The fluid dynamics around and inside an OWC-type wave energy device is studied using Direct Numerical Simulations of the multiphase air-water two-dimensional vertical Navier-Stokes equations. A schematic rectangular shaped OWC device placed against the vertical ending wall of the numerical rectilinear wave flume is considered. Owing to numerical constraints, reduced scale simulations are carried out with a focus on the near field of the device over a couple of wavelengths. The power take-off system is simply modelled by an opening through the roof of the device. A parametric study on incident wave period is performed to determine the fluid-structure interactions. Beyond the efficiency predictions, which agree fairly well with different values found in the literature, specific behaviours related with an enhancement of free-surface non-linearities around the resonance frequency are observed. Water flow vorticity is found mainly produced in the vicinity of the end of the semi-immersed frontal wall of the device. Significant energy dissipation results from shear-layer and vorticity associated with the air flow passing through the turbine opening. A one-dimensional vertical analytical model is used to compute the instantaneous bottom shear stress and the induced-bedload sediment transport rate which appears impacted by OWC device.