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Investigation of multi-physics problems such as flow-structure interaction (FSI) in free surface is very important in mechanical engineering, whereas numerical simulations of such problems have been widely conducted by researchers. The implementations of CFD in engineering applications are most of the time based on the Eulerian description. In this method, one can focus on flows at a fixed spatial point x at time t and any flow variable Φ is expressed as Φ (x, t). This description has been studied for over fifty years and is clearly understood. Most of commercial codes have been developed by using finite difference, finite element and finite volume approaches. Simulating free surface flow with most Eulerian CFD methods is potentially very difficult as explicit treatment of the free surface is required. Moreover, The problems of most Eulerian and mesh-based numerical methods for complex free surface deformations involves difficulties and complexities of various boundaries remeshing as well as moving boundaries and exact determination of free- surface fluid. Another description of study of CFD is the Lagrangian method where one can follow the history of an individual fluid parameter through the time. In the Lagrangian methods, any flow variable is expressed as Φ (x0, t), where the point vector x0 of the particle at the reference time t = 0. Smoothed Particle Hydrodynamics (SPH) is a meshless and fully Lagrangian method which is able to simulate the FSI problems due to its simplicity and capability, as there is no special treatment needed for the free surface. The current problem in hydrodynamic science and fluid engineering is studied as a complex phenomenon in free-surface flow. Smoothed Particle Hydrodynamics (SPH) is a flexible Lagrangian and meshless technique for CFD simulations initially developed by Lucy (1977) and Gingold and Monaghan (1977) to simulate the nonaxisymmetric phenomena in astrophysics. In recent years, the SPH method has been very popular in fluid mechanics, e.g. multiphase flows,3 heat conduction,4 underwater explosions, free-surface flows, etc. In this method, each particle carries an individual mass, position, velocity, internal energy and any other physical quantity. The Lagrangian nature of SPH would lead this method to be well suited to problems with large deformations and distorted free surfaces. Simplicity, robustness and relative accuracy in comparison with other numerical methods are the main advantages of using SPH.10 Moreover, the SPH method can handle fully nonlinear, multiply-connected free-surface problems and extend computations beyond wave breaking, which need complex treatments in other grid-based methods, e.g. Volume of Fluid (VoF).In this approach the computational domain is formed by a set of particles. Each particle represents macroscopic volume of fluid conveying information about the mass, density, pressure, speed, position and the other parameters related to the nature of the flow. However, the computational cost is a disadvantage of SPH because the time step is small because of the explicit integration scheme in a weakly compressible formulation. This method has been successfully applied to a range of free-surface problems which involve breaking and splashing up There is a choice of SPH formulation in the literature mostly expressed in weakly compressible forms where pressure is obtained from the equation of state In this research, SPH is used to investigate the flow-structure interaction in free surface. First, the simulation of dam break problem on a dry and infinite bed is shown and compared with the experimental data. Then, and after implementing the governing equations, the vibration of a beam is studied. Finally, the dam break problem on an elastic gate is shown. Comparison between the SPH results and available numerical and experimental data shows that the SPH method is useful method for simulating the FSI problem.

Keywords: Computational fluid Dynamics, Smoothed Particle Hydrodynamics, dam break problem, Flow-Structure Interaction.

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