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Resistance estimation of ships using GEKO turbulence model in ANSYS Fluent

A ship's resistance or drag at a given speed is defined as the force required to tow the ship at that speed in calm water. Resistance of a ship can be estimated using towing tank experiments, empirical formulas, and computational methods, of which the most reliable is the towing tank method. The experiments in the towing tank suffer from one major drawback, i.e., the scaling effects because of the non-feasibility of meeting the dynamic similarity condition of keeping the Reynolds number (Re) the same for both the model and the prototype. Furthermore, because of the high costs, manpower requirements, and time constraints, the feasibility of exploring various hull parametric designs using a towing tank is not viable during the initial ship design phase. These days, ship design offices employ computational fluid dynamics (CFD) for resistance estimation and optimization studies as it is financially feasible and less time-consuming. Usually, two-equation turbulence models like k-ε and k-ω are used in turbulence modelling in CFD resistance studies. The 2-equation turbulence models are the workhorses for the CFD studies; however, with a plethora of turbulence models now available, the designer is in a dilemma as to selecting the most optimum turbulence model. These 2-equation models have usually five coefficients, which have been calibrated based on the physics of the flow over a flat plate boundary layer, some selected free shear flows (like plane mixing layer, plane jet, etc) and decaying turbulence in free stream with the assumption that flow simulation over all other bodies can be carried out with these set of global constants, which may not always be the case.To overcome these lacunae and come out with a 'universal' 2-equation turbulence model, ANSYS Fluent has developed a new approach for 2-equation turbulence modelling, the Generalized k-Omega (GEKO) model, which modifies the k-ω model by including free parameters. The GEKO model offers six free parameters: two are for tuning wall-bounded flows, two for the calibration of free shear flows, one coefficient to improve corner flow simulations (comer separation), and finally, a curvature correction term. The setting of these free parameters does not affect the law of the wall calibration. The six free parameters can be thought of as switching functions that can change flow behavior by formulating flow attributes that can have specific changes in the turbulent and free stream regimes. Though the exact structure of these switching functions is proprietary, the effect of the free parameters on the flow characteristics is identifiable.This paper aims at the calculation of resistance of DTMB 5512 using GEKO turbulence model. The volume of fluid method is used to model multiphase flows to estimate resistance in ANSYS Fluent. Numerical simulation studies on the two ship models are carried out by tuning the free parameters of the GEKO turbulence model. The results obtained are validated with the experimental tests to get the most optimum free parameters in the GEKO model for ship resistance.