USING GROOVED SURFACES TO IMPROVE THE EFFICIENCY OF AIR INJECTION DRAG REDUCTION METHODS IN HYDRODYNAMIC FLOWS

A summary of experiments using grooved surfaces to trap and hold (via surface tension forces) an injected airstream in a low-speed (1.25 to 5 m/s) water flow is presented. The purpose of creating a low-volume n ear-wall air sheet is to possibly enhance the efficiency of current ai r injection drag reduction methods in terms of unit gas volume per % d rag reduction. Flow visualization and preliminary quantitative data ar e included for a laminar channel flow, a disturbed laminar channel flo w, and a flat plate turbulent boundary-layer flow. A stable convecting low-volume, near-wall gas film is produced in several instances. Groo ve dimension and the presence of anti-wetting surface coatings are sho wn to greatly affect the formation and stability of the gas sheet. Dee per, narrower grooves, anti-wetting surface coatings, and shallow-angl e gas injection increase the stability of the attached gas layer. Conv ected disturbances are shown to increase the interfacial instability o f the attached sheet. It is not known if a gas sheet can be held under a turbulent boundary layer over 3 m/s, or if the groove sizes needed to do so would become too small to be of use in a practical high-speed hydrodynamic flow.