Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019 3.2


RESULTS OBTAINED FROM THE DRAG FORCE VALUES


The drag force values of the SALINA at the desired speeds in both laden and ballast conditions (drafts of 8 and 16.5cm) were extracted from the post-processing section of the software. The values are indicated in Table 6 and Figure 14.


Table 6. Drag force values of the Saline tanker at desired speeds in both laden and ballast conditions Draft (cm)


Ship’s speed (m/s) 0.643


8 16.5


4.2928 (N)


6.36 (N)


4.876 (N)


7.28 (N)


0.6945 0.7459


5.53 (N)


8.31 (N)


0.7974 0.8488


6.17 (N)


9.4 (N)


6.89 (N)


- 3.3 VALIDATION CFX. Drag Result - Calm Water Simulation / Salina 10


0 5


0.6 0.65 0.7 0.75 Speed ( m/s ) Balast ( 8 m. draft )


Figure 14. Drag force variation in response to speed and draft variations


0.8 0.85 0.9


Comparing the results of previous studies, predicting the numerical and laboratory values of a ship drag force revealed that the calculated drag of the ship’s hull in the towing tank, including frictional resistance and wave resistance, provided a more realistic estimate (Noblesse, et al, 2013 & Huang, et al, 2013). The computational error of this method for a large part of ships with different weights has been considered to be within the acceptable range of 10% (Yang, et al, 2013). The results of a previous research on a bulk carrier indicated that the results of numerical simulation using the ANSYS FLUENT software and results of the experimental model at low and high speeds presented differences up to 5 and 13%, respectively. Moreover, the CFD-derived drag force value has always been greater than the values obtained from the experimental model of the bulk carrier (Ebrahimi, 2012). Table 7 compares the results of the numerical modeling and experimental simulation of SALINA, which are in line with the results of previous studies. In the case of 16.5cm draft, the difference between the drag force value of the numerical and experimental model in its maximum status is about 7%; however, this difference was observed to be 15% in the 8 cm draft. The reason for the observed error may be related to the weakness of the CFX software during simulation of the wave resistance of the model at higher speeds. There was less difference at lower speeds. In both drafts, the drag force value obtained from the numerical model was always higher than that of the experimental model (Figure. 15). Furthermore, the trend of results obtained from the experimental model tests and CFD simulation tests in the draft of 16.5cm were more consistent than the results obtained in the 8cm draft.


10 12


Table 7. Differences between drag values calculated in the experimental and numerical models (CFX) Speed (m/s) 0.64 0.69 0.75 0.80 0.85


Draft of 16.5cm.


Model & CFX deviation (%)


Draft of 8cm.


Model & CFX deviation (%)


Exp. model 6


5.7 6.76 7.79 8.87 10.79 CFX 6.36 7.28 8.31 9.4 7.1 6.3 5.6


Exp. model 4.14 4.62 4.68 5.49 5.92 CFX 4.29 4.87 5.53 6.17 6.89


3.5 5.1 15.4 11 14.1


0 2 4 6 8


0.6 0.7 0.8 Speed ( m/s ) Loaded ( Exper. )


Figure 15. Comparison of the total resistance values of SALINA presented by the CFX and the experimental model


The results of a study conducted by Barrass (2004) addressing the very large crude carriers reveal that the ratio of frictional resistance to total resistance or drag is about 90%. Equation (4).


= %90


(4) 0.9


Drag - Speed curve / CFX. Result & LAB. Results Comparision


A-464


©2019: The Royal Institution of Naval Architects


Drag ( N )


Drag ( N )


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