decisions. Moreover, experiment tools and equipment can be both expensive and require longer labour. Researchers know that the model experiments are more accurate and more reliable than other methods. However, using software is more practical than doing model experiment at the preliminary study stage. The resistance is the force that arises due to the motion of an object inside a fluid. Calculation of the resistance is probably the most significant parameter in the long process of designing a submarine. Therefore, obtaining an optimal form in terms of the resistance characteristics is an important stage. Designers use several methods to calculate the resistance force. These methods consist of three main approaches: first, use of data from previously designed similar forms, second use of results from model experiments, and finally use of numerical techniques via available softwares. The method of using data from previously designed similar forms has the disadvantage of obtaining accurate results for the exact form in hand. Despite of the fact that the experimental model tests give the most reliable resistance values, the designers tend to use softwares because of the realtively higher expense and duration of conducting experiments. Fast numerical solutions by the use of softwares provide a convenient way of analyzing the resistance characteristics of different types of submarine forms. For this reason, an optimum submarine form in terms of resistance characteristics may be obtained by repeatedly using a software for different forms. After calculating the resistance values of a generic submarine form, by introducing relatively minor changes to the shape, an optimum submarine form with least possible resistance could be achieved. The resistance characteristics of a vessel is particularly important as it determines the fuel consumption characteristics of the vessel for its entire life of service. Therefore, a fuel-efficient optimum hull form is a quite essential problem to solve. For practical purposes then employing a software to calculate the resistive force of a submarine is an acceptable approach. Thanks to Computational Fluid Dynamics (CFD) methods, researchers can obtain important relatively reliable information about their model at the design stage. Thus, virtually unlimited data may be obtained for models without resorting to costly and time consuming model experiments. Therefore, CFD methods are most preferred by designers. Studies are usually conducted by first performing tests for the numerical results. For a form with known experimental data the numerical computations are done and compared with experimental values to determine the reliability of the numerical results and to fix the adjustable parameters, if needed. Following such tests the calibrated numerical model may be run for other forms. As the title of this work implies the subject matter of this thesis is to obtain an optimum submarine hull form in terms of resistance characteristics by introducing slight variations to a given generic submarine form, preferably built before. It must be emphasided that the form of the intended submarine has no rudder, appendances, and propeller. It consists of an entirely bare hull form. In the first chapter, information about the previous studies first is given. The experinmental resistance results of the Darpa Suboff model, which is the selected generic model in this work, are then compared with the numerical calculations. The comparisons indicate around 10% of differences between the two approches. However, such differences are bot inevitable and acceptable for the purposes of comparing different forms. Although the absolute results do contain some errors these errors approximately the same for all the forms tried therefore a comparatively better hull form may be selected by comparing the resistance values of these different forms used. This simple approach is basically the method followed in this work. In the second chapter, Darpa Suboff model form is generated from the equations given in the previous studies with the help of Rhinoceros modeling software. The hull form is divided into four parts due to its axisymmetric form. After this division, new forms are generated by introducing slight variations to the bow and the stern patterns of the selected Darpa Suboff model. Basically, three different bow forms and three different stern forms have been generated. These different bow and stern patterns are combined with each other so that a total of 9 new submarine forms are created. After generating the new forms, the control volumes are drawn for all of them. The control volumes are then imported into ANSYS Workbench software. Then, the mesh generation is carried out by the use of ANSYS Workbench software. In the third chapter, the control volumes with generated mesh systems are imported to the ANSYS-Fluent software. SST k-ω turbulence model is chosen in order to model the turbulence in the flow. SST k-ω turbulence model is based on k-ε and k-ω turbulence models. This selection means that the turbulence model acts as k-ω model near wall regions and k-ε model faraway the wall regions. The flow speeds are set as 5.14 m/s, 6.10 m/s and 7.16 m/s as taken from the published experimental data for making comparisons of different cruise velocities. Momentum, turbulent kinetic energy and specific dissipation rate are calculated by using second order upwind scheme. After these steps, the resistance of Darpa Suboff model has been calculated by Ansys-Fluent. Results of calculations are given in tables and graphs are drawn. Then CFD results are compared with the published experimental measurements of the Darpa Suboff model. For the velocity 5.14 m/s, the difference is about 8%, for velocities 6.10 and 7.16, the difference is slightly higher and about 9%. These differences are deemed to be within acceptable limits. Afterwards, using the same cruise speeds of 5.14 m/s, 6.10 m/s and 7.16 m/s, the resistance values for the newly created submarine forms are calculated again by using the same CFD programme with previously set parameter values. The obtained resistance coefficients are shown in the graphs. In the fourth chapter, all the resistance values of the newly generated and generic submarine forms have been compared. According to these comparisons made based on the numerical calculations, it is observed that the "Form 13" is the best among all. It is inferred that if the model experiments are to be done for all the generated forms, the "Form 13" is expected to perform best hence give the optimum form sought in this study.