Sea waves are usually irregular and of random nature. Therefore, no two waves are of the same wave height and period and move at different speeds. Investigating the effect of sea waves on offshore structures as well as absorbing energy from sea waves requires a thorough recognition of the waves and their features. Using numerical wave substrates being developed over the last two decades is an effective technique to simulate waves in different situations.For the first time Havelock (1929) [1] and Hieu (1976) [2] assuming the non-viscous flow through using linear wave theory provided an analytical solution for flap and piston wave generators. However, Ursell (1960) [3], after conducting numerous experiments and examining waves with different sharpness, showed that the height of the waves generated by the piston wave generator is 10 percent less than the values obtained from analytical solutions based on the linear theory. Inability of analytical solutions for modeling waves in solid objects led high-order numerical models be presented based on the stream function wave theory to simulate the waves.In one of the leading research studies conducted in 2008, Lee and Elanggovan [4] modeled linear regular waves by finite volume method. In 2015, Finnegan and Goggins [5], using the same method and Fast Fourier transformation, modeled linear irregular waves.All afore-mentioned research studies used linear boundary conditions and problems are solved in a nonlinear form. Especially, in cases considering the wave contact with moving objects and absorbing their energy and when the drag force acting on the object cannot be ignored, linear boundary conditions will reduce the accuracy for the free surface. Thus, using non-linear theories to simulate waves and to track the free surface of the liquid is inevitable. For the first time in 1965, Harlow and Welch [6] introduced marker and cell (MAC) method for free surface flows. In 1970, Chan and Street [7] improved this method at the Stanford University Modified Marker and Cell SUMMAC. In 2005, Suea et al. [8], through solving the Navier-Stokes equations and boundary conditions by volume of fluid method, investigated the impact of non-linear waves on cubic structures submerged in the water.For the actual mechanism of generating waves, height and wavelength are a function of range and course of the wave-generator motion as well as water depth. To this end, the linear wave-generating theory proposed by Dean and Darimpel [9] in 1984 and second-order wave-generating theory proposed by Madson [10] in 1971 were employed. Accordingly, the development of a numerical model by assuming the viscous flow which models the actual conditions of the wave-generator seems necessary. In this study, Anbarsooz et al. numerical model [11] is used to model the real conditions of irregular nonlinear waves are produced flap wave generator.

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