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Numerical simulation of cushioning problem for blunt bodies using boundary element method

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Barjasteh M. , Zeraatgar H.

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tom S 1

85--93

EN

Induced air pressure and resulting free surface profile due to air cushioning layer is studied. The study is mainly focused on 2D blunt circular bodies with constant downward speed. The problem is first solved for the air flow between the body and the free surface of the water. Then the results are employed to solve the problem for the water problem, numerically. Both air and water problem are assumed to be governed by Laplace potential equation. Depending on the induced pressure and velocity of the escaping air flow from cushioning layer, compressibility of the air is also included in the modeling. Gravitational acceleration is also included in the model. An iterative boundary element method is used for numerical solution of both air and water problems. Instantaneous pressure distribution and free surface profile are evaluated for different bodies. The results of calculation for large blunt bodies show that inviscid potential method can fairly approximate the problem for large blunt bodies. Additionally, the behavior of the air pressure for the very blunt body is impulsive and the magnitude of the peak pressure is in order of impact pressure of water entry. The obtained results are compared with analytical method. The comparison shows that as the bluntness of a body increases, the better agreement is concluded.

2

Parametric comparison of rectangular and circular pontoons performance as floating breakwater numerically

100%

Tabatabaei S. M. R. , Zeraatgar H.

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nr Special Issue S1

EN

Rectangular and circular pontoons are one of the most widely cross-sections used as floating breakwaters (FB). Although, there are several articles on comparison of behavior of rectangular and circular floating breakwaters however, the Authors try to show some details of difference between these two types where they have not been addressed before. To do so, transmission coefficient (Ct), as a measure of merit, of similar rectangular and circular sections is numerically compared. A computer code is developed for two-dimensional hydrodynamic analysis of floating breakwater based on diffraction theory in frequency domain in regular waves with any configuration of mooring line. The numerical method is the finite element method and validated by comparing with experimental and numerical results. Three types of rectangular sections are defined equivalent to circular section and a numerical comparison is made between 100 similar sections. The Ct versus wave frequency has been considered in detail and three new points called LMinF, LMaxF and LMaxCt are introduced. It has been shown that LMinF and LMaxF of circular section are greater and LMaxCt is much smaller than equivalent rectangular section. The LMaxCt of both sections are very dependent to new non-dimensional parameter B/D (Breadth/Draft). Although, rectangular sections are more common for floating breakwater, however the results of this study show that possibility of using circular sections must be also considered

3

ANAFIS and neural network for modeling and prediction of ship squat in shallow waters: a new approach

80%

Zeraatgar H. , Akbari Vakilabadi K. , Salmalian K. , Haghi A. K.

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tom Vol. 16, no 1

211-231

EN

The reduction of the distance between ship floor and seabed, while the ship is moving forward, is called squat. In this research, squat is determined for vessels with Series-60 hull forms in various depths by experimental methods and then different numerical methods are employed for squat modeling. For this reason, a set of facilities for testing the ship movement in shallow waters is prepared. A series of models of the vessel is manufactured and many tests are carried out. The aim of the present study is to demonstrate the usefulness of an adaptive-network-based fuzzy inference system (ANFIS) for modeling and predicting the squat parameter for ships in shallow waters. It is also shown how dimensionless squat (S*) varies with the variation of important parameters, namely: block coefficient (CB), dimensionless distance between the seabed and ship floor […] and hydraulic Froude Number (Fnh). The results obtained through the ANFIS are also compared with those of a multiple linear regression and GMDH-type neural network with multi-layered feed forward back propagation algorithm. The results show that the ANFIS-based squat has higher predictability function than other numerical methods.