The numerical investigation of the passive hydrodynamic performance of the manta ray beneath the water surface is presented. The main aim is to investigate the effect of attack angle on hydrodynamic forces of the manta ray near the water's surface. Three dimensional incompressible Navier-Stokes equations are resolved with RNG k-e turbulence closure model. The undulation of the free surface is captured by the volume of fluid (VOF) method. Numerical simulations are carried out with the manta ray oriented with the longitudinal axis at attack angles of -5, -2.5, 0, +2.5 and +5 degrees with various swimming speeds ranging from 0.5m/s to 4.5m/s. The drag coefficient and the lift coefficient of the manta ray are measured and compared. Results show obvious differences between negative and positive angles of attack. Additionally, the deformation of the free surface induced by the gliding manta ray is also captured and compared.

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