Numerical investigations of the effects of ocean water waves on the structures and also the devices designed to capture energy from waves, primarily need proper generation of desired water waves with Specific features. In this study, a numerical method for generation of nonlinear irregular waves is proposed for viscous flow simulations based on Navier-Stokes equations. The numerical method is based on a control-volume approach where a two-step projection method is used to solve the governing equations. In this regard, the motion of the flap-type wavemaker inside the water is simulated using the fast-fictitious domain method, the VOF method is used to capture the free surface evolutions and high-viscosity regions are employed to damp the reflecting waves. First, various methods of wave generation for the numerical wave tanks, available in the literature, are reviewed and next, three waves with different wave steepness are simulated to demonstrate the capabilities of the proposed method. Results show that the method can effectively produce irregular waves, from linear waves up to the steep nonlinear ones. Furthermore, shallow to deep water waves can be generated with reasonable accuracy using the proposed method.