The interaction between water waves and artificial rectangular bars under Bragg scattering is investigated both numerically and experimentally. A 2-D Reynolds Averaged Navier-Stokes (RANS) model is applied to simulate the entire vortex evolution process as water waves pass over a series of artificial bars. Meanwhile, the Particle Image Velocimetry (PIV) technique is used to experimentally measure the flow field. The numerical model is therefore validated through the comparisons of water elevations and velocity field with the measurements. A good agreement between the calculated results and the measured data is found. Based on the present results, the mechanism of vortex generation and dissipation due to flow separation is analyzed.
In the past decades, the phenomenon of Bragg scattering as waves propagate over a patch of bottom ripples has been investigated by means of experimental, theoretical, or numerical methods. Their results indicate that the mechanism of a resonant Bragg reflection occurs as the wavelength of the bottom undulation being one half the wavelength of the surface gravity wave (Davies and Heathershaw, 1984; Mei, 1985). For a practical application on coastal engineering, a series of artificial bars could be installed to protect shores or offshore structures from wave attack with the concept of Bragg scattering (Mei et al., 1988; Kirby and Anton, 1990). Nevertheless, both the numerical and theoretical studies are made based on the potential flow theory in which the viscosity and rotational effect of fluid is neglected (e.g., Dalrymple and Kirby, 1986; Kirby and Anton, 1990; Hsu and Wen, 2001; Hsu et al., 2003). In recent years, the Navier-Stokes equations based numerical models and their derivatives have been developed to investigate the interaction between the periodic waves and the obstacles. Huang and Dong (2002) simulated the propagation of water waves over rigid ripples by solving Navier-Stokes equations directly.