Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019 2.4 BOUNDARY CONDITIONS


Figure 8. Accumulated areas of grid 2.3


GENERATION OF THE SOLUTION FIELD


In performing the analyses using computational fluid dynamics, the construction and quality of the computational grid generated for the model was of great value since it had great effects on the convergence, accuracy, and precision of the obtained results.


In the present analysis, one single solution field was used to construct an unstructured grid and to perform numerical analyses. The advantage of applying this approach was grid integrity and increased stability of the solution (Figures. 9 & 10).


First, the solution grid was called in the pre-processing section of the software. The boundary conditions were applied to the grid and the problem was about to be solved. For the entrance boundary, flow with the steady state was used as the boundary condition of the momentum equations. The boundary condition in the output of the grid was considered as the output flow type. In other words, distributions of the hydrostatic pressure in the water phase and the stable atmospheric pressure in the air phase were used as the boundary conditions for the output of the momentum equations.


Identical volume fraction profile was used in both the input and output boundaries. For turbulence equations, the development condition (zero gradient) was used. The free slip condition, whose shear stress was zero, was used for the side walls of the solution field and the upper and lower boundaries. In this case, the perpendicular velocity to the surface was zero and the tangential velocity was exactly equivalent to the calculated value in the first node after the wall. In the solver section of the software, the time step for free flow was selected based on the ratio of the length scale (length of the vessel) to the speed scale. The residual values were one of the most fundamental indicators in the convergence of repetitive numerical calculations, which directly determined the error rate in the solving equations. Over the time of performing all the equations, the convergence index in all numerical calculations was considered to be 10−4.


2.5


SIMULATION OF THE EXPERIMENTAL MODEL


Figure 9. The domain and it’s dimensions


Figure 10. Status of the domain’s boundary in relation to the ship’s model


The purpose of performing the model tests in the towing tank was to determine the drag force values of the SALINA and to compare the results with those of the numerical simulation. To meet the mentioned objective, the model of SALINA was designed and constructed at the scale of 1:100 and geometric accuracy of 0.05 mm with the length of 2.74 meters, width of 0.5 meters, draft of 0.17 meters, and height of 0.23 meters. The constructed model was in accordance with ITTC standards. To measure the drag force of the model, tests were performed in accordance with the numerical model in the towing tank at five different speed values ranging from 0.65 to 0.85 m/s (equivalent to the actual speeds of the vessel, i.e. 12.5, 13.5, 14.5, 15.5, and 16.5 knots). The process of conducting the hydrodynamic tests inside the towing tank involved determining test implementation scenario, preparing the model to perform the test, adjusting the force measurement system, installing the model into the towing carriage, performing the tests according to the predetermined scenario, extracting the data from the system, analyzing the data, and presenting the obtained results. The towing tank of subsea R & D center of Isfahan University of Technology (Figure. 11) was used to carry out the experimental simulation. The length, width, and depth of the mentioned tank were 108, 3, and 2/2 meters, respectively.


A-462


©2019: The Royal Institution of Naval Architects


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