Boundary-layer transition on a shallowly submerged vehicle

As the underwater vehicle travels beneath the free surface, the surface gravity waves are generated, which may have a significant effect on the boundary-layer transition. The boundary-layer transitions on a shallowly submerged vehicle are investigated by coupling the transition model, wave system model, and volume of fluid model. The numerical method is validated through the experimental data and theory. It is found that the free surface has a significant influence on the boundary-layer transition of the upper surface, whereas no such influence is observed on the transition of the lower surface. On the upper surface, the wave system of the free surface results in a fuller velocity profile and a more extended region of the laminar flow, which causes a delay in the boundary-layer transition. The underlying mechanism of the transition delay phenomenon on a shallowly submerged vehicle is discussed. It is observed that the region of the favorable pressure gradient generated by the interaction between the bow and aft shoulder waves of the free surface becomes larger. Furthermore, as the submergence depth increases, the transition delay effect weakens rapidly, which is closely associated with the sharp reduction in amplitude of the free surface wave and in favorable pressure gradient.