A Study of Flow over Ventilated Discontinuities in a Boundary Layer

At present hydrodynamicists are critically interested in ventilation as a means of vessel guidance and control. The injection of air into the boundary layer can also produce significant drag reduction benefits and cushion an abruptly changing flow boundary against the severity of cavitation damage in high velocity flows. This report describes an experimental investigation of the characteristics of ventilated cavities situated in a boundary layer and their effects, including drag reduction. Study cases include free surface flows over ventilated boundary discontinuities in the form of a step, a slot, and a break-in-grade. The observed cavities are generally similar to those found in the wake of supercavitating hydrofoils. The quantitative results refer to cavity length, air supply coefficient, and ventilation parameter, The maximum drag reduction obtained with a ventilated slot in a boundary layer over the smooth surface in the fully wetted case was on the order of fifteen per cent. The average flow velocity for the drag reduction experiment was from ten to nineteen feet per second. Empirical cavity results are also compared with irrotational linearized potential flow theory in the appendix. Only the step experiment showed good agreement with theory.